Pathology of theileriosis in roan antelope (Hippotragus equinus)

Abstract

The published literature on schizont-“transforming” Theileria species/genotypes that cause theileriosis in African wild artiodactyls is dated and based on limited information. Here the authors review the taxonomy, diagnosis, epidemiology, haematology, pathology, and aspects of control in various species. Molecular studies based on 18S and 16S rRNA gene sequences have shown that African wild artiodactyls are commonly infected with diverse Theileria spp., as well as nontheilerial haemoprotozoa and rickettsia-like bacteria, and coinfections with pathogenic and non-pathogenic Theileria species are often recorded. Although theileriosis is still confusingly referred to as cytauxzoonosis in many species, the validity of a separate Cytauxzoon genus in artiodactyls is debated. The epidemiology of theileriosis is complex; the likelihood of lethal disease depends on the interplay of parasite, vertebrate host, tick vector, and environmental factors. Roan calves (Hippotragus equinus) and stressed animals of all host species are more susceptible to lethal theileriosis. Even though regenerative anaemia is common, peripheral blood piroplasm parasitaemia does not correlate with disease severity. Other than anaemia, common gross lesions include icterus, haemorrhages (mucosal, serosal, and tissue), fluid effusions into body cavities, lung oedema, and variably sized raised creamcoloured foci of leucocyte infiltration in multiple organs. Histopathologic findings include vasocentric hyperproliferation and lysis of atypical leucocytes with associated intracellular schizonts, parenchymal necrosis, haemorrhage, thromboembolism, and oedema. Immunophenotyping is required to establish the identity of the schizont-transformed leucocytes in wild ungulates. Throughout the review, we propose avenues for future research by comparing existing knowledge on selected aspects of theileriosis in domestic livestock with that in African wild artiodactyls.