Comparison of heartworm antigen test kit performance in dogs having low heartworm burdens

Beyond Borders: Dirofilaria immitis Infection in Dogs Spreads to Previously Non-Enzootic Areas in Greece-A Serological Survey

Although Dirofilaria immitis in dogs is considered enzootic in northern Greece, the available data on the occurrence of infection in southern parts of the country demonstrate its scarcity. The aim of this study was to update the current knowledge on D. immitis infection in dogs in areas of Greece previously considered non-enzootic (Central Greece, Attica, Peloponnese, North Aegean, South Aegean, Crete and the Ionian islands). In total, 1528 dog blood samples were collected from the aforementioned areas and examined by Dirochek® ELISA (Synbiotics). Additionally, data published until 2022 on the prevalence of infection in these areas were compared to the data of this study. The 'Wilson' Score interval method (Epitools) was employed. Overall, 10.8% of dogs were positive for D. immitis. In detail, the prevalence was 21.7, 13.7, 10.7, 5.4, 4.7, 6.2 and 17.0% for D. immitis, in Central Greece, Attica, Peloponnese, North Aegean, South Aegean, Crete and the Ionian islands, respectively. Infection with D. immitis is recorded for the first time in Crete. The probability of a dog becoming infected has increased 4.1 times since 2022 in previously non-enzootic areas. This study denotes the spread of D. immitis and highlights the necessity for preventive measures.

Can nematode infection cause internal bleeding in dogs? A case of Dirofilaria immitis infection in cavitary fluids

A 9-year-old dog was presented with weight loss, respiratory effort, and an enlarged abdomen. Imaging studies and exploratory surgery showed pulmonary and splenic masses and bi-cavitary effusion, later classified as hemorrhage. Cytology of the peritoneal and pleural fluids also revealed several microfilariae. Immunologic and molecular analyses confirmed Dirofilaria immitis infection and histopathology of the spleen indicated a cavernous endothelial proliferation with undefined etiology (hemangiosarcoma vs reaction to parasite infestation). The nematode larvae are speculated to have entered body cavities via erratic migration or via hemorrhage and visceral lesions to be related to parasitism. Nematode infection should be considered as a differential diagnosis for internal bleeding of undetermined origin.

[Overview on utility of in-house tests for detection of systemic infectious diseases in dogs]

For detection of infectious diseases, several point-of-care (POC) tests are on the market in addition to methods performed in commercial laboratories. These POC tests are based on enzyme-linked immunosorbent assay (ELISA) or other immunochromatographic technologies and present results within few minutes in veterinary practice. This article gives an overview of the utility of numerous POC tests of different manufacturers for detection of parvovirus antigen in feces, Dirofilaria (D.) immitis antigen in blood as well as antibodies against Borrelia (B.) burgdorferi, Anaplasma (A.) spp., Ehrlichia (E.) spp., Leptospira (L.) spp. and Leishmania (L.) infantum in blood (single or in different combinations). Sensitivity and specificity of these tests are important for their usefulness in veterinary practice. Furthermore, presence of antibodies or detection of antigen has to correlate with the presence of clinical signs. POC tests for detection of canine parvovirus antigen have a very high specificity, the sensitivity of all evaluated POC tests, however, is very low. POC tests for detection of D. immitis antigen have a very high sensitivity and specificity. As they detect antigen from the uterus of female adult parasites, test results are negative when only very few female or only male adults are present. POC tests for detection of antibodies against B. burgdorferi only indicate contact with Borrelia spp. and do not prove clinical Lyme disease, as the infection only extremely rarely causes clinical signs. POC tests for detection of antibodies against A. phagocytophilum are also not suitable for diagnosis of clinical anaplasmosis. Infections with A. phagocytophilum only lead to clinical disease in very rare cases and in these, clinical signs occur before the development of antibodies. POC tests for detection of antibodies against E. canis have a very high sensitivity as well as specificity. POC tests for detection of antibodies against L. infantum and Leptospira species (spp.) show a very high specificity and a high sensitivity. However, Leptospira spp. antibody-positive results may occur following vaccination, as the POC tests cannot distinguish between field and vaccination strains.

Relative diagnostic accuracy of point-of-care tests to rule-in Dirofilaria immitis infection in clinically suspect dogs: A systematic review and meta-analysis

Canine heartworm, Dirofilaria immitis, can cause severe disease and sometimes death of the host. Associated clinical signs, lack of preventative usage and regional endemicity are unlikely sufficient by themselves to reach a definitive diagnosis. Several point-of-care (POC) diagnostic tests are commercially available to aid in-clinic diagnosis, however, there is variable diagnostic accuracy reported and no synthesis of published evidence. This systematic review aims at meta-analysing the likelihood ratio of a positive result (LR⁺) to inform the selection and interpretation of POC tests in practice to rule-in heartworm infection when there is clinical suspicion. Three literature index interfaces (Web of Science, PubMed, Scopus) were searched on November 11th, 2022, for diagnostic test evaluation (DTE) articles assessing at least one currently commercialised POC test. Risk of bias was assessed adapting the QUADAS-2 protocol and articles with no evidence of high risk of bias were meta-analysed if deemed applicable to our review objective. Substantial between DTE heterogeneity was investigated including potential threshold or covariate effects. A total of 324 primary articles were sourced and 18 were retained for full text review of which only three had low risk of bias in all four QUADAS-2 domains. Of the nine heartworm POC tests evaluated, only three, IDEXX SNAP (n DTEs = 6), Zoetis WITNESS (n DTEs = 3) and Zoetis VETSCAN (n DTEs = 5) could be analysed. Both WITNESS and VETSCAN DTEs showed substantial heterogeneity due to a putative threshold effect and no summary point estimates could be reported. SNAP DTEs showed acceptable heterogeneity, and a summary LR⁺ was estimated at 559.0 (95%CI: 24.3-12,847.4). The quality and heterogeneity of heartworm POC test DTEs was highly variable which restricted our summary of the diagnostic accuracy to only the SNAP test. A positive result from the SNAP test provides strong evidence of the presence of an infection with adult heartworm(s) in a dog patient and this test is warranted to rule-in clinical suspicion(s) in clinics. However, our review did not appraise the literature to assess the fitness of SNAP test, or any other POC tests, to rule-out heartworm infection in dogs without clinical suspicion or following heartworm therapy.

Prevalence of canine heartworm infection in Queensland, Australia: comparison of diagnostic methods and investigation of factors associated with reduction in antigen detection

BACKGROUND

The prevalence of Dirofilaria immitis infection in dogs is increasing globally and spreading into new areas. Prevalence of dirofilariosis in the state of Queensland, Australia, was as high as 90% before the introduction of macrocyclic lactones. Limited research on prevalence of D. immitis infection in dogs in Queensland has been reported in the last 30 years. Antigen testing is the most common method for detection of dirofilariosis but its accuracy is reduced by antigen getting trapped (blocked antigen) in immune complexes (ICs). The objectives of this research were to determine the prevalence of D. immitis infection in dogs from two geographical areas (Brisbane and Townsville) in Queensland, to determine the extent to which blocked antigen affects the validity of antigen testing, and to explore whether this was associated with microfilaraemia, location, age or sex.

METHODS

Blood samples from Brisbane (sub-tropical climate) and Townsville (tropical climate) shelter dogs were evaluated for the presence of D. immitis antigen before (conventional antigen testing, CAT) and after dissociation of ICs by heat treatment (antigen testing after heat treatment, ATHT), using a commercially available test. Microfilariae were detected using modified Knott's test (MKT). Test proportions were compared with McNemar's test and the association between antigen test-discordant results (positive for antigen after dissociation of ICs) and microfilaraemia, location, sex and age was modelled using logistic regression.

