About bovine TB
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Many species of non-bovine farmed (e.g. South American camelids (SAC), captive deer, goats, pigs and sheep) companion (e.g. cats, dogs and ferrets) zoo and wild mammals are susceptible to M. bovis infection. Only a relatively small number of animals are identified as infected each year through scanning surveillance. Evidence suggests that non-bovine species other than the badger are generally ‘spillover’ hosts and appear to pose a very small risk of spreading M. bovis to cattle and badgers.
Wild mammals other than badgers can act as maintenance hosts for M. bovis and vectors of the infection for cattle, as illustrated by the experiences of New Zealand (brush-tailed possum) Australia (Asiatic water buffalo) Michigan (white-tailed deer) South Africa (Cape buffalo) the Central and Southern Iberian Peninsula (wild boar and red deer) and some départements of France (wild boar and red deer in addition to badgers). However, the existing evidence from wildlife surveys and quantitative risk models carried out by APHA (formerly Fera) in GB indicates that in this country the badger remains the principal and possibly the only wildlife maintenance host of M. bovis. Whilst M. bovis infection has been found in other wild mammals in England (notably deer and more rarely wild boar, fox and some rodents) the data on the prevalence of infection, pathology, abundance and ecology suggest that fallow deer and possibly muntjac and red deer are the only other wild mammals that could act as potential sources of M. bovis for cattle in the South West of England and Wales. Even in these deer species the effect is localised and the risk of transmission to cattle much lower than that posed by badgers, primarily due to differences in behaviour and contact levels with cattle. Additionally, once detected, deer infection is often controlled locally by additional culling.
Wild deer surveillance is carried out by private stalkers who are aware of the need to submit suspicious lesions for bacteriological examination. Where there is a suspicion of deer-related infection in cattle, this surveillance can be intensified and additional radial surveillance of cattle in an area can be initiated by APHA when considered appropriate. APHA will continue to monitor the results of scanning surveillance in wild mammals.
There is sparse information available about feral populations of wild boar in England. They are currently thought to be present in a limited number of localised populations at low population densities.
They are normally only infected with TB in areas where infection is widely found in cattle and badgers and a small number of isolations are made from feral wild boar each year and the genotypes are usually identical to those isolated from the local cattle.
In England, feral wild boar are thought to act as spill-over, sentinel hosts of M. bovis and current evidence suggests that the risk of transmission of M. bovis between cattle and feral wild boar is very low. In other countries such as France and Spain, there is however evidence that they can act as reservoirs of infection and transmit TB to cattle. Widespread tuberculous lesions have been found in feral wild boar in Spain suggesting that these animals can shed the bacterium and contaminate the environment. Therefore, feral wild boar in England, should be regarded as potential amplifier hosts for M. bovis.
If evidence emerges that feral wild boar are playing a significant role in the epidemiology of TB in England, measures could be taken to control the population.
Wild deer are highly susceptible to TB infection. In GB they are generally considered to be spill-over hosts of TB i.e. they are unlikely to sustain the infection within their own population in the absence of infected cattle or a wildlife reservoir. The wild deer population in GB has been estimated to be well over two million with an annual cull of over 300,000.
There are usually only a few isolations of M. bovis from wild deer each year in GB.
There is evidence that wild deer can be a reservoir of TB and can transmit TB both to other deer, and to cattle. This is where wild deer live or congregate at high population density and commonly interact with cattle e.g. in SW England and the northern USA.
As for wild boar, deer are not a protected species so if evidence emerges that wild deer are involved in the spread of TB in a particular location, measures could be taken to control the population.