Facts About TB
The range of animals susceptible to bTB infection is very broad. All land mammals are susceptible to infection to varying degrees, depending on the exposure level, innate resistance, predominant immunological pathways and type of husbandry.
An animal’s opportunity to transmit the disease to other animals (particularly cattle) varies according to their ability to maintain TB within their own populations. There is a recognised classification method which is used to describe the ability of an animal to maintain TB within their own population:
• Maintenance hosts (e.g. bovines, badgers and occasionally farmed and wild deer), where the infection persists by vertical, pseudo-vertical or horizontal transmission within the species, without the need for input from other species; and
• Spillover hosts, where TB occurs within the species only as long as there is input from an external source. Spillover hosts may in turn be either ‘dead-end’ hosts (if the incidence and pathology of the disease indicates they play no significant role in its onward transmission) or ‘amplifier’ hosts if they appear capable of increasing the prevalence of TB in livestock or other species.
Generally, pigs, horses, sheep, cats, dogs and camelids are all considered spillover hosts. In other words, these species become infected only when the challenge level around them is particularly high, and they cannot sustain the infection within their own populations in the absence of infected cattle or a wildlife reservoir. This does not mean that, if infected, they cannot transmit the infection to other animals or humans as some of them may on occasion act as amplifier hosts (e.g. pigs, farmed wild boar, dogs, cats and camelids).
There are also different strains of bacterium within the mycobacterium family and care must be taken not to confuse one strain and its natural host species with another.
Human TB can be caused by both Mycobacterium tuberculosis and Mycobacterium bovis (the bacterium which causes bovine TB). Cattle only show signs of disease when infected by M.bovis. M.avium is the cause of TB in birds and can cause disease in other species including cattle, deer, pigs and man. Other mycobacteria such as M.microti, M.smegmatis, M.kansasii and M.phlei may occasionally be isolated from a range of species.
There are strict meat hygiene laws in the UK which protect food safety and control disease. Where a meat inspector at a slaughterhouse believes that any part of the carcase or offal of any animal is infected with tuberculosis, he will insist that the carcase is split and will visually inspect the ribs, sternum, spinal cord and if necessary the brain and kidney. In the case of a pig carcase, the lymph nodes will also be examined in detail.
If generalised TB is found, the whole carcase, blood and offal will be condemned as unfit for human consumption. If the inspector is satisfied that the tuberculosis is only localised, he will condemn the part of the carcase infected, including any infected or associated lymph nodes and organs.
Under the TB (England) Order 2007, TB is notifiable in carcasses in pigs and other non-bovine species, so if suspicion of TB is found at slaughter, Animal Health has to be told. They could then impose movement restrictions on any susceptible animals remaining on the holding. AH would then come out and discuss each case on an individual basis, which may mean that further testing is carried out, before the movement restrictions get lifted.
The NFU is concerned at the numbers of non-bovine species which are picking up bTB, and see this as further indication of the growing infection levels in the environment. We do continually express these concerns to Defra as well as stressing the financial implications and compensation need of such cases on the farming families affected. There are some biosecurity measures that may be appropriate for individual farmers and we can help to advise if you require our help with these.