Canine hepatozoonosis: comparison of lesions and parasites in skeletal muscle of dogs experimentally or naturally infected with Hepatozoon americanum

We report previously undescribed, early lesions in skeletal muscle of dogs experimentally infected with Hepatozoon americanum by ingestion of laborato ry-reared, infected Amblyomma maculatum. The earliest muscle lesion was rec ognized at the first interval of examination 3 weeks following exposure. Th e lesion consisted of a large, modified host eel whose cytoplasm frequently contained a demonstrable parasite, In skeletal muscle, the cell was consis tently located between muscle fibers or in loose connective tissue adjacent to those fibers. Evidence suggesting that the parasite arrives in muscle a nd other tissue within the host cell cytoplasm is presented. Mucopolysaccha ride encystment of the host cell, absent at this early stage, was acquired gradually and approached maximal development 26 weeks post exposure. Comple tion of the asexual cycle as evidenced by the presence of parasites enterin g vascular lumens within granulomas and also by the presence of gamonts in peripheral blood leukocytes, occurred within 28-32 days postexposure. Progr ession of the parasite cycle from meront to passage of zoites into vessel l umens of granulomas can occur in 11 or fewer days. The density with which p arasitic lesions occur in one named skeletal muscle compared to other named muscles, although somewhat variable, was not significantly different in ei ther experimentally induced or natural infections. The distribution of deve lopmental stages of the parasite/lesion in four experimental infections (96 9 lesions) is compared with those in eight dogs with natural infections (55 7 lesions). (C) 1999 Elsevier Science B.V. All rights reserved.