ABSTRACT: Both dogs and cats can be infected by protozoal parasites. Signs of disease, however, are not always evident and some will live within the host undetected. Where they cause disease, it is often in debilitated or young animals. Intestinal forms are more likely to cause diseases resulting in diarrhoea, which can range from mild and chronic, to acute and severe. Some protozoal diseases have the potential to be zoonoses. Increased attention is now being paid to the risk of protozoal diseases, which are endemic in Europe, spreading to the UK through the growing numbers of pets travelling within the Pet Travel Scheme.

Protozoa are single-celled organisms, many of which are found free-living in the environment and are innocuous.

Some are capable of infecting mammals, including cats and dogs, where they may be asymptomatic and tolerated by the host, or they may cause disease. Disease is more common in young or debilitated animals and can vary from diarrhoea and weight loss, to debilitation and even death. They are commonly classified as either gastrointestinal or extraintestinal.

Perhaps the greatest challenge in managing these parasitic infections lies in their diagnosis. Many can be difficult to detect, shedding of infective forms may be intermittent and, where identified, their significance can sometimes be unknown.

Protozoan life cycles can be complex and it is outside the remit of this article to detail these. Briefly, in many, the stages of parasitic protozoa that actively feed and multiply are called trophozoites, although other terms can be used for these stages.

Cysts are stages with a protective membrane or thickened wall, and are found either in the body tissues, where they rely on being eaten for transmission, or are passed into the faeces. They can survive for long periods in the environment where they are ingested by host species. Some protozoa can have two host species; others complete their life cycle within a single host.

In this article we will discuss the more common protozoa.

Protozoa in cats

Most coccidian protozoal infections are harmless in cats and the body will eliminate them without any clinical signs of infection being evident. More serious infections cause severe watery or bloody diarrhoea. These are often seen in confined situations where there is a high- stocking density and hygiene may be sub- optimal. Good hygiene measures are necessary to prevent re-infection.

The extra-intestinal Toxoplasma gondii is perhaps the most well-known coccidian, because of its zoonotic potential. Infection of both cats (20-60% prevalence, www.fabcats.org) and humans is common, but disease is rare – with developing foetuses, young children or immuno compromised individuals being at higher risk. Cats are the definitive host for the organism (meaning sexual reproduction of the organism only takes place in this species) but intermediate hosts include man, rodents, bird, sheep, pigs and cattle.

In man, consumption of raw meat significantly increases the risk of acquiring this infection,1 as does the accidental ingestion of oocysts from the environment, such as litter trays. Cats themselves are infected by eating raw meat contaminated with oocysts, most commonly from prey species. Shed oocysts sporulate and become infective in the environment for a very long time (up to 18 months).

Prevention of the disease in people is straightforward by means of good hygiene measures and thorough cooking of meat.

Isospora (now known as Cystoisospora) is one of the most common organisms to infect kittens. In transient or chronic disease very little may be seen; but acute, severe infections have the potential to cause bloody diarrhoea, and even death. The life cycle is complex, and multiple asexual replications may cause severe damage to the gut. Infection can be via the ingestion of an infected oocyst in the environment or a paratenic host, such as a mouse.

Cryptosporidia infection in cats follows a similar pattern. Infection is common but disease is rare. Transmission between cats is via mutual grooming and ingestion of faeces, sharing litter trays, ingesting contaminated food and water, and possibly by ingestion of prey species.2 It’s now thought (through molecular studies) that cats are usually infected with the host-specific Cryptosporidium felis3_5. The diarrhoea caused usually originates in the small intestine.

