Anaplasma phagocytophilum infection in sheep: investigating disease outbreaks in the Netherlands and development of an experimental tick transmission model
MetadataShow full item record
Background: Anaplasma phagocytophilum is the etiologic agent of tick-borne fever (TBF) in sheep and of human granulocytic anaplasmosis (HGA) in humans. Anaplasma is transmitted by Ixodes ricinus ticks (in Europe), causes a sudden onset of high fever and eventually leads to immunosuppression. This study was initiated, after an outbreak of Anaplasma was reported in sheep introduced into the Bargerveen nature reserve in the Netherlands. The study consisted of two parts. A field study with sentinel sheep, in which Anaplasma was collected from natural clinical cases in the Bargerveen nature reserve. The second part of the study consisted of the development of an experimental tick transmission model, in order to study the tick-host-pathogen transmission dynamics. Methods: In the sentinel study, a total of 16 Anaplasma-negative sheep were introduced into the Bargerveen nature reserve. Sheep were monitored by full physical examination and blood samples were taken once a week in order to perform both blood smears and PCR/RLB. In the experimental study, Anaplasma-negative sheep kept under laboratory conditions were experimentally inoculated with an Anaplasma strain collected in 1988 from sheep on the North Sea Island of Ameland. Sheep were monitored by daily temperature measurement and PCR/RLB for the presence of Anaplasma. Ticks, which were subsequently placed on the sheep, were analyzed after they completed their blood meal by PCR/RLB to determine the infection rate of ticks. Results: In the sentinel study, blood stabilates were made and stored for later use in an experimental model. Analysis of data, gathered from both clinical examination and blood sample analysis, revealed a possible relationship between tick-borne fever on one hand and both lymphadenopathy and a rectal temperature of ≥ 40,5 ⁰C on the other hand. A specificity of 100% was found for blood smears compared to PCR/RLB, indicating a positive blood smear is a suitable diagnostic tool to confirm the presence of Anaplasma in a clinical setting. However a negative blood smear does not rule out the presence of Anaplasma, as a sensitivity of 40% was found in this study. After 8 days already 8 out of 16 (50%) introduced sheep were positive for Anaplasma. At the end of this fieldstudy, 51 days after introduction, 13 out of 16 (81%) sheep were positive for Anaplasma. In the experimental tick transmission study, a fever peak was observed which started at approximately day 4 after inoculation and lasted for 4 to 6 days. Infection rates in ticks still had to be determined at the time of writing this paper. Conclusion: A high infection rate of Anaplasma was found in newly introduced sheep into the Bargerveen nature reserve. The high infection rate observed in sheep demonstrates that sheep are suitable as sentinel animals for TBF. It is not known whether the high infection rate in the Bargerveen nature reserve is just a local characteristic, or whether the presence of Anaplasma is widespread in sheep pastures or nature reserves all over the Netherlands. Further research should give insight into the spread and impact of Anaplasma in different nature reserves (and/or sheep pastures) in the Netherlands. In the experimental model, a self-limiting infection in sheep was observed. The model is suitable for producing infected ticks. This model as well as these ticks can subsequently be used in further studies. Results of these studies could eventually lead to a One-Health approach of Anaplasma as an emerging zoonosis.