The GI tracts of animals are subject to infection by many pathogens (see the table Common Infections of the Gastrointestinal Tract). Such infections can cause illness, suboptimal performance, and death in animals, often resulting in major economic losses. GI tract infections are transmitted via direct contact or fecal-orally. Some infections are the result of overgrowth of specific organisms in the normal GI microbiome (eg, Clostridium perfringens infection in dogs; salmonellosis in horses after transportation, extended anesthesia, or surgery).
Common Infections of the Gastrointestinal Tract
Pathogen | Cattle, Sheep, and Goats | Pigs | Horses | Dogs and Cats |
|---|---|---|---|---|
Viruses | Bovine viral diarrhea, rotavirus, coronavirus, rinderpest, malignant catarrhal fever, bluetongue, foot-and-mouth disease | Transmissible gastroenteritis, porcine circovirus type II, porcine epidemic diarrhea virus, rotavirus, foot-and-mouth disease, vesicular stomatitis, vesicular exanthema | Rotavirus, vesicular stomatitis, coronavirus | Canine parvovirus, canine coronavirus, feline panleukopenia virus, feline enteric coronavirus, canine and feline rotaviruses, canine and feline astroviruses |
Rickettsiae | Neorickettsia risticii (Potomac horse fever [equine monocytic ehrlichiosis]) | Neorickettsia helminthoeca (salmon poisoning disease in dogs) | ||
Bacteria | Enterotoxigenic Escherichia coli, Salmonella spp, Mycobacterium paratuberculosis, Fusobacterium necrophorum, Clostridium perfringens (types B, C, and D), Actinobacillus lignieresii, Yersinia enterocolitica, Campylobacter jejuni | Enterotoxigenic E coli, Salmonella spp, Brachyspira hyodysenteriae, Clostridium perfringens types B and C, Lawsonia intracellularis, Clostridium difficile | Enterotoxigenic E coli, Salmonella spp, Rhodococcus equi, Actinobacillus equuli, Clostridium perfringens, Clostridium difficile, Lawsonia intracellularis | Salmonella spp, Campylobacter jejuni, Clostridium spp, adherent invasive E coli |
Protozoa | Eimeria spp, Cryptosporidium spp | Eimeria spp, Isospora suis | Eimeria spp, Cryptosporidium spp | Isospora spp, Toxoplasma sp, Giardia sp, Tritrichomonas spp, Cryptosporidium spp, Neospora sp |
Fungi | Candida spp (cattle) | Candida spp | Aspergillus fumigatus | Histoplasma capsulatum, phycomycetes |
Algae | Prototheca spp | Prototheca spp | Prototheca spp | Prototheca spp |
Parasites (helminths) |
Definitive etiological diagnosis of infectious disease of the GI tract requires demonstrating the infectious organism's presence in the affected animal's GI tract or feces. In herd epidemics, such as outbreaks of acute undifferentiated diarrhea in newborn calves or piglets, the best way to establish a diagnosis is to select untreated animals in the earliest stage of the disease and submit them for necropsy and detailed microbiological examination of the intestinal microbiota.
When selective necropsy is not an option, specific culture techniques, depending on the infectious disease suspected, should be performed on a series of carefully collected daily fecal samples. Molecular technologies, including ELISA and PCR assay, have been developed to demonstrate the presence of parasitic, viral, bacterial, or protozoal proteins or nucleic acids in feces, which can provide a definitive diagnosis (eg, canine parvovirus, salmonellosis, cryptosporidiosis).
Gastrointestinal Parasitism
The GI tract can be inhabited by many species of parasites. Parasite life cycles may be direct, in which case eggs and larvae are passed in the feces, and stadial development occurs to the infective stage, which is then ingested by the final host. Alternatively, the immature stages may be ingested by an intermediate host (usually an invertebrate) in which further development occurs, and infection is acquired when the intermediate host, or the free-living stage shed by that host, is ingested by the final host. Sometimes, there is no development in the intermediate host, in which case it is known as a transport or paratenic host, depending on whether the larvae are encapsulated or in the tissues.
The clinical severity of GI parasitism depends on the number and pathogenicity of the parasites, which depend on the biotic potential of the parasites or, when appropriate, their intermediate host and the climate and management practices. In the host, resistance, age, nutrition, and concomitant disease also influence the course of parasitic infection.
