Parasitic infections are very common among household pets, predominantly dogs and cats. Up to 50 percent of dogs are infected with at least one intestinal parasite, and 15 percent of adult dogs actively excrete Toxocara canis, the source of toxocariasis, visceral larva migrans.52 Despite its prevalence in dogs, human toxocariasis is infrequently diagnosed probably because infection is often subclinical. Typically, the only indication of infection is eosinophilia. Children may display fever, cough, and nonspecific rash, as well as an inability to gain weight. Rarely, pulmonary infiltrates, hepatosplenomegaly, and seizures may occur. Diagnosis is by either biopsy of infected tissue or by ELISA. Treatment in the symptomatic patient consists of oral diethylcarbamazine (2 mg/kg tid for 10 days) or albedazole (400 mg bid for 5 days). Corticosteroids can be used to control the allergic component.
Other intestinal parasites that may be transmitted to humans from household pets include Ancylostoma caninum (cutaneous larva migrans), Echinococcus granulosus (echinococcosis), and Dipylidium caninum (dipylidiasis or dog and cat tapeworm).52 Cutaneous larva migrans is often a self-limiting, pruritic, erythematous serpiginous rash caused as the larva migrates through the skin and is often acquired from fecal-contaminated soil. Topical thiabendazole is effective therapy in shortening the disease course.3 52 Though dogs and other carnivores are the definitive hosts for E. granulosus, echinococcosis is most common in areas of cattle and sheep ranching. This zoonosis involves multiple organ systems: liver, lung, muscle, bone, kidney, and brain. Typically, there is a unilocular cyst containing multiple larvae that enlarge over time. Diagnosis and treatment usually occur at the time of surgical resection. Aspiration of the cyst is contraindicated, because leakage of the cystic fluid can spread the infection and cause an anaphylactic reaction. 52 Dipylidiasis, a tapeworm common to both dogs and cats, is found worldwide. Human infection is rare; when infection does occur, however, it is often in children and presents with the nonspecific symptoms of diarrhea and pruritus ani. Occasionally, the cucumber-shaped proglottides are seen moving in the child's stool. Treatment is with oral niclosamide. Infection with Dirofilaria immitis (dog heartworm) is extremely rare in humans.52
Cats are the host of the intracellular protozoan Toxoplasma gondii, which causes toxoplasmosis. Human toxoplasmosis can occur in three ways: by ingestion of uncooked or raw meat, especially pork or mutton containing the Toxoplasma cysts; by ingestion of the oocysts from cat and wild-animal feces; and transplacentally.303 52 Transplacental transmission can result in congenital abnormalities of retinochoroiditis, hydrocephalus, hepatosplenomegaly, and thrombocytopenia in 10 percent of the children infected. The majority of children transplacentally infected with toxoplasmosis display no significant abnormalities. Nevertheless, pregnant women should limit their contact to only indoor cats and avoid contact with cat feces. The encysted trophozoite can become reactivated in a previously infected host if the host becomes immunocompromised.54
Bacterial zoonoses from household pets include, but are not limited to, brucellosis, leptospirosis, salmonellosis, and campylobacteriosis. Brucellosis ( Brucella canis) is an uncommon infection in humans and is most often acquired from dogs. Pigs, cattle, and goats are less frequent transmitters. The typical human course for brucellosis is self-limited, with fever, headache, myalgias, and nonspecific laboratory findings. Tetracycline is the standard therapy in treating human brucellosis, with streptomycin added to tetracycline in the treatment of severe cases.30,34 Leptospirosis (Leptospira canicola) infects almost all mammals, but dogs are the principal vector for humans. Humans become infected through exposure to body fluids, particularly urine, of an infected animal. The acute phase of leptospirosis is characterized by headache, malaise, myalgias, and fever. Nonspecific rash, meningitis, uveitis, myositis, and leptospiruria follow the acute phase. Doxycycline (100 mg PO bid for 10 to 14 days) and high dose penicillin (penicillin G 20 to 24 million units IV qd for 10 to 14 days) are the mainstays of treatment for leptospirosis. 3 52
Campylobacteriosis (Campylobacter jejuni) is a major cause of infectious diarrhea in humans. This zoonotic infection also occurs in dogs, cats, pigs, poultry, cattle, and horses. Human infection from a pet is uncommon, but the presence of a puppy or kitten with a diarrheal illness in the house and human contacts experiencing diarrhea should raise the suspicion of pet-acquired campylobacteriosis. Though the disease is often self-limiting, treatment with erythromycin can facilitate resolution in protracted cases. Nontyphoidal salmonellosis is typically a self-limiting gastrointestinal illness prevalent among dogs. Human transmission is rare. Diarrhea in any pet should alert the pet owner to use increased personal hygiene and to dispose of the pet's feces properly. 3 34
The ticks and fleas inhabiting dogs and cats can transmit the zoonotic infections of tularemia, plague, and Rocky Mountain spotted fever. Tularemia ( Francisella tularensis) is a zoonotic infection occurring in humans exposed to infected animals or the vectors of ticks or deer flies. Endemic in the wild-animal population of the southeastern and north-central United States, cats are the primary reservoir of tularemia in human infections. Transmission is often by cat bite, tick bite, or scratch. Dogs do not directly transmit tularemia but do serve as carriers of the ticks that can transmit this zoonosis. Tularemia presents in four different forms, depending on the site of infection. The ulceroglandular form is the most common, followed by oculoglandular tularemia, oropharyngeal tularemia, and pneumonic tularemia. The infrequent but most serious pneumonic form is most common among laboratory workers and animal handlers. This form has the highest mortality. Treatment is with either streptomycin or tetracycline.3034,52
Plague (Yersinia pestis) is endemic in the rodent population of the southwestern United States. Dogs and cats can also be sources of this zoonosis. Transmission occurs when bitten by fleas inhabiting an infected rodent, dog, or cat or by eating infected rodents. Plague has three forms: bubonic or suppurative lymphadenopathy (the most common), the pneumonic form, and the septicemic form. Because of the aggressive nature of Y. pestis, treatment should not be delayed. Aggressive treatment with gentamicin or streptomycin should be initiated at the first indication of infection. Doxycycline or chloramphenicol are effective alternative antibiotics.3 ,43,5.2.
Rocky Mountain spotted fever (Rickettsia rickettsii) is a tick-transmitted, systemic zoonosis, with the tick Dermacentor sp. as principal vector and reservoir. Animal reservoirs consist of rodents, rabbits and, infrequently, dogs. Children incur two-thirds of the documented infections of RMSF. With a characteristic rash, this systemic zoonosis often presents with fever and headache. Treatment is with tetracycline or chloramphenicol in young children and pregnant women. 2 3 54
Zoonotic fungal infections are occasionally acquired by humans. The most common of these infections is a dermatophytosis from Microsporum canis. It is estimated that up to 30 percent of human dermatophytoses have a zoonotic origin.52 Treatment is often with topical antifungals or griseofulvin.
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