CHAPTER 19 Parasitic and Protozoal Diseases
Young animals are commonly at risk of a variety of parasitic infections, and morbidity and mortality rates are typically higher than in adults. Resistance to parasitic infection is weak in pediatric patients, as the immune system is not fully developed and the relative parasite burden is high. Successful management depends on a working knowledge of parasites and their life cycles, diagnostic techniques, appropriate therapeutics, and preventative strategies. The information in this chapter is intended as a brief overview of parasitic diseases encountered in puppies and kittens. It is not intended as a comprehensive list of all possible parasites. The list of Suggested Readings at the end of the chapter is a guide to more complete discussions. General guidelines concerning parasite control in dogs and cats are summarized in Box 19-1.
Roundworms are relatively large nematodes (3 to 18 cm long) found in the small intestine. Toxocara canis affects virtually all puppies because of prenatal infection. Many kittens are infected with Toxocara cati soon after birth by transmammary transmission. Toxascaris leonina affects both dogs and cats but is less common.
Pregnant bitches have arrested (dormant) T. canis larvae in somatic tissues that become activated after day 42 of gestation. These larvae migrate to the uterus and infect developing fetuses. Postnatal infection may occur as a small amount of larvae are excreted in the milk. Older puppies acquire T. canis after ingestion of embryonated eggs, and larvae may undergo hepatotracheal migration before being coughed up and swallowed, leading to maturation in the small intestine. Pregnant queens also have arrested T. cati that migrate during gestation to mammary glands. Neonates commonly acquire larvae by this route, whereas older kittens are infected by ingestion of larvae or paratenic hosts such as rodents. T. leonina larvae are acquired by ingestion of embryonated ova or paratenic hosts.
Mild infections may be asymptomatic, whereas moderate numbers of worms cause vomiting, diarrhea, lethargy, and an unthrifty appearance. A rounded, fluid-filled abdomen (“pot belly”) is common with heavier worm burdens. Severe complications including intestinal obstruction, intussusception, and death are possible. Larvae migrating through the liver, lungs, and other organs occasionally result in tissue damage.
A number of effective anthelmintics are available (Table 19-1). Deworming is ideally started at 2 weeks of age and repeated every 2 weeks for four treatments. Following this, certain heartworm preventatives will control roundworms if continued year-round (see Table 19-1). Prenatal infection with T. canis can be prevented in puppies by daily administration of fenbendazole to pregnant bitches starting at day 40 of gestation and continuing through day 14 postpartum. Selamectin also prevents prenatal transmission in puppies if given at days −40, −10, +10, and +40 before and after whelping. Similar studies have not been reported in cats. Roundworm ova persist in the environment for months and possibly years. Removal and proper disposal of feces at least twice a week will decrease the risk of soil contamination.
T. canis and, to a lesser extent, T. cati cause visceral larval migrans and ocular larval migrans in humans. Accidental ingestion of soil contaminated with embryonated ova is considered to be the main source of infection, but roundworm ova have also been found in hair samples clipped from the perianal areas of dogs. Households with children or immunosuppressed individuals should be informed of the risk of toxocariasis.
Hookworms: Ancylostoma caninum, Ancylostoma tubaeforme, Ancylostoma braziliense, Uncinaria stenocephala
Hookworms are small bloodsucking nematodes (5 to 15 mm long) that attach to the mucosa of the small intestine. Perinatal infections are common in puppies, whereas kittens generally acquire infections after several weeks of age. Ancylostoma caninum is found in dogs, Ancylostoma tubaeforme in cats, and both dogs and cats can be infected with Ancylostoma braziliense and Uncinaria stenocephala.
Both dogs and cats acquire hookworms through larval ingestion and skin penetration. Rodents may contain larvae in tissues and serve as paratenic hosts for cats. As with roundworms, an arrested larval stage in somatic tissues is activated during pregnancy in bitches. A. caninum larvae migrate to mammary glands and infect puppies through nursing.
Asymptomatic cases are possible, but diarrhea and weight loss are common with mild to moderate infections. Puppies and kittens with heavy worm burdens become anemic and may die peracutely. Hemorrhagic diarrhea caused by hookworms may resemble other causes of enteritis such as parvovirus. Dermatitis is occasionally reported as a result of larval skin penetration.
Many anthelmintics treat both hookworms and roundworms (see Table 19-1). An appropriate schedule for puppies and kittens is every 2 weeks starting at 2 weeks of age for four treatments. Monthly deworming may be continued using heartworm preventatives. Perinatal infection in puppies can be avoided using daily fenbendazole in pregnant bitches from day 40 of gestation through day 14 postpartum.
Humans can acquire larvae through skin penetration, leading to cutaneous larval migrans. A. braziliense is the most common hookworm recovered from humans, although A. caninum is occasionally reported.
Whipworms are nematodes found in older puppies and adult dogs rather than in neonates. The usual length is 4 to 7 cm, and they are found in the large intestine, where they attach and absorb blood. In the United States, cats are unaffected by Trichuris vulpis. Feline whipworms (Trichuris felis) have been reported in Australia and Central and South America.
Dogs are infected by ingestion of ova from contaminated environments. Perinatal transmission does not occur. Development of whipworms takes place in the intestines without tissue migration or arrested larval stages. The prepatent period is approximately 3 months, so puppies less than 8 to 12 weeks of age are generally not at risk.
Several anthelmintics are effective against whipworms (see Table 19-1). Fenbendazole and febantel are commonly used, although monthly heartworm preventatives containing milbemycin and moxidectin can be used for treatment and control of recurrent infections.
The common tapeworms found in puppies and kittens are large cestodes (15 to 60 cm long) that are anchored to the mucosa of the small intestine. The terminal segments (proglottids) containing eggs or egg packets are able to crawl out of the anus onto the haircoat or are excreted along with feces.
Tapeworms are acquired through ingestion of intermediate (paratenic) hosts rather than by direct fecal-oral exposure. The larval stage of Dipylidium caninum (cysticercoid) is found in fleas, and dogs and cats become infected after swallowing infected fleas. A mature worm develops approximately 2 to 4 weeks later. Taenia spp. larvae occur in rodents and rabbits. Cats are more likely to acquire Taenia as a result of hunting behavior, and adult worm development takes 1 to 3 months.
Other than anal pruritus associated with shedding of proglottids, tapeworms are considered to be harmless parasites. There is one report of acute small intestinal obstruction in an adult cat from a mass of T. taeniaeformis.
Direct observation of tapeworm segments is possible. A close examination of the perianal region may reveal proglottids (resembling grains of rice) on the skin or haircoat. Segments are also seen in feces or in the environment. To identify the type of tapeworm, a proglottid is placed in a drop of saline or tap water on a glass slide and squashed or teased apart to release eggs. D. caninum occurs in egg packets, whereas Taenia ova are spherical and contain six-hooked larvae. Fecal flotation rarely reveals tapeworm ova.
Praziquantel and epsiprantel are effective one-time treatments for tapeworms. Fenbendazole treats Taenia but not Dipylidium (see Table 19-1). To prevent recurrence, flea control and avoidance of hunting are necessary.