Decreased Clinical Severity of Strangles in Weanlings Associated with Restricted Seroconversion to Optimized Streptococcus equi ssp equi Assays

Background

Streptococcus equi ssp. equi causes characteristic clinical signs that are most severe in young horses, including fever, purulent nasal discharge, and lymph node abscessation in the head region.

Hypothesis/Objectives

Clinical, serologic, and microbiologic factors related to unexpectedly mild disease severity in a natural outbreak of strangles in immunologically naïve weanlings were investigated.

Animals

One‐hundred and twelve warmblood weanlings.

Methods

Prospective longitudinal observational study of a natural outbreak of strangles. The entire cohort was examined at the peak of the outbreak by deep nasal swabs for culture and quantitative PCR (qPCR) for the presence of S. equi and clinically and serologically in a sequential manner by an optimized ELISA from the index case throughout the outbreak until resolution. Descriptive statistics were calculated and comparisons made using a nondirectional Wilcoxon signed‐rank test.

Results

Outbreak morbidity was 53%, with 9 of 14 horses culture positive and 26 of 53 horses qPCR positive for S. equi lacking clinical signs characteristic of strangles. By resolution, 91 of 112 had seroconverted to Antigen A by ELISA but seroconversion to antigen C (part of the SeM protein) was minimal. Sequencing of the isolates detected no alterations in the SeM protein, but identified a 61 bp deletion in the gene SEQ_0402.

Conclusions and Clinical Importance

Absence of clinical signs alone in naïve horses may be an insufficient criterion to release horses from strangles quarantine measures. Restricted seroconversion to antigen C may have been associated with decreased clinical severity. The role of a minor gene deletion in SEQ_0402 in the virulence of S. equi warrants further investigation.

Development and application of a standardized assay for chemical disinfection of coccidia oocysts

The development and application of a standardized model for testing of anticoccidial disinfectants are described. Due to its economic impact, tenacity of oocysts, and reproducibility of the course of infection Eimeria tenella has been chosen as test organism. Oocysts of the Houghton strain were more susceptible to disinfection with 4% TP4 (Preventol) than oocysts of a field isolate (FI 292/1) as determined by sporulation inhibition and lysis. Scoring of intestinal lesions and of oocyst numbers in mucosal scrapings in chicken infected with various doses of oocysts were found unsuitable for assessment of disinfectants. Because strain differences were observed only Houghton strain oocysts were applied for further testing. Guidelines for standardized in vivo testing of disinfectants have been stipulated by the German Veterinary Society (DVG) on the basis of these studies. When applied for testing of Neopredisan (NP) in two separate laboratories similar results were obtained. Inhibitory activity (IA; proportion of inactivated oocysts) of 92.9 and 90.6% were calculated for 3% NP and of 95.2 and 96.8% for 4% NP after treatment with the disinfectant over 120 min. According to the guidelines IA of at least 95% is required for certification of sufficient disinfecting efficacy by the DVG.

The 18 kDa antigen of Theileria equi is a specific but less abundant protein also expressed by parasites cultured in vitro

A universally applicable test has not yet been developed for the reliable diagnosis of equine theileriosis, mainly because of the paucity of specific antigens and antigenic variation between different isolates. In this study, we used Theileria equi parasites cultured in vitro to identify potential diagnostic antigens. Using preparative isoelectric focusing to resolve the proteins in a lysate of infected erythrocytes, we identified an 18 kDa component of the parasite as a specific but poorly expressed antigen. This antigen also appears to have conserved epitope(s) between the isolates from the New and the Old World, as positive sera from both European and South American horses recognize it. The recombinant replica of this antigen might be a valuable tool for inclusion in the development strategy for a diagnostic test.

