Estimating the True Prevalence of Mycobacterium Bovis in Hunter-Harvested White-Tailed Deer in Michigan

Abstract

Apparent prevalence, although useful as a consistent index, may underestimate the true prevalence of disease. In Michigan, the ability to estimate the true prevalence of bovine tuberculosis (TB; caused by Mycobacterium bovis) in free-ranging white-tailed deer (Odocoileus virginianus) will become increasingly important to accurately assess progress towards eradication. Our objectives were threefold: to estimate the true prevalence of M. bovis in free-ranging deer in Michigan, to evaluate the effectiveness of existing TB surveillance methods, and to indirectly assess whether TB epidemiologic data from captive cervid herds can be meaningfully extrapolated to free-ranging populations. The study population consisted of all free-ranging deer submitted for TB testing in 2001 from six townships in northeastern Lower Michigan. Tissue samples of tonsil and cranial lymph nodes were collected bilaterally from all deer eligible for the study that did not have gross lesions suggestive of TB (nϭ701). Samples were subjected to histopathologic, acid-fast (AF) staining, mycobacterial culture, and polymerase chain reaction (PCR) testing. Seven deer cultured positive for M. bovis that would not have been detected by current surveillance, yielding apparent and true prevalence estimates (95% confidence limits) of 2.7% (1.6, 3.8) and 3.6% (2.3, 4.9), respectively. The sensitivity, specificity, and positive and negative predictive values of the current surveillance protocol were 75, 100, 100, and 99%, respectively. Histologic lesions were present only in tonsils, and ranged from simple necrosis to caseation, suppuration, and granuloma formation. Acid-fast staining and PCR detected M. bovis in only one of the seven culture-positive deer. Our study provides the first estimate of the true prevalence of M. bovis in Michigan's free-ranging deer population and suggests modest underestimation of that prevalence by current surveillance. This study also suggests that caution is warranted when extrapolating epidemiologic data on TB in captive cervids to free-ranging populations and confirms the pivotal role of the tonsil in early infections.

Wildlife Tuberculosis: A Systematic Review of the Epidemiology in Iberian Peninsula

“…This introduces a bias and it was shown that the sensitivity of gross pathology was 72,2% of that obtained from bacteriology in the wild boar (Santos et al, 2010). The same trend has been reported elsewhere for deer (Rohonczy et al, 1996;O'Brien et al, 2004).…”

Epidemiology of Mycobacterium Bovis Infection in Wild Boar (Sus Scrofa) From Portugal

“…In this study, as well as in the study by Aranaz et al (2004), M. bovis infection rates in wild boar were estimated by systematically employing bacteriology and not relying on gross pathology as the screening test. This is of relevance because gross pathology alone can substantially underestimate disease prevalence, as shown in free-ranging cervids (O'Brien et al, 2004). Vicente et al (2006) estimated a TBL prevalence rate of 43% in wild boar in Sierra Morena, southwestern Spain.…”

“…This is usually explained by sexual differences in behavior. For example, M. bovis may be transmitted during fights in breeding season in white-tailed deer (Odoicoleus virginianus) (O'Brien et al, 2006). Vicente et al (2006) reported that TBL prevalence tended to be higher in male wild boar on intensively managed populations with apparently high overall prevalence.…”

Appraisal of Interpretation Criteria for the Comparative Intradermal Tuberculin Test for Diagnosis of Tuberculosis in Cattle in Central Ethiopia

“…We found a significant difference in apparent prevalence rates at the two cutoff points. Although it is useful as a consistent index and is used routinely to measure disease frequencies, apparent prevalence may underestimate the true prevalence of disease because the test used to determine apparent prevalence does not capture all infected animals (26,30). The true prevalence of a disease in a population can be calculated using the apparent prevalence (as detected by some screening test) together with the sensitivity and specificity values of the test (30).…”