In Europe, A. phagocytophilum transmitted transstadially by I. ricinus (castor-bean tick) causes pasture fever and tick-borne fever in domestic ruminants (20). Although A. phagocytophilum also occurs in the United States, clinical cases have not been reported in cattle and it seems these strains are of low pathogenicity in ruminants (21).
Pasture fever occurs in dairy cattle that have over-wintered in barns and are turned out onto tick-infested pastures in spring. There is a severe drop in milk yield, high fever, mild depression, anorexia, and occasional coughing.
Tick-borne fever occurs in sheep, cattle, and very occasionally in goats that are moved from tick-free to tick-infested areas. Infections are usually subclinical but there may be fever, anorexia, coughing, and mild weight loss. Infected lambs may have reduced weight gain (22). Although clinical signs in lambs are usually mild, there are marked hematological changes including high parasitemias (80%) of neutrophils and lymphopenia, neutropenia, and throm-bocytopenia. Up to 50% of pregnant animals abort or give birth to weak premature offspring, whereas the leucopenia predisposes to a variety of severe concurrent bacterial (lamb pyemia due to Staphylococcus aureus, pasteurellosis, and listeriosis) and viral infections (louping ill). Affected animals, especially sheep, usually become carriers for months to years, and relapses can occur with sudden leucopenia, parasitemia, and transient fever. Deer can also be infected and might also be reservoirs of infection.
Diagnosis usually depends on the observation of inclusion in granulocytes and may be supported by serology. Tetracyclines reduce parasitemias and clinical signs and may lessen losses from decreased milk production, weight gain, and secondary infections. Disease is best prevented by avoiding tick-infested areas. If this is not possible, animals should be treated with appropriate acaricides, and long-acting tetracyclines can be given prophylactically. Abortions may be prevented by exposing animals to infected ticks before they are bred.
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