Prevalence and possibility of vertical transmission of Anaplasma phagocytophilum in field sheep in Drenthe

Abstract

Background The intracellular, tick-borne bacterium Anaplasma phagocytophilum (A. phagocytophilum) is wide spread and causes disease in a wide range of animals including sheep. In Drenthe (the Netherlands) several sheep flocks have severe health problems. These flocks are faced with high morbidity (15-20% show fever, locomotion disturbance, emaciation and arthritis) and variable mortality (0-15%). The symptoms, bacteriological findings and timing of the illness point in the direction of Anaplasma infections. The aim of this study was to investigate the prevalence of A. phagocytophilum infections in pregnant ewes and to investigate whether these ewes vertically transmit the bacterium to their newborn lambs under field condition.

Methods The study included 127 ewes before lambing and 44 ewes after lambing, all selected randomly. Blood samples of these ewes were taken once. In addition, general impression, body condition score and tick presence were recorded. The lambs of the postpartum sampled ewes were included as well. Directly after birth blood samples were collected from the living lambs (n = 53) and spleen samples were collected from death lambs (n = 12). Blood and spleen samples were analyzed with real-time PCR (qPCR) targeting mps2 gene. The q-PCR positive samples were analysed further by PCR targeting groEL gene and by sequencing this DNA-fragment.

Results The overall antepartum prevalence of A. phagocytophilum was 16.5% in ewes. The ewes of Hijkerveld showed an increase of prevalence during the last month of gestation (antepartum: 14.3%, postpartum: 29.5%), but this was not significant. No cases of vertical transmission were observed in this study. Calculation of probability showed that this results is highly unlikely if the presumption of a transmission rate of 30%, based on results of a previous study, is true under field conditions.

Conclusion A substantial number of ewes is infected with A. phagocytophilum and are potential sources of infection for the unborn lamb under field conditions. However, no evidence for vertical transmission was found. Calculation of probability showed that it is highly probable that the rate of transmission is lower than 20% under field conditions in Drenthe. A definite answer whether colostrum might be a source of infection of A. phagocytophilum for the lambs in the field is still unknown. In comparison to previous experimentally studies, the importance of vertical transmission seems limited under field conditions.