Purification of a 28 kDa Babesia (Theileria) equi antigen and a 29 kDa spurious erythrocyte antigen from in vitro culture through ion exchange chromatography

An extract of in vitro cultivated Babesia equi was fractionated using a MonoQ anion exchange column. Separation of a 28 kDa B. equi antigen from a 29 kDa spurious erythrocyte antigen, both of which were intensely immunoreactive, was achieved by chromatography of the infected erythrocyte proteins. Using tricine-SDS-PAGE, the 28 kDa antigen of B. equi showed multiple band resolution, while the 29 kDa antigen was consistently resolved as a single band. The 29 kDa antigen was identified in both infected and non-infected erythrocyte extracts. Moreover, B. equi antiserum recognised this antigen in the non-infected erythrocyte extract, and conversely serum from horses not infected with babesia detected the antigen in infected erythrocyte extract. This 29 kDa antigen could represent a horse erythrocyte isoantigen. The purified 28 kDa antigen is specifically recognised by B. equi antisera and therefore could be useful for the production of the recombinant replica and to employ these in further test systems.

Seroepidemiologic studies on Babesia equi and Babesia caballi infections in Brazil

Horses from six stud farms representing the most frequent types of horse breeding in Brazil were tested for Babesia antibodies by the IFA test. The farms are located at the tropic of Capricorn at an altitude of 472-715 m where temperatures below 0 degrees C may occur. Horses of conventional stud farms were infested with Dermacentor nitens, Amblyomma cajennense, and Boophilus microplus. Infestation with Bo. microplus was associated with direct or indirect contact of horses with cattle, and was not detected at professional stud farms. At one large professional stud farm, only D. nitens was observed. Prevalence of Babesia equi correlated positively (p<0.001) with contact of pastured horses to cattle (67.1% versus 17.5%). The IFA test was validated using sera from 92 non-infected horses and from 18 ponies infected experimentally with the USDA strains of B. equi or B. caballi or with the Brazilian isolates from the study area. Differences in test results obtained using antigens from USDA strains or Brazilian isolates were not significant. The specificity was 100% except for the IFA test using Brazilian antigen of B. caballi (98%). The sensitivity was 100% except for the IFA test using the USDA antigen of B. caballi. Accuracy of the tests ranged from 98 to 100%, and predictive values from 99 to 100%. Only 59% (132/224) of sera, positive by the IFA test for B. equi, tested positive by CF test, and 45% (232/515) of sera, positive by the IFA test for B. caballi, also tested positive by CF test. In all, 740 field sera from 140 horses, including 63 mares and their foals, were tested. Prevalence and incidences of B. caballi infections were significantly higher than those of B. equi infections: 49.2% (31/63) of the mares were infected with B. equi, but 79.4% (50/63) with B. caballi; 36% (18/50) of the foals became infected with B. equi within 12 months, but 100% (50/50) with B. caballi within 10 months. Maternal antibodies against B. equi and B. caballi in foals were 44 (22/50) and 68% (34/50), respectively. Titers persisted for 1-5 months for B. equi and 1-4 months for B. caballi.

Epidemiological aspects of equine babesioses in a herd of horses in Brazil

Epidemiological studies of Babesia equi and B. caballi were undertaken in a herd of 120 pastured horses in Rio de Janeiro, Brazil. The area where the horses were held was shown to be highly endemic for both Babesia spp., i.e. the prevalence of B. equi antibodies in horses aged 6 months or older ranged from 90.6% to 100% as determined by the immunofluorescence antibody (IFA) test, and the prevalence of B. caballi antibodies as determined by Western blot ranged from 59.4% to 65.5%. From the herd, 20 foals and their dams were selected to estimate the degree of tick infestation and the foals were bled at monthly intervals to determine the incidence of antibodies to B. equi and B. caballi. The incidence of B. equi was 100% by about 127 days of age as determined by IFA of B. caballi was 100% by about 150 days of age as determined by Western blot. Tick infestation of the horses estimated by using a semiquantitative key ranged from at least five ticks on every horse to more than 100 ticks on many horses throughout the year. Except for three Boophilus microplus female ticks, they were identified as Amblyomma cajennense and Anocentor nitens. A. cajennense had one generation per year, whereas An. nitens had three. Kinetes of B. caballi were detected in the haemolymph of two of 68 An. nitens female ticks and in the ovary and eggs of one of these, suggesting that this tick is a significant vector of B. caballi.(ABSTRACT TRUNCATED AT 250 WORDS)

