Helminthic transmission and isolation of Ehrlichia risticii, the causative agent of Potomac horse fever, by using trematode stages from freshwater stream snails

Diagnostic approaches, aetiological agents and their associations with short-term survival and laminitis in horses with acute diarrhoea admitted to referral institutions

BACKGROUND

An international description of the diagnostic approaches used in different institutions to diagnose acute equine diarrhoea and the pathogens detected is lacking.

OBJECTIVES

To describe the diagnostic approach, aetiological agents, outcome, and development of laminitis for diarrhoeic horses worldwide.

STUDY DESIGN

Multicentre retrospective case series.

METHODS

Information from horses with acute diarrhoea presenting to participating institutions between 2016 and 2020, including diagnostic approaches, pathogens detected and their associations with outcomes, were compared between institutions or geographic regions.

RESULTS

One thousand four hundred and thirty-eight horses from 26 participating institutions from 4 continents were included. Overall, aetiological testing was limited (44% for Salmonella spp., 42% for Neorickettsia risticii [only North America], 40% for Clostridiodes difficile, and 29% for ECoV); however, 13% (81/633) of horses tested positive for Salmonella, 13% (35/262) for N. risticii, 9% (37/422) for ECoV, and 5% (27/578) for C. difficile. C. difficile positive cases had greater odds of non-survival than horses negative for C. difficile (OR: 2.69, 95%CI: 1.23-5.91). In addition, horses that were positive for N. risticii had greater odds of developing laminitis than negative horses (OR: 2.76, 95%CI: 1.12-6.81; p = 0.029).

MAIN LIMITATIONS

Due to the study's retrospective nature, there are missing data.

CONCLUSIONS

This study highlighted limited diagnostic investigations in cases of acute equine diarrhoea. Detection rates of pathogens are similar to previous reports. Non-survival and development of laminitis are related to certain detected pathogens.

Infectious Causes of Equine Placentitis and Abortion

A variety of infectious agents including viral, bacterial, and fungal organisms can cause equine abortion and placentitis. Knowledge of normal anatomy and the common pattern distribution of different infectious agents will assist the practitioner in evaluating the fetus and/or placenta, collecting appropriate samples for further testing, and in some cases, forming a presumptive diagnosis. In all cases, it is recommended to confirm the diagnosis with molecular, serologic, or microbiological testing. If a causative agent can be identified, then appropriate biosecurity and vaccination measures can be instituted on the farm.

Potomac Horse Fever

Potomac horse fever (PHF) is a common cause of equine colitis in endemic areas. Until recently, the only causative agent known to cause PHF was Neorickettsia risticii. However, N. findlayensis has been isolated from affected horses. Horses typically become infected upon ingestion of Neorickettsia spp.-infected trematodes within aquatic insects. The most common clinical signs include diarrhea, fever, anorexia, lethargy and colic. The diagnostic test of choice for PHF is PCR of blood and feces. Tetracyclines remain an effective treatment. Supportive care, including fluid therapy, colloid administration, NSAID and anti-endotoxin medication, and digital cryotherapy, is also necessary in some cases.

Bacterial and viral enterocolitis in horses: a review

Enteritis, colitis, and enterocolitis are considered some of the most common causes of disease and death in horses. Determining the etiology of these conditions is challenging, among other reasons because different causes produce similar clinical signs and lesions, and also because some agents of colitis can be present in the intestine of normal animals. We review here the main bacterial and viral causes of enterocolitis of horses, including Salmonella spp., Clostridium perfringens type A NetF-positive, C. perfringens type C, Clostridioides difficile, Clostridium piliforme, Paeniclostridium sordellii, other clostridia, Rhodococcus equi, Neorickettsia risticii, Lawsonia intracellularis, equine rotavirus, and equine coronavirus. Diarrhea and colic are the hallmark clinical signs of colitis and enterocolitis, and the majority of these conditions are characterized by necrotizing changes in the mucosa of the small intestine, colon, cecum, or in a combination of these organs. The presumptive diagnosis is based on clinical, gross, and microscopic findings, and confirmed by detection of some of the agents and/or their toxins in the intestinal content or feces.

Potomac horse fever in Ontario: Clinical, geographic, and diagnostic aspects

Clinical findings, geographic locations, laboratory diagnoses, and culture isolation of Neorickettsia spp. in Potomac horse fever (PHF) cases diagnosed in Ontario between 2015 and 2019 are described. Forty-six confirmed PHF cases occurred from late June to early September. Of 41 horses admitted to the Ontario Veterinary College, 28 (68%) survived and 13 (32%) were euthanized due to poor prognosis or financial constraints. Most cases were in southern Ontario along the Canada-USA border. Blood and fecal samples from 43 suspect PHF cases were submitted to 2 laboratories for polymerase chain reaction (PCR) testing for Neorickettsia risticii. Agreement between both laboratories for detection of N. risticii DNA was excellent for feces [κ = 0.932, 95% confidence interval (CI): 0.80 to 1], and fair for blood samples (κ = 0.494, 95% CI: 0.13 to 0.85). Neorickettia spp. were isolated from 16 of 41 (39%) blood samples. DNA analysis confirmed 14 isolates were N. risticii and 2 were N. findlayensis, a novel species of Neorickettsia recently demonstrated to cause PHF.

