Parasitology in the Animal Shelter

With large populations of closely housed cats and dogs, especially kittens and puppies, parasitic pathogens are pervasive in animal shelters. Young animals are more vulnerable than adults to primary infection and secondary sequelae. Adult animals in poor condition, often served by shelters, may be significantly affected by invertebrate vectors and the diseases they carry.^1,2 Regardless of the age of the animal, parasite control—whether prophylactic or therapeutic—is an essential component of the shelter intake process.³

The Parasitology series is brought to you by Merck Animal Health, the makers of Bravecto® (fluralaner) and Sentinel® (milbemycin oxime/lufenuron).

Preventive treatments for enteric parasites control infection and decrease transmission by reducing environmental load. Many nematode eggs and protozoan cysts carry the potential for zoonotic infection and are virtually impossible to kill with disinfectants.⁴ Keys to maintaining a clean environment include treatment, prompt removal of feces, and mechanical scrubbing. Inexpensive and safe preventives should be given prophylactically at shelter intake for many of the parasites in this article.^1,3

Even with good prophylactic deworming and ectoparasite intake protocols, juvenile and adult shelter animals may still be affected by parasitic diseases. For these cases, shelters should seek veterinary advice, perform diagnostic tests to rule in or out specific pathogens, and provide specific and supportive care.³ Some parasitic pathogens are seen more frequently in shelter animals and less often in privately owned pets; below is a brief review of common parasites of particular concern in shelters.

Endoparasites

Roundworms

Clinical signs and/or transmission: Adult animals with ascarid infestations are frequently asymptomatic, although juveniles with heavy worm burdens may exhibit vomiting, diarrhea, hyporexia, and poor growth. Puppies infected in utero may exhibit respiratory symptoms associated with larval migration. Routes of transmission depend on parasite species and include transplacental (Toxocara canis), transmammary (Toxocara cati), and fecal ingestion of infective-stage larvae (T canis, T cati, Toxascaris leonina, and Baylisascaris procyonis).^1,4

Prevention and/or treatment: Products effective against roundworms include pyrantel pamoate, fenbendazole, and macrocyclic lactone products (ivermectin, selamectin, moxidectin, milbemycin oxime). One dose of pyrantel pamoate at shelter intake is recommended for adults; juveniles 2 to 3 weeks of age should receive treatment at or before intake and every 2 weeks until at least 12 weeks of age.^1,4

Hookworms

Clinical signs and/or transmission: As with roundworms, infected adults are frequently asymptomatic. Juveniles may exhibit anemia (can be severe enough to cause death), diarrhea, weight loss, coughing, dyspnea, and dermatitis associated with larval migration. Transmission route depends on species and includes transcutaneous (Ancylostoma caninum, Ancylostoma tubaeforme, Uncinaria stenocephala, and Ancylostoma braziliense), transmammary (A caninum), and ingestion of paratenic rodent or insect hosts (A caninum, A tubaeforme, U stenocephala, and A braziliense).^1,4

Prevention and/or treatment: Products effective against roundworms are also effective against hookworms.

Whipworms (Dogs)

Clinical signs and/or transmission: Puppies infected with heavy Trichuris vulpis worm burdens may exhibit severe clinical signs; infection in adults is often inapparent. Signs include bloody, mucoid diarrhea; inflammation of the large bowel; anemia; and weight loss. Infections are contracted through direct environmental contact or ingestion of embryonated eggs, which larvate 1 to 3 weeks after eggs are deposited in feces.^1,4

Prevention and/or treatment: The most effective preventive measure is removal of fecal material from the environment before larvation. Treatment with fenbendazole, febantel, or macrocyclic lactone products is recommended when eggs are observed in fecal flotation. Repeated or ongoing treatment may be needed monthly to treat unencysting larvae.^1,4

Coccidia

Clinical signs and/or transmission: In juvenile hosts, these small protozoa may cause diarrhea, hematochezia, anorexia, or weight loss, depending on parasite burden and host immunity. Transmission is through direct ingestion of cysts in feces or vertebrate prey.^1,4

Prevention and/or treatment: Shelters routinely provide a single prophylactic dose of ponazuril (or toltrazuril) at intake for animals younger than 5 months; animals showing signs of coccidiosis should receive treatment with ponazuril daily for 3 to 5 days or sulfadimethoxine for 14 to 20 days (TABLE 1).^1,4

Ectoparasites

Ectoparasites are commonly found on juvenile and adult animals, although they may cause more significant secondary issues (e.g., vector-borne illnesses, anemia) in the young. Some ectoparasites survive in indoor environments, making effective sanitation indispensable.

Fleas

Clinical signs and/or transmission: Ctenocephalides felis fleas may cause no clinical signs, may cause primary signs (e.g., pruritus, dermatitis, anemia), secondary signs (e.g., allergic reactions), or may cause disease from pathogens for which they are a vector.^2,4

Prevention and/or treatment: Many effective preventives are available. At intake, shelters often administer rapid-acting oral adulticide treatment to animals with severe flea infestations as well as a long-acting topical or oral flea preventive.^2,4

Ear Mites (Cats)

Clinical signs and/or transmission: Otodectes cynotis mites are a frequent cause of pruritic ears in cats of all ages, especially kittens. Infected ears have dark, gritty “coffee grounds” debris; directly examining debris on a microscope slide for adult mites, nymphs, and/or eggs is diagnostic.^2,4

Prevention and/or treatment: Otodectes species mite infestation is readily treated with many different medications and treatment schedules, including macrocyclic lactones biweekly and isoxazolines once or monthly, depending on the product.^2,4