Current state and future trends in the diagnosis of babesiosis

An overview is given of the currently available methods to diagnose babesiosis in livestock. Microscopic techniques are still the only appropriate techniques to diagnose acute disease. Thin or thick blood films stained with Giemsa's stain are sufficient. The sensitivity ranges from 10(-5) to 10(-6), i.e. one parasite per 10(5)-10(6) erythrocytes can be detected. Thick films stained with acridine orange (sensitivity approximately 10(-7)) and the Quantitative Buffy Coat (QBC) analysis tube system (sensitivity approximately 10(-7)-10(-8)) are applicable for diagnosis in the laboratory. DNA probes are very specific tools to identify haemoparasites in organs post mortem and in ticks. For the identification of carrier animals the sensitivity (approximately 10(-5)-10(-6)) is generally not sufficient. For the latter the polymerase chain reaction (PCR) technique is a very powerful tool (sensitivity approximately 10(-9)). Many different serodiagnostic tests have been described; however, the immunofluorescence antibody test is the most widely used, while the enzyme-linked immunosorbent assay (ELISA) is the test system which holds the greatest promise for the future. Thus far, improvements to the ELISA have been limited as the quality of antigen preparations made from infected blood is generally poor with a few exceptions (Babesia bovis, Babesia caballi). Potentially, most of the problems associated with crude antigens can be overcome by the production of recombinant antigens. Several ELISAs based on highly defined recombinant antigens have been described and show promise. None of these tests has been validated to the extent that it could be applied globally. Future research requirements as well as the need for coordination of the research effort and collaboration between institutions involved in the diagnosis of babesiosis are discussed.

Cloning and expression of two genes from Babesia equi merozoites and evaluation of their diagnostic potential

High-titre equine immune sera were used to screen a lambda gt 11 expression library of Babesia equi cDNA fragments. Two cDNA clones which did not cross-hybridize to each other were studied. Both clones hybridized specifically to DNA from B. equi but not to DNA from B. caballi, B. divergens or B. ovis. Recombinant proteins were expressed as glutathione S-transferase (GST) fusion proteins with apparent molecular weights of 40 kDa and 75 kDa. Polyclonal antibodies directed against the 40 kDa and 75 kDa recombinant proteins detected native antigens of 55 kDa and 50 kDa respectively in crude lysates of B. equi merozoites indicating that neither cDNA clone was full length (GST = 26 kDa). In Western blotting experiments the 75 kDa protein showed cross-reactivity with sera from horses infected with B. caballi and was not further investigated. The 40 kDa protein was additionally tested in an enzyme-linked immunosorbent assay (ELISA). A test was developed which had a calculated specificity of 99% and a sensitivity of 88% with sera from horses infected with the homologous strain of B. equi. The ELISA did not recognize sera from horses infected with B. equi strains from Brazil and Morocco.

Polyclonal antibody characterization of Babesia caballi antigens

In a previous study diagnostic B. caballi antigens with apparent molecular mass of 50 and 48 kDa were identified. Another antigen of 141 kDa was recognized by most but not all B. caballi sera tested. Here a further characterization of the three antigens is reported. Rabbits were vaccinated with gel-purified antigens and monospecific antibodies were obtained for the 141 and 48 kDa antigens. Antibodies raised against the 50 kDa antigen cross-reacted with the 48 kDa antigen, suggesting that these two antigens bear unique as well as common epitopes. After two-dimensional electrophoresis the 50 and 48 kDa antigens were present as horizontal bands over a pH range from approximately 5.0-7.0 with focused spots at a pH of 5.5 and 5.9, respectively. The 141 kDa antigen was not present after two-dimensional electrophoresis. None of the three antigens could be identified as a glycoprotein. Judging from the immunofluorescence antibody test staining pattern obtained with the rabbit sera the 141 kDa antigen is present on the surface of infected erythrocytes. The 50 and 48 kDa antigens are located in the parasite itself and probably not on the surface of infected erythrocytes. The punctate staining pattern observed with the 48 kDa antiserum suggests that this antigen might be located in or associated with the apical complex of the parasite.