An Ecotype of Neorickettsia risticii Causing Potomac Horse Fever in Canada

Neorickettsia (formerly Ehrlichia) risticii is an obligatory intracellular bacterium of digenetic trematodes. When a horse accidentally ingests aquatic insects containing encysted trematodes infected with N. risticii, the bacterium is transmitted from trematodes to horse cells and causes an acute and often fatal disease called Potomac horse fever (PHF). Since the discovery of N. risticii in the United States in 1984, using immunofluorescence and PCR assays, PHF has been increasingly recognized throughout North America and South America. However, so far, there exist only a few stable N. risticii culture isolates, all of which are from horses within the United States, and the strain diversity and environmental spreading and distribution of pathogenic N. risticii strains remain poorly understood. This paper reports the isolation of N. risticii from the blood of a horse with acute PHF in Ontario, Canada. Intracellular N. risticii colonies were detected in P388D1 cells after 47 days of culturing and 8 days after the addition of rapamycin. Molecular phylogenetic analysis based on amino acid sequences of major surface proteins P51 and Ssa1 showed that this isolate is distinct from any previously sequenced strains but closely related to midwestern U.S. strains. This is the first Canadian strain cultured, and a new method was developed to reactivate dormant N. risticii to improve culture isolation.

IMPORTANCE

Neorickettsia risticii is an environmental bacterium that lives inside flukes that are parasitic to aquatic snails, insects, and bats. When a horse accidentally ingests insects harboring flukes infected with N. risticii, the bacterium is transmitted to the horse and causes an acute and often fatal disease called Potomac horse fever. Although the disease has been increasingly recognized throughout North and South America, N. risticii has not been cultured outside the United States. This paper reports the first Canadian strain cultured and a new method to effectively culture isolate N. risticii from the horse blood sample. Molecular analysis showed that the genotype of this Canadian strain is distinct from previously sequenced strains but closely related to midwestern U.S. strains. Culture isolation of N. risticii strains would confirm the geographic presence of pathogenic N. risticii, help elucidate N. risticii strain diversity and environmental spreading and distribution, and improve diagnosis and development of vaccines for this dreadful disease.

Germs within Worms: Localization of Neorickettsia sp. within Life Cycle Stages of the Digenean Plagiorchis elegans

Neorickettsia spp. are bacterial endosymbionts of parasitic flukes (Digenea) that also have the potential to infect and cause disease (e.g., Sennetsu fever) in the vertebrate hosts of the fluke. One of the largest gaps in our knowledge of Neorickettsia biology is the very limited information available regarding the localization of the bacterial endosymbiont within its digenean host. In this study, we used indirect immunofluorescence microscopy to visualize Neorickettsia sp. within several life cycle stages of the digenean Plagiorchis elegans Individual sporocysts, cercariae, metacercariae, and adults of P. elegans naturally infected with Neorickettsia sp. were obtained from our laboratory-maintained life cycle, embedded, sectioned, and prepared for indirect immunofluorescence microscopy using anti-Neorickettsia risticiihorse serum as the primary antibody. Neorickettsiasp. was found within the tegument of sporocysts, throughout cercarial embryos (germ balls) and fully formed cercariae (within the sporocysts), throughout metacercariae, and within the tegument, parenchyma, vitellaria, uteri, testes, cirrus sacs, and eggs of adults. Interestingly, Neorickettsia sp. was not found within the ovarian tissue. This suggests that vertical transmission of Neorickettsia within adult digeneans occurs via the incorporation of infected vitelline cells into the egg rather than direct infection of the ooplasm of the oocyte, as has been described for other bacterial endosymbionts of invertebrates (e.g.,Rickettsia and Wolbachia).

Transmission rates of the bacterial endosymbiont, Neorickettsia risticii, during the asexual reproduction phase of its digenean host, Plagiorchis elegans, within naturally infected lymnaeid snails

BACKGROUND

Neorickettsia are obligate intracellular bacterial endosymbionts of digenean parasites present in all lifestages of digeneans. Quantitative information on the transmission of neorickettsial endosymbionts throughout the complex life cycles of digeneans is lacking. This study quantified the transmission of Neorickettsia during the asexual reproductive phase of a digenean parasite, Plagiorchis elegans, developing within naturally parasitized lymnaeid pond snails.

METHODS

Lymnaea stagnalis snails were collected from 3 ponds in Nelson County, North Dakota and screened for the presence of digenean cercariae. Cercariae were identified to species by PCR and sequencing of the 28S rRNA gene. Neorickettsia infections were initially detected using nested PCR and sequencing of a partial 16S rRNA gene of pooled cercariae shed from each parasitized snail. Fifty to 100 single cercariae or sporocysts were isolated from each of six parasitized snails and tested for the presence of Neorickettsia using nested PCR to estimate the efficiency at which Neorickettsia were transmitted to cercariae during asexual development of the digenean.

