Severity of bovine tuberculosis is associated with co-infection with common pathogens in wild boar

Abstract

Co-infections with parasites or viruses drive tuberculosis dynamics in humans, but little is known about their effects in other non-human hosts. This work aims to investigate the relationship between Mycobacterium bovis infection and other pathogens in wild boar (Sus scrofa), a recognized reservoir of bovine tuberculosis (bTB) in Mediterranean ecosystems. For this purpose, it has been assessed whether contacts with common concomitant pathogens are associated with the development of severe bTB lesions in 165 wild boar from mid-western Spain. The presence of bTB lesions affecting only one anatomic location (cervical lymph nodes), or more severe patterns affecting more than one location (mainly cervical lymph nodes and lungs), was assessed in infected animals. In addition, the existence of contacts with other pathogens such as porcine circovirus type 2 (PCV2), Aujeszky's disease virus (ADV), swine influenza virus, porcine reproductive and respiratory syndrome virus, Mycoplasma hyopneumoniae, Actinobacillus pleuropneumoniae, Haemophilus parasuis and Metastrongylus spp, was evaluated by means of serological, microbiological and parasitological techniques. The existence of contacts with a structured community of pathogens in wild boar infected by M. bovis was statistically investigated by null models. Association between this community of pathogens and bTB severity was examined using a Partial Least Squares regression approach. Results showed that adult wild boar infected by M. bovis had contacted with some specific, non-random pathogen combinations. Contact with PCV2, ADV and infection by Metastrongylus spp, was positively correlated to tuberculosis severity. Therefore, measures against these concomitant pathogens such as vaccination or deworming, might be useful in tuberculosis control programmes in the wild boar. However, given the unexpected consequences of altering any community of organisms, further research should evaluate the impact of such measures under controlled conditions. Furthermore, more research including other important pathogens, such as gastro-intestinal nematodes, will be necessary to complete this picture.

Figure 1. The study area is located in mid-western Spain where wild boar is the most important reservoir of bovine tuberculosis.

Figure 2. Wild boar showing localized (bTB like lesion in submandibular lymph node) (a) or generalized bTB lesion patterns (bTB like lesions in lungs) (b).

Figure 3. Relationships between exposure to viral infections (PCV2, arrow in pink, and ADV, in blue), nematode infection (arrow in black) and age on PLSr component describing disease severity (arrow in yellow) in Mycobacterium bovis infected wild boar.

This plot represents the PLSr model shown in Table 5. Arrow way indicates either an increase or decrease in the component value. Arrow thickness directly indicates the contribution of each variable to PLSr X's component. Since swine influenza virus and Mycoplasma hyopneumoniae explained less than 10% of the PLSr X's component, they were not represented in this plot. The wild boar silhouettes summarize those pathogen combinations linked to severe bovine tuberculosis (represented by several circles in yellow). The more yellow circles a wild boar has, the more severe is the disease. The rest of coloured circles represent co-infections by different pathogens. Each colour matches with species represented by the arrows (i.e., pink for PCV2 or blue for ADV). Nematodes, however, are represented by the black short lines within the silhouettes.