RESULTS

Dirofilaria immitis prevalence in dogs from Townsville (22% by CAT, 32.1% by ATHT and 16.7% by MKT) was significantly higher than in dogs from Brisbane (1.1% by CAT and MKT and 1.7% by ATHT) [Formula: see text]. Dissociation of ICs allowed detection of significantly more D. immitis infected dogs than either conventional antigen testing or microfilariae detection, or the combined antigen and microfilariae detection [Formula: see text]. The odds of dogs being positive for antigen after dissociation of ICs were significantly higher for microfilaraemic, 3-4-year-old female dogs from Townsville.

CONCLUSIONS

The high prevalence of infection with D. immitis in dogs from Townsville poses a health risk for local susceptible host species, including humans. Dissociation of ICs increases antigen detection and should be considered in dogs suspected of D. immitis infection but negative on routine testing.

High prevalence of canine heartworm, Dirofilaria immitis, in pet dogs in south Texas, USA, with evidence of Aedes aegypti mosquitoes contributing to transmission

Background

The canine heartworm Dirofilaria immitis, a filarioid nematode of dogs and other carnivores, is widespread in the USA and the world. Over 20 different mosquito species serve as intermediate hosts of D. immitis, but their contribution to transmission varies according to factors like host feeding patterns, geographic locations and climatic conditions. The yellow fever mosquito, Aedes aegypti, is a competent vector of D. immitis but is often dismissed as a vector of veterinary relevance given its anthropophilic feeding behavior. We evaluated the prevalence of D. immitis in pet dogs along the USA-Mexico border and assessed whether Ae. aegypti in the area are naturally infected with heartworm and are potentially acting as a vector.

Methods

A total of 200 whole blood samples collected from pet dogs in the Lower Rio Grande Valley in south Texas from 2016 to 2019 were included in this study. Canine serum samples for D. immitis were tested using the DiroCHEK® Canine Heartworm Antigen Test Kit pre- and post-immune complex dissociations (ICD) and blood samples were tested using high-resolution melt (HRM) quantitative PCR (qPCR) and a probe-based qPCR. Additionally, mosquito specimens were collected and identified, and Ae. aegypti heads, abdomens and pools were tested using conventional PCR (cPCR) and HRM qPCR.

Results

Overall, heartworm prevalence in dogs aged > 6 months was 40.8% (64/157) when the results from all testing modalities were considered. Heartworm antigen was detected in 33.5% and 40.7% of the dogs using DiroCHEK® pre- and post-ICD, respectively. By molecular screening, 20.1% of dogs tested positive with probe-based qPCR, while only one tested positive with HRM qPCR. Of the Ae. aegypti abdomens from blood-fed Ae. aeygpti tested, 20 (21.7%) from mosquitoes that fed on dogs and four (7%) from those that fed on humans tested positive for heartworm. Among Ae. aegypti heads from blood-fed Ae. aeygpti, two (1.1%) were positive based on cPCR and four (2.5%) were positive based on HRM qPCR. No D. immitis DNA was detected in the 208 pools of whole bodies (358 individuals) of Ae. aegypti gravid females.

Conclusions

Our study highlights a high prevalence of heartworm in dogs in south Texas and provides evidence that Ae. aegypti could be contributing to heartworm transmission in canine populations in this region.

Graphical Abstract

Selection of new diagnostic markers for Dirofilaria repens infections with the use of phage display technology

Dirofilaria repens is a parasitic nematode causing vector-borne disease (dirofilariasis), considered an emerging problem in veterinary and human medicine. Although main hosts are carnivores, particularly dogs, D. repens shows high zoonotic potential. The disease spreads uncontrollably, affecting new areas. Since there is no vaccine against dirofilariasis, the only way to limit disease transmission is an early diagnosis. Currently, diagnosis depends on the detection of microfilariae in the host bloodstream using modified Knott's test or multiplex PCR. However, the efficacy of tests relying on microfilariae detection is limited by microfilariae periodic occurrence. Therefore, a new reliable diagnostic test is required. Our study aimed to select new diagnostic markers for dirofilariasis with potential application in diagnostics. We focused on single epitopes to ensure high specificity of diagnosis and avoid cross-reactivity with the other parasite infections common in dogs. Using phage display technology and 12-mer peptides library, we selected epitopes highly reactive with IgG from sera of infected dogs. Additionally, our study presents the possibility of detecting D. repens specific cell-free DNA in dogs with no microfilaria but high IgG and IgM antibody levels against parasite somatic antigen.

Multiple diagnostic tests demonstrate an increased risk of canine heartworm disease in northern Queensland, Australia

BACKGROUND

Canine heartworm (Dirofilaria immitis) is a life-threatening infection of dogs with a global distribution. Information on the prevalence of D. immitis and associated risk factors for canine heartworm antigen positivity-and thus disease-in Australia is scarce or outdated. The current reference method for D. immitis diagnosis in dogs is via the detection of heartworm antigen in blood using commercially available microwell-based enzyme-linked immunosorbent assays (ELISAs). Heat treatment of canine plasma prior to testing has been suggested to increase test sensitivity. The aim of the current study was to estimate the prevalence of D. immitis in dogs confined to shelters in Queensland, Australia. The impact of heat treatment on antigen test results was also assessed.

METHODS

Blood samples (n = 166) were collected directly from dogs in seven shelters across Queensland (latitudinal span of approx. 1700 km) into EDTA blood collection tubes. A commercially available ELISA (DiroCHEK®) was used to detect canine heartworm antigen in untreated and heat-treated plasma. Whole blood was concurrently tested for the presence of microfilariae and D. immitis DNA using a modified Knott's test and real-time PCR, respectively. Risk factors (age, gender, source, location) associated with the odds of positivity for canine heartworm were assessed using binary logistic regression models.

RESULTS

A total of 16 dogs (9.6%; 95% confidence interval [CI]: 5.9-15.2%) were positive for canine heartworm based on combined test results. Heat treatment did not impact on the positivity of D. immitis antigen within samples (Cohen's kappa = 0.98), but the optical density was significantly increased in paired plasma samples for D. immitis antigen-positive samples (Wilcoxon matched-pairs signed rank test, two-tailed P < 0.01). Location of the dog in a shelter in northern Queensland was the only risk factor significantly associated with the odds of a dog being more likely to be D. immitis antigen positive (odds ratio: 4.39; 95% CI: 1.26-13.51). All samples positive for the modified Knott's test were also positive for D. immitis DNA by PCR.

CONCLUSIONS

This study demonstrated the presence of heartworm-positive dogs in shelters in Queensland, with positive animals significantly more likely to occur in northern Queensland than southern Queensland. Sustained testing for the presence of D. immitis microfilariae and antigen remain important diagnostic tools in areas with known and re-emerging canine heartworm activity.

Heartworm disease - Overview, intervention, and industry perspective

Dirofilaria immitis, also known as heartworm, is a major parasitic threat for dogs and cats around the world. Because of its impact on the health and welfare of companion animals, heartworm disease is of huge veterinary and economic importance especially in North America, Europe, Asia and Australia. Within the animal health market many different heartworm preventive products are available, all of which contain active components of the same drug class, the macrocyclic lactones. In addition to compliance issues, such as under-dosing or irregular treatment intervals, the occurrence of drug-resistant heartworms within the populations in the Mississippi River areas adds to the failure of preventive treatments. The objective of this review is to provide an overview of the disease, summarize the current disease control measures and highlight potential new avenues and best practices for treatment and prevention.

