Is it tuberculosis?! Differential diagnoses for caseous lung lesions in deer
As a wildlife pathologist, probably the last thing you want to see when you open the chest cavity of a white-tailed deer are white, firm lung nodules. At that point you have to break out the PAPR (powered air-purifying respirator) or N95 respirator, because there is a distinct possibility that the deer might be infected with Mycobacterium bovis, a zoonotic (an infectious agent that can be passed from animals to humans) bacteria that causes bovine tuberculosis (bTB).
Bovine TB is an OIE reportable disease that is very closely related to Mycobacterium tuberculosis, which is the most common cause of TB in humans. In the past bTB was most often transmitted to people through unpasteurized milk. These days, humans most commonly contract it via the respiratory route through close contact with infected cattle. Deer are also commonly infected via the respiratory route, often either from infected cattle or from other infected deer, especially if they are concentrated at artificial feeding sites in the winter. Bovine TB has not been reported in white-tailed deer in Ontario since 1958 (Belli 1962, Wobeser 2009), but is more common in Michigan where feeding or baiting deer was a common practice. Tuberculosis is a chronic disease, causing the formation of granulomas or abscesses in the lungs and mesenteric or mediastinal lymph nodes. These lesions can progress to extensive consolidation of lung tissue and the development of purulent bronchopneumonia in deer. Under the microscope, you would expect to see aggregates of inflammatory cells with large macrophages, rarely containing acid-fast Mycobacterium bovis bacteria, surrounded by a fibrous capsule. Although these gross and microscopic lesions are highly suggestive of bTB, there are other infectious agents that can cause similar lesions. To definitively diagnose bTB, culture with subsequent polymerase chain reaction (PCR) is required.
In the past six months, the Ontario-Nunavut node of the CWHC has had two white-tailed deer cases that were suggestive of bTB on gross examination. The first case was an adult male submitted in November by two hunters who found multiple lung abscesses while dressing the carcass. This deer had abscesses and granulomas throughout the lungs, surrounding the heart and in the thoracic wall (Figure 1).
The lungs and heart were firmly adhered to the body wall and the diaphragm and the lung also contained an 18 cm diameter sequestrum (dead lung tissue, Figure 2).
Microscopically, these were typical abscesses that contained no acid-fast bacteria, making Mycobacterium bovis a less likely cause. At this point we were able to submit lung tissue for culture (the bacteriologists prefer not to be culturing zoonotic agents), which isolated pure Mycoplasma bovis. Like Mycobacterium bovis, Mycoplasma bovis is typically a disease of cattle causing pneumonia. However, Mycoplasma bovis is not a zoonotic agent and has not been previously reported in free-ranging white-tailed deer. It has been previously reported as a cause of pneumonia and death in 10-week-old farmed white-tailed deer. In cattle, the characteristic lesion of Mycoplasma bovis pneumonia is caseonecrotic bronchopneumonia with sequestra formation, similar to what was seen in our affected deer. We suspect that this deer may have contracted the disease through close contact with infected cattle.
The second case was a yearling male submitted in January by the Ministry of Natural Resources and Forestry with a history of emaciation, weakness and hypersalivation. On gross examination, we found that the right cranial lung lobe was almost entirely replaced by coalescing abscesses, which displaced the heart and other lung lobes (Figures 3 and 4).
Microscopically, we again found no acid-fast bacteria, so we submitted lung tissue for bacterial culture. This time, a variety of bacteria were isolated, including large numbers of Escherichia coli, Streptococcus gallolyticus, and Acinetobacter lwoffii. This combination of bacteria is suggestive of aspiration pneumonia, or perhaps an underlying viral infection with secondary opportunistic bacterial colonization. This deer tested negative for Mycoplasma spp. and Ureaplasma spp. via culture, negative for Mycobacterium spp. via PCR on paraffin embedded fixed tissues, and negative for chronic wasting disease via ELISA.
Overall, the moral of this story is that if you find pulmonary or lymph node abscesses in a white-tailed deer, you should stop what you are doing and submit the carcass to the CWHC or put on a respirator if you are a wildlife pathologist. Although the risk of the deer having bTB in Ontario is rare, it’s better to be safe than sorry. And perhaps you will find something even more rare than a deer with bTB, such as a deer with Mycoplasma bovis!
Belli LB. Bovine tuberculosis in a white-tailed deer (Odocoileus virginianus). Can Vet J 1962;3:356–358.
Wobeser G. Bovine tuberculosis in Canadian wildlife: an updated history. Can Vet J. 2009 Nov;50(11):1169-76.
Submitted by: Christina McKenzie, DVM