Serological Detection of Ovine Gammaherpesvirus 2 Antibodies in Dairy Farms from Southern Brazil

Abstract

Sheep-associated malignant catarrhal fever (SA-MCF) is a severe lymphoproliferative vascular disease of cattle that is caused by ovine gammaherpesvirus 2 (OvGHV2), which is a Macavirus within the Gammaherpesvirinae subfamily. SA-MCF occurs worldwide in several mammalian hosts. Alternatively, alcelaphine gammaherpesvirus 1 (AlGHV1) is a Macavirus that causes wildebeest-associated malignant catarrhal fever (MCF), which principally occurs in cattle from Africa. Previous serological assays to evaluate the presence of MCF in mammals used a competitive inhibition enzyme-linked immunosorbent assay (CI-ELISA). This CI-ELISA is based on the 15A antigenic epitope that is common to all Macavirus associated with the development of MCF in their respective hosts. This study evaluated an indirect MCF-specific ELISA assay based on the AlGHV1 C500 strain to detect antibodies against OvGHV2 in 43 closed dairy cattle farms from Southern Brazil. These farms are located in a region where subclinical infections by OvGHV2 have been detected in free-ranging wild boars (Sus scrofa). Sheep or goats were not reared at these farms or within the proximity of these farms. Risk factors associated with seropositivity to OvGHV2 were evaluated, while the possible participation of subclinically infected wild boars in the dissemination of OvGHV2 was estimated using spatial analysis. Sera from 29 dairy cows from 16 farms demonstrated sample/positive (S/P) values considered positive with this MCF-specific ELISA (cutoff S/P, 0.063). The S/P values for the positive dairy cows varied between 0.0633 and 0.2510 (mean, 0.0998; standard deviation, 0.0476). At least one cow was seropositive in 16/43 (37.2%) of these farms, with seropositivity identified in 29/367 (7.9%) of dairy cows maintained at these farms. Additionally, dairy cows raised within the intensive system had a more than threefold higher chance of being seropositive to OvGHV2 relative to those reared within the semi-intensive system. Furthermore, the spatial evaluation revealed that cows on dairy farms within a 50 km radius of the home range of subclinically infected wild boars had an increased risk of being seropositive to this assay. These findings demonstrated that the AlGHV1 C500-specific MCF ELISA can be efficiently used to monitor the occurrence of OvGHV2 in cattle. In addition, the occurrence of subclinically infected free-ranging wild boars within a radius of 50 km from susceptible cattle may be a possible risk factor for the occurrence of OvGHV2-related infections in these animals from Southern Brazil. These initial results are fundamental to understanding the epidemiology of OvGHV2-associated infections and clinical SA-MCF in mammals in Brazil.

Keywords: Macavirus; epidemiology; malignant catarrhal fever; serodiagnosis; spatial analysis; wild boars.

Figure 1

The distribution of the MCF ELISA S/P values identified in dairy cows in Southern Brazil. Panel (A) shows the S/P values of all cows evaluated, while panel (B) focuses on the S/P values of the seropositive cows. The identification of each cow at the respective dairy farm is provided; there is a 0.063 cutoff for this assay.

Figure 2

Geographical locations of the dairy farms and subclinically infected wild boars within the municipalities of the central–eastern mesoregion of Paraná State, Southern Brazil. The overview of the study location is provided (1) with details of the positive and negative dairy herds and OvGHV2 positive free-ranging wild boars (2). Legend: (A) Municipalities of Arapoti and Jaguariaíva; (B) Carambeí; (C) Castro; and (D) Palmeira [37,54].

Figure 3

Spatial relationship between dairy farms and subclinically infected wild boars. The buffer zone for the home area of wild boars is shown (50 km radius). The statistical chi-squared analysis of the relationship between dairy farms and subclinically infected wild boars is provided in the contingency table [37,54].