The Incidence of Dirofilaria immitis in Shelter Dogs and Mosquitoes in Austria

To estimate the incidence of Dirofilaria immitis in Austrian shelter dogs and mosquitoes trapped in their proximity, 115 shelter dogs from fourteen animal shelters located in five different Austrian states were examined. Blood samples were screened for D. immitis using ELISA antigen-testing, PCR and microscopical examination for microfilariae. In total, 91% of the dogs originated from countries endemic for dirofilariosis. Eleven dogs (9.6%), all originating from Hungary, tested positive for D. immitis. None of the dogs examined showed microfilaremia. Eight dogs showed no or only mild clinical signs (e.g., infrequent coughing), and three dogs showed frequent coughing, dyspnea, exercise intolerance, blunt fur or weight loss. In total, 205 Mosquitoes of ten different species were caught at five different shelter sites in four different Austrian states, using CO₂-baited mosquito traps set once a month (June–September 2019) for 24 h. All 205 mosquitoes tested negative for Dirofilaria spp. via PCR. The risk of endemisation of D. immitis in Austria (and other non-endemic countries in a similar situation) is very serious and its zoonotic potential should be communicated more strongly. To monitor a possible transmission of microfilariae from untreated or even untested positive dogs, e.g., in animal shelters, to mosquitoes in the near surroundings, frequent screening for Dirofilaria in mosquitoes should be used more intensively. Current knowledge on D. immitis should be integrated into daily veterinary practice and dog owners should be proactively educated, especially before traveling to endemic areas or adopting dogs from endemic countries. Animal shelters and animal welfare organizations should be provided with appropriate education and veterinary guidance regarding the testing and treatment of dogs imported from high-risk areas.

Optimizing heartworm diagnosis in dogs using multiple test combinations

Background

Various heartworm (HW) diagnostic testing modalities detect products of, or reactions to, different life cycle stages of Dirofilaria immitis. Microfilariae (Mf) can be directly visualized in blood, antigen (Ag) from immature and adult heartworms may be detected on commercial assays, and antibody (Ab) tests detect the host immune response to larval stages. Ag and Mf tests are commonly used in dogs, which frequently carry adult HW infections, but Ab tests have only been validated for use in cats. In some HW-infected dogs, Ag is blocked by immune complexing leading to false-negative results. Heat-treatment (HT) to disrupt these complexes can increase the sensitivity of HW Ag tests. The aim of this study was to compare different methods for diagnosing HW infection in dogs at high risk using individual and paired diagnostic tests, including an exploration of using Ab tests designed for cats to test canine samples.

Methods

One hundred stray adult (≥ 2-year-old) dogs in Florida shelters were tested using Mf, HW Ag, and HW Ab tests (feline HW Ab tests currently not commercially validated/approved for use in dogs); two versions of each test platform were used.

Results

Fourteen dogs tested positive using point-of-care (POC) Ag tests; an additional 2 dogs tested positive with microtiter well assay, and an additional 12 dogs tested positive using HT Ag testing. For individual tests, Ag test sensitivity/specificity compared to HT Ag was 50–57%/100%, and Ab tests were 46–64%/82–94%. Sensitivity estimates for individual tests were higher when comparing to non-HT Ag. Pairing POC Ag tests with Mf tests improved sensitivity without loss of specificity, while pairing POC Ag and Ab tests modestly increased sensitivity at the expense of specificity.

Conclusions

Screening dogs for HW infection using both POC Ag and Mf detection, which is recommended by the American Heartworm Society, improved diagnostic performance in this study compared to single Ag test use, but may have missed more than one in four infected dogs. The need to improve access to highly accurate, rapid, and inexpensive large-scale HW testing for dogs in animal shelters remains largely unmet by current testing availability. The development of practical and validated protocols that incorporate heat or chemical treatment to disrupt Ag-Ab complexes in POC testing or decreasing the cost and time required for such testing in reference laboratories might provide solutions to this unmet need. Similar studies performed in countries where the prevalence of parasites such as D. repens or A. vasorum is different to the USA could potentially yield very different positive predictive values for both HT and non-HT Ag tests.

Graphic abstract

Prevalence of Dirofilaria immitis in shelter dogs in Bucaramanga metropolitan area, Colombia

The heartworm Dirofilaria immitis is the causative agent of cardiopulmonary dirofilariosis in dogs and cats and also infects humans. The current knowledge about this parasite in South America is scarce compared to North America and Europe. Nevertheless, this parasite has been reported in some regions of Colombia. The objective of this study was to determine the prevalence and risk factors associated with D. immitis infection in shelter dogs from Bucaramanga metropolitan area. We performed a cross-sectional study using blood samples from shelter dogs. Microfilariae testing of whole blood included a blood smear and modified Knott's test. Antigen testing of serum was performed using the Anigen Rapid HW Ag Test Kit 2.0 ®. A descriptive analysis and univariable logistic regression analysis were applied. A total of 207 whole blood samples, from 7 shelters dogs in the Bucaramanga metropolitan area, were utilized. The overall prevalence of D. immitis observed in shelter dogs in the Bucaramanga metropolitan area by blood smears and modified Knott's test was 6.3% (13/ 207) and 0.5% (1/207) by immunochromatography test kit. Regarding the prevalence by municipalities, there was a statistical association (P < .05), indicating that the prevalence was higher in dogs in Girón municipality. Likewise, there was a statistical association (P < .05) between the prevalence with respect to heartworm knowledge and mosquito control. For prevalence by gender, age group, dog breed, and hair length, there was no statistical association (P > .05). For Girón municipality there was a 7.1 times (OR = 7.1; CI95% 0.8-59.2; P < .05) increased infection risk. This study provides current data and the first known report of the prevalence of D. immitis microfilaremia and antigenemia in shelter dogs in the Bucaramanga metropolitan area, and these results could be useful for designing new control measures for this infection. We observed that the prevalence of both microfilaremia and antigenemia was significantly lower in these sampled dogs compared to previous reports.

Heartworm control in Grenada, West Indies: Results of a field study using imidacloprid 10% + moxidectin 2.5% and doxycycline for naturally-acquired Dirofilaria immitis infections

Canine heartworm disease (CHD) results from infection with Dirofilaria immitis and while it is of global concern, it is most prevalent in tropical climates where conditions support the parasite and vector life cycles. Melarsomine dihydrochloride is the sole treatment for CHD recommended by the American Heartworm Society. However, in cases where cost or access to melarsomine precludes treatment of an infected dog, therapeutic alternatives are warranted. This randomized, controlled field study evaluated the adulticidal efficacy of a combination therapeutic protocol using 10 % imidacloprid + 2.5 % moxidectin spot-on and a single 28-day course of doxycycline and compared with that of a 2-dose melarsomine dihydrochloride protocol. Of 37 naturally-infected domestic dogs with class 1, 2 or early class 3 CHD enrolled in the study, 30 were evaluated for a minimum of 12 months. Seven dogs were withdrawn due to canine ehrlichiosis, non-compliance, or wrongful inclusion. Dogs were randomly assigned to a control (CP, n = 15) or investigational (IVP, n = 15) treatment group. CP dogs received two injections of melarsomine dihydrochloride (2.5 mg/kg) 24 -hs apart and maintained on monthly ivermectin/pyrantel. IVP dogs were treated with oral doxycycline (10 mg/kg twice daily for 28 days) and topical 10 % imidacloprid + 2.5 % moxidectin once monthly for 9 months. Dogs were evaluated up to 18 months - monthly for the first 9 months, then every 3 months. Parasiticidal efficacy was based on antigen status using the IDEXX PetChek® 34 Heartworm-PF Antigen test. By month 18, antigen was not detected in any study dog except one from the IVP group. One other IVP dog was persistently antigenemic and treated with melarsomine at month 12 according to the initial study protocol. Mean antigen concentration (based on optical density) decreased more rapidly in the CP group and by month 15 was 0.11 for the IVP and 0.07 for CP groups, with equivalent median concentrations (0.04) in both groups. Conversion following heat-treatment of antigen-negative samples occurred frequently and at similar rates in both treatment groups. Based on the bias of diagnostic tests towards detection of female worms, we conclude that monthly application of 10 % imidacloprid + 2.5 % moxidectin for 9 months combined with a course of doxycycline twice daily for 28 days resulted in effective therapy against female adults in CHD. This therapeutic option may be particularly useful in cases where financial constraint or access to melarsomine precludes treatment of an infected individual. This study was supported by Bayer Animal Health.