RESULTS

A total of 616 L. stagnalis were collected and 240 (39%) shed digenean cercariae. Of these, 18 (8%) were Neorickettsia-positive. Six Neorickettsia infections were selected to determine the transmission efficiency of Neorickettsia from mother to daughter sporocyst and from daughter sporocyst to cercaria. The prevalence of neorickettsiae in cercariae varied from 11 to 91%. The prevalence of neorickettsiae in sporocysts from one snail was 100%.

CONCLUSION

Prevalence of Neorickettsia infection in cercariae of Plagiorchis elegans was variable and never reached 100%. Reasons for this are speculative, however, the low prevalence of Neorickettsia observed in some of our samples (11 to 52%) differs from the high prevalence of other, related bacterial endosymbionts, e.g. Wolbachia in Wolbachia-dependent filariid nematodes, where the prevalence among progeny is universally 100%. This suggests that, unlike the Wolbachia-filaria relationship, the Neorickettsia-digenean relationship is not obligatory mutualism. Our study represents the first quantitative estimate of the Neorickettsia transmission through the asexual phase of the digenean life cycle.

Pesquisa de trematódeos digenéticos em Heleobia spp. (Mollusca: Hydrobiidae) em área de ocorrência da Ehrlichiose monocítica equina, no Rio Grande do Sul, Brasil

A ehrlichiose monocítica equina (EME) na região Sul do Rio Grande do Sul tem demonstrado ser importante nas criações de cavalos Crioulos. A enfermidade foi relatada e diagnosticada como causa de diarreia, prejuízos com tratamentos e com a morte de equinos não estabulados, sendo apontada como um fator limitante na criação de equinos em algumas regiões. O modo de transmissão pela via oral, intermediada por trematódeos em ambientes aquáticos, tem sido sustentado. Caracóis dulciaquícolas estão envolvidos como hospedeiros intermediários de trematódeos albergadores de Neorickettsia risticii. Um total de 16.846 caracóis Heleobia foi coletado nos municípios de Arroio Grande, Rio Grande, Palmares do Sul e Santa Vitória do Palmar, 92,2% dos quais foram encontrados nas raízes de aguapés (Eichornea spp.). A frequência de trematódeos presentes nos caracóis variou de 2,3 a 12,8% nas propriedades coletadas. Foram encontrados três tipos de cercárias, morfotipo 1, morfotipo 2 e morfotipo 3, e dois morfotipos de metacercárias nos caracóis. Um total de 357 insetos da ordem Odonata foi coletado, fases de metacercárias foram encontradas no tegumento da subordem Anisoptera com frequência de 5,3%. Mais estudos são necessários para identificar as fases larvais encontradas, bem como para conhecer o hospedeiro definitivo, identificar o parasito adulto e a relação de seu ciclo de vida com a ocorrência da ehrlichiose monocítica equina.

Historical aspects of Potomac horse fever in Ontario (1924-2010)

In the summer of 1924 Dr. Frank W. Schofield conducted investigations into an endemic disease of horses in the Kent and Essex counties of Ontario. According to farmers in these counties the disease had existed in this region for at least 50 years previously. The clinical, pathological, histopathological, and epidemiological findings outlined in Schofield's detailed report strongly suggest that this endemic disease was what was designated in 1979 as "Potomac horse fever" (PHF). This assumption is further substantiated by transmission experiments involving horses and laboratory animals that were conducted by Schofield utilizing horse feces, whole blood, and mayflies. The aim of this paper is to present Schofield's detailed investigations and findings and to compare these with PHF research conducted from 1979 to 2010 that ultimately led to the discovery of Neorickettsia risticii as the etiological agent and to elucidation of the organism's complex life cycle.

Prevalence and molecular characterization of Anaplasmataceae agents in free-ranging Brazilian marsh deer (Blastocerus dichotomus)

Anaplasmataceae organisms comprise a group of obligate intracellular gram-negative, tick-borne bacteria that can infect both animals and humans. In the present work we investigate the presence of Ehrlichia, Anaplasma, and Neorickettsia species in blood samples from Brazilian marsh deer (Blastocerus dichotomus), using both molecular and serologic techniques. Blood was collected from 143 deer captured along floodplains of the Paraná River, near the Porto Primavera hydroelectric power plant. Before and after flooding, marsh deer were captured for a wide range research program under the financial support of São Paulo State Energy Company (CESP), between 1998 and 2001. Samples were divided into four groups according to time and location of capture and named MS01 (n=99), MS02 (n=18) (Mato Grosso do Sul, before and after flooding, respectively), PX (n=9; Peixe River, after flooding), and AGUA (n=17; Aguapeí River, after flooding). The seroprevalences for Ehrlichia chaffeensis and Anaplasma phagocytophilum were 76.76% and 20.2% in MS01, 88.88% and 5.55% in MS02, 88.88% and 22.22% in PX, and 94.12% and 5.88% in AGUA, respectively. Sixty-one animals (42.65% of the total population) were PCR-positive for E. chaffeensis PCR (100.0% identity based on 16S rRNA, dsb, and groESL genes). Seventy deer (48.95% of the total population) were PCR-positive for Anaplasma spp. (99.0% of identity with A. platys, and in the same clade as A. phagocytophilum, A. bovis, and A. platys based on 16S rRNA phylogenetic analysis). Our results demonstrate that Brazilian marsh deer are exposed to E. chaffeensis and Anaplasma spp. and may act as reservoirs for these rickettsial agents, playing a role in disease transmission to humans and other animals.

