Toxoplasma gondii, Neospora caninum, Hammondia hammondi and Hammondia heydorni are closely related coccidian parasites with similarly sized oocysts

hammondi and Hammondia heydorni are closely related coccidian parasites with similarly sized oocysts [1]. Cats (felids) are definitive hosts for T. gondii and H. hammondi, and dogs (canids) are definitive hosts for N. caninum and H. heydorni. However, these species have important biological and ultrastructural differences. Whereas T. gondii causes a disseminated infection in humans and animals, and N. caninum is one of the most important causes of abortion in cattle, H. hammondi and H. heydorni are not known to be of clinical significance. Recently, Mehlhorn and Heydorn [2] and Heydorn and Mehlhorn [3] questioned the validity of the genera Hammondia and Neospora and argued that, in this group, only T. gondiiand T. heydorni are valid. In a rebuttal paper, Frenkel and Dubey [4] stressed the ultrastructural, biological and genetic differences between T. gondii and H. hammondi. They gave reasons why Hammondia and Toxoplasma should be deemed separate genera, and why T. gondii should be regarded as a species distinct from H. hammondi. This discussion is not repeated here; instead, this article discusses the relationships between N. caninum, T. gondii and H. heydorni. N. caninum and T. gondii In 1984, Bjerkås et al. [5] first discovered a toxoplasmosis-like disease of Norwegian dogs that had no demonstrable antibodies to T. gondii. In 1988, Dubey et al. [6] described in detail a similar neurological disease of dogs in the USA, distinguished the parasite from T. gondii based on antigenic and ultrastructural differences, and proposed the genus Neospora with N. caninum as the type species. Isolation and in vitro cultivation of N. caninum [7,8] led to the development of serological and immunohistochemical tests [7,9], which confirmed that N. caninum is distinct from T. gondii. Today, N. caninum is recognized as a major cause of abortion in cattle worldwide [10,11]. In 1998, Marsh et al. [12] described another species of Neospora, N. hughesi, as a cause of neurological disease in horses. The full life cycle of N. caninum was not discovered until 1998, when McAllister et al. [13] found N. caninum oocysts in the feces of dogs fed tissues from experimentally infected mice. When these oocysts were inoculated into immunodeficient mice [13] and normal gerbils [14], neosporosis was induced. Recently, Lindsay et al. [15] found that dogs fed tissues of mice infected with cloned N. caninum tachyzoites produced N. caninum oocysts, thus confirming the dog as a definitive host of N. caninum. Since the discovery of N. caninum in 1988, >500 papers have been published on this parasite in peer-reviewed scientific journals. The description of numerous morphological, biological, antigenic and molecular differences [10,16–20] between N. caninum and T. gondii leave no doubt that they are separate species, and this issue will not be discussed further in this communication.