Problems associated with intestinal parasites in cats

May 01, 2002

Most parasitologists would agree that feline internal parasites, particularly helminths, have received less attention as disease agents or as potential causes of zoonotic human diseases than their canine counterparts. This is due in part to the perception that many feline internal parasites, particularly Toxocara cati and Ancylostoma spp. are uncommon. However, the few fecal and/or necropsy surveys that have been conducted in cats in the United States do not support this presumption. In fact, results of these surveys indicate that feline roundworms and hookworms represent the more common internal helminth parasites of cats, regardless of the geographic region in which the studies were conducted. It is also interesting that although effective anthelmintics have been available for many years, worldwide prevalences of feline internal parasites do not appear to have changed significantly. In this article, I will discuss several potentially pathogenic internal protozoal and helminth parasites of cats. Some of these parasites also are capable of causing disease in humans. This latter point will be emphasized given the recent initiatives by governmental agencies and professional associations to prevent transmission of certain helminth parasites from pets to people. I will also review available anthelmintics, antiprotozoals and recommended strategies for controlling common feline internal parasitic diseases (see Suggested Reading, end of article).

Giardiasis Giardia infections in cats are caused by Giardia intestinalis (also called G. lamblia). The parasite usually resides in the small intestine, although exceptional infections in the lower bowel cannot be ruled out. Giardia is a dimorphic parasite in that it exists as a fragile flagellated binucleate trophozoite and a quadrinucleate cyst. The trophozoite attaches to the surface of epithelial cells in the small intestine; encystment (formation of cysts) occurs in the ileum, cecum or colon. Although the mechanism(s) of Giardia-induced disease remain unknown, evidence suggests that the disease is likely multifactorial involving inhibition of brush border enzymes or other factors such as altered immune responses, nutritional status of the hosts, presence of intercurrent disease agents, and the strain of Giardia involved in the infection. Although many infected animals remain asymptomatic, the most common presenting sign is small bowel diarrhea. Feces are usually semi-formed, but may be liquid. Blood usually is not present. Feces have been described as pale (often gray or light brown), fetid and containing large amounts of fat. Cats with giardiasis may present with poor body condition, and weight loss. Vomiting or fever are not common presenting signs.

Photo 1: Iodine-stained cysts of Giardia from fecal flotation.
As mentioned previously, it is not unusual to find Giardia present with other gastrointestinal diseases such as inflammatory bowel disease. Giardiasis is best diagnosed by fecal flotation using zinc sulfate (specific gravity = 1.18) as the flotation solution. Centrifugation of the preparation increases the likelihood of recovering cysts. Also, the addition of a small amount of Lugol's iodine to the slide prior to placement of the coverslip containing the concentrated cysts will aid in visualizing the small (10-12 um) cysts (Photo 1).

Table 1: Treatment of feline giardiasis
Use of barium sulfate, anti-diarrheals or enemas prior to sampling feces may interfere with detection of cysts and should be avoided if possible. Other diagnostic techniques that can be used to recover trophozoites, cysts, or proteins produced by the parasite include direct examination of feces (wet-mount), immunofluorescent procedures, and ELISA techniques. These techniques are either too insensitive (direct examination) or impractical for the practicing veterinarian because of cost, required equipment or because of the effort required to conduct the test. Some controversy exists surrounding the potential of some animal strains of Giardia to infect humans. Although it is known that some host specificity among animal strains of Giardia does exist, I believe that it is best to be conservative about the potential for human infections with animal strains of Giardia. Consequently, it is my view that all animals that are positive by fecal examination or suspected of having giardiasis should be treated. Several options are available for treatment of Giardia infections in cats (see Table 1). Cats are best treated with metronidazole as indicated. Use of metronidazole in cats is generally safe if the total daily dose remains below 50 mg/kg. Additional attributes of metronidazole are its antibacterial effects, its activity against other protozoans, and its possible immune modulating effects. Studies documenting the efficacy of the benzimidazole anthelmintics such as fenbedazole against Giardia in cats have not been conducted. However, fenbendazole administered at 50 mg/kg daily for three to five days, as recommended for giardiasis in dogs, is also likely to be safe and effective in cats. Veterinarians now have an available vaccine to assist in the control of feline giardiasis. Based on available data, vaccinated cats are less likely to get infected with Giardia than nonvaccinated cats. Also, vaccinated cats that do get infected generally experience less severe diarrhea and shed fewer organisms for a shorter period of time. Veterinarians should assess each situation to determine whether a particular animal or group of animals are potential vaccine candidates.

Coccidial Coccidial infections in cats are caused by Isospora spp., also called Cystoisospora (see Table 2). The principal agents in the cat are I. felis and I. rivolta. These parasites reside in the posterior small intestine or in the large intestine depending on the species. Their life cycles are generally self-limiting, after which the infection is terminated. The parasites replicate first asexually by schizogony resulting in destruction of many host enterocytes in which they develop. Asexual development is followed by production of gametes that fuse to produce non-infective oocysts that are passed in feces. The developmental cycles in the feline host require four to 11 days depending upon the species.

Table 2: Developmental information on common feline coccidia
Development to the infective stage (sporulation) usually requires one to several days in the animal's environment. Only sporulated oocysts are infective to susceptible hosts. Clinical signs of coccidiosis include hemorrhagic or mucoid diarrhea, abdominal pain, dehydration, anemia, weight loss, emesis, as well as respiratory and neurologic signs.

Photo 2: Isospora felis (right) and I. rivolta (above) oocysts from fecal flotation.
Death can result from extreme cases, particularly in young kittens. Nursing animals, recently weaned animals, or those that are immunocompromised are more likely to develop clinically apparent infections. Diagnosis of coccidiosis is based on signalment (usually kittens), clinical signs and recovery of oocysts in feces (Table 2 and Photo 2). Fecal flotation remains the most practical means of recovering oocysts. A point to remember is that recovery of oocysts alone in feces is not sufficient proof to implicate coccidia as the cause of clinical signs. I have observed coccidial oocysts in the feces of many animals without evidence of intestinal disease.

Table 3: Treatment of feline coccidiosis
Although sulfadimethoxine is the anticoccidial medication most commonly used in cats, several other agents have been used with success (see Table 3). Little can be done to disinfect environments because of the ability of the oocysts to withstand chemicals and adverse environmental conditions. Good sanitation, including prompt removal of feces to prevent development of oocysts to the infective stage and treatment of queens with anticoccidial agents prior to parturition, have been shown to reduce the occurrence of coccidiosis in young animals.