Neorickettsial endosymbionts of the digenea: diversity, transmission and distribution

Digeneans are endoparasitic flatworms with complex life cycles and distinct life stages that parasitize different host species. Some digenean species harbour bacterial endosymbionts known as Neorickettsia (Order Rickettsiales, Family Anaplasmataceae). Neorickettsia occur in all life stages and are maintained by vertical transmission. Far from benign however, Neorickettsia may also be transmitted horizontally by digenean parasites to their vertebrate definitive hosts. Once inside, Neorickettsia can infect macrophages and other cell types. In some vertebrate species (e.g. dogs, horses and humans), neorickettsial infections cause severe disease. Taken from a mostly parasitological perspective, this article summarizes our current knowledge on the transmission ecology of neorickettsiae, both for pathogenic species and for neorickettsiae of unknown pathogenicity. In addition, we discuss the diversity, phylogeny and geographical distribution of neorickettsiae, as well as their possible evolutionary associations with various groups of digeneans. Our understanding of neorickettsiae is at an early stage and there are undoubtedly many more neorickettsial endosymbioses with digeneans waiting to be discovered. Because neorickettsiae can infect vertebrates, it is particularly important to examine digenean species that regularly infect humans. Rapid advances in molecular tools and their application towards bacterial identification bode well for our future progress in understanding the biology of Neorickettsia.

Neorickettsia risticii surface-exposed proteins: proteomics identification, recognition by naturally-infected horses, and strain variations

Neorickettsia risticii is the Gram-negative, obligate, and intracellular bacterial pathogen responsible for Potomac horse fever (PHF): an important acute systemic disease of horses. N. risticii surface proteins, critical for immune recognition, have not been thoroughly characterized. In this paper, we identified the 51-kDa antigen (P51) as a major surface-exposed outer membrane protein of older and contemporary strains of N. risticii through mass spectrometry of streptavidin-purified biotinylated surface-labeled proteins. Western blot analysis of sera from naturally-infected horses demonstrated universal and strong recognition of recombinant P51 over other Neorickettsia recombinant proteins. Comparisons of amino acid sequences for predicted secondary structures of P51, as well as Neorickettsia surface proteins 2 (Nsp2) and 3 (Nsp3) among N. risticii strains from horses with PHF during a 26-year period throughout the United States revealed that the majority of variations among strains were concentrated in regions predicted to be external loops of their β-barrel structures. Large insertions or deletions occurred within a tandem-repeat region in Ssa3. These data demonstrate patterns of geographical association for P51 and temporal associations for Nsp2, Nsp3, and Ssa3, indicating evolutionary trends for these Neorickettsia surface antigen genes. This study showed N. risticii surface protein population dynamics, providing groundwork for designing immunodiagnostic targets for PHF.

Neorickettsia helminthoeca and salmon poisoning disease: a review

Neorickettsia helminthoeca is an obligate intra-cytoplasmic bacterium that causes salmon poisoning disease (SPD), an acute, febrile, fatal disease of dogs. The complex life-cycle of this pathogen involves stages in an intestinal fluke (Nanophyetus salmincola), a river snail (Oxytrema silicula), in fish, and in fish-eating mammals. This complexity has created confusion with respect to the various bacterial and parasitic infections associated with the disease and its significance in dogs in specific geographical locations has likely to have previously been under-estimated. This paper addresses the history, taxonomy, microbiology of N. helminthoeca and summarises the pathogenesis, clinical signs and pathological features associated with infection. Furthermore, the biological cycles, treatment, control, and both public and veterinary health impacts associated with this pathogen and the intestinal fluke N. salmincola are discussed.

Bacteria isolated from parasitic nematodes--a potential novel vector of pathogens?