Evaluation of the SNAP® 4Dx® plus test for the detection of Dirofilaria immitis antigen and characterization of exposure to tick-borne pathogens in wild canids in southern Ontario

Population-level surveys for Dirofilaria immitis in wild canids typically rely on identification of the parasite at necropsy. More recently, some studies have employed the use of the SNAP® 4Dx® Plus test. However, since the assay was designed for use with domestic dogs it needs to be validated for use with wild canids for accurate interpretation of results. We therefore evaluated the performance of the SNAP® 4Dx® Plus test for detection of D. immitis in wild canids in southern Ontario. Overall, 199 wild canid carcasses were collected from across the region and assessed for the presence of D. immitis parasites at necropsy; ten were infected. Lung tissue extract (LE) and thoracic fluid filter paper extract (TFE) prepared from each wild canid were tested via the SNAP® 4Dx® Plus test, which simultaneously tests for the presence of D. immitis antigen and antibodies to Borrelia burgdorferi, Anaplasma spp., and Ehrlichia spp. The prevalence adjusted bias adjusted kappa (PABAK) and Gwet's first-order-agreement coefficient (AC1) were used to assess the level of agreement between sample pairs. The PABAK and AC1 between LF and TFE applied to the SNAP® 4Dx® Plus test and the necropsy-confirmed D. immitis status indicated a very high level of agreement for all sample pairs. Compared to necropsy-confirmed D. immitis status, the estimated sensitivity and specificity of the SNAP® 4Dx® Plus test for D. immitis antigen in LE was 80 % (95 % CI 44.4-97.5%) and 98.9 % (95 % CI 96.2-99.9%), respectively. For the TFE, the sensitivity was 70 % (95 % CI 34.8-93.3%) and the specificity was 97.9 % (95 % CI 94.7-99.4%). With respect to the tick-borne pathogen components, 1.5 % (3/199; 95 % CI 0.3-4.5%) of wild canids tested positive for B. burgdorferi antibody; 1.0 % (2/199; 95 % CI 0-3.8%) of LE samples were positive and 0.5 % (1/199; 95 % CI 0-3.1%) of TFE samples were positive. No samples tested positive for antibody to Anaplasma spp. (95 % CI 0-2.3%) or Ehrlichia spp. (95 % CI 0-2.3%). Collectively, the results suggest that the SNAP® 4Dx® Plus test may be a suitable test for use with LE and TFE for the detection of D. immitis antigen in wild canids from southern Ontario.

Comparison of Diagnostic Tools for the Detection of Dirofilaria immitis Infection in Dogs

In the last two decades, reports of canine heartworm (HW) infection have increased even in non-endemic areas, with a large variability in prevalence data due to the diagnostic strategy employed. This study evaluated the relative performance of two microtiter plate ELISA methods for the detection of HW antigen in determining the occurrence of Dirofilaria immitis in a dog population previously tested by the modified Knott's test and SNAP 4Dx Plus test. The prevalence of this infection in the sheltered dog population (n = 363) from a high-risk area for HW infection was 44.4% according to the modified Knott's test and 58.1% according to a point-of-care antigen ELISA. All serum samples were then evaluated by a microtiter plate ELISA test performed with and without immune complex dissociation (ICD). The prevalence increased from 56.5% to 79.6% following ICD, indicating a high proportion of samples with immune complexing. Comparing these results to that of the modified Knott's test, the samples negative for microfilariae (mfs) and those positive only for D. repens mfs demonstrated the greatest increase in the proportion of positive results for D. immitis by ELISA following ICD. While the ICD method is not recommended for routine screening, it may be a valuable secondary strategy for identifying HW infections in dogs.

Heartworm prevalence in dogs versus cats: Multiple diagnostic modalities provide new insights

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Prevalence of adult heartworm (HW) infection was 4 % in cats and 28 % in dogs.

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Combining antigen and antibody testing led to an overall 19 % positive cats.

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Prevalence did not differ between dogs and cats with added feline antibody testing.

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Dirofilaria repens microfilariae were identified in one dog and one cat.

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Acanthocheilonema reconditum microfilariae were identified in four dogs.

The cornerstones of diagnosis of heartworm (HW) in dogs are the detection of circulating antigen from adult female Dirofilaria immitis or the visualization of microfilariae in whole blood. These tests are less sensitive in cats because of the feline immune response leading to low numbers of adult worms, but heartworm antibody tests are also licensed for use in cats. HW antibodies in cats are detectable when there has at least been larval development in the tissues, but positive antibody tests cannot distinguish between current and previous larval infections; thus, cats with positive antibody test results are considered currently or previously infected with D. immitis. The aim of the present study was to use multiple HW diagnostic modalities to maximize detection of infection in dogs and cats at high risk of infection and to compare infection prevalence between these two hosts. Blood samples collected from 100 stray dogs and 100 stray cats at Florida animal shelters were tested for HW antigen (before and after heat treatment) and microfilariae; cats were also tested for HW antibody. Dogs were significantly (P = 0.0001) more likely to be diagnosed with adult HW infection (28 %; 95 % CI: 20.1–37.6%) when compared with cats (4 %; 95 % CI: 1.6–10.2%) on the basis of positive antigen and microfilariae test results. Cats with current or previous adult, immature adult, or larval HW infections comprised 19 % (95 % CI: 12.4%–27.9%) of the feline population, which was not significantly different (P = 0.1) from the prevalence of adult D. immitis infection in dogs. Testing unprotected cats for heartworm antibodies demonstrated a similar, high risk of infection to the matched unprotected dog population in Florida, which supports the use of HW preventives in cats in areas where HW transmission occurs.

Current trends in canine dirofilariosis in Austria-do we face a pre-endemic status?

A retrospective study based on cases of canine dirofilariosis presented to the University of Veterinary Medicine, Vienna or diagnosed by private practitioners throughout Austria, from 1998 to 2018 was conducted to investigate the long-term development and current state of canine dirofilarial infections in Austria. Included in this study were 146 dogs which were tested positive for D. immitis and/or D. repens. The most commonly used diagnostic methods and the probable geographical origins of the infections were evaluated and the treatment protocols applied were compared with each other and with the literature. The results show that most infections were found due to screening for common travel infections using antigen-ELISA or PCR-testing, or by the incidental finding of microfilariae. Remarkably, only 24.3% of all cases presented showed clinical signs indicating canine dirofilariosis. Regarding the origin and travel history of the dogs, thirteen different countries could be identified. The three treatment protocols used showed a similar outcome after 8 months of treatment and minor side effects, which is consistent with the literature. An alarming increase in reported infections with both D. immitis and D. repens in Austria was noted since 2014. The number of documented cases had almost tripled by 2018, raising severe concerns about the threat of it becoming endemic in Austria. Therefore, the existing recommendations in current guidelines regarding canine dirofilariosis should be widely publicised and more strictly enforced. Prophylactic measures for dogs travelling abroad and diagnostic and therapeutic strategies for dogs imported from endemic countries should be obligatorily established throughout Europe, to reduce the risk of further spread of canine filarial infections to non-endemic regions.

Design, statistical analysis and reporting standards for test accuracy studies for infectious diseases in animals: Progress, challenges and recommendations

The quality of diagnostic accuracy studies (DAS) for infectious diseases of animals has improved over the last 20 years because of international educational efforts, use of design and reporting standards to guide researchers and test developers, and acceptance of the use of latent class models to account for imperfect reference tests. In this review, we focus on measurement of diagnostic sensitivity and specificity as a measure of clinical validity, describe the leadership role of the World Organisation of Animal Health (OIE) in setting standards for test validation in the context of fitness-for-purpose, and describe how design and reporting quality have facilitated the increased use of systematic reviews and meta-analysis of DAS. Ongoing challenges for design, conduct, analysis and reporting of DAS are identified; and we make recommendations for improvements in these areas for OIE-listed and non-listed infectious diseases.

