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 e ither a low or high dose of N. caninum tachyzoites and temperature response s monitored daily for the first 2 weeks after inoculation. Blood samples we re collected before inoculation, and at regular intervals thereafter for 1 year. Serological analysis was achieved using an indirect fluorescent antib ody test (IFAT), an enzyme-linked immunosorbent assay (ELISA) and an IgG av idity 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 inocu lation increased in size following injection in all animals, in both infect ed groups, before returning to normal by day 14 after injection. Both group s 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 ex periment. The specific Neospora antibodies produced did not cross-react in an IFAT for the detection of antibodies to Toxoplasma gondii. IgG avidity i ncreased 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 differen t doses of N. caninum produced a similar antibody response, and that the hi gher dose also induced a febrile reaction. The IgG avidity ELISA was succes sful at distinguishing between recent and long-standing infection in this s tudy. However, in both groups, there was fluctuation in the levels of speci fic antibody throughout the yearlong study, which accords with similar expe riments in pregnant cattle, where it has been suggested that fluctuation ma y indicate periodic recrudescence of infection and a re-stimulation of anti body production by antigen. (C) 2001 Elsevier Science B.V. All rights reser ved.