Surveillance of bovine tuberculosis and risk estimation of a future reservoir formation in wildlife in Switzerland and Liechtenstein

Abstract

Bovine tuberculosis (bTB) caused by Mycobacterium bovis or M. caprae has recently (re-) emerged in livestock and wildlife in all countries bordering Switzerland (CH) and the Principality of Liechtenstein (FL). Comprehensive data for Swiss and Liechtenstein wildlife are not available so far, although two native species, wild boar (Sus scrofa) and red deer (Cervus elaphus elaphus), act as bTB reservoirs elsewhere in continental Europe. Our aims were (1) to assess the occurrence of bTB in these wild ungulates in CH/FL and to reinforce scanning surveillance in all wild mammals; (2) to evaluate the risk of a future bTB reservoir formation in wild boar and red deer in CH/FL. Tissue samples collected from 2009 to 2011 from 434 hunted red deer and wild boar and from eight diseased ungulates with tuberculosis-like lesions were tested by direct real-time PCR and culture to detect mycobacteria of the Mycobacterium tuberculosis complex (MTBC). Identification of suspicious colonies was attempted by real-time PCR, genotyping and spoligotyping. Information on risk factors for bTB maintenance within wildlife populations was retrieved from the literature and the situation regarding identified factors was assessed for our study areas. Mycobacteria of the MTBC were detected in six out of 165 wild boar (3.6%; 95% CI: 1.4-7.8) but none of the 269 red deer (0%; 0-1.4). M. microti was identified in two MTBC-positive wild boar, while species identification remained unsuccessful in four cases. Main risk factors for bTB maintenance worldwide, including different causes of aggregation often resulting from intensive wildlife management, are largely absent in CH and FL. In conclusion, M. bovis and M. caprae were not detected but we report for the first time MTBC mycobacteria in Swiss wild boar. Present conditions seem unfavorable for a reservoir emergence, nevertheless increasing population numbers of wild ungulates and offal consumption may represent a risk.

Figure 1. Map of Switzerland and Liechtenstein depicting the origin of samples and microbiological results.

Animal species: red deer (square); wild boar (triangle); other species (circle). Microbiological results: survey samples negative for mycobacteria of the M. tuberculosis-complex (MTBC; green); MTBC-positive survey samples (red); scanning surveillance samples (all MTBC negative; orange). Study areas (dark grey): Geneva (GE); Thurgovia (TG); Saint Gall (SG); Principality of Liechtenstein (FL); Grisons (GR); Tessin (TI). Further: cantonal borders (grey lines); main lakes (blue areas).

Figure 2. Bilateral tuberculosis-like lesions in the mandibular lymph nodes of a wild boar.

This sample was positive for mycobacteria of the Mycobacterium tuberculosis complex on tissue material, but yielded only atypical mycobacteria in culture. Scale: centimeters.

Figure 3. Macroscopic and microbiological results, wild boar.

Real-time PCR from tissue material (PCR_T) and real-time PCR from culture (PCR_C) for the detection of mycobacteria of the Mycobacterium tuberculosis-complex (MTBC). Positive mycobacterial cultures negative by PCR_C were classified as atypical mycobacteria. Cultures were considered positive if typical growth occurred and acid-fast bacilli were subsequently detected with Ziehl-Neelsen staining.

Figure 4. Macroscopic and microbiological results, red deer.

Real-time PCR from tissue material (PCR_T) and real-time PCR from culture (PCR_C) for the detection of mycobacteria of the Mycobacterium tuberculosis-complex (MTBC). Positive mycobacterial cultures negative by PCR_C were classified as atypical mycobacteria. Cultures were considered positive if typical growth occurred and acid-fast bacilli were subsequently detected with Ziehl-Neelsen staining.

Figure 5. Risk factors favoring the maintenance of bovine tuberculosis (bTB) in reservoir host populations.

Figure 6. Increase of population numbers and hunting bags of red deer and wild boar in Switzerland.

Red deer counts (black line); red deer hunting bag (dashed line); wild boar hunting bag (dotted line). No wild boar counts available. (Source: Swiss hunting statistics: http://www.wild.uzh.ch/jagdst/).