Bacterial diarrhea (enteritis) in swine continues to be a daily challenge for pork producers and veterinarians in treating affected pigs. Some bacteria causing diarrhea have been consistent problems over the years, while other types have declined in case numbers only to be replaced by newly identified bacteria.
Reviewed in this article are some of the common bacteria currently and historically associated with diarrhea in pigs. The frequency of the different types of bacteria associated with diarrhea in pigs may vary by site and by production system.
When trying to identify the bacteria causing diarrhea, it is important to select pigs in the early stages of the disease that have not been treated with antibiotics. Testing pigs in the later stages of diarrhea, or pigs that have been treated with antibiotics, may yield negative or inappropriate results. Most diagnostic efforts involve testing for multiple agents.
The types of bacteria that produce diarrhea are commonly age associated. For pigs still in farrowing crates, Clostridium perfringens Type A and Escherichia colibacillosis (E. coli) represent the more common agents. Clostridium difficile and Clostridium perfringens Type C are identified less often.
Clostridium perfringens Type A represents one of many types of Clostridium perfringens causing diarrhea in young animals (Figure 1). Clostridium perfringens Types A through E are differentiated by the type(s) of toxins (specifically one or more of four exotoxins) each produces. Pigs most likely to get this diarrhea are infected with Clostridium perfringens by oral exposure to feces.
Clostridium perfringens isolates are currently typed by polymerase chain reaction (PCR) technique to identify the gene responsible for toxin production. Many of the Clostridium perfringens Type A isolates associated with diarrhea are also positive for the beta-2 toxin gene.
Clostridium perfringens Type A is commonly found in most farms. The spore form of the organism easily survives in the environment.
Diarrhea most commonly occurs within the first three days of life and may last for up to five days. Clostridium perfringens Type A infection results in a watery-to-pasty, white-to-yellow diarrhea. Pigs will lose some body condition and growth slows. The affected intestine may look normal or slightly thickened.
The number of pigs affected may be high, but death from Clostridium perfringens Type A alone is considered to be low.
Clostridium perfringens can be isolated from the small intestine of affected pigs by anaerobic culture. Histopathologic lesions (structural tissue changes viewable with a microscope) also help define it as a causative agent.
E. coli diarrhea in nursing pigs usually occurs within the first seven days of life and, even more commonly, between three hours and four days of age. Pigs from gilt litters are more commonly affected due to less colostral antibody protection.
Other factors that may influence the development and severity of E. coli diarrhea in young pigs include dirty crates at farrowing and low air temperature.
Most E. coli attach to the surface of the small intestine by fimbriae (a filamentous bacterial cell wall appendage that binds to the surface of the pig's small intestinal cells). The types of receptors vary and have been given different designations such as K99, F41 and 987P. The E. coli attach to small intestine cells and produce toxins, causing fluid loss into the intestine and the resultant diarrhea.
Clinical signs usually include severe watery diarrhea, dehydration, sunken eyes, lethargy and death, if left untreated. The affected intestine may be fluid-distended and thin-walled or empty.
The E. coli that is infecting the pigs can be cultured from small intestine as well as attachment to intestinal cells identified by histopathology. The E. coli can be further typed by PCR characterization of the fimbrial (pilus) and toxin genes.
Clostridium difficile is one of the more recently recognized bacteria associated with baby pig diarrhea. Like Clostridium perfringens, Clostridium difficile is also commonly found in the environment.
Clostridium difficile has been associated with diarrhea in pigs up to 10 days old. Affected pigs show a mildly distended abdomen, with or without diarrhea; lethargy; and temporary loss of appetite. The affected colon is commonly edematous (thickened by clear fluid). The number of sick pigs may be high, but mortality is generally low.
As with Clostridium difficile infection in other species, development of infection may be linked to prior oral antibiotic treatment that changes the normal intestinal bacterial population.
Diagnosis of Clostridium difficile infection is usually evident by typical lesions in the colon and identification of toxin in colon contents. Clostridium difficile culture requires special conditions.
Postweaning E. coli diarrhea is usually associated with two groups of bacteria designated by their fimbrial type, K88 and F18. As with E. coli infection in preweaned pigs, the E. coli bacteria binds to intestinal cells and secretes toxins, resulting in fluid loss(Figure 2).
In addition, some postweaning E. coli may secrete a toxin known as shiga-like toxin, which is responsible for the development of central nervous system signs such as staggering, uncoordination, poor balance, paddling and convulsions.
Both K88 and F18 E. coli may exist and cause clinical disease in the same group of pigs. In some cases, K88 E. coli infections precede F18 infections.
Postweaning E. coli infections can be distinguished by a variety of clinical signs, including watery red/bloody diarrhea; loss of appetite; central nervous system signs; and sudden, unexpected death. Pig eyelids may appear remarkably swollen and intestinal contents are liquid. The gastrointestinal tract in some pigs may be completely empty at death.
The type of E. coli involved in the diarrhea can be further characterized by PCR typing. This process can identify the genes in the bacteria that code for the fimbrial type as well as the genes responsible for toxin production.
It is important to properly identify the E. coli responsible for disease in order to develop appropriate treatment and control plans.
There are many types of salmonella that can infect pigs; however, the two types associated with clinical diarrhea problems in pigs are Salmonella typhimurium and, less commonly, Salmonella cholerasuis.