Diagnosis of Babesia caballi infections in horses by enzyme-linked immunosorbent assay (ELISA) and western blot

From Babesia caballi in vitro cultures a preparation of 100% infected erythrocytes was obtained. From this, B. caballi antigens were extracted with the detergent 3-[(3-Cholamidopropyl)-dimethylammonio]-1-propane-sulfonate (CHAPS) and used as ELISA antigens. A control antigen of normal erythrocytes from the same donor horse was prepared in an identical manner. The ELISA and Western blot were validated by testing of sera from horses experimentally infected with B. caballi or B. equi or not infected with Babesia spp. ELISA and Western blot results were compared with those obtained by the immunofluorescence antibody test (IFAT) and complement fixation test (CFT). The sensitivity of the ELISA of 98.3% obtained for sera from day 14 after infection was superior to the Western blot (94.9%), the IFAT (96.6%) and the CFT (28.8%). No positive results were obtained in the ELISA and Western blot with 106 sera from horses not infected with Babesia spp. resulting in a calculated specificity of 100% for both tests. Cross reactions of B. equi-positive sera did occur to a larger extent in the ELISA (20%) than in the IFAT (4%). No cross reactions were observed with the Western blot and the CFT. The higher sensitivity of the ELISA was also demonstrated by testing of 132 field sera: more positive results were obtained by ELISA (112) as compared to IFAT (92) or CFT (41). The validity of these results was confirmed by testing of sera by Western blot. The ELISA as the most sensitive test provides the best method for the identification of carrier horses to prevent the introduction into non-endemic areas (export testing).(ABSTRACT TRUNCATED AT 250 WORDS)

Cryopreservation of Babesia caballi cultures

Babesia caballi cultures were cryopreserved with a solution of 10% (w/v) polyvinylpyrrolidone 40 as cryoprotectant. Samples were cooled at rates of 1, 10, 30 and 100 degrees C min-1 using a programmable freezer. Additionally, a styrofoam box designed to cool samples at an approximate rate of 10 degrees C min-1 when placed in a -80 degrees C freezer was used. Samples were stored in liquid nitrogen, thawed rapidly and inoculated into cultures. Although, a high loss of infectivity was observed after cryopreservation, cultures could be initiated reliably from cryo-stabilates frozen at a rate of 10 and 30 degrees C min-1 or frozen with the styrofoam box.

Identification of diagnostic antigens for South American Babesia caballi infections

Sera from 60 horses held in breeding herd in Brazil were examined monthly by ELISA, immunofluorescence antibody test (IFAT) and Western blot. All foals had maternal antibodies detectable by ELISA and IFAT, and sero-conversion took place between the 2nd and 5th month of age. The 48 and 50 kDa antigens were recognized first in the course of infection. Of 79 sera taken after sero-conversion 78 reacted with the 48 kDa antigen, 76 with the 50 kDa, 50 with the 70 kDa, 54 with the 112 kDa, 72 with the 141 kDa antigen. In general, sera from horses older than 1 year reacted with all 5 diagnostic antigen bands, but sera from horses older than 3 years only weakly. The antigens of 48 and 50 kDa appear to be conserved among all strains of B. caballi examined so far and are consistently recognized by all infected horses. They are the target antigens for a serological test based on antigens produced by recombinant DNA techniques.

Identification of antigens diagnostic for European isolates of Babesia equi by two-dimensional electrophoresis and western blotting

A lysate of Babesia equi-infected erythrocytes (USDA strain) was separated by two-dimensional electrophoresis and analysed by Western blotting. Nine major antigens or antigen groups with mol. wts. ranging from 43 to 19 kDa were recognized by sera from horses experimentally infected with the USDA strain. Four antigens or antigen groups were also recognized by some or all sera from horses infected with B. caballi or not infected with Babesia spp. Of the remaining five antigens, four were recognized by all sera from field-infected horses from Europe. Thus, four antigens with mol. wts. of 33, 31, 19 and 20 kDa were identified as diagnostic antigens for European isolates of B. equi. None of the antigens diagnostic for European isolates was recognized by sera from field-infected horses from Brazil.

