Volume 24, Number 6—June 2018
Mycobacterium bovis Infection of Red Fox, France
Mycobacterium bovis, a member of the Mycobacterium tuberculosis complex (MTBC), is the main etiologic agent of bovine tuberculosis (TB). In France and other countries in Europe, this ancient zoonotic disease is regarded not just as a problem of cattle but as a concern for multihost communities that include wildlife species such as wild boar (Sus scrofa), red deer (Cervus elaphus), and badgers (Meles meles) (1). The red fox (Vulpes vulpes) is usually considered a spillover host of TB. Sparse numbers of infected foxes had been found in highly prevalent TB regions, such as Great Britain (2). Until recently, the number of infected foxes in TB-endemic zones in France was low (1/68 in Brotonne Forest, Normandy, northwestern France, and 2/61 in Côte d’Or, Burgundy, central-eastern France) (3). However, recently an increasing number of reports have described the presence of a substantial number of infected red foxes in Mediterranean habitats in Spain (4) and Portugal (5), where the disease is highly prevalent in wildlife and livestock. In these cases, the prevalence was high (14% and 26.9%, respectively). Strikingly, most animals did not have TB-like visible lesions in any of these reports.
In a recent study, Payne demonstrated that in Burgundy, France, a region where TB is prevalent in cattle and wildlife, the red fox is the species that most often visits cattle environments (3). In 2015, we conducted a small-scale investigation on trapped red foxes (n = 6) in a municipality of Dordogne, a TB-endemic region in southwestern France, where multihost cycles involving cattle, badgers, wild boar, and even red deer and roe deer (Capreolus capreolus) exist (6). The necropsy examination included detailed macroscopic inspection of lymph nodes and abdominal and thoracic viscera (6). We found no TB-like visible lesions; pooled tissue samples (retropharyngeal, tracheobronchial, mediastinal, and mesenteric lymph nodes) were submitted bacterial culture and molecular diagnosis (6); urine, feces, and oropharyngeal swabs were taken, and DNA was extracted as previously described (7). We performed molecular diagnostic testing using MTBC PCR (IS6110 and IS1081) and the regions of difference and through spoligotyping (6), the latter 2 enabling differentiation of the MTBC members when used on highly concentrated DNAs. Of the 6 red foxes, 4 were bacteriology positive; molecular tools enabled identification of M. bovis spoligotype SB0120 and MTBC or M. bovis excretion in feces of all infected animals (Table). One red fox (RN5) also gave positive results on oropharyngeal swab and urine samples, suggesting M. bovis excretion through several routes.
Although molecular detection does not prove that bacilli are viable, this method is particularly useful for monitoring shedding in environmental samples—especially fecal material—where bacterial culture has poor sensitivity. Molecular quantification on these samples seems to correlate strongly with the animal’s infection level (7).
Fecal excretion may result from digestive infection (8). Infection in mesenteric lymph nodes, observed in other carnivores, suggests that the primary route of transmission of infection is through the digestive tract and strongly suggests fecal excretion (5). Fox RN5 may be considered a super-shedder or super-excretor (9) because shedding of the tuberculous bacillus by several routes was observed (e.g., oropharyngeal swab samples, feces, and urine). Super-shedders have been described as responsible for a disproportionately large amount of M. bovis excretion from the infected animal with a substantial role in the transmission and maintenance of TB in multihost pathogen systems (9), raising the question of the role of the fox in the epidemiology of TB.
Infection in foxes has been suggested to result from scavenging on infected wild ungulate carcasses (4). Alternatively, because foxes may inhabit disused badger setts and vacant parts of occupied setts, they could acquire infection from the contaminated environment (10). This possibility may constitute a risk regarding TB because the fox has been described as the most frequently observed species directly interacting with cattle and visiting farm facilities (3).
The presence of visible lesions alone does not appear to be a good indicator of M. bovis infection in carnivores such as foxes, which often lack macroscopic lesions. The nature of M. bovis infection and the host response are likely to vary widely among species, making simple generalizations about pathology difficult to determine (2). The prevalence of kidney lesions has been reported to be low in wildlife species, which may be explained by the difficulty of detecting TB lesions in organs with a large parenchyma or the presence of microscopic lesions often missed by gross pathology (3,8). Further studies are needed to investigate urinary excretion and the prevalence of kidney TB lesions in red fox and in wildlife in general. Together with this study’s results, these observations highlight that the role of red fox in the epidemiology of TB needs further investigation.
Dr. Michelet is a molecular biologist and junior researcher working at the Animal Health Laboratory of Maisons-Alfort, Anses, France, since 2011. Her research interests include genomic epidemiology.
We thank the hunting federation, hunters and trappers from Dordogne, and Celia Lesage for the samples and Victoria Boschiroli for her useful comments on this article.
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Original Publication Date: 5/7/2018
1Current address: Service Régional de l’Alimentation, Limoges, France.
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