Other types of salmonella may be implicated in short-lived episodes of diarrhea, but generally don't have the serious implications of S. typhimurium and cholerasuis infections. Some salmonella infections in pigs are asymptomatic.
Salmonella diarrhea usually affects pigs from postweaning to 4 or 5 months of age; however, it is occasionally seen in adult swine.
Salmonella typhimurium infection results from contact with infected pigs or a contaminated environment. Once contaminated, some facilities may remain the source of infection for new groups of pigs.
Pigs infected with Salmonella typhimurium may be subclinically (inapparent) infected or develop diarrhea. The bacteria infect both the small and large intestine, inducing fluid loss that results in diarrhea, and damaging the intestinal surface.
At first, affected pigs will have a watery diarrhea that may contain flecks of blood and mucous. Pigs are also febrile (feverish), dehydrated and show signs of weight loss. The diarrhea quickly spreads to other pigs in the pen and is easily tracked to other locations. Pigs may have recurring episodes of diarrhea over several weeks. The colon and intestine in affected pigs may be thick-walled with an ulcerated surface lined by small to large pieces of debris.
Salmonella cholerasuis infection may also result in diarrhea and intestinal damage if the pigs survive the severe, initial septicemic (invasion of the bloodstream by infective organisms) stage of the disease.
Salmonella diarrhea can be diagnosed by culture and examination of tissues by histopathology.
Porcine proliferative enteritis (PPE) caused by Lawsonia intracellularis is an intestinal disease of swine occurring in pigs from 6 to 20 weeks of age. But it also occurs in adult hogs.
Infection can take two diverse routes. It can result in severe intestinal disease and death, or occur as an inapparent infection with disease presented only as decreased performance (Figure 3).
The bacteria are shed in the feces of all clinically infected pigs, but also in pigs with inapparent infections. Transmission between pigs occurs through direct contact with feces from infected pigs. Ingestion of the bacteria results in infection of small and large intestinal cells, and clinical diarrhea usually starts 2-3 weeks later. Diarrhea may occur in less than half of the infected animals, however.
Clinical disease may take a chronic form or an acute, hemorrhagic form. In the chronic disease form, pigs may have a soft-gray/brown stool that alternates with normally-formed stools on other days. Appetite usually decreases and pigs become thin. There may be size variation within the group. Pigs will normally recover in four to 10 weeks.
Both the large and small intestine may be affected, but changes are usually more severe in the small intestine. The intestinal wall appears slightly to markedly thicker and the surface may be lined by a thick layer of yellow/white debris.
The acute, hemorrhagic form of PPE or ileitis is more common in young adults and is characterized clinically by loose-to-formed black (bloody) feces and a pale body color (due to blood loss) (Figure 4). Some pigs die suddenly without showing any signs of diarrhea or blood loss in the stool. The intestine in this form of the disease is usually thick-walled (Figure 5) and the cavity contains blood or blood clots.
Diagnosis of PPE by histopathology is usually straightforward in pigs with obvious intestinal changes. The bacteria can be demonstrated with special stains under the microscope or by PCR. However, the bacteria are very difficult to grow; culture is not even attempted in routine testing.
Subclinical forms of the disease can be even more challenging to diagnose. The bacteria can be identified in the feces of clinically affected pigs by PCR but not in all subclinical infections. Serologic testing is available, but the number of pigs seroconverting in a group may move slowly, with only 5% to 50% positive.
Swine dysentery, primarily a disease of growing and finishing pigs, causes severe catarrhal (abundant mucous) and hemorrhagic colitis. The number of cases of swine dysentery has decreased dramatically, with success for control and elimination credited to antibiotics and modern production practices.
Transmission of the bacteria is through fecal shedding by infected pigs to susceptible pigs. Contaminated pit waste, mice, birds and dogs are other potential sources of infection and disease spread.
Disease usually spreads slowly within a herd. Affected animals develop diarrhea, dehydration and weight loss, and death losses may be as high as 50%. The bacteria grow in the colon (not the small intestine), producing toxins that result in damage to the colon.
Diarrhea is associated with malabsorption rather than fluid loss. The diarrhea associated with swine dysentery is initially a watery, yellow diarrhea that progresses to large amounts of blood-flecked, mucous-filled stool. At necropsy, the colon wall is markedly thickened and the surface is covered by a thick layer of mucous, blood and cell debris. The lumen or organ cavity may be filled with blood and cell debris forming luminal casts.
Diagnosis is by histopathology, culture and isolate verification using PCR analysis.
Brachyspira pilisicoli is related to, but distinct from, the causative agent of swine dysentery. The bacteria are identified with a syndrome of mild diarrhea caused by a mild colitis. Production losses are associated with treatment costs and a loss of body condition. Mortality is very low.
Infection is thought to occur by the fecal-to-oral route. The incubation period may be up to 20 days and infected pigs develop a self-limiting, soft-to-watery diarrhea that lasts from two to 14 days. Colon and cecum (opening to the large intestine) in affected pigs appear thin-walled and filled with soft, green feces.
Diagnosis is by culture, histopathology or immunohistochemistry and PCR-based typing.
The bacteria causing diarrhea in pigs will continue to be recurring problems for pigs, producers and veterinarians. In some cases, antibiotics are no longer an option due to resistant bacteria. Sanitation plays an important part in controlling intestinal diseases of swine.