Prime detection of Dirofilaria immitis: understanding the influence of blocked antigen on heartworm test performance

Detection of circulating antigen of Dirofilaria immitis has been a mainstay of identifying heartworm infection in clinical practice for the past three decades. Several validated commercial antigen tests have very good sensitivity, specificity, and positive predictive values, especially when used in patients for which heartworm infection is likely. In some dogs and cats infected with heartworm, antigen may not be available for detection although present in the patient sample; heat pretreatment of these samples reveals the antigen, changing the false negative to positive. This phenomenon was documented in the literature in the 1980s but subsequently overlooked by the heartworm research community for many years. In this review, we provide a summary of the current understanding of the role of heat reversal in diagnosing heartworm infection. This additional diagnostic step is most important for patients in which heartworm infection is likely, such as dogs or cats in an endemic area with an inconsistent history of heartworm preventive use, or dogs with a prior diagnosis of heartworm infection that were recently treated. To illustrate the concept, we share a summary of results from canine samples tested at the state veterinary diagnostic laboratory in Oklahoma, USA in 2017 by modified Knott test and by commercial antigen test before and after heat treatment of samples; in this sample set, heat treatment changed all D. immitis microfilaria-positive but antigen-negative samples to antigen-positive. Pet dogs with a history of consistent preventive use are unlikely to become positive with heat pretreatment; for that reason, routine pretreatment of all samples tested in a veterinary practice is not recommended. We also review known causes of false negative and false positive results on heartworm antigen tests that, although uncommon, can complicate accurate diagnosis in individual patients. Together, this review provides a primer to aid understanding of strategies that can enhance accurate diagnosis of heartworm infection in veterinary practice and clinical research.

Comparison of six commercial antigen kits for detection of Dirofilaria immitis infections in canines with necropsy-confirmed heartworm status

Patient-side test kits for detecting antigenemia in dogs associated with sexually mature female heartworms (Dirofilaria immitis) have been available for three decades, and these tests are continually updated and improved. To define the sensitivity (Se) and specificity (Sp) of contemporary antigen detection tests against cardiopulmonary D. immitis burden, we evaluated five patient-side kits-Anigen Rapid One Step^® (Bio note), SNAP^® 4Dx Plus Test Kit (IDEXX), WITNESS^® Heartworm Canine Heartworm Antigen Test Kit (Zoetis), VetScan^® Canine Heartworm Rapid Test (Abaxis), and Solo Step^® CH Canine Heartworm Antigen Test (Heska), and one microplate ELISA (DiroCHEK^®; Zoetis), using archived canine sera divided into five subclasses of female worms (0, 1-5, 6-20, 21-40, and >40). The patient-side tests were performed once, side-by-side according to each manufacturer's protocol by personnel blinded to the D. immitis status of each dog. The overall Se and Sp of the patient-side kits was ≥97.5 and =94.0%, respectively. For samples from dogs with 1-5, 6-20, and 21-40 D. immitis, the Se was between 96 and 100%, with a slight increase in Se in dogs with ≥41 worms. The agreement between tests for all subclasses of D. immitis burden was between 99 and 100%. The Se and Sp for the ELISA compared with the necropsy results of dogs was 99 and 96%, respectively. Agreement between each patient-side test and the ELISA was between 97 and 100%. All commercially available tests can give practitioners excellent patient-side information, allowing them to make informed decisions on the need for additional diagnostic work-up before instituting new or continuing D. immitis prophylaxis.

Evaluation of a rapid immunochromatographic test for the detection of low burden Dirofilaria immitis (heartworm) in dogs and cats

The performance of a rapid immunochromatographic test for the detection of Dirofilaria immitis antigens (Speed Diro™; BVT-Virbac, France) was assessed in 49 experimentally infected dogs and in 244 naturally infected animals; 142 dogs and 102 cats. In experimentally infected dogs, Speed Diro™ showed a sensitivity of 90.9% in dogs infected with one adult female worm and 100% in dogs infected with more than one female worm. Specificity was 100%. For naturally infected dogs, the Knott test and PetChek^® HTWM PF served as reference methods for microfilaremia and antigenemia, respectively. All microfilaemic dogs (55/142) were positive with Speed Diro™. Importantly, none of the 21 dogs infected with D. repens were positive. The results of Speed Diro™ for the detection of antigenemia were compared with two in-house tests, SNAP^® HTWM and Witness^® Dirofilaria, and all three tests were 100% specific and sensitive in comparison to PetChek^® HTWM PF. For the evaluation of feline samples, 102 cats were examined by echocardiography. Sera from 87 heartworm-infected cats were tested by Speed Diro™ and SNAP^® HTWM. The results of Speed Diro™ were equivalent to SNAP^® HTWM, with a sensitivity of 98.9% and a specificity of 100%.

Impact of heat treatment on Dirofilaria immitis antigen detection in shelter dogs

Background

The diagnosis and management of canine heartworm disease is a growing concern for shelter veterinarians. Although the accuracy of commercial antigen test kits has been widely studied, recent reports have renewed interest in antigen blocking as a causative factor for false “no antigen detected” results. The objectives of this study were to determine the prevalence of false “no antigen detected” results in adult dogs entering shelters in northern, southern, and western regions of the country and to identify historical and clinical risk factors for such results.

Methods

Serum samples were evaluated for Dirofilaria immitis antigen using a commercially available point-of-care ELISA; samples in which no antigen was detected underwent a heat treatment protocol and repeat antigen testing. Whole blood samples underwent Knott testing to identify the presence of microfilariae. Historical and clinical findings were analyzed using exact logistic regression.

Results

A total of 616 samples were analyzed. Overall prevalence of positive antigen test results (prior to heat treatment) was 7.3% and frequency of false “no antigen detected” results due to antigen blocking (ie, samples with no antigen detected prior to heat treatment and positive after heat treatment) was 5.2%. Among dogs that had no detectable antigen on the initial tests, dogs that had microfilariae detected via modified Knott testing (OR = 32.30, p-value = 0.013) and dogs that previously received a heartworm preventive (OR = 3.81, p-value = 0.016) had greater odds of antigen blocking than dogs without these factors. Among dogs that were heartworm positive, those without microfilariae detected had greater odds of antigen blocking than dogs with this factor (OR = 11.84, p-value = 0.0005). Geographic region of origin was significantly associated with occurrence of antigen blocking (p = 0.0036); however, blocking occurred in all regions sizably contributing to heartworm diagnoses. Of the 74 dogs found to be infected with heartworms in this study, 39.2% (29) had no detectable antigen prior to heat treatment.

Conclusions

Heat treatment of serum samples should be considered to improve diagnostic test accuracy, particularly in dogs that reportedly received a heartworm preventive prior to antigen testing regardless of region of origin.

Electronic supplementary material

The online version of this article (10.1186/s13071-017-2443-7) contains supplementary material, which is available to authorized users.

Heat treatment and false-positive heartworm antigen testing in ex vivo parasites and dogs naturally infected by Dirofilaria repens and Angiostrongylus vasorum

Background

Heartworm antigen testing is considered sensitive and specific. Currently available tests are reported as detecting a glycoprotein found predominantly in the reproductive tract of the female worm and can reach specificity close to 100%. Main concerns regard sensitivity in the case of light infections, the presence of immature females or cases of all-male infections. Research and development have been aimed at increasing sensitivity. Recently, heat treatment of serum prior to antigen testing has been shown to result in an increase in positive antigen test results, presumably due to disruption of natural antigen–antibody complexes. Cross-reactions in dogs with both natural and experimental infections with Angiostrongylus vasorum and Spirocerca lupi have been reported, but cross-reactions with other helminths have not been extensively studied. In order to evaluate potential cross-reactivity with other canine and feline parasites, two studies were performed. Study 1: Live adults of Dirofilaria immitis, Dirofilaria repens, Toxocara canis, Toxocara cati, Dipylidium caninum, Taenia taeniaeformis and Mesocestoides spp. larvae were washed and incubated in tubes with saline solution. All worms were alive at the time of removal from the saline. Saline solutions containing excretory/secretory antigens were then tested for heartworm with six different, commercially available antigen tests. All results were evaluated blind by three of the authors. Study 2: Sera from dogs with natural infections by A. vasorum or D. repens, living in areas free of heartworm disease, were tested with the same tests before and after heat treatment (103 °C for 10 min).

