Swine dysentery (SD)

This disease affects all ages. The key clinical signs are watery, bloody diarrhoea; wasting body condition; and dehydration. Mortality can occur if dehydration persists.

Background and history

Swine dysentery (SD) is caused by a small, spiral shaped bacterium called Brachyspira hyodysenteriae. It was first identified in the late 1960s and previously known as Treponema hyodysenteriae and Serpulina hyodysenteriae. If it takes hold in your herd it can be one of the most expensive and possibly impossible diseases to eradicate especially in the growing pigs, causing a severe, red blood cell destroying (haemolytic), mucus producing, bloody diarrhoea with a severe inflammation of the large intestine (colitis) - which is the primary site for the absorption of water into the body.

There are four other Brachyspira species in pigs:

  1. Brachyspira innocens which is considered non-pathogenic.
  2. Brachyspira pilosicoli which is often associated with a less severe colitis and may also cause disease in chickens and humans.
  3. Brachyspira intermedia which is commonly found in poultry and its significance in pigs remains unclear.
  4. Brachyspira murdochii which was thought to be commensal (normal inhabitant) to the intestine but is now considered to be a low-pathogenic species that interestingly has also been isolated from chickens and rodents – who infects who remains unclear.

There have also been recent reports of an emergence of two newly described also strongly haemolytic pathogenic species, Brachyspira suanatina and Brachyspira hampsonii both of which appear to have reservoirs of infection in migratory water birds, and which may be transmitted to and between pigs. B. suanatina seems to be confined to Scandinavia, whereas B. hampsonii has been reported in North America and Europe, both causes a disease in pigs indistinguishable from SD without laboratory analysis.

Colitis may be caused by other pathogens such as Escherichia coli, coccidian parasites, Clostridium perfringens type A, heavy Whip worm infestations and Salmonella infections. Dietary factors are also thought to play a role in some cases of colitis eg, pelleted feed is much more likely to be associated with the disease than meal, even when pellets are ground back into meal it still results in colitis, demonstrating an effect of the pelleting process. There is further evidence that diet can play a prominent role on the severity of disease; studies have shown that even when given large doses of B. hyodysenteriae experimentally, pigs fed on cooked white rice show few clinical signs and that diets with a high digestibility in the small intestine e.g. meal, produce less solid matter passed through undigested into the large intestine for the bacteria to thrive on and so less damage occurs.

Clinical signs


The incubation period (time from exposure to SD to exhibiting clinical signs) is normally 2 to 14 days but can be much longer, with up to 60 days being reported. The pig may develop a sub-clinical carrier state (shedding the bacteria in their faeces but not showing any signs) first before breaking down with clinical disease, when put under stress or when there is a change of feed.

Post-mortem examinations have shown that the lesions are confined to the large intestine and in severe cases the greater curvature of the stomach.

For individual pigs that survive an infection, their prognosis ranges from fully recovered to chronically restricted growth with permanent diarrhoea. For keepers that take notes of feed conversion rates (FCR) may see long-term effects in final carcass weights in their growers and finishers.


Diagnosis in a large breeding and/or grower-finisher unit will be based upon on the history of how the disease has spread and for how long, the clinical signs, post-mortem examinations, laboratory tests on faecal smears and the isolation and identification of B. hyodysenteriae by serological or biochemical tests and DNA analysis. In a large unit it is essential that the disease is quickly distinguished from colitis caused by other organisms or dietary factors, due to the sheer numbers of pigs involved.

In the small-scale pig situation the vet will primarily use clinical signs and the case history eg, numbers affected, recently bought in a new boar with/without following isolation procedures, lack of or good on-farm worming protocols or a change of feed from meal to pellet for example, before prescribing the appropriate treatment. Obviously if this does not cure the problem then further laboratory investigations will be required.


Transmission is always via the faecal-oral route, from one pig to another or from another source including mechanically eg, on your wellies and equipment. So if you think about a showing ring where the pigs often walk in the faeces of another pig and then go back into a small pen, any sub-clinical cases of SD will spread through the exhibited pigs to take home with them, and they may even show no clinical signs until long after the isolation period has finished.

Once in your herd, clinical signs will spread steadily within and between pens over a number of days/ weeks as the organism builds up in the environment. Untreated and uncontrolled, the disease could persist for prolonged periods of time, if not permanently in breeding herds.

In a naïve herd (not been exposed to SD before) the disease can occur in all age groups of pigs, even as young as two to three weeks of age. Most typically it’s a disease of the weaned, growing pig (12 to 75 kg) and in chronically infected herds disease is often restricted to this age group. Infected pigs that recover develop a low level immunity and rarely suffer from the disease again. The disease often appears in cycles as previously affected pigs will still transmit infection to susceptible ones for at least 10 to 12 weeks. SD can be spread by flies, mice, birds and dogs, all these species have been reported to shed SD for up to 21 days.

In the environment the organism survives outside the pig for up to eight weeks in cold moist conditions but it perishes in two to three days in dry warm environments. So if you have a SD case, do not use their pen again for other pigs within these time frames.





Various antibiotics available on veterinary prescription only (POMv) have been shown to have activity against B. hyodysenteriae including Lincomycin, Tiamulin, and Tylosin. These are licenced for use in water, as an injection or as an in-feed option depending upon the vets’ desired approach. Often with the first signs of SD the water is medicated with only severely infected individuals being given the medicine by injection. Occasionally in-feed medication is used but often with SD the pigs are not eating.

It should be noted that certain strains of B. hyodysenteriae have shown antimicrobial resistance to some of these medicines, but your vet will be able to advise you.