Giardia spp. are flagellate protozoans, also capable of causing zoonotic disease. Where they cause disease in cats, diarrhoea is a common presentation. They have a direct life cycle and infection is caused by ingestion of infective cysts in the environment. Although the diarrhoea is small intestine in origin (from trophozoites found in the jejunum and ileum), mucus is commonly found. Perhaps the greatest challenge with Giardia is in knowing how significant a positive result is, as shedding of cysts can be intermittent and the finding of Giardia doesn’t necessarily confirm disease. The subclinical carrier state varies from two-10 percent.6

Tritrichomonas foetus is another flagellated protozoan that causes intestinal disease in cats and is increasingly being recognised (Figure 1). It infects and colonises the large intestine, and can cause prolonged and intractable diarrhoea. Colitis is seen with fresh blood or mucus and some cases can be severe, with faecal incontinence. It is most commonly seen in young cats and kittens, but can infect cats of all ages. Affected cats in most instances are from rescue shelters and pedigree breeding colonies. Treatment is unrewarding although most cats eventually overcome the infection.

Figure 1: Tritrichomonas foetus. Image courtesy of FAB (www.fabcats.org)

Protozoa in dogs

In dogs, intestinal protozoa include Cystoisospora spp. (more commonly causing disease in puppies), and increasingly recognised Neospora caninum. This is recognised as an important agent in abortion of cattle, although the dog is considered the definitive host.

In dogs, clinical signs can include neurological and locomotor disturbances, caused by the cysts affecting neurological tissue. The dog is also considered the definitive host for Sarcocystis spp. Clinical signs are not commonly seen, though control (preventing dogs from eating uncooked meat) is important to prevent disease in other hosts, including sheep and catde where it can cause illness and a knock-on economic effect.

Cryptosporidium canis causes a similar disease to that in kittens, with debilitated and young animals more commonly affected. Both C. canis and C. felis can infect humans and, therefore, present a zoonotic risk.

Giardia spp can also cause diarrhoea in dogs, particularly puppies, and foul¬smelling faeces may be evident. Diagnosis is best done by faecal flotation techniques, although because of intermittent shedding of cysts, it’s recommended to pool together faeces from three days’ outputs.7 Ideally three individual examinations should be carried out. A recent abstract presented at the BSAVA Congress in 2011, showed good concordance of the SNAP Giardia antigen test with microscopic examination after flotation.8

Extra-intestinal protozoa in the dog can include Toxoplasma gondii infection, which has the potential to cause neuromuscular or generalised clinical signs such as pyrexia, vomiting and diarrhoea and pneumonia.

Extra-intestinal species, endemic in other parts of the world, are worth noting because of the increased risks posed by many pets travelling in and out of the UK under the Pet Travel Scheme. Canine leishmaniasis has been described i
n the UK in imported animals, as well as some cases originating here. Transmission is through bites from sandflies, the vector for the parasite (Figure 2).

Figure 2: Sandfly – vector for the protozoa Leishmania spp. Image courtesy of Merial Animal Health.

Tick-borne diseases represent a serious risk in the UK now with diseases including babesiosis caused by Babesia canis and B. gibsoni, and hepatozoonosis caused by Hepatozoon canis (Figure 3). Worryingly, the European Meadow Tick, Dermacentor reticulates, which is an important vector of canine babesiosis in Europe has been identified as having become established in Southern England.9

Figure 3: Babesia   canis   in red blood cells Image courtesy of Merial Animal Health.


Infection by protozoal organisms in the UK is not uncommon, and clinical signs can range from none to severe diarrhoea, illness and death. Some protozoan diseases are zoonotic. Prevention is straightforward in most cases, with good hygiene and parasite control measures being key. However, continuing vigilance for more serious non-endemic diseases spread by vectors is important as they have the potential to change the overall picture of protozoal disease in this country. 


Libby Sheridan MVB MRCVS

Libby graduated as a vet from University College Dublin in Ireland and spent 10 years working in mixed and small animal practice, followed by seven years as the UK and Ireland Veterinary Affairs Manager for Hill's Pet Nutrition. She founded Mojo in 2008 (www.mojoconsultancy.com). an agency providing marketing, project and PR consultancy to the veterinary and petcare market.

To cite this article use either

DOI: 10.1111/j.2045-0648.2011.00096.x or Veterinary Nursing Journal Vol 26 pp 355-357.


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Veterinary Nursing Journal • VOL 26 • October 2011 •