Submitted to: Veterinary Parasitology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/7/2000
Publication Date: N/A
Citation: Interpretive Summary: Neosporosis is a disease primarily of dogs and cattle caused by the protozoan parasite Neospora caninum. The disease causes neurologic disorders in newborn animals due to a propensity of the parasite for nervous tissue. Neosporosis is also a major cause of abortion in dairy cattle worldwide. Studies on the epidemiology of neosporosis are needed if fcontrolling the spread of the parasite is to be realized. The present study was carried out to gain an understanding of the immune response in dairy cows in a herd that was experiencing a major outbreak of N. caninum- associated abortion. Serum was obtained from all cows in the dairy herd and tested in a variety of diagnostic assays for antibody against the parasite. The results showed that serum from cows that aborted early in the epidemic recognized different antigens compared to cows that aborted later. These studies indicate that abortion epidemics may be due to two types of infection- one, a reactivation of an already-existing latent infection, the other, an infection by a form of the parasite found in the environment. This data also indicates that preventing abortion will require blocking of reactivation of an existing infection and transmission from environmental forms of the parasite.
Technical Abstract: An outbreak of Neospora caninum-associated abortion occurred in a South Carolina dairy wherein 10% of the cows aborted over a 4 month period. Immunohistochemistry of brain from a subset of aborted fetuses confirmed N. caninum as the causative agent of abortion. A variety of serological assays, indirect fluorescence antibody test (IFAT), recombinant enzyme- linked immunosorbent assay (rELISA), iscom-ELISA, avidity ELISA, and Neospora agglutination test (NAT), were used to evaluate sera collected during the outbreak from 240 cows for antibodies (Ab) to N. caninum. IFAT and iscom-ELISA testing showed that nearly 80% of the dairy cows had Ab to N. caninum. Depending on cut-off value chosen, NAT and rELISA had similar levels of seropositivity relative to IFAT and iscom-ELISA. These latter two assays also had the best agreement in identifying seropositive and seronegative cows. The agreement between rELISA or NAT and IFAT or iscom- -ELISA was also high, but was dependent on cut-off value used to determine seropositivity. The IFAT, iscom-ELISA, and NAT all had an inverse relationship between presence of N. caninum-specific Ab and cow age. No significant relationship was observed between rELISA results and cow age. In addition to these tests, an avidity ELISA was performed on sera with a positive iscom-ELISA reading. The avidity ELISA testing showed that cows aborting in the first 2 months of the outbreak had high avidity index (AI > 50), indicative of a chronic N. caninum infection, whereas cows aborting in the second 2-month period had low AI (< 35), likely reflecting an acute N. caninum infection. As a group, the aborting cows had a significantly lower AI (P > 0.05) relative to non-aborting cows.