Tuberculosis (Mycobacterium microti) in wild field vole populations

Abstract

Vole tuberculosis (TB; Mycobacterium microti) is an understudied endemic infection. Despite progressing slowly, it causes severe clinical pathology and overt symptoms in its rodent host. TB was monitored for 2 years in wild field voles in Kielder Forest, UK. The prevalence of characteristic cutaneous TB lesions was monitored longitudinally at 4 sites, with individuals live-trapped and repeatedly monitored. A prevalence of 5.2% of individuals with lesions was recorded (n=2791). In a cross-sectional study, 27 sites were monitored bi-annually, with TB assessed by post-mortem examination for macroscopic lesions, and by culture and histopathology. Seventy-nine voles (10.78%; n=733) were positive for mycobacteria, with the highest prevalence in spring (13.15%; n=327). TB prevalence varied, with between 0% and 50% of voles infected per site. Prevalence increased with age (mass), and apparent seasonality was due to a higher proportion of older animals in spring. Survival analysis supported this result, with cutaneous lesions only manifesting in the advanced stages of infection, and therefore only being found on older voles. The body condition of individuals with lesions declined at the time when the lesion was first recorded, when compared to individuals without lesions, suggesting there may be an acute phase of infection during its advanced stage. Although predicted survival following the appearance of a cutaneous lesion was lower than for uninfected individuals, this was not significant.

Fig. 1

Prevalence of voles positive for cutaneous TB lesions (raw data) per month based on the total years combined. No trapping was undertaken in February.

Fig. 2

Predicted increase in risk of external tuberculosis lesions with increasing individual mass for voles in 2002 based on the model estimates. The dotted grey lines indicate the 95% confidence limits for the estimates.

Fig. 3

Frequency of median changes in BC for uninfected voles matched to the 47 TB voles in the 100 replicate Mann-Whitney U-tests. TB voles had a median change in BC of −1.

Fig. 4

Predicted increase in risk of infection with Mycobacterium microti with increasing individual mass for each dependent variable (A) presence of cutaneous lesions; (B) cutaneous and internal lesions detected post-mortem; (C) culture positive for mycobacteria but with no macroscopic lesions; (D) total infection based on all methods combined. The dotted lines are the 95% confidence limits on the predictions.