Results

Results suggest that antigens detected by currently available tests are not specific for D. immitis. They may give positive results through detection of different parasites’ antigens that are normally not released into the bloodstream or released in a low amount and/or bound to antibodies. Tests may even detect antigens released by male D. immitis adult worms. D. repens appears to release more detectable antigens than the other worms studied.

Conclusions

Cross-reaction with A. vasorum and D. repens does occur in the field and could potentially occur with other helminths. Heat treatment decreases specificity by enhancing cross-reactivity.

Pharmacokinetics and efficacy of an ivermectin implant for long-term prevention of Dirofilaria immitis infection in dogs

An exploratory study was carried out to assess the in vivo efficacy of different prototypes of subcutaneous implants containing ivermectin (IVM) for the prevention of canine Dirofilaria immitis infection. The implants consisted of an ethylcellulose matrix containing IVM (3.0, 4.5, and 6.0 mg/implant; from 0.29 to 0.63 mg/kg bw) as active ingredient designed to release approximately 0.1 ng of IVM/mL in the plasma for at least 12 months. Six dogs were implanted on day -365. On day -12, three heartworm-free dogs were included in the study as a control group. All nine dogs were examined on day -7 and day 0 for circulating D. immitis microfilariae and by an antigen ELISA kit to confirm that the dogs were heartworm-free. The dogs were artificially infected with 75 D. immitis infective larvae (L3) each on day 0. Dogs in the control group were humanely euthanized on day 153 to verify the infectivity of the larvae, while implanted dogs were further examined for circulating D. immitis microfilariae and antigen on days 153, 195, and 246. The treated dogs were not necropsied. The pharmacokinetic profile of the IVM implant was assessed in plasma samples taken on day -364, then at different times until the infection day, and again on days, 15, 30, 60, 90, 120, and 153. Throughout the study, body weights were measured during clinical examination on days 0, 30, 60, 90, 120, and 153. At necropsy, all control dogs were found infected, each with 10-11 adult heartworms. Implanted dogs were negative at both microfilaria and D. immitis antigen examinations until day 246 (8 months from the infection). IVM plasma levels ranged 0.06-0.16 ng/mL on day 0 and remained stable until day 60, afterward they gradually decreased under the limit of quantification of the method. Throughout the study, no side effect was observed. On the basis of these results, it was possible to conclude that the different prototypes of implants were able to protect the dogs from D. immitis artificial infection for at least 12 months.

Prevalence of Dirofilaria immitis infection in dogs in Henan province, central China

The heartworm Dirofilaria immitis is the causative agent of cardiopulmonary dirofilariosis in dogs and cats, and also infects humans. However, there has been no study on dirofilariasis in dogs in central China. From March 2015 to February 2016, sera from 1176 randomly selected household dogs from Henan province, central China were examined for D. immitis antigen using the Canine Heartworm Antigen Test Kit. The overall seroprevalence of D. immitis in dogs in Henan province was 13% (155/1176). The prevalence was significantly higher in older dogs and dogs kept outdoors, compared to the younger ones and those sheltered indoors. No significant difference of prevalence was observed between sexes. The results suggest that the risk of exposure to D. immitis in dogs is high in Henan, and prophylaxis against the parasite is advisable to decrease the incidence of canine dirofilariosis in this region.

Dirofilaria immitis and Angiostrongylus vasorum: the contemporaneous detection in kennels

BACKGROUND

The cardiopulmonary nematodes Dirofilaria immitis and Angiostrongylus vasorum are increasingly reported in dogs and are responsible for two diseases with overlapping endemic areas, especially in Europe: dirofilariosis and angiostrongylosis. The reasons for their apparent emergence are unknown, but several factors (e.g. increased disease awareness, better diagnostic tools, climatic changes, seasonal population dynamics and movements of animals) may play a role in the recent rise in reports of infection in the various countries of Europe. The aim of this study was to investigate the seroprevalence of D. immitis (by DiroCHECK® ELISA) and the fecal presence of first stage larvae (L1) of A. vasorum (by FLOTAC) in dogs from 68 kennels of the Campania region (southern Italy). The fecal samples were collected from pooled samples using the box as epidemiological unit. To the authors's knowledge, this is the first cross-sectional survey conducted at regional-scale in Italy and in Europe on the contemporaneous detection of D. immitis antigens and A. vasorum L1 in kennels.

RESULTS

Antigens of D. immitis were detected in 24/537 (4.4%; 95% Confidence Interval = 3.0-6.7) dogs in 6 out of the 68 kennels (8.8%; 95% CI = 3.6-18.9). The 24 positive samples for D. immitis antigen were tested also with AngioDetect® and only 1 sample was seropositive for A. vasorum with a prevalence of 4.2%. A. vasorum L1 were detected in dogs from 9 out of the 68 kennels (13.2%; 95% CI = 21.8-44.9). Pooled fecal samples from 25 boxes out of the 1360 analyzed resulted positive to A. vasorum L1 (1.8%; 95% CI = 1.2-2.7).

CONCLUSIONS

The present study indicates that cardiopulmonary nematodes are present in Campania region in symptomatic dogs as well as in asymptomatic ones. Therefore, regular parasitological surveillance, appropriate treatment strategies and high quality standard of hygiene are required to guarantee the health and welfare of kennel dogs.

Current surveys on the prevalence and distribution of Dirofilaria spp. and Acanthocheilonema reconditum infections in dogs in Romania

During the last decades, Dirofilaria spp. infection in European dogs has rapidly spread from historically endemic areas towards eastern and northeastern countries, but little or no information is available from these geographical regions. The present study provides a picture of filarial infections in dogs from Romania and compares two tests for the diagnosis of Dirofilaria immitis. From July 2010 to March 2011, blood samples were collected from 390 dogs from nine counties of Romania and serological SNAP tests were performed for the detection of D. immitis antigen. The remaining blood clots were subsequently used for DNA extraction followed by multiplex PCR for assessing filarioid species diversity (i.e. D. immitis, Dirofilaria repens and Acanthocheilonema reconditum). Based on molecular detection, an overall prevalence of 6.92 % (n = 27; 95 % confidence interval (CI) 4.70-10.03 %) for D. repens, 6.15 % (n = 24; 95 % CI 4.07-9.14 %) for D. immitis and 2.05 % (n = 8; 95 % CI 0.96-4.16 %) for A. reconditum was recorded, with significant variations according to sampling areas. Coinfections of D. immitis and D. repens were recorded in 23.91 % (n = 11) positive dogs. A slightly higher prevalence for D. immitis was detected at the SNAP test (n = 28, 7.17 %; 95 % CI 4.91-10.33 %), but this difference was not statistically significant (p = 0.66). However, only 53.57 % (n = 15) of antigen-positive dogs were confirmed by PCR, while other dogs (n = 9) PCR positive for D. immitis were negative at the serology. The present study shows that Dirofilaria species are endemic in the southern and southeastern areas of Romania, This article also provides, for the first time, an epidemiological picture of the distribution of A. reconditum in Romania.

Evaluation of a rapid device for serological in-clinic diagnosis of canine angiostrongylosis

Background

Angiostrongylus vasorum is a potentially fatal canine nematode. Due to the high variability of clinical signs and the often chronic and subtle course of the infections, the diagnosis is particularly challenging. A rapid in-clinic assay (Angio Detect™ Test, IDEXX Laboratories, Westbrook, Maine, USA) for the serological detection of circulating antigen and intended for routine in-clinic diagnosis has been evaluated.

