Emergency Vaccination as a Control Strategy against Sheeppox Outbreak in a Highly Susceptible Population

Abstract

This study aimed to investigate a sheeppox outbreak in a highly susceptible naive sheep population in Kharsit village, Gharbia Governorate, Egypt. Moreover, to compare commercial sheeppox vaccines, the Romanian strain and RM-65 vaccines, as emergency vaccination against sheeppox under field conditions. In December 2018, a sheeppox outbreak occurred in a flock of 65 sheep upon the purchase of an apparently healthy ewe from outside the village. This ewe showed a systemic disease with cutaneous lesions after a few days, thereafter more cases began to appear. Cutaneous lesions in other sheep in the flock in the form of macules, papules, and scabs were common in wool-less areas of the body, in addition to fever and respiratory disorders. Postmortem findings revealed the congestion of visceral organs with apparent gross pathology of the lung. Biopsies of cutaneous lesions and visceral organs were collected, and sheeppox was identified by histopathology and transmission electron microscopy, which showed the existence of sheeppox cells and intracytoplasmic brick-shape sheeppox virions. The Romanian strain and RM-65 vaccines were used for the emergency vaccination for two different groups of animals and the third group was left as a control group. Serum samples were collected before vaccination as well as 21 days post-vaccination, and serum protein fractionation analysis was performed for all groups. The outbreak ended after 2.5 months, the cumulative incidence was 66.2%, and the overall case fatality was 51.1%. There was significantly higher protection against sheeppox infection and mortalities among RM-65 vaccine immunized group compared to Romanian strain vaccine-immunized animals at p < 0.05. RM-65-vaccinated animals did not show sheeppox cases or mortalities, compared to Romanian strain-vaccinated animals, which had mild pox signs in 78% of animals and case fatality of 35.7%. The serum protein analysis also indicated the superior performance of the RM-65 vaccine; it increased the level of α1-globulin and β-globulin compared to the Romanian strain, which increased the level of β-globulin only. The current study shows a better performance of the tested RM-65 than the Romanian strain vaccine for emergency vaccination against sheeppox under field conditions. These findings point to the validity of emergency vaccination against sheeppox and the importance of the comparative field evaluation of vaccines; however, wide-scale studies are required for further evaluation. Future investigation of whether the Romanian strain itself or vaccine-production-related issues are responsible for these findings is required.