The economic importance of subclinical parasitism in farm animals is also determined by the factors just described, and it is well established that lightly parasitized animals showing no clinical signs of disease perform less efficiently in the feedlot, dairy, or finishing house. Feed conversion in animals with light to moderate parasitism is adversely affected and is primarily due to decreased appetite and poor use of absorbed protein and energy. Carcass quality and size also decrease, which negatively affects financial returns.
Endoparasites in companion animals can cause severe disease or unthriftiness and have aesthetically undesirable consequences. Furthermore, some of these parasites also infect humans (see the table Zoonotic Diseases: Parasitic Diseases).
To estimate parasite load, see Parasitology in Veterinary Practice.
Treatment of Infectious Diseases of the Gastrointestinal Tract
Supportive care
Antimicrobials or anthelmintics, depending on the diagnosis
Supportive care for infectious diseases of the GI tract may include the following:
oral or parenteral fluids and electrolytes to correct imbalances caused by vomiting or diarrhea
nutritional support
antiemetics
probiotics to help restore the normal intestinal microbiome
Treatment of bacterial GI infections is with antimicrobial agents; parasitic infections are treated with anthelmintics or sometimes only with changes in management and housing or environment. There is no specific treatment for most viral infections; however, in 2023, a monoclonal antibody product became available for the treatment of canine parvovirus that may shorten the duration of illness.
Antimicrobials for specific bacterial pathogens are commonly administered at least until recovery is apparent. The use of antimicrobials as nonspecific first-line therapy is not recommended. Instead, prebiotics, probiotics, and diet changes are recommended initially. Antimicrobials are reserved for cases that do not respond to such therapy or for severe cases in which the risk of sepsis is a concern. Overdosage or prolonged oral treatment with antimicrobials may have adverse effects on the GI tract itself (eg, intestinal dysbiosis, villous atrophy). Parenteral administration of antimicrobials is indicated when septicemia is apparent or may occur. Choice of antimicrobial agent depends on the suspected disease, previous results, and cost.
Pearls & Pitfalls
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In herd epidemics, antimicrobials may be added to feed or water supplies at therapeutic concentrations for several days, followed by preventive levels for an extended period, depending on the infection pressure in the population. The feed and water supplies of in-contact animals may also be medicated in an attempt to prevent new cases from developing.
Also see Systemic Pharmacotherapeutics of the Digestive System.
Control of Infectious Diseases of the Gastrointestinal Tract
Measures to effectively control common infectious diseases of the GI tract include the following:
practicing good sanitation and hygiene
developing and maintaining nonspecific resistance in the animal
in certain cases, providing specific immunity by vaccinating the pregnant dam or susceptible animal
using GI parasite preventatives
Effective sanitation and hygiene is achieved primarily by providing adequate space for animals, regularly cleaning pens, and efficiently removing feces from the immediate environment.
Developing and maintaining nonspecific resistance to pathogens requires both genetic selection of animals with a reasonable degree of inherent resistance and provision of adequate nutrition and housing to minimize stress and allow animals to grow and behave normally. The occurrence of infected but clinically healthy animals, which can shed pathogens for weeks or months, is a major problem with some infectious GI diseases (eg, salmonellosis). Ideally, these carrier animals should be identified by microbiological means and isolated from the rest of the herd until free of infection or culled.
Certain diseases (eg, enterotoxic colibacillosis in calves and piglets) can be controlled by vaccinating the pregnant dam several weeks before parturition. This method requires achieving a protective level of antibodies in the colostrum. There are exceptions, but systemic immunity provides little protection against infectious enteritides in most cases. Effective immunity against GI disease depends on stimulation of local intestinal immunity after the neonatal period. During the neonatal period, protection can be provided through the local action of maternally derived antibodies. For example, secretory IgA progressively increases in sow’s milk from the time of farrowing until weaning, which provides the piglet with daily protection during the nursing period.
See also Parasite Control in Horses, Parasite Control in Beef Cattle, Parasite Control in the Management of Miniature Pet Pigs, and Gastrointestinal Parasites of Small Animals.
For More Information
Learn more about parasitic infections in humans in the MSD Manual for medical professionals.
Also see pet owner content regarding digestive disorders of cats, dogs, and horses.
