(When sourcing stock IT IS UP TO YOU to ask the breeder if they have suffered any diseases within their herd if you don’t ask you don’t know and the breeder will and should not be offended by your question as this shows due diligence on your behalf.)
Despite a control programme spanning decades, M. bovis infection levels in cattle in Great Britain (GB) have continued to rise over recent years. As the incidence of infection in cattle and wildlife may be linked to that in pigs, data relating to infection of pigs identified at slaughter are being examined. Data suggest that pigs raised outdoors or on holdings with poor biosecurity may be more vulnerable to infection with M. bovis. In the majority of cases, the same strains of M. bovis were found in pigs and cattle, despite the fact that direct contact between these species was rarely observed. Genotyping and geographical mapping data indicated that some strains found in pigs may correlate better with those present in badgers, rather than cattle. Given the potential implications of this infection for the pig industry, and for the on-going effort to control bovine TB, the importance of understanding the epidemiology and pathogenesis of M. bovis infection, as well as monitoring its prevalence, in pigs should not be underestimated.
Tuberculosis is transmitted through animal to animal contact or by ingestion of contaminated food, water or soil. Mycobacterium avium ss hominisuis and M. avium ss avium have been isolated from peat and kaolin and should not be used as feed additives since they are considered high-risk factors. Early in the 20th century when Tb in cattle and man was more prevalent, disease in pigs was due mainly to M. bovis or M. tuberculosis. The bacteria is most often of environmental origin. M. avium and other mycobacteria abound in the environment and occur in food and drinking water; therefore it is not surprising that they are present in the human alimentary tract. Rhodococcus equi infection in pigs also produces a granulomatous lesion that resembles Tb microscopically. The earliest reports of R. equi infection in pigs were made during the 1930’s, and isolation of the organism has been reported frequently since then. Rhodococcus equi is common in the soil of pig pens, and infection with this organism occurs about as often in pigs with or without mycobacterial disease. In summary, although other bacteria can cause diseases resembling pig Tb, M. avium ss hominisuis is responsible for a large number of reported cases in commercial herds in countries with M. bovis eradication programs. Peat when added to feed or when used as bedding material is considered a very high risk factor for TB in pigs.
Pigs usually are infected with M. avium by ingesting the organism. After ingestion, the organism penetrates the wall of the pharynx near the tonsils or the wall of the small intestine and becomes localised in the mandibular and/or mesenteric lymph nodes. Small lesions develop in these lymph nodes. The health and condition of the infected pig are usually not affected, therefore it is often impossible to establish a clinical diagnosis in these animals. It should also be noted that in herds in which Tb has been diagnosed M. avium has been isolated from the lymph nodes of pigs that were negative to skin tests, presented no lesions tissues, and had no signs of illness.
Because diagnosis of Tb in the live animal is usually impossible, the prevalence of the disease must be determined from postmortem. The actual infection rate may be higher since mycobacteria can be cultured from lymph nodes with no visible lesions and because some lesions may go undetected. Pathogenic mycobacteria may survive for more than 4 years in soil and litter contaminated by chickens with Tb. Studies have shown that sawdust or wood shavings used for bedding are a source of M. avium ss hominisuis in pigs. Mycobacterium avium complex is often found in samples of sawdust and wood shavings where it survives for long periods. The mycobacteria may multiply under proper conditions of moisture and temperature which could explain the seasonal occurrence of disease in some herds. It has been suggested that seasonal changes may produce less favourable conditions for survival of bacteria in wood shavings and cause the infection rate to decrease. The presence of lesions in the intestinal wall with a subsequent pig to pig transmission probably is due to shedding of mycobacteria in the feaces. Granulomatous lesions of lungs, mammary glands, and uterus also may occur with the potential for transmission of organisms from these sites.
The addition of infected breeding stock could introduce the disease within a herd, and transmission from infected sows to their litters may maintain the disease within a herd.
Detection of mycobacterial disease in a live animal is often very difficult, therefore the presence of disease must be determined by post-mortem examination. Infection in pigs exposed to M. avium is usually associated with the lymph nodes of the head and the digestive tract and rarely spreads to other locations. Diagnosis of Tb by physical examination of the live pig is usually impossible. Visual examination of infected sites at slaughter cannot differentiate lesions of Tb from those caused by other microorganisms or conditions; a confirmed diagnosis should be based on mycobactereologic examination from these sites.