Development of Trypanosoma (M.) theileri in tabanids

Thus far the life cycle of Trypanosoma (Megatrypanum) theileri has not been studied. We collected tabanids during the mass hatching, when only few tabanids are infected with trypanosomes. Tabanids were caught immediately after attacking a bait cow to serve as controls or after they had been allowed to engorge on the Trypanosoma (M.) theileri-infected cow. Tabanids were kept in the laboratory and used to study the developmental cycle of T. (M.) theileri in the tabanid gut. From day 1 to day 10 the presumably unfed controls and the engorged tabanids were dissected and cytological smears made from the mid- and hindgut. In total 2.6% (1/38) of the controls and 39% (23/59) of the engorged tabanids were positive for trypanosomes in the 1991 season. From day 1 to day 4 after engorgement trypanosomes were found in the midgut. Epimastigotes with a length of 29 microm on day 1 after infection multiplied by inequal division to form smaller epimastigotes of 26 microm on day 3. On day 4 morphologically indistinguishable trypanosomes of 21 microm total length were found in both mid- and hindgut. From day 5 to day 10 trypanosomes were found only in the hindgut in which the transformation to metacyclics was demonstrated, i.e., epimastigotes transformed to amastigote stages of 5 microm in total length.

Cryopreservation of Babesia divergens from jirds as a live vaccine for cattle

A Babesia divergens live vaccine can be produced in jirds (Meriones unguiculatus). The major drawback of this live vaccine is the short shelf-life. We evaluated different methods for the cryopreservation of this vaccine. Blood from jirds infected with B. divergens was frozen to -196 degrees C using cooling rates of 1, 10, 30, 100, 196 and 250 degrees C min-1, and a two-step cooling rate. The cryoprotectants dimethyl sulfoxide (DMSO), polyvinylpyrrolidone 40 and glycerol were used at different concentrations. Aliquots were stored in liquid nitrogen for 10-20 days and rapidly thawed in a water bath at 40 degrees C. Infectivity of blood before and after cryopreservation was tested in jirds by i.p. inoculation. The prepatent periods recorded were used to calculate the infectivity of the inocula. The highest infectivity of 46% was recorded from blood cryopreserved with 3 M DMSO and cooled at 10 degrees C min-1. Infectivity of the frozen vaccine was tested in 4 heifers by inoculation of 2.5 x 10(7) parasites. All animals showed Ab titres 12 days after inoculation.

Characterization of Megatrypanum trypanosomes from European Cervidae

Megatrypanum trypanosomes have been isolated from a number of different European Cervidae, but on the basis of morphology it has not been possible to define the species to which these isolates belong. We isolated Trypanosoma (Megatrypanum) theileri from 10 cattle, and Megatrypanum trypanosomes from 11 fallow deer (Cervus dama), 9 red deer (Cervus elaphus), and 4 roe deer (Capreolus capreolus) by blood culture on a biphasic medium (NNN agar slopes). Trypanosomes were propagated in Schneider's Drosophila medium and characterized by isoenzyme analysis and molecular karyotyping. Isocitrate dehydrogenase and phosphoglucomutase were visualized after starch gel electrophoresis of trypanosome lysates. By cluster analysis of this data all isolates from deer were clearly separated from the T.(M.) theileri isolates from cattle. Isolates from roe deer were different not only from T. (M.) theileri but also from the other deer isolates. Isolates from fallow deer and red deer were grouped together. Thus, there are probably at least two different species of Megatrypanum trypanosomes in the three Cervidae. One parasitizing roe deer, the other, apparently less host specific species, infecting red deer and fallow deer. Separation of the chromosomes of Megatrypanum trypanosomes by pulsed-field gradient gel electrophoresis (PFGE) showed that each isolate contained a large number (> 18) of chromosomes ranging in size from 300 to > 2200 kb. The molecular karyotypes were similar for all isolates, although no isolate was identical to another.