Bacterial pathogens are ubiquitous in soil and water - concurrently so are free-living helminths that feed on bacteria. These helminths fall into two categories; the non-parasitic and the parasitic. The former have been the focus of previous work, finding that bacterial pathogens inside helminths are conferred survival advantages over and above bacteria alone in the environment, and that accidental ingestion of non-parasitic helminths can cause systemic infection in vertebrate hosts. Here, we determine the potential for bacteria to be associated with parasitic helminths. After culturing helminths from fecal samples obtained from livestock the external bacteria were removed. Two-hundred parasitic helminths from three different species were homogenised and the bacteria that were internal to the helminths were isolated and cultured. Eleven different bacterial isolates were found; of which eight were indentified. The bacteria identified included known human and cattle pathogens. We concluded that bacteria of livestock can be isolated in parasitic helminths and that this suggests a mechanism by which bacteria, pathogenic or otherwise, can be transmitted between individuals. The potential for helminths to play a role as pathogen vectors poses a potential livestock and human health risk. Further work is required to assess the epidemiological impact of this finding.

The science and fiction of emerging rickettsioses

As newly recognized rickettsial diseases and rickettsial pathogens increase in scope and magnitude, several elements related to the concept of emerging rickettsioses deserve consideration. Newly identified rickettsiae may be mildly pathogenic, or perhaps even nonpathogenic, and have little direct impact on human or animal health, yet nonetheless wield considerable influence on the epidemiology and ecology of historically recognized diseases. In this context "new" rickettsioses provide a lens through which "old" rickettsioses are more accurately represented. Predicting pathogen from nonpathogen is not an exact science, particularly as so few rickettsiae have been broadly accepted as nonpathogenic by contemporary rickettsiologists. However, various factors relating to specific physiologic requirements and molecular machinery of the particular rickettsia, as well as characteristics of its invertebrate host that either position or exclude the rickettsia from infecting a human host, must be considered. Close inspection of mild or atypical forms of historically recognized rickettsioses and a greater emphasis on culture- and molecular-based diagnostic techniques are the keys to identifying future rickettsial agents of disease.

Digenetic trematodes,Acanthatriumsp. andLecithodendriumsp., as vectors ofNeorickettsia risticii, the agent of Potomac horse fever

Neorickettsia(formerlyEhrlichia)risticii, the agent of Potomac horse fever (PHF), has been recently detected in trematode stages found in the secretions of freshwater snails and in aquatic insects. Insectivores, such as bats and birds, may serve as the definitive host of the trematode vector. To determine the definitive helminth vector, five bats (Myotis yumanensis) and three swallows (Hirundo rustica,Tachycineta bicolor) were collected from a PHF endemic location in northern California. Bats and swallows were dissected and their major organs examined for trematodes and forN. risticiiDNA using a nested polymerase chain reaction (PCR) assay. Adult digenetic trematodes,Acanthatriumsp. and/orLecithodendriumsp., were recovered from the gastrointestinal tract of all bats and from one swallow. The intestine of three bats, the spleen of two bats and one swallow as well as the liver of one swallow tested PCR positive forN. risticii. From a total of seven pools of identical digenetic trematodes collected from single hosts, two pools ofAcanthatriumsp. and one pool ofLecithodendriumsp. tested PCR positive. The results of this investigation provide preliminary evidence that at least two trematodes in the family Lecithodendriidae are vectors ofN. risticii. The data also suggest that bats and swallows not only act as a host for trematodes but also as a possible natural reservoir forN. risticii.

Ehrlichiose monocítica eqüina no Rio Grande do Sul: aspectos clínicos, anátomo-patológicos e epidemiológicos

Descreve-se um surto natural de Ehrlichiose mono-cítica eqüina (EME) ocorrido nos meses de novembro/2001 a fevereiro/2002, em uma propriedade no município de Arroio Grande, localizado à margem oeste da Lagoa Mirim, Rio Grande do Sul. De um total de 50 eqüinos, 13 foram afetados pela enfermidade e seis morreram. A morbidade foi de 26% e a letalidade 46,1%. Os animais acometidos tinham idades entre um e cinco anos e o quadro clínico mais evidente foi o desenvolvimento de diarréia aguda aquosa e profusa. Um dos animais que morreram foi necropsiado e as lesões observadas localizavam-se principalmente no trato intestinal, caracterizando-se por intestino grosso com conteúdo líquido e áreas hemorrágicas difusas na mucosa. Em todo o segmento de intestino delgado observaram-se áreas com mucosa congesta e hemorrágica alternadas com áreas de mucosa normal. Na mucosa de jejuno e íleo observou-se a presença de nódulos contendo secreção purulenta. Os achados histológicos em todo trato intestinal caracterizaram-se pela presença de discreto infiltrado mononuclear restrito a mucosa do órgão, caracterizando uma enterite linfo-histiocitária com predominância de macrófagos. No jejuno e íleo também havia regiões com lesões granulomatosas na lâmina própria, relacionadas a invaginações das criptas das vilosidades intestinais. Para o diagnóstico da infecção por Neorickettsia (Ehrlichia) risticii amostras de sangue de sete eqüinos doentes foram testados pela reação em cadeia da polimerase (PCR), resultando em três amostras positivas (42,8%) com a formação de uma banda de 529pb, típica para N. risticii.

