Protozoa are single-celled organisms that can sometimes cause disease. They may be free-living or parasites. Some of the more common diseases caused by protozoa in dogs are discussed below.

Amebiasis

Amebiasis is a disease caused by the amoeba Entamoeba histolytica. It causes inflammation of the large intestine, which leads to persistent diarrhea. Amebiasis is common in tropical and subtropical areas worldwide. Its prevalence has declined in the US over the past several decades, but the disease is still important in many tropical areas, particularly in times of disasters. It is common in people, sometimes seen in dogs and cats, and rare in other mammals. Humans are the natural host for this species and are the usual source of infection for domestic animals. Dogs become infected by ingesting food or water contaminated with feces that contain infective cysts, which are a dormant form of the parasite.

Entamoeba histolytica lives inside the large intestine and will sometimes produce no obvious signs; on the other hand, it can invade the intestinal lining and produce inflammation, wounds, and bleeding. In short-term disease, severe diarrhea may develop. This disease may be fatal, become long-lasting, or improve spontaneously. Signs in longterm cases include weight loss, loss of appetite, straining to defecate, and diarrhea. Any of these signs may be continuous or may come and go. In addition to the large intestine, the infecting amoebae may invade skin around the anus, genitals, liver, brain, lungs, kidneys, and other organs. Signs may resemble those of other diseases of the large intestine.

The disease is diagnosed by the presence of active or dormant E. histolytica in feces. If infection has occurred outside of the intestine, the parasites may be difficult to detect. In some cases, your veterinarian may examine the large intestine directly using specialized instruments. This procedure is called a colonoscopy.

Medication may be prescribed to treat this illness, although the effectiveness of treatment has not been fully evaluated. Dogs may continue to shed amoebae in the feces even after drug treatment. Be sure to follow treatment and care instructions fully.

Coccidiosis

Coccidiosis is an invasion and destruction of intestinal tissues by any of several protozoa. It can cause illness in dogs but while infections are relatively common, most infected dogs show no signs.

Coccidia tend to infect dogs or puppies that have been weakened in some way. Therefore, signs of illness caused by coccidiosis are most prevalent under conditions of poor nutrition, poor sanitation, or overcrowding, or after stresses such as weaning. Infection results when a dog ingests infective eggs (oocysts). These oocysts enter the environment in the feces of an infected dog, but they are unable to cause infection until they develop further under the right environmental conditions.

The most common signs of infection in severe cases are diarrhea (sometimes bloody), weight loss, and dehydration. The veterinarian will combine the results of fecal examinations with observations of signs and intestinal abnormalities to confirm the diagnosis.

Infection usually ends on its own within a few weeks unless reinfection occurs. Medication may lessen the likelihood of reinfection and spread. Sick dogs should be isolated to prevent exposure of other animals.

Sanitation is important, especially in kennels or where large numbers of dogs are housed. Feces should be removed frequently and fecal contamination of feed and water should be prevented. Runs, cages, and utensils should be disinfected daily. Raw meat should not be fed. Insect control is also important.

Giardiasis

Giardiasis is a longterm, intestinal protozoal infection caused by species of Giardia. It is seen worldwide in most domestic and wild mammals, many birds, and people. Infection is common in dogs. Giardia has been reported to be found in up to 39% of fecal samples from pet and shelter dogs, with a higher rate of infection in younger animals.

Giardia protozoa live in the small intestine, where they attach and multiply. They produce cysts that are passed in the feces. Transmission occurs by spread from feces to the mouth. Shedding of cysts by an infected dog may be continuous over several days and weeks but is often intermittent. It is currently unclear whether the same species of Giardia can infect both domestic animals and people. It appears that some Giardia species can infect a variety of mammals, while others only infect a single species.

Giardia infection in dogs sometimes causes no signs. In other cases, it causes weight loss and longterm diarrhea, which can be continual or intermittent, particularly in puppies. Feces are usually soft, poorly formed, pale, and foul-smelling. Watery diarrhea is unusual, and blood is not present in feces. Occasionally vomiting occurs. Giardiasis causes malabsorption of nutrients and must be distinguished from other conditions that also affect the dog's ability to absorb nutrients properly (see Digestive Disorders of Dogs: Malabsorption). A diagnosis is usually made by identification of Giardia cysts or Giardia antigen in stool samples.

To treat giardiasis, your veterinarian will likely prescribe a drug or combination of drugs that are effective against protozoa. A vaccine is available for dogs in some countries. Some studies indicate that the vaccine may reduce signs as well as the number and length of time cysts are shed into the environment.

Giardia cysts in the feces are a source of infection and reinfection for dogs, particularly those in crowded conditions such as kennels. Prompt removal of feces from cages, runs, and yards limits environmental contamination. Cysts are inactivated by disinfectants such as quaternary ammonium compounds, household bleach (1 part bleach to 16 or 32 parts water), steam, and boiling water. To increase the effectiveness of disinfectant treatment, solutions should be left for 5 to 20 minutes before being rinsed off kennel or run surfaces. Disinfection of grass yards or runs is impossible. These areas should be considered contaminated for at least 1 month after infected dogs last had access. Cysts are susceptible to drying, so areas should be allowed to dry thoroughly after cleaning. Shampooing and rinsing the dogs well can help remove cysts from hair.

Dogs may be infected with a type of Giardia species that normally infects cats. They may occasionally be infected by a type that also infects people. Dog feces should be disposed of promptly. It is important to wash hands properly after handling infected animals or their feces.

Last full review/revision July 2011 by Dana G. Allen, DVM, MSc, DACVIM; Ben H. Colmery, DVM, DAVDC; Lisa E. Moore, DVM, DACVIM; Stanley I. Rubin, DVM, MS, DACVIM; Sharon Campbell, DVM, MS, DACVIM; James G. Fox, DVM, MS, DACLAM; Carlton L. Gyles, DVM, PhD, FCAHS; Walter Ingwersen, DVM, DVSc, DACVIM; Sofie Muylle, DVM, PhD; Sharon Patton, MS, PhD; Andrew S. Peregrine, BVMS, PhD, DVM, DEVPC; H. Carolien Rutgers, DVM, MS, DACVIM, DECVIM-CA, DSAM, MRCVS; Jörg M. Steiner, DrMedVet, PhD, DACVIM, DECVIM-CA; Thomas W. Swerczek, DVM, PhD