Serological analysis of calves experimentally infected with Neospora caninum: a 1-year study

A serological study was conducted with calves experimentally infected with the protozoan parasite Neospora caninum. The animals were inoculated with either a low or high dose of N. caninum tachyzoites and temperature responses monitored daily for the first 2 weeks after inoculation. Blood samples were collected before inoculation, and at regular intervals thereafter for 1 year. Serological analysis was achieved using an indirect fluorescent antibody test (IFAT), an enzyme-linked immunosorbent assay (ELISA) and an IgG avidity ELISA. Injection of Neospora produced a significant rise in rectal temperature in the high dose group. In addition, the lymph node draining the site of inoculation increased in size following injection in all animals, in both infected groups, before returning to normal by day 14 after injection. Both groups given N. caninum produced specific antibody that was detected by the IFAT and the ELISA, which remained elevated for the 12-month duration of the experiment. The specific Neospora antibodies produced did not cross-react in an IFAT for the detection of antibodies to Toxoplasma gondii. IgG avidity increased 2 weeks after inoculation, in both infected groups, until week 12 when infection was well established. There was a little difference between the two infected dose groups. This study demonstrates that the two different doses of N. caninum produced a similar antibody response, and that the higher dose also induced a febrile reaction. The IgG avidity ELISA was successful at distinguishing between recent and long-standing infection in this study. However, in both groups, there was fluctuation in the levels of specific antibody throughout the yearlong study, which accords with similar experiments in pregnant cattle, where it has been suggested that fluctuation may indicate periodic recrudescence of infection and a re-stimulation of antibody production by antigen.