[A laboratory infection with tick bite fever].
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Anaplasma marginale is an intraerythrocytic rickettsia that affects the cattle and constitute a constraint for its production. The transplacental transmission of A. marginale could be of importance under field conditions, but until now no data is available to interpret its relevance in Venezuela. The goal of this work was to evaluate both the presence of A. marginale and the antibody response against the rickettsia in new born calves. Fourteen calves, one week old, were simultaneously bled for parasitological tests, packed cell volume measurement and to obtain sera for immunoenzymatic assays. All the animals showed parasitemia between seventeen and thirty days old. The antibody IgM and IgG response at this time was low. Probably, these calves acquired A. marginale infection through transplacental route and a very low level of immunity by the colostrum.
A serologic survey was conducted in yearling cattle imported into Alberta feedlots from Montana during October 2001 to estimate the prevalence of antibodies to bluetongue virus (BTV) and Anaplasma marginale in Montana yearling cattle. The apparent prevalence of antibodies to BTV when the competitive enzyme-linked immunosorbent assay (cELISA) was used was 0.37% (21/5608). Test positive cELISA samples were also all positive when tested by virus neutralization (VN) and they reacted to 1 or more BTV serotypes, including 2, 10, 11, 13, and 17. The apparent prevalence of antibodies to A. marginale when a recombinant cELISA (rcELISA) was used with a positive cutoff at 30% inhibition was 1.93% (108/5608). When the rcELISA positive cutoff was at 42% inhibition, the apparent prevalence was 0.73% (41/5608). After the reported sensitivity and specificity of the test had been accounted for, the A. marginale antibody results were consistent with a population that was either free of exposure or had a very low prevalence for A. marginale.
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The evaluation of newly developed diagnostic tests (tests) commonly involves the comparison of the test outcomes (pos/neg.) of a sample of animals to those of a reference test (gold standard) in order to derive sensitivity and specificity estimates. Often, however, new tests have to be evaluated against an imperfect reference test since a true gold standard test is either too expensive or too costly to apply. This results in bias in the test characteristic estimates. To solve this problem, latent class and Bayesian models can be used to estimate sensitivity and specificity when evaluating a diagnostic test in the absence of a gold standard. They require at least two imperfect reference tests applied to all individuals in the study. In our approach we used a two-test two-population scenario. Both the gold standard and these modelling approaches rely on various assumptions. When violated, biased results will be obtained. The analysis of field data from an Anaplasma marginale outbreak in cattle in Switzerland with four diagnostic procedures (detection of the agent, serology, PCR and hematocrit measurements) was used as a practical example to demonstrate and critically discuss the approaches taken. In this relatively small data set (n = 275) the estimates for the test characteristics obtained by the different methods were quite similar. Overall, the bias in the point estimates depended mainly on the chosen estimation approach. All tests showed a non-negligible correlation mainly in the test sensitivities. This emphasizes the importance of taking into account test dependence even if it seems not biologically plausible at first thought.
In a serologic survey of Montana-source weaned calves and yearling cattle, the apparent prevalence of antibodies to Bluetongue virus was 0.68% and 1.26% in 2002 and 2003, respectively, and to Anaplasma marginale at a positive cutoff at 30% inhibition it was 1.82% and 1.35% in 2002 and 2003, and at a positive cutoff at 42% inhibition it was 0.76% and 0.55% in 2002 and 2003, respectively, suggesting that the risk of importing infected animals was very low.
Tick-borne rickettsioses are diseases which remain comparatively unknown to field veterinarians. The authors present a literature review, focusing on taxonomic aspects and on laboratory diagnosis.