There are several species of Mycobacteria in the group known as the Mycobacterium tuberculosis complex (MTBC), including Mycobacterium tuberculosis, Mycobacterium bovis, Mycobacterium microti, Mycobacterium caprae and others; the most important species in deer is M. bovis. In free-living wild deer, deer in enclosed parks and on deer farms, M. bovis infection presents challenges for the veterinary surgeon. It is a notifiable disease in the jurisdictions of most developed countries. In the United Kingdom, infection may remain undetected in a herd for a long time because deer are not routinely screened. Wild deer and park deer may only receive cursory postmortem inspection by non-veterinarians and infection may be missed. Deer farms may only consign young deer to the abattoir; these may be free of visible lesions when older breeding stock is infected.

Infection with M. bovis has been confirmed in all species of wild deer in the United Kingdom and Europe, with the exception of European elk (Alces alces; Gavier-Widén et al. 2012) and water deer (Hydropotes inermis) (R. de la Rua-Domenech, personal communication). The gregarious species of deer (red, fallow and sika) have the capacity to become maintenance hosts of the infection in Europe, both in the wild and in enclosed herds (Delahay et al. 2007; Fink et al. 2015; Kelly et al. 2021; Justus et al. 2024). There is no routine compulsory statutory testing programme for M. bovis infection (tuberculosis [TB]) in farmed deer herds in Great Britain (GB). In the endemic bovine TB areas in the West Country of England, there are TB hotspots in red and fallow deer, usually associated with higher densities of deer. White-tailed deer have proven to be maintenance hosts in the United States (Palmer 2013) and the topic is discussed in Chapter 28. Infected deer herds have significant implications for the statutory control of the infection in domestic cattle.

In the United Kingdom, testing for M. bovis is managed by the Animal Plant and Health Agency (APHA), an Executive Agency of the Department of the Environment, Food and Rural Affairs (Defra). Passive surveillance of wild deer for TB has been undertaken by APHA with some prospective studies focused on particular areas (including Exmoor, Oxfordshire and Pembrokeshire). A large-scale study of the prevalence of TB in wild mammals was undertaken by APHA; selected results for deer are given in Table 14.1. These data must be interpreted with caution because of sampling bias and geographical location: for instance, few red deer were submitted from the known TB hotspot on Exmoor and the prevalence is lower than was expected. At the time of sampling, muntjac were mainly present in the fallow deer hotspot in Gloucestershire, which gave an artificially high prevalence.

Deer appear to excrete infection far more in oro-nasal secretions than in faeces or urine; sputum super-shedders have been identified (Santos et al. 2015). Where bovine TB is endemic in cattle or wildlife reservoirs, communal feeding of deer is a high-risk practice, especially in wild deer populations and in deer parks (Palmer et al. 2001, 2004). The bacterium can survive for weeks on partially consumed feedstuffs, especially root crops and for more than a year in soil (Palmer and Whipple 2006; Duffield et al. 1985; Ghodbane et al. 2014).

The clinical signs of M. bovis infection in deer are variable, vague and non-specific. Infected individuals may remain in good condition for a very long time, even for years, after acquiring the infection. The pathogenesis of the disease in deer seems to be similar to that encountered in domestic camelids: the affected deer continue to appear and behave normally until infection is widespread with multiple internal abscesses. They then decline over a period of a few weeks. Infected wild deer with advanced disease become inappetant, lose body condition, become recumbent and are either killed by predators or die. Wild deer tend to fail in this way during the winter months when food is scarce and the weather is inclement. Enclosed deer on farms and in parks are usually culled before they become so depleted.

Deer with advanced disease may occasionally have profuse loose faeces, especially if the infection is concentrated within the abdomen, but this is not invariable. External abscessation is uncommon but is sometimes encountered. If it does occur, the abscesses usually discharge from the lymph nodes of the head, throat and neck. Deer with severe pulmonary TB do not cough. They may have evidence of dyspnoea and there may be a muco-purulent or serosanguinous nasal discharge arising from abscesses discharging into the pharynx or respiratory tree.

In the overwhelming majority of M. bovis infections, the disease is detected as an unexpected finding as a culled or slaughtered deer is being eviscerated and the carcase prepared for consignment to the human food chain.

The cardinal sign of M. bovis infection in deer is internal abscessation and the character of the abscesses is extremely variable (see Figures 14.1–14.16). In most cases, abscesses first form in lymph nodes. The predilection sites for these initial abscesses are the lymph nodes of the head and throat (submandibular, retropharyngeal), the lymph nodes of the thorax (bronchial, mediastinal) or the lymph nodes of the bowel (mesenteric, ruminal and hepatic). However, isolated or multiple tubercular abscesses may occur anywhere, including in skeletal muscle, maxillary sinuses and in the substance of the liver, lungs or spleen, with no local lymph node involvement. It is therefore important to palpate the organs, especially the lungs, when undertaking postmortem examination (PME) to detect tubercular abscesses deep in the tissues. Because deer maintain body condition and apparent biological vigour even when infected with M. bovis, abscesses may be discovered in carcases that are in excellent condition, replete with fat and with no other signs of ill health. In such cases, the abscesses may be obscured by fat deposits, especially in the abdomen.

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