Demonstration of the humoral immune response of horses to Babesia caballi by western blotting

Babesia caballi-infected or normal equine erythrocytes were solubilized in sodium dodecyl sulfate (SDS) buffer and analyzed by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting. Antigens were allowed to react with sera from horses experimentally or field-infected with B. caballi and with sera from non-infected horses. Major babesial antigens recognized by immune sera had apparent mol. wts of 141, 112, 70, 50, 48, 34, and 30 kDa. The polypeptides at 50 and 48 kDa were recognized earliest and throughout infection, but also weakly by 3/100 equine sera tested negative and 1/33 sera tested false positive by the complement fixation test (CFT) and immunofluorescence antibody test (IFAT). Thus, further characterization and purification of B. caballi antigens are required to identify target antigens for an improved enzyme immuno assay. Until such an assay is available, Western blotting can provide a specific tool for the diagnosis of B. caballi infections, particularly in cases of contradicting CFT and IFAT results.

Demonstration of extrachromosomal DNA from Babesia equi merozoites

An extrachromosomal nucleic acid element was detected in high-molecular-weight DNA preparations form Babesia equi merozoites. This extrachromosomal element was shown to be DNA rather than RNA and had an apparent fragment size of about 9 kilobase-pairs (kb). Hybridization experiments using purified 9-kb DNA as a probe revealed sequence homologies with extrachromosomal DNA from two other Babesia species.

Lipoptena cervi (Diptera), a potential vector of Megatrypanum trypanosomes of deer (Cervidae)

In three different areas in northern Germany, hippoboscids were collected from red deer (Cervus elaphus). Typanosomatids were demonstrated in the midgut and hindgut of 9/37 Lipoptena cervi. The trypanosomatids were morphologically similar to vector forms of the subgenus Megatrypanum that had been found in tabanids at the same location 5 years previously. Hippoboscids are therefore potential vectors of Trypanosoma (Megatrypanum) spp. that infect red deer.

An improved ELISA for the detection of antibodies against Babesia bovis using either a native or a recombinant B. bovis antigen

Two new enzyme-linked immunosorbent assays (ELISA) for the diagnosis of Babesia bovis in cattle are described. The ELISA using a native antigen is more sensitive and less laborious than the assays described previously, because it does not require adsorption of sera with bovine erythrocytes. The second ELISA, using a recombinant B. bovis antigen expressed in Escherichia coli, was both sensitive and specific. It is suitable to replace the native antigen, thus avoiding large batch-to-batch variations in antigen preparations and the need to sacrifice experimental cattle.

Transmission of Megatrypanum trypanosomes to Cervus dama by Tabanidae

Four fallow deer, Cervus dama, became infected with Trypanosoma (megatrypanum) sp. by oral application of triturated guts from tabanids collected in an area with deer but without any cattle; four control calves remained negative. Upon challenge with triturated guts from tabanids from an area with pastured cattle, the four calves became infected with Trypanosoma (M.) theileri. The prepatent period in deer was five days or less. Haematopota spp. and Tabanus spp. were identified as vectors of the deer trypanosomes. It is concluded that the trypanosomes of C. dama belong to a Megatrypanum species that is not identical with T. theileri.

Transmission of Trypanosoma theileri to cattle by Tabanidae

Tabanids were collected in an area in northern Germany, where pastured cattle were abundant. Trypanosomatidae were identified in 14% of tabanids examined. Twelve cattle became infected with Trypanosoma theileri after applying usually 2-5 infected tabanids, to the intact oral mucosa. Haematopota pluvialis, Haematopota italica, Hybomitra micans and Tabanus bromius were identified as vectors. Infective stages of Trypanosoma theileri were identified in the gut and in the faeces of tabanids by transmission experiments. The minimum prepatent period was less than 4 days. No apparent signs of disease were observed in the infected cattle.