Sequential pathology of tick-borne fever

The sequential pathology and clinical responses of 12 sheep infected with the Sourhope strain of Cytoecetes phagocytophila are described. During the experimental period of 3 weeks, the clinical effects were mild, as is usual. There was transient leucopenia, affecting mainly lymphocytes, eosinophils and neutrophils. Parasitaemia was detected from days 5 to 16, approximately. A mild but consistent histopathological response up to day 21 comprised (i) pulmonary alveolitis with minimal shedding of cells, (ii) follicular and especially parafollicular lymphoreticular reactions in lymph nodes and spleen, (iii) mesangial glomerulitis, and (iv) in the cranial nervous system marked choroiditis but minimal reaction in the brain. Two forms of intracytoplasmic inclusions were found, a smaller, granular basophilic structure occurring in clumps and an intravacuolar pleomorphic form. In tissue sections they were mainly associated with lymphoid and macrophagic cells. The significance of these findings is discussed including the problems of differential diagnosis in complex infections.