Methods

Sensitivity was calculated with sera from 39 naturally infected dogs confirmed by Baermann-Wetzel analysis, while sera of 38 experimentally infected dogs were used for follow-up analyses, of which 10 were treated with imidacloprid/moxidectin. Cross-reactivity was tested with a total of 123 samples from dogs with proven parasitic infections with Toxocara canis (n = 21), Ancylostoma caninum (n = 4), Crenosoma vulpis (n = 18), Oslerus osleri (n = 3), Eucoleus aerophilus, (n = 6), Dirofilaria immitis (n = 28), Dirofilaria repens (n = 20), Acantocheilonema reconditum (n = 10) or Dipetalonema dracunculoides (n = 10) or multiple infections (n = 3). All sera were tested with the Angio Detect™ Test and with an ELISA for detection of circulating antigen of A. vasorum.

Results

The sensitivity of the Angio Detect™ Test was 84.6% (95% C.I. 69.5 - 94.1%), while specificity was 100% (95% C.I. 97.6 - 100%). The sensitivity of the ELISA (94.9%, 95% C.I. 82.7 – 99.3%) was comparable with previous evaluations. In experimentally infected dogs, earliest positive results with the Angio Detect™ Test were observed 9 weeks post inoculation and 5 weeks later all sera were Angio Detect™ Test positive. After anthelmintic treatment, seropositive dogs turned negative again within 3 to 7 weeks after treatment. The evaluation of the colour intensity of the test strips confirmed the delay of approximately 3-4 weeks for antigen detection by the Angio Detect™ Test compared to the ELISA and its correlation with the time after infection.

Conclusions

This study provided evidence of a good sensitivity and a very high specificity of the rapid device Angio Detect™ Test for detection of circulating A. vasorum antigen in dogs with suspected canine angiostrongylosis, representing a very simple and useful tool to be broadly applied in veterinary practices. The rapid detection of infected dogs is a key point for initiating an indispensable and urgent therapy.

Pre-treatment with heat facilitates detection of antigen of Dirofilaria immitis in canine samples

Diagnosis of Dirofilaria immitis infection in dogs is largely dependent on detection of antigen in canine serum, plasma, or whole blood, but antigen may be bound in immune complexes and thus not detected. To develop a model for antigen blocking, we mixed serum from a microfilaremic, antigen-positive dog with that of a hypergammaglobulinemic dog not currently infected with D. immitis and converted the positive sample to antigen-negative; detection of antigen was restored when the mixed sample was heat-treated, presumably due to disruption of antigen/antibody complexes. A blood sample was also evaluated from a dog that was microfilaremic and for which microfilariae were identified as D. immitis by morphologic examination. Antigen of D. immitis was not detected in this sample prior to heating but the sample was strongly positive after heat treatment of whole blood. Taken together, our results indicate that blood samples from some dogs may contain factors that inhibit detection of antigen of D. immitis, and that heat treatment of these samples prior to testing could improve the sensitivity of these assays in some patients.

Heat treatment prior to testing allows detection of antigen of Dirofilaria immitis in feline serum

Background

Diagnosis of Dirofilaria immitis infection in cats is complicated by the difficulty associated with reliable detection of antigen in feline blood and serum samples.

Methods

To determine if antigen-antibody complex formation may interfere with detection of antigen in feline samples, we evaluated the performance of four different commercially available heartworm tests using serum samples from six cats experimentally infected with D. immitis and confirmed to harbor a low number of adult worms (mean = 2.0). Sera collected 168 (n = 6), 196 (n = 6), and 224 (n = 6) days post infection were tested both directly and following heat treatment.

Results

Antigen was detected in serum samples from 0 or 1 of 6 infected cats using the assays according to manufacturer’s directions, but after heat treatment of serum samples, as many as 5 of 6 cats had detectable antigen 6–8 months post infection. Antibodies to D. immitis were detected in all six infected cats by commercial in-clinic assay and at a reference laboratory.

Conclusions

These results indicate that heat treatment of samples prior to testing can improve the sensitivity of antigen assays in feline patients, supporting more accurate diagnosis of this infection in cats. Surveys conducted by antigen testing without prior heat treatment of samples likely underestimate the true prevalence of infection in cats.

Effects of diagnostic test accuracy and treatment efficacy on the occurrence of suspected failure of heartworm prophylaxis in dogs

BACKGROUND

Food and Drug Administration Center for Veterinary Medicine (FDA/CVM) cited concern regarding failure of heartworm prophylaxis. The positive and negative predictive value of the heartworm antigen test is an estimate of the probability of adult heartworm infection.

HYPOTHESIS/OBJECTIVES

Assess the sensitivity, specificity, and predictive value of heartworm antigen tests. Explore the role of heartworm test accuracy and treatment with immiticide to generate reports of suspected failure of heartworm prophylaxis.

METHODS

Literature searches for published information on the accuracy of heartworm antigen tests and efficacy of immiticide for treatment of the adult heartworm.

RESULTS

Weighted averages for heartworm antigen test sensitivity and specificity were 78.2 and 97.3%, respectively. Efficacy of immiticide by 2-injection or alternate dose protocols were 88.3 and 89.1%, respectively. Depending on prevalence, the positive predictive value of the heartworm antigen test ranged from 15 to 54% and negative predictive value from 99 to 99.9%. For a hospital testing 1,000 dogs per year, false-positive test results may vary from 24 to 27 dogs. If these dogs were on heartworm preventive, they may undergo treatment and be classified as prophylaxis failures. Ten percent of dogs who were treated and continued or placed on prophylaxis at the time of treatment may have adult heartworms when tested 1 year later and be presumed to represent failure of prophylaxis.

CONCLUSIONS

When interpreting heartworm antigen test results, clinical signs, history, and regional prevalence of heartworm disease should be considered in estimating the predictive value of the test result. Limitations of test and treatment should be discussed with clients as part of the decision-making process.

Cross-reactions of sera from dogs infected with Angiostrongylus vasorum in commercially available Dirofilaria immitis test kits

Background

Dirofilaria immitis and Angiostrongylus vasorum are both important potentially fatal canine nematodes with overlapping endemic areas, especially in Europe. The preadult and adult stages of both species are living in the Arteria pulmonalis and the right heart, and diagnostically detectable circulating parasite antigens have been demonstrated for both species. For the detection of D. immitis infections, a variety of commercial tests have been developed, however, they have not been evaluated for cross-reactions against circulating antigens of A. vasorum.

Methods

In this study, potential cross-reactions of sera from 16 dogs, which were experimentally infected with A. vasorum and which had circulating antigens as confirmed by a species-specific ELISA, were evaluated for the detection of A. vasorum antigen in six commercially available D. immitis test kits.

Results

In three fast tests (Witness® Dirofilaria, SensPERT® Canine Heartworm, SNAP® 4Dx® Plus), all sera were negative. One fast membrane ELISA (SNAP® HTWM RT Test) was positive with four sera (25%), and one serum delivered a non-valid result twice. In the PetChek® HTWM PF Test, depending on the interpretation protocol, 5 or 8 dogs (31.2 – 50%) were positive. With the DiroCHEK®-ELISA, a single A. vasorum-infected dog (6.2%) tested positive.

Conclusions

Due to potential cross-reactions with A. vasorum in commercially available test kits for the detection of D. immitis antigen, the simultaneous use of highly specific diagnostic methods for the differentiation of these two canine heart worms is recommended.

First analysis of the secretome of the canine heartworm, Dirofilaria immitis

Background

The characterization of proteins released from filariae is an important step in addressing many of the needs in the diagnosis and treatment of these clinically important parasites, as well as contributing to a clearer understanding of their biology. This report describes findings on the proteins released during in vitro cultivation of adult Dirofilaria immitis , the causative agent of canine and feline heartworm disease. Differences in protein secretion among nematodes in vivo may relate to the ecological niche of each parasite and the pathological changes that they induce.

Methods

The proteins in the secretions of cultured adult worms were run on Tris-Glycine gels, bands separated and peptides from each band analysed by ultra mass spectrometry and compared with a FastA dataset of predicted tryptic peptides derived from a genome sequence of D. immitis.

Results

This study identified 110 proteins. Of these proteins, 52 were unique to D. immitis . A total of 23 (44%) were recognized as proteins likely to be secreted. Although these proteins were unique, the motifs were conserved compared with proteins secreted by other nematodes.

