A field study evaluating the humoral immune response in Mongolian sheep vaccinated against sheeppox virus

Sheeppox is a transboundary disease of sheep caused by infection with the capripoxvirus sheeppox virus (SPPV). Sheeppox is found in Africa, the Middle East and Asia and is characterised by fever, multifocal cutaneous raised lesions, and death, with substantial negative impact on affected flocks. Vaccination with live attenuated capripoxvirus (CPPV) strains is an effective and widely used means of controlling sheeppox outbreaks, however there are few reports of post-vaccination field surveillance studies of sheeppox. This study used a commercially available ELISA and a fluorescence-based neutralisation assay (FVNT) to examine quantitative and temporal features of the humoral response of sheep vaccinated with a live attenuated CPPV strain in Mongolia. 400 samples were tested using the ELISA, and a subset of 45 also tested with the FVNT. There was substantial agreement between the FVNT and ELISA tests. Antibodies to CPPV were detected between 40 and 262 days post vaccination. There was no significant difference between serological status (positive / negative) and sex or age, however an inverse correlation was found between the length of time since vaccination and serological status. Animals between 90 and 180 days post-vaccination were more likely to be positive than animals greater than 180 days post vaccination. This data provides temporal parameters to consider when planning sheeppox post-vaccination monitoring programmes. In summary, our results show a commercial CPPV ELISA kit is a robust and reliable assay for use in resource-restricted low and low-middle income countries for post CPPV vaccination surveillance on a regional or national level.

Methods
Blood samples from sheep and associated data were collected as part of the post-vaccination surveillance programme for sheeppox implemented by the Mongolian General Authority for Veterinary Services (GAVS) in 2016.