Ticks

Clinical signs and/or transmission: These arachnids are frequently found embedded in animals’ skin at shelter intake. Although blood meals by numerous ticks can lead to anemia, infestation is more concerning with regard to the vector-borne pathogens transmitted by ticks. Ixodes scapularis and Ixodes pacificus (deer or black-legged) ticks can transmit the agents of Lyme disease and anaplasma; Rhipicephalus sanguineus (brown dog or kennel) ticks can transmit Babesia, Hepatozoon, Rickettsia, and Ehrlichia; Amblyomma americanum (lone star) ticks can transmit Ehrlichia, Rickettsia, Cytauxzoon, and Francisella; Dermacentor variabilis (American dog) ticks and Dermacentor andersoni (Rocky Mountain wood) ticks can transmit Rickettsia and Francisella.^2,4

Prevention and/or treatment: Many available products are effective for killing ticks. Shelters may administer these products to all animals at intake, or they may elect to reserve administration for animals with tick infestations at intake or those at risk for infestations (i.e., walked outside). When examining recently adopted dogs, obtaining their transport/travel history is critical as prevalence of many of the agents mentioned above varies significantly from state to state.^2,4

Demodex (Dogs)

Clinical signs and/or transmission: Diagnosed by microscopic examination of skin scraping or trichogram, demodectic mange can appear in localized, generalized, or adult-onset patterns. Localized demodicosis is characterized by small discrete areas of hair loss in puppies; frequently self-limiting, it resolves as the immune system matures. Generalized demodicosis is characterized by hair loss over much of the body, often accompanied by bacterial and yeast dermatitis.^2-4

Prevention and/or treatment: Most cases of generalized demodicosis require treatment. Until recently, high-dose oral ivermectin was used; however, given the superior safety margin and dosing schedule of isoxazolines, these products are now the treatment of choice. In adult dogs, overgrowth of Demodex species should prompt further investigation into endocrine and immune system disorders.^2,4

Lice

Clinical signs and/or transmission: These insects are diagnosed by recognizing adults or their egg sacs (nits) cemented to hair shafts during physical examination; their presence is confirmed with microscopy.⁴

Prevention and/or treatment: Although extremely contagious within species, lice are not transmitted between species and are easily treated with most ectoparasiticides.^2,4

Cheyletiella

Clinical signs and/or transmission: These fur mites are diagnosed by visualizing adults or their eggs on acetate tape preparations.^2,4

Prevention and/or treatment: Flea medications provide effective off-label treatment. Because mites survive for weeks in the environment, animals in close contact with infected dogs or cats should also be treated.^2,4

Tritrichomonas foetus (Cats)

Clinical signs and/or transmission: These protozoa, diagnosed by culture or polymerase chain reaction, are acquired through fecal–oral transmission and cause chronic diarrhea.^2-4

Prevention and/or treatment: Treatment is challenging because trophozoites hide in intestinal crypts and reemerge when antibiotic pressure is lifted. Ronidazole at 30 mg/kg q24h PO for 14 days has been suggested, but treatment failures and neurological symptoms are frequent concerns.^2-4

Tapeworms

Clinical signs and/or transmission: Dipylidium caninum tapeworm infection is transmitted by ingestion of fleas. Infection can be diagnosed by visualizing proglottid segments in the feces or on fecal flotation.

Prevention and/or treatment: Infection is prevented by routinely applying flea preventive to dogs and cats and can be treated with praziquantel or epsiprantel.^2,4

Heartworm (Dogs, Rarely Cats)

Clinical signs and/or transmission: Dirofilaria immitis heartworms are extremely prevalent in some parts of the United States (South and Midwest) and extremely uncommon in others (Northwest, Northeast, and Mountain West).⁴ The mosquito vectors thrive in warm, humid climates, often the same geographic settings where shelter dog populations exceed local adoption capacity. Interstate dog transport is a critical life-saving tool for these dogs and provides adoptable animals in areas where demand is high. However, transport without attention to heartworm prevention has increased prevalence in some areas where it was once close to zero.⁵ Fortunately, attention to a few basic principles can prevent transmission and give infected dogs a chance for a healthy life.

Inexpensive point-of-care tests detect antigen of adult female heartworms. Infection can be confirmed by observing circulating microfilaria on a blood smear or in the buffy coat of a hematocrit tube.^4,5 Because diagnosis may be missed for some dogs (those receiving monthly heartworm prevention may have no microfilariae, and those in early stages of infection will test negative for the disease), dogs—especially those from areas of high heartworm prevalence—should be retested 6 months after an initial negative point-of-care test or when 6 months of age.^4,5

Prevention and/or treatment: Regardless of their test results, shelter dogs and cats should receive an effective heartworm preventive at appropriate dosing intervals starting at intake.⁵ These medications prevent heartworm disease from developing in uninfected animals and significantly decrease circulating microfilariae in heartworm-positive dogs, making them much less contagious.⁵ Information regarding specific treatment of heartworm disease is beyond the scope of this article, but veterinarians should be familiar with the many methods, each with various benefits and drawbacks.

Conclusion

More than 40% of all owned animals in the United States have been through an animal shelter.⁶ For some animals, such as feral cats, preventive medications received at shelter intake may represent the only veterinary care they will ever receive; for many others, it is just the start. By providing preventive medications and treating parasitic infestations when diagnosed, shelters play a critical role in keeping animals, humans, and our communities safe and healthy.