NEORICKETTSIA (EHRLICHIA) RISTICII NO SUL DO BRASIL: HELEOBIA SPP. (MOLLUSCA: HYDROBILIDAE) E PARAPLEUROLOPHOCECOUS CERCARIAE (TREMATODA: DIGENEA) COMO POSSÍVEIS VETORES

RESUMO Para pesquisa de possíveis vetores envolvidos na transmissão da erliquiose monocítica eqüina (EME) no Rio Grande do Sul, Brasil, caramujos dulciaquícolas dos gêneros Drepanotrema, Physa, Pomacea, Biomphalaria e Heleobia foram coletados em propriedades com histórico de ocorrência de EME localizadas nos Municípios de Rio Grande, Arroio Grande e Santa Vitória do Palmar, extremo sul do Estado. A Neorickettsia (Ehrlichia) risticii foi detectada através de PCR em uma freqüência de 13,33%, em caramujos do gênero Heleobia (H. piscium, H. parchappei e H. davisi). Nesses caramujos, estágios de cercárias e rédias de um trematódeo não identificado foram encontrados. Cercárias emergidas de H. piscium foram classificadas como Parapleurolophocercous cercariae. Esses trematódeos testados por PCR foram positivos para N. risticii em 20% das amostras. Com esses resultados pode-se afirmar que a N. risticii está no ambiente veiculada por caramujos do gênero Heleobia presentes em canais de irrigação e rios da região costeira da Lagoa Mirim, e em trematódeos portados por eles.

Two cases of Neorickettsia (Ehrlichia) risticii infection in horses from Nova Scotia

Two horses from Nova Scotia were diagnosed with Potomac horse fever (PHF). Polymerase chain reaction analysis was performed on formalin-fixed colon tissue or whole blood to show the presence of Neorickettsia risticii DNA, the causative agent of PHF. These are the first reported cases of PHF in the Maritime Provinces.

Identification of trematode cercariae carrying Neorickettsia risticii in freshwater stream snails

We provide evidence of Neorickettsia (Ehrlichia) risticii Holland, the agent of Potomac horse fever, in trematode larval stages found in aquatic snails and insects collected from a stream in Korea, using nested polymerase chain reaction (PCR) and sequence analysis of the 16S rRNA gene fragment amplified from several cercaria species. It was observed that of 423 (13.1%) of 3,219 snails infected with cercariae, 77 (20.8% of the 371) were infected with N. risticii. Five families of trematode cercariae, Schistosomatidae, Echimostomatidae, Heterophyidae, Microphallidae, and Acanthocopidae were identified morphologically within Semisulcospira libertina, Radix auricularia coreana, and S. gottschei snails. Echinostoma cinetorchis, E. hortense, and Metagonimus sp. were identified based on both the cercarial morphology as well as by phylogenetic analysis of the amplified 18S rRNA gene sequences. Adult aquatic insects were also collected from the same sites and were sorted into five species, Ischnura asiatica in Coenagrionidae and Calopteryx japonica, Sympetrum darwinianum, Symptrum eroticum, and Symptrum parvulum in Calopterygoidae. One thousand and two hundred eighty five metacercariae (classified into groups A through F) were isolated from 310 adult aquatic insects, and the average number of metacercariae per aquatic insect was 4.1. However, there was no amplification of N. risticii from these metacercariae.

Detection of Neorickettsia (Ehrlichia) risticii in tissues of mice experimentally infected with cercariae of trematodes by in situ hybridization

Neorickettsia (Ehrlichia) risticii was demonstrated to occur in cercariae developing in Juga yrekaensis snails by experimental transmission, genetic detection and histopathology. Cercariae were isolated from the digestive glands of snails collected in a fresh stream water area of Siskiyou County, CA, and inoculated into CF1 mice. Mice developed clinical signs, splenomegaly and histopathologic abnormalities. The agent was maintained by serial passages of whole blood in CF1 mice. A 527-bp product of the 16S rRNA gene of N. risticii was serially detected by nested PCR in blood, feces, salivary gland, suprarenal gland, spleen, intestine and bone marrow of inoculated mice. N. risticii DNA was detected by in situ hybridization with DIG-labeled probe in PCR-positive salivary gland, intestine and spleen tissue sections of experimental mice on day 30 after inoculation. Infection in mice was established when cercariae were inoculated by either IP or SC routes but not established following intraoral route. N. risticii was detected by PCR in spleen, intestine and bone marrow even after 73 days post-inoculation whereas blood from the same animals became negative at 58 days. N. risticii was observed by in situ hybridization in salivary gland, spleen and intestine of mice infected by IP or SC inoculation. This ISH protocol should aid investigations on the host range of the Neorickettsiosis and pathogenesis of neorickettiosis in vector, animal or human.

Molecular analysis of Neorickettsia risticii in adult aquatic insects in Pennsylvania, in horses infected by ingestion of insects, and isolated in cell culture

Upon ingestion of adult aquatic insects, horses developed clinical signs of Potomac horse fever, and Neorickettsia risticii was isolated from the blood. 16S rRNA and 51-kDa antigen gene sequences from blood, isolates, and caddis flies fed to the horses were identical, proving oral transmission of N. risticii from caddis flies to horses.

