Blood and mucus are evident in watery feces.
Blood and mucus are evident in watery feces.

Swine dysentery, commonly known as bloody scours, seems to be making a comeback. It was first described in 1921 in the Midwest. By the early 1970s the disease was prevalent in up to 38 percent of U.S. herds, with an estimated annual cost to the industry near $130 million.

In 1971, the work of my Iowa State University research team produced the discovery and naming of the cause of swine dysentery, Treponema hyodysenteriae, now called Brachyspira hyodysenteriae.  At about that same time, Cambridge University researchers made the same discovery, which helped validate our research.

Thanks to worldwide research swine dysentery became a disease of the past by the 1980s and had been mostly forgotten until its re-emergence at the turn of this century.

Recently, however, swine dysentery has been making a resurgence. Typically affecting weaned/grower pigs, symptoms include severe diarrhea with blood and mucus.

Looking Back to Move Forward

In the 1950s and 1960s, swine dysentery caused economic havoc worldwide. In the United States, its spread was exacerbated by the movement of 30- to 50-pound feeder pigs into Iowa from small farrowing operations in the south. 

These farrowing operations were unknowingly infected with B. hyodysenteriae, and apparently healthy feeder pigs were placed in finisher buildings, subsequently breaking with swine dysentery a few weeks later. 

Pigs in farrow-to-feeder operations and even small herds of purebred breeders were infected with B. hyodysenteriae, but the disease went undetected because there is good immunity in adult animals once they have recovered from the disease. 

Only when young pigs and young breeding animals were transported and commingled did the disease emerge with a vengeance. The result was severe diarrhea containing blood and mucus, with death rates reaching as high as 40 percent in some cases.

Individual herd disease-eradication strategies were created, and control methods were developed without having to depopulate herds. 

Based on serologic prevalence studies in the early 1970s, the disease incidence on Midwestern farms was as high as 38 percent. By the early 1980s, swine dysentery appeared to be of little significance worldwide.  Numerous patents describing vaccines to prevent it had been granted — most are now expired — and a commercial vaccine was sold in the United States for a time but has since been removed from the market.

Where Are We Today?

I first became aware of swine dysentery’s re-emergence in 2004 as I attended the International Pig Veterinary Society meeting in Germany. An owner of a large integrated operation in Spain asked me to investigate an outbreak.  Of note, there were no presentations on swine dysentery at the 2004 IPVS meeting. 

Since then, reports of swine dysentery occurrence in the United States and Canada appear to have increased.  In my opinion, two facts play heavily in the re-emergence:

  1. Mice are a known reservoir for B. hyodysenteriae, which affects mice without causing disease so they can remain infected for more than a year.
  2. Single-site farrow-to-isowean operations appear to be uninfected due to B. hyodysenteriae immunity in the sow herd and colostral immunity in suckling pigs, making it difficult to detect infected farms. The large increase in movement of 3-week-old isowean pigs from apparently subclinically affected farms has increased the occurrence of swine dysentery-positive isowean pigs entering wean-to-finish units in the Midwest. 

Other factors include difficulties in eradicating B. hyodysenteriae from manure pits due to the difficulty in effectively cleaning and disinfecting all areas of the pit. Animals exposed to pit or lagoon water can be infected by the organisms still present.

Also, research in Australia has shown that dietary components can influence whether clinical signs of swine dysentery are easily recognizable. The organism also survives in feces for 60 days at refrigerator temperature, so it can remain viable throughout the cold periods of the year.

Here’s a look at some of the past options involved with swine dysentery infections and the potential financial impact.
-----------Per 100 kg-----------
Farm Status Cost of Production Profit
Swine Dysentery-free $101.09 $7.44
Endemic Swine Dysentery $106.85 $1.67
Medically Eliminated $103.60 $4.93
Depopulation/Repopulation $109.52 $0.07
Source: Polson, Marsh, Harris - 1992

Knowing the Symptoms

Swine dysentery affects pigs of all ages, but it’s most frequent in 8- to 14-week-old pigs. With a one- to three-week incubation period, symptoms include sunken flanks, depression and diarrhea of loose to watery feces with blood and mucus evident. Lesions are restricted to the large intestine, cecum, colon, rectum, edematous mesentery and intestinal wall. The colon contains mucus, fibrin and blood.

Rectal swab samples are not a good way to diagnose swine dysentery, as cultures are very specific if the organism can be detected but they lack sensitivity. Using rectal swab material in the qPCR test can give misleading results due to a lack of both sensitivity and specificity.

The most accurate determination of swine dysentery is to post pigs with clinical signs. A complete necropsy is recommended, and tissues should be submitted to a diagnostic laboratory. Cultures of intestinal material from pigs with lesions are both specific and sensitive for diagnosis confirmation.

Swine dysentery is often subclinically present in sow herds, providing maternal antibody immunity to baby pigs until they are weaned, then the disease breaks when they are moved to grow/finish barns. The best indicator of whether a sow farm is subclinically infected is whether isowean pigs break soon after placement in wean/finish barns. In those barns, swine dysentery will occur if no drugs are present in feed or water to mask it.

Strategies to Consider

Based on costs in the 1980s, it was more economically feasible to eliminate the disease without depopulation versus to live with it or to depopulate. (See table.)  Currently, depopulation may be a viable option if additional diseases can be eradicated simultaneously.

Several drugs used in the 1980s to prevent, treat and eliminate swine dysentery are no longer available.  Drugs currently approved for treatment and/or prevention can be found at  Research is underway to determine if any of the drugs can be used at approved levels to eliminate the disease without depopulation programs.  Consult a veterinarian for assistance in developing effective control programs.

Control in the future will depend on improved detection of asymptomatically affected sow farms and vaccine development to prevent swine dysentery in growing pigs. Research has shown that pigs that have recovered without medication are immune to re-challenge, making vaccine therapy a viable control option.

Pregnant gilts that recovered from swine dysentery transmitted B. hyodysenteriae to their suckling pigs. Those pigs did not exhibit symptoms while nursing but became ill with the disease post weaning.

A commercial subunit vaccine previously approved by USDA has been removed from the market. An RNA particle-based vaccine is now in development and is expected to be available soon.

Swine dysentery is difficult to diagnose definitively, but if clinical signs and lesions are observed, intestinal tissue samples should be sent to a diagnostic lab for examination.

Swine dysentery is too costly a disease to live with, and biosecurity measures can prevent it from entering a farm in the first place. Though difficult, it can be eliminated via isowean pigs in multiple isolated-site production practices.