Hammondia heydorni: Oocyst shedding by dogs fed in vitro generated tissue cysts, and evaluation of cross-immunity between H. heydorni and Neospora caninum in mice

Hammondia heydorni is a coccidian parasite believed to be nonpathogenic for naturally-infected animals, but it is biologically and genetically related to Neospora caninum, a worldwide cause of abortion in cattle. The major aim of the present work was to determine whether dogs shed H. heydorni oocysts after consuming in vitro generated tissue cysts of the parasite. In addition, we investigated cross-immunity between H. heydorni and N. caninum in mice. Two dogs were fed cultured cells containing tissue cysts of H. heydorni mixed with canned dog food, and a third dog (negative control) received only non-infected cells mixed with canned food. The two dogs that consumed in vitro produced tissue cysts shed high numbers of oocysts, which were induced to sporulate and tested positive for H. heydorni by a species-specific PCR. The third uninfected dogs did not shed H. heydorni oocysts in the feces. Oocysts shed by the dogs induced the formation of encysted bradyzoites of H. heydorni on KH-R cells. Nineteen BALB/c mice were employed in the cross-immunity study. Nine mice were orally inoculated with 1 × 105 sporulated oocysts of H. heydorni and challenged with N. caninum tachyzoites 30 days after infection with H. heydorni. Other ten mice, which did not receive H. heydorni oocysts, were infected with 2 × 105 N. caninum tachyzoites. Thirty days after challenging with N. caninum, all mice were euthanized and N. caninum DNA in their tissues was quantified by real time PCR. No statistically significant difference in N. caninum DNA concentrations were observed between the two groups. We concluded that in vitro generated cysts of H. heydorni are biologically active, because they induced oocyst shedding in dogs. As no cross-protection occurred in mice inoculated with H. heydorni and challenged with N. caninum, it is suspected that these parasites do not express significant numbers of homologous proteins during infection, or the immune response of BALB/c mice after H. heydorni infection was not sufficient.