Equine monocytic Ehrlichiosis (Potomac horse fever) in horses in Uruguay and southern Brazil

A disease named locally as churrío or churrido equino (i.e., equine scours) has occurred for at least 100 years in Uruguay and southern Brazil in farms along both shores of the Merín lake. This report describes cases of churrido equino and provides serologic, pathologic, and DNA-based evidence indicating that the disease is in fact equine monocytic ehrlichiosis (Potomac horse fever). Results of an epidemiological investigation conducted on an endemic farm are also presented. Clinical signs in 12 horses were fever, depression, diarrhea, dehydration, and sometimes colic and distal hind limb edema. Postmortem findings of 3 horses were of acute enterocolitis. Inclusion bodies containing ehrlichial organisms were found in the cytoplasm of macrophages of the large colon of 1 horse. Eleven of the 12 horses were serologically positive to Ehrlichia risticii (indirect fluorescent antibody assay) and, of 3 paired samples, 2 showed seroconversion. Ehrlichia risticii DNA was identified by a nested polymerase chain reaction in peripheral blood of an affected horse. A healthy horse inoculated with peripheral blood from an affected horse developed the disease and antibodies to E. risticii. The disease had a peak incidence in March (summer) and was statistically associated with a marshy ecosystem near the Merín lake, where large numbers of Pomacea spp. (Ampullariidae) snails were found. Incidence density was almost 8 times higher in nonnative horses than in native horses. It was concluded that the previous diarrheic disease of horses known in Uruguay and southern Brazil as churrido equino is equine monocytic ehrlichiosis.

Ehrlichial diseases

Equine granulocytic and monocytic ehrlichiosis caused by Ehrlichia equi and E. risticii, respectively, are seasonal diseases in horses that occur throughout the United States E. equi is transmitted by lxodes ticks and causes high fever, depression, anorexia, limb edema, petechiation, icterus, ataxia, and stiffness in gait. E. risticii, also known as the agent of Potomac horse fever, causes a febrile illness with a colitis of variable severity. Its occurrence is associated with aquatic habitats. The natural route of transmission is oral, through the ingestion of E. risticii infected trematode stages either free in water or in an intermediate host, such as aquatic animals.

Molecular detection of an Ehrlichia-like agent in rainbow trout (Oncorhynchus mykiss) from Northern California

Ehrlichia DNA was identified by nested PCR in rainbow trout (Oncorhynchus mykiss) collected from a creek in northern California where Potomac horse fever is endemic. Ehrlichia DNA was found in tissues from several organs including the gills, heart, spleen, liver, kidneys and intestine of trout and from three different adult digenetic trematodes (Deropegus sp., Crepidostomum sp., Creptotrema sp.) parasitizing the gallbladder and/or the intestine of the trout. Sequencing of PCR-amplified DNA from the 16S rRNA gene indicated that the source organism was most closely related to the sequences of E. risticii (level of sequence similarity 96.0%), the SF agent (95.9%), E. sennetsu (95.8%), and Neorickettsia helminthoeca (95.3%). The data suggest that trout and parasitic trematodes may be involved in the epidemiology of an Ehrlichia-like agent belonging to the E. sennetsu genogroup. Whether the fish agent infects horses, dogs, or human beings, and whether it causes disease, remain to be determined.

Infection rate of Ehrlichia risticii, the agent of Potomac horse fever, in freshwater stream snails (Juga yrekaensis) from northern California

Juga yrekaensis freshwater snails were tested for trematode stages and for Ehrlichia risticii DNA using a nested PCR assay. Snails were collected monthly from two Potomac horse fever (PHF) endemic locations in northern California (Montague and Weed). The trematode infection rate varied between 40 and 93.3% in large snails (shell size >15mm) and between 0 and 13.3% in small snails (<15mm). The highest trematode infection rate for large and small snails was recorded in September and the lowest infection rate for large snails was recorded in June (Weed) and October (Montague). The E. risticii PCR infection rate among small snails from both sites was similar and varied monthly between 0 and 3.3%. The PCR infection rate for large snails from Weed was high in May (20.0%) and decreased progressively until November (10.0%). The PCR infection rate for large snails from Montague was 5.0% in May, 26.3% in August and 16. 7% in October. PCR-positive snails were always related to the microscopic detection of trematode stages (virgulate cercariae). This study provides evidence that J. yrekaensis are infected with trematode cercariae that harbor E. risticii. The number of snails harboring trematode stages and the number of PCR positive snails varied with the size of the snails, the month of collection, and the geographic origin.

