Risk factors associated to a high Mycobacterium tuberculosis complex seroprevalence in wild boar (Sus scrofa) from a low bovine tuberculosis prevalence area

Abstract

Animal tuberculosis is a worldwide zoonotic disease caused principally by Mycobacterium bovis, a member of the Mycobacterium tuberculosis complex (MTC). In southern Iberian Peninsula, wild reservoirs such as the wild boar, among other factors, have prevented the eradication of bovine tuberculosis. However, most of the studies have been focused on south-central Spain, where the prevalence of tuberculosis is high among wild ungulates and cattle herds. In northern regions, where wild boar density and bovine tuberculosis prevalence are lower, fewer studies have been carried out and the role of this species is still under debate. The aim of this study was to describe the temporal and spatial distribution of antibodies against MTC in wild boar from the Basque Country, northern Spain. Sera from 1902 animals were collected between 2010 and 2016. The seroprevalence was determined with an in house enzyme-linked immunosorbent assay and the search of risk factors was assessed by Generalized Linear Models. Overall, 17% of wild boars (326/1902; 95%CI, [15.5%-18.9%]) showed antibodies against MTC. Risk factors associated with seropositivity were the year and location of sampling, the number of MTC positive cattle, the distance to positive farms and the percentage of shrub cover. Younger age classes were associated with increased antibody titres among seropositive individuals. The seroprevalence detected was higher than those previously reported in neighbouring regions. Hence, further studies are needed to better understand the role of wild boar in the epidemiology of tuberculosis in low tuberculosis prevalence areas and consequently, its relevance when developing control strategies.

Fig 1. Spatial distribution of MTC seroprevalence (%) detected in wild boar and bovine TB positive cattle.

Dot sizes and intensity of colour increase with the seroprevalence detected in municipalities where more than 10 wild boars were analysed. Labels inside these dots indicate the number (N) of animals analysed. Rhombuses indicate municipalities where less than 10 wild boars were analysed. Those in white mean they were negative and greys mean at least one animal was positive. Blue fill colour intensity increase with the number of TB positive cattle detected in each region: No (zero), low (1–3), medium (8–22) and high (> 78).

Fig 2. Descriptive statistics of the ELISA index of positive boars according to continuous variables.

“*” indicates a significant association between the response and the explanatory variable at the univariate analysis (p <0.05). Variables with a p-value lower than 0.25 were included in the gamma model.