Phylogenetic analysis based on full-length large subunit ribosomal RNA gene sequence comparison reveals that Neospora caninum is more closely relatedto Hammondia heydorni than to Toxoplasma gondii

Since its first description in the late 1980s, Neospora caninum has been re cognised as a prominent tissue cyst-forming parasite due to its ability to induce congenital disease and abortion in animals, especially cattle. It is found worldwide and is a cause of significant economic losses for the live stock industry. However, its place within the family Sarcocystidae, like th at of several other taxa, remains unresolved. Neospora caninum shares sever al morphological and life cycle characters with Hammondia heydorni, althoug h it is most commonly thought of as being a dose relative of Toxoplasma gon dii. This study presents information regarding the phylogenetic relationshi p of N. caninum to species currently classified into the genus Hammondia, a s well as to two strains (RH and ME49) of T. gondii based on the full-lengt h large subunit ribosomal RNA gene. Phylogenetic analyses using two alignme nt strategies and three different tree-building methods showed that the two species in the genus Hammondia are paraphyletic. Neospora caninum was show n to form a monophyletic clade with H. heydorni instead of T. gondii, which in turn was shown to be most closely related to H. hammondi. The finding t hat N. caninum and H. heydorni are closely related phylogenetically may aid the elucidation of currently unknown aspects of their biology and epidemio logy, and suggests that H. heydorni should be considered in the differentia l diagnosis of N. caninum from other apicomplexan parasites. (C) 1999 Publi shed by Elsevier Science Ltd on behalf of the Australian Society for Parasi tology Inc. All rights reserved.