Conclusion

The present data indicate that D. immitis secretes proteins that are unique to this species, when compared with Brugia malayi. The two major functional groups of molecules represented were those representing cellular and of metabolic processes. Unique proteins might be important for maintaining an infection in the host environment, intimately involved in the pathogenesis of disease and may also provide new tools for the diagnosis of heartworm infection.

Diagnosis of imported canine filarial infections in Germany 2008 - 2010

Filarial infections of dogs are attracting attention across Europe because of the risk of spread into previously non-endemic areas (e.g. Dirofilaria repens with Culicidae as vectors) and as emerging zoonotic agents. The occurrence of filarial infections in German dogs has been analysed based on 8,545 samples collected either from imported animals or following travel into endemic regions. All samples were tested by means of modified Knott's test and heartworm antigen assay within the period 2008 - 2010. Heartworm antigen was detected in 127 samples (1.49 %; 95 % CI: 1.25 - 1.77 %), but only 38 dogs also had microfilariae in their blood samples. On the other hand, 125 animals (1.46 %; 95 % CI: 1.23 - 1.74 %) were only positive in the Knott's test. For discrimination by means of PCR and sequencing a total of 73 blood samples as well as two samples of adult worms were included, which have been sent by veterinarians during 2008 - 2010. A mono-infection caused by D. repens was detected in 35 cases, while D. immitis was proven in 15 samples, with 6 of these showing a combination of D. immitis and D. repens. Imported Dipetalonema dracunculoides (transmitted by Rhipicephalus sanguineus or Hippobosca longipennis) or Acanthocheilonema reconditum (fleas and lice serve as intermediate hosts) infections were diagnosed in 24 cases and in a single sample a co-infection of A. reconditum and D. repens was evident. D. repens was the most common filarial infection imported and it was introduced into Germany from eleven European countries. Slovenia and Hungary are reported for the first time as endemic for D. repens and A. reconditum, respectively. Furthermore this study reports, to the best of our knowledge, for the first time import of D. dracunculoides from the Canary Islands, A. reconditum from Majorca, D. immitis from Corfu and a co-infection of D. repens and A. reconditum from Spain as well as mixed infections of D. repens and D. immitis from Corfu, Sardinia and Bulgaria. Co-infections with other arthropod-borne infections as well as therapeutical follow-up were also considered. Selamectin (as spot-on formulation) was not able to clear microfilaraemia in dogs infected with either D. repens, A. reconditum or D. dracunculoides, whereas a topical moxidectin/imidacloprid formulation was able to eliminate microfilariae in one dog infected with A. reconditum.

Canine vector-borne disease in travelled dogs in Germany--a retrospective evaluation of laboratory data from the years 2004-2008

When importing dogs from various Mediterranean countries into Western Europe canine vector-borne infections are often considered as a major issue. Several diseases including babesiosis, leishmaniosis, hepatozoonosis, canine heartworm disease or ehrlichiosis can potentially be endemic in this region and pose a potential health risk for travelling dogs. Information on such infections in travelled dogs is scarce and therefore this study has been undertaken to examine the frequency of vector-borne infections in travelled dogs from the years 2004-2008. A total of 997 samples were screened by direct and/or indirect methods. Total seroprevalence was 7.5% with individual seroprevalence for the 3 species Leishmania spp., Ehrlichia canis and Babesia canis spp. ranging from 3.1 to 4.9%. Total detection rate for pathogens by direct methods was 3.5%. Nineteen Giemsa-stained blood smears were positive for large Babesia. None of the samples screened for microfilariae by Knott's test or for Dirofilaria immitis antigen by DiroChek(®) were positive. Using PCR methods Leishmania-DNA was detected in 1/42 samples but none of 59 animals screened for E. canis-DNA was positive. The prevalence values as established by indirect and direct pathogen detection are considered as rather low.

Survey of Canine Dirofilaria immitis Infection in New Caledonia

Canine dirofilariosis is a frequent parasitic disease in New-Caledonia. A survey of canine heartworm (Dirofilaria immitis) infection among dogs from the cities of Tontouta, Nandaï and Nouméa, was performed in March 2009 using two antigen test kits; the microwell ELISA test: DiroCHE (Synbiotics Europe) and the Rapid Immuno Migration (RIM) test: WITNESS DIROFILARIA (Synbiotics Europe). Blood samples were collected from 64 dogs: 49 strays and 15 military working dogs. The military dogs received a permanent chemoprophylaxis (moxidectin). In 11 stray dogs, both tests were positive (22.4%). All the military dogs were negative, showing efficiency of chemoprophaxis. Results were discrepant in 6 dogs, negative with one test and doubtful with the other. Antigen heartworm test kits are available and reliable diagnostic tools. They are useful to evaluate the efficiency of chemoprophylaxis and to detect infected animals in order to treat them and to prevent the spreading of the disease.

Evaluation of a new in-clinic method for the detection of canine heartworm antigen

Canine heartworm is endemic in many parts of the world, and veterinarians rely on rapid in-clinic antigen tests to screen for this infection. Recently, an in-clinic, instrument-based rotor employing a colloidal gold agglutination immunoassay was launched in the marketplace (VetScan VS2(®) Canine Heartworm (HW) Antigen Test Kit; Abaxis, Inc.). Because of the widespread use of heartworm prevention and possible false negative test results in dogs with low heartworm burdens, the performance of the VetScan VS2(®) HW test and a commercially available in-clinic, membrane-based ELISA test (SNAP(®) Heartworm RT Test; IDEXX Laboratories) was compared using samples from dogs with low heartworm burdens and/or low levels of circulating antigen. Ninety serum samples were evaluated using the two methods. Testing was performed according to the manufacturer's product insert by personnel blinded to sample status. The samples were derived from two populations: dogs with necropsy-confirmed heartworm status (40 with 1-4 female worms, 30 with no worms), and field dogs (20) confirmed positive for antigen by microtiter plate ELISA (PetChek(®) Heartworm PF Antigen Test; IDEXX Laboratories). All 40 dogs with heartworms on necropsy were also confirmed to have circulating antigen by the PetChek HW ELISA. In necropsy-negative dogs (n=30), neither the VetScan VS2 HW nor SNAP HW tests detected heartworm antigen. Of the samples testing positive for antigen by PetChek HW (n=60), the VetScan VS2 HW and SNAP HW tests detected antigen in 15 and 56 samples, respectively. Percent agreement (plus 95% confidence interval) for each test relative to the PetChek HW qualitative result was 50% (40-60%) for VetScan VS2 HW and 96% (89-98%) for SNAP HW. Relative to the presence or absence of female worms at necropsy, agreement was 61% (50-72%) for VetScan VS2 HW and 99% (92-99.6%) for SNAP HW tests. It is clinically important that dogs with low heartworm burdens and/or low levels of circulating heartworm antigen be correctly identified by veterinarians in order to ensure prompt treatment, and the VetScan(®) VS2 HW test does not appear to be as accurate as the SNAP HW or PetChek HW tests when performed on this subset of patients.

Clinical relevance of annual screening using a commercial enzyme-linked immunosorbent assay (SNAP 3Dx) for canine ehrlichiosis

Eighty-six dogs were selected based upon Ehrlichia (E.) canis SNAP 3Dx positive results to determine clinical relevance of annual E. canis screening. Immunofluorescence assay showed 72 (84%) of 86 dogs were seroreactive for E. canis. Polymerase chain reaction (PCR) revealed that 12 (14%) of 86 dogs had Ehrlichia deoxyribonucleic acid; seven had E. canis, four had E. ewingii, and one was coinfected with E. chaffeensis and E. ewingii. Thrombocytopenia (<164,000 platelets/microL) was found in 28 (39%) of 72 dogs. In this study, thrombocytopenia was frequently detected in healthy Ehrlichia SNAP 3Dx-positive dogs, whereas active infection was infrequently confirmed by PCR. Therefore, treatment based upon screening results alone is not recommended.