Evaluation of experimental and naturally acquired anaplasmosis in California sheep

Anaplasma phagocytophilum and A. ovis are important tick-borne pathogens in Old World sheep that cause morbidity, in some cases mortality, and potentially devastating loss of production to the sheep industry. There is the potential for these pathogens to adversely affect the dynamic sheep industry in California, yet little is known about their prevalence and clinical course in sheep flocks of the U.S. Additionally, A. marginale is a relatively common pathogen of cattle in California and this pathogen has not been evaluated in sheep. This study was designed to determine the potential for infection of California sheep by experimental infection with two different strains of A. phagocytophilum and by surveillance of free-ranging sheep in flocks in the Sierra Nevada foothills and northern coast range mountains. Experimental infection was performed on 10 6-year old ewes in a tick free environment. Four sheep were experimentally infected with horse origin MRK strain, four were infected with human origin Webster strain, and two sheep were used as controls. Sheep did develop mild infection based on serology and PCR. Over the course of the two month study period A. phagocytophilum was detected by PCR in 3 MRK strain sheep and 1 Webster strain sheep, and two sheep showed seroconversion by IFA, one of which had tested PCR positive. Mean maximum temperature of sheep infected with Webster strain was significantly higher at 40.4°C then MRK strain sheep at 39.8°C. Field surveillance revealed that sheep tested from locations within the Sierra Nevada foothills had a 22% seroprevalence and sheep from northern coast range mountains as having 6.6% seroprevalence, and within positive flocks a total seroprevalence of 62.5% was detected. These results indicate that sheep have the potential to become infected with Anaplasma spp. pathogens, and high prevalence infection areas are consistent with the reported geographical distribution of anaplasmosis in dogs, horses, and humans.