Pathology of <i>Lankesterella</i> sp. infection in a cliff swallow from California, USA

<i>Lankesterella</i> spp., apicomplexan coccidian protozoa historically associated with amphibians, are increasingly reported in birds. Here, we describe lesions associated with infection from, and depict phylogenetic relationships of, a <i>Lankesterella</i> sp. in a cliff swallow (<i>Petrochelidon pyrrhonota</i>) from California, USA. An 8-wk-old male, hand-raised, cliff swallow housed in an outdoor aviary had progressive nonspecific clinical signs for 12 d. The bird was euthanized because of poor prognosis. Histologically, histiocytic, lymphocytic, and plasmacytic myocarditis and necrotizing pectoral myositis were noted, with intralesional protozoa in macrophages, myocytes, and the interstitium. Mononuclear mural arteritis in the ascending aorta had similar intralesional protozoa. Ovoid-to-spherical, pale-eosinophilic structures, presumed to be meronts, were found within the endothelial cells of the ascending aorta. Sequence analysis of the 18S rRNA gene identified the parasite as a <i>Lankesterella</i> sp., similar to those reported from other <i>Hirundinidae</i> species in Europe. Comorbidities included upper respiratory and conjunctival cryptosporidiosis and vitamin E deficiency, likely contributing to the clinical deterioration. Transmission electron microscopy of the heart revealed intra- and extracellular zoites with ultrastructural features compatible with <i>Lankesterella</i> spp. Most <i>Lankesterella</i> surveys have included healthy birds in Europe; disease is rarely reported. Additional research is needed on the pathogenic potential of <i>Lankesterella</i> infections in birds.