American canine hepatozoonosis. An emerging disease in the New World.

Hepatozoon canis was first described from dogs in 1905 in India and Rhipicephalus sanguineus was identified as the vector. Dogs on the Texas Gulf Coast were recognized in 1978 to have hepatozoonosis, and it was thought that H. canis had entered the New World. Later, it was realized that American canine hepatozoonosis (ACH) is more debilitating than its Old World counterpart, often resulting in death. When the malady and parasite were characterized, a new species, H. americanum, was described, in 1997. Phylogenetic analysis, based on 18S rRNA gene sequence and classical taxonomic features, revealed that the two dog parasites are closely related. Amblyomma maculatum, the Gulf Coast tick (GCT), has been demonstrated to be an excellent vector; nymphal ticks were readily infected and oocysts from newly molted adults were uniformly infectious for dogs. The merogonic cycle of H. americanum in dogs and the sporogonic development in the invertebrate host have been described. ACH is diagnosed primarily by histologic examination of skeletal muscle. Curative therapy is not available, but anti-protozoal and anti-inflammatory drugs may prolong life. Naturally infected coyotes have been found in Oklahoma and Texas, and experimental infections have been produced in this canid. Additional effort is needed to determine the vertebrate host range of H. americanum and to define the enzootic cycle of which dogs have become a part; likewise, more work is required to determine whether larval GCTs can acquire infection and transmit it as nymphs.