Age affected: Growers/finishers, gilts, sows, boars, human risk.
Causes: Avian (mostly), bovine or human types of the bacteria – Mycobacterium bovis/ tuberculosis; contact with birds.
Effects: Abattoir condemnations (especially heads and offals).
Tuberculosis in pigs is caused by a range of slow growing waxy mycobacteria. Most important for zoonotic reasons are infections with Mycobacterium tuberculosis:bovis group and the infections with M. avium, its subspecies, and M. intracellulare. All can now be identified to species by their DNA sequences. They can be demonstrated easily in tissue and can be cultured by specialist laboratories.
Infections with avian bacteria come from infected poultry or from contamination of feeders, etc. with infected bird droppings. M. intracellulare, M. avium and M. fortuitum infections appear to originate in sawdust or peat in some outbreaks and are not readily transmitted from pig to pig. M. xenopi has been recorded in water. M. bovis infections arise from contact with tuberculous cattle wildlife (principally badgers) or access to infected milk and may spread from pig to pig. Infection in wild boar has been identified in several parts of Europe and this poses a risk to outdoor pigs. Infection is generally oral and lesions may first be detected by microscopy in mesenteric and submadibular lymph nodes after 14 days. More often they require 6-10 weeks or even 90-115 days (some M. avium strains). Antibodies are produced early in infection but immunity to infection is cell-mediated.
Mode of transmission
Infection is oral or by the respiratory route. In most cases it follows the consumption of infected tissue, infected cow’s milk or water, food or bedding, but can be by direct contact with infected pen mates or with contaminated pen furniture. Transmission between farms can be by the transfer of infected pigs, but is often the result of contact with free-living wildlife. Infected food or bedding may be the vehicle.
These are usually only seen in generalised tuberculosis and usually result from infection with mammalian tuberculosis species such as M. bovis. The incubation period is at least 6-12 weeks after which loss of weight, chronic pneumonia or local sinus (deep infected tracts oozing pus) formation may be seen. Chronic diarrhoea has been recorded in enteric infections. Most infections are only detected at slaughter.
Tuberculosis is usually detected as slaughter or by the post-mortem examination of wasted pigs. Tuberculosis must be notified to the veterinary authorities in most countries as it is a zoonosis. ELISA tests have been developed to detect serum antibodies to recombinant mycobacterial proteins but are not yet widely used. Intradermal tuberculin testing may be used on a herd basis with either mammalian or avian tuberculins given intradermally on the ear. Pigs may be negative 6 weeks after infection but positive at 12 weeks. Desensitisation may occur in late cases.
As clinical tuberculosis is uncommon, the post-mortem examination of wasted pigs is a possible but unusual method of detection. Most cases are identified from the presence of lesions suspected to be those of tuberculosis. Lesions (granulomas, chronic pneumonia) may be local or general and can be confirmed as such by demonstration of the organism in smears by Ziehl Neelsen staining. Localised tuberculosis usually occurs only in the pharyngeal, cervical and mesenteric lymph nodes. The lesions vary between small yellowish-white caseous lesions and generalised enlargement of the node. Histological examination confirms the granulomatous nature of the lesions and acid-fast (ZN+ve) organisms may be seen. The organism may be demonstrated by culture in specialist laboratories. These laboratories can now use the Polymerase Chain Reaction (PCR) to identify mycobacterial DNA in tissue or clinical and environmental samples and DNA probes to identify mycobacteria in histological sections.
Treatment and prevention
Treatment is technically possible for valuable individuals (e.g. using rifampicin) but is not practised commercially. Prevention involves the removal of the source of infective material e.g. sawdust and prevent access by birds, wild boar and badgers. Pasteurise peat used for piglets. Affected carcases may still be found at slaughter until unexposed stock reach slaughter age. Where M. bovis is involved, skin or ELISA testing may reveal infected animals which may then be culled.
As tuberculosis infects humans and the exact species concerned cannot be determined by visual inspection of the lesions, carcasses with these lesions (or parts of them) may be detained and destroyed to safeguard human health.