Analysis of 16S rRNA and 51-kilodalton antigen gene and transmission in mice of Ehrlichia risticii in virgulate trematodes from Elimia livescens snails in Ohio

Operculate snails (the family Pleuroceridae: Elimia livescens) were collected between June and October 1998 from a river in central Ohio where repeated cases of Potomac horse fever (PHF) have occurred. Of collected snails, consistently 50 to 80% carried a combination of cercariae and sporocysts of digenetic virgulate trematodes. The trematodes obtained from each snail were pooled and tested for Ehrlichia risticii, the agent of PHF, by nested PCR using primers specific to the 16S rRNA gene. Out of a total of 209 trematode pools, 50 pools were found to be positive by PCR. The DNA sequence of the 16S rRNA gene identified in one trematode pool was identical to that of the type strain of E. risticii, and the sequence of the gene identified in another pool differed from that of the type strain by 1 nucleotide. Comparison of the deduced amino acid sequence of the partial 51-kDa antigen gene from various sources revealed that Maryland, Ohio (except Ohio 081), and Kentucky strains are in a cluster distinct from the sequences obtained from sources in California and Oregon. Ohio 081 was shown previously by antigenic composition analysis to be distinct from other groups. However, all sequences examined were not segregated according to their sources: horse blood or infected trematodes. E. risticii was found to be transmittable from trematodes to mice and was subsequently passaged from infected mice to additional mice, as determined by PCR analysis. Our findings suggest the evolution of E. risticii in the natural reservoir in separate geographic regions and persistent infection of trematode populations with E. risticii during summer and early fall. The study also suggests that the mouse can be used to isolate E. risticii from the infected trematode.

Infection of aquatic insects with trematode metacercariae carrying Ehrlichia risticii, the cause of Potomac horse fever

We provide evidence of Ehrlichia risticii Holland, the agent of Potomac horse fever, in trematode stages found in aquatic insects collected from a pasture stream in northern California, using nested polymerase chain reaction (PCR) amplification and sequence analyses of the 16S rRNA, 51 kDa major antigen and groEL heat shock protein genes. E. risticii was detected in metacercariae found in the immatures and adults of the following insects: caddisflies (Trichoptera), mayflies (Ephemeroptera), damselflies (Odonata, Zygoptera), dragonflies (Odonata, Anisoptera), and stoneflies (Plecoptera). The prevalence of E. risticii was 31.9% (n = 454 individuals) in aquatic insects (13 of 17 species were positive). Prevalence within orders was as follows: 43.5% (n = 207) in caddisflies, 15.2% (n = 92) in mayflies, 13.9% (n = 115) in damselflies, 10.0% (n = 10) in dragonflies, and 80.0% (n = 30) in stoneflies. This study demonstrates a broad intermediate host range for trematodes that act as vector for E. risticii. Insects are likely to play an important role in the epidemiology of this disease.

Transmission of Ehrlichia risticii, the agent of Potomac horse fever, using naturally infected aquatic insects and helminth vectors: preliminary report

Ehrlichia risticii, the agent of Potomac horse fever (PHF), has been recently detected in trematode stages found in snail secretions and in aquatic insects. Based on these findings, horses could conceivably be exposed to E. risticii by skin penetration with infected cercariae, by ingestion of infected cercariae in water or via metacercariae in a second intermediate host, such as an aquatic insect. In order to test this hypothesis, horses were challenged with infectious snail secretions and aquatic insects collected from a PHF endemic region in northern California. Two horses stood with their front feet in water harbouring E. risticii-infected cercariae, 2 horses drank water harbouring E. risticii-infected cercariae, and 6 horses were fed pools of different aquatic insects harbouring E. risticii-infected metacercariae. In this preliminary study, only the one horse infected orally with mature caddisflies (Dicosmoecus gilvipes) developed the clinical and haematological disease syndrome of PHF. The agent was isolated from the blood of the infected horse in a continuous cell line and identified as E. risticii by characterisation of the 16S rRNA gene. Therefore, E. risticii is maintained in nature in a complex aquatic ecosystem and transmission to horses can occur through accidental ingestion of insects such as caddisflies containing infected metacercariae. At present, the small number of horses used in this study does not exclude other insects and free trematode stages as potential sources of infection.

Detection and quantitation of Ehrlichia risticii genomic DNA in infected horses and snails by real-time PCR

A real-time quantitative PCR using the TaqMan fluorogenic detection system (TaqMan PCR) was established for identification of Ehrlichia risticii, the agent of Potomac horse fever (PHF). The TaqMan PCR identified an 85 base pair section of the 16S rRNA gene by use of a specific fluorogenic probe and two primers. This technique was specific for eight tested E. risticii strains. The TaqMan system identified 10 copies of a cloned section of the 16S rRNA gene of E. risticii. The sensitivity and specificity of the TaqMan PCR were similar to those of conventional nested PCR. The TaqMan PCR was evaluated on horses with infectious colitis and on freshwater stream snails collected from regions with a history of PHF. E. risticii could be detected in 22 of 153 (14.4%) horses with infectious colitis and in 25 of 234 (10.7%) snails in the TaqMan PCR. The same results were obtained in the conventional nested PCR. The Ehrlichia-load was in the range of 10,000-9,000,000 and 35,000-680, 000,000 Ehrlichia equivalents per microg leukocyte DNA and snail DNA, respectively.