Diagnosis of Tb in pigs on a herd basis is important and usually depends on detection of infected lymph nodes from swine at slaughter. When Tb has been confirmed by microscopic and bacteriologic examinations, the producer should work with a veterinarian to determine potential sources of the infection and alter management practices to eliminate the source if possible.
Tuberculin skin testing has been used to identify pigs exposed to pathogenic mycobacteria. The amount of tuberculin used and the site of injection have varied depending on the investigator. The recommended method for a tuberculin skin test in pigs is an intradermal injection of 0.1 ml M. avium purified protein derivative (PPD) in the dorsal surface of the ear. The response (induration) to injection of PPD is observed and recorded at 48 hours. Positive reactions usually include swelling and redness, and they may vary in size and intensity. Haemorrhage and ulceration may occur at the injection site. The reliability of the tuberculin test, when used on individual swine, has been questioned.
The tuberculin test can be used successfully as a herd test although false positive reactions occur. Biologically balanced PPD’s of M. avium and M. bovis may be injected at separate sites on the dorsal surface of the ear to gain useful information on exposure to M. tuberculosis complex or to M. avium complex organisms. The responses (mm) at the injection sites should be measured (mm) and compared at 48 hours post-injection of PPD. Enzyme-linked immunosorbent assays have been described for obtaining information on the presence of mycobacterial antibodies in the sera of the Pig naturally exposed to clinically significant mycobacteria. However, these tests have not come into widespread use since some animals fail to develop detectable antibodies in the sera for several weeks or months following natural exposure.
Prevention and Control
Control of mycobacterial infection in pigs is difficult since no vaccine is available and the preventive use of anti–tuberculosis drugs in feed is either illegal or of unknown value. Preventing the disease in noninfected herds is more effective than trying to eliminate the disease from infected herds. It is important not to:
1. Raise pig and poultry in close proximity on the same premises.
2. Feeding kitchen waste, unpasteurised milk, or other materials that might contain viable mycobacteria to pigs must be avoided.
3. Breeding stock should be purchased from herds free of Tb (those in which no lesions of Tb are found in slaughter).
4. Efforts should be made to prevent all contact between pigs and wild birds. The potential for transmission of M. avium complex from infected wild birds to pigs is probably low but must be considered.
5. Pigs should not be housed in old poultry buildings unless they have first been thoroughly cleaned and disinfected.
6. Wood shavings should be kept dry and protected from contamination by wild birds and invertebrates.
There are few options for eliminating Tb from infected herds:
1. Producers should not use peat or kaolin as feed supplements.
2. Breeders may depopulate the herd and then repopulate with stock from Tb- free herds. Little is known about decontamination of infected soil since mycobacteria can survive in this environment for at least 4 years. To avoid such problems, concrete lots should be used whenever possible.
Concrete surfaces and equipment including farrowing areas and feeders which must be disinfected with a suitable disinfectant as described in earlier posts from AHDB Pork webinar recently.
Ammonium disinfectants or halogens (e.g., Chlorine) will not kill mycobacteria. A mycobacterial infection will recur if the source of infection cannot be effectively decontaminated or if replacement stock is not separated from the source.
Producers may choose to endure the 6-month period until all exposed pigs have been slaughtered if the source of infection can be determined and eliminated. Mycobacterial disease increases the need for mandatory identification of slaughter pigs. The ability to trace pigs with mycobacterial infection to the herd of origin is useful to solve this problem. IT IS UP TO YOU TO ASK THE BREEDER IF THEIR HERD HAS SUFFERED TB IN THE LAST TWO TO THREE YEARS. YOU WILL NOT OFFEND A BREEDER YOU ARE SHOWING DUE DILIGENCE.
The Proposed compensation rates in place by DEFRA already in place in Scotland and Wales. UK to be confirmed.
Breeding female (Gilt or Sow) – £250
Breeding male – £350
Suckler (a pig weighing under 25kg) – £30
Weaner (a pig weighing from 25kg to 35kg) – £40
Grower or Finisher (a pig weighing over 35kg) – £90
Tuberculous lesions in various tissues of pigs at slaughter
Bronchial lymph node (A)
Submandibular lymph nodes (B, C)
Enlarged mediastinal lymph node (D).
Photo above shows retropharyngeal lymph node of a pig with TB. The lymph node shows many tubercles, some of which are merging (image source: APHA)
Map Bovine TB checker http://www.ibtb.co.uk
Photos and information courtesy of BMC Veterinary research, DEFRA, Harvard University