Characterisation of the first Australian isolate of Neospora caninum from cattle

Abstract

Objective To isolate Neospora caninumfrom a congenitally infected calf. Procedure A calf was obtained from a N caninum infected dam maintained in a dairy herd of Holstein-Friesian cattle located on the south coast of NSW near Nowra. The calf was euthanased and samples collected for serology and pathology. Samples of brain and spinal cord of the calf were homogenised and injected into immunocompromised mice in an attempt to recover protozoa by in vivo culture. Sequential passage of brain homogenate through IFNgR KO mice was performed and tissue culture flasks were inoculated with brain homogenate. Parasites were identified by electron microscopy and DNA sequencing. The antigen profile of the isolate was analysed using Western blotting. Pathogenicity was examined in BALB/c mice and transmission of the parasite during pregnancy was examined in Qs mice. Results The calf was seropositive for N caninumand histopathological examination of sections of cerebrum identified lesions consistent with a very mild infection with N caninum. The parasites isolated using tissue culture were identified as N caninum, based on the sequence of the ribosomal DNA and electron microscopy. The antigen profile of the new isolate was similar to that of the NC-Liverpool isolate, but quite different from that of Toxoplasma gondii. In BALB/c mice inoculated with the new isolate, severe clinical signs developed in only three of ten infected mice, compared with six of ten mice infected with NC-Liverpool. Mild to moderate nonsup-purative encephalitis was observed in BALB/c mice infected with the new isolate, compared with mice infected with NC-Liverpool, that developed severe nonsuppura-tive encephalitis. Transplacental transmission of the isolate arising from an acute infection during pregnancy occurred in about 87% of pups. Conclusion This is the first isolation of bovine Neospora caninum in Australia. This isolate, called NC-Nowra, appears to be a less virulent form and may prove to be a suitable candidate for vaccine development.