Zoonotic Infections: Your Pets Can Make You Sick

US Pharm. 2011;36(8):41-50. 

Zoonoses are diseases that are transmitted from animals to humans. It is estimated that 75% of emerging infectious diseases are zoonoses.¹ Zoonoses are a huge problem in agriculture with food-producing animals. Diseases such as mad cow disease, H1N1 flu, and avian flu are well-known zoonoses that have proved to be fatal in humans. This article, however, is going to focus on diseases transmitted by household pets. There are over 30 significant diseases that can be contracted from pets.² According to the results from a 2007 survey by the American Pet Products Manufacturers Association, 63% of American households (71.1 million homes) had one or more pets.³ 

Up to 50% of dogs are infected with one or more intestinal parasites.² Cats, birds, rodents, fish, reptiles, and exotic pets also put their owners at risk. Despite the significant number of afflicted animals, humans are rarely infected. The very young, the very old, and the immunocompromised are at greatest risk for developing symptomatic infection. 

Since zoonotic infections are relatively rare, they are often not recognized.² Probably the most common diseases contracted from pets are parasitic infections, with fungal infections being the next most prevalent.⁴ The zoonoses most commonly encountered by practitioners are salmonellosis, toxoplasmosis, roundworm, and hookworm,⁵ although a number of other infections are likely to be seen as well (TABLE 1).

Salmonellosis

Along with campylobacteriosis (Campylobacter), salmonellosis (Salmonella) is probably the most widespread bacterial zoonotic infection,⁴ and it is one of the most common causes of bacterial gastroenteritis.⁶ Approximately 6% of all salmonella cases are caused by direct or indirect contact with reptiles.^3,7 Less frequently cats, dogs, rodents (e.g., hamsters, mice), and birds (e.g., chicks, ducks) can be reservoirs.^1,3-5 There have even been cases of human infection from handling contaminated pet food and treats.³ 

Salmonella organisms are a normal part of reptile intestinal flora, and approximately 90% of reptiles carry the bacteria.^7,8 It should be assumed that all reptiles carry salmonella, and preventive measures should be taken when handling these animals.¹ 

In 1975, the federal government banned the sale of turtles with shell lengths <4 inches.^6,7 The only exceptions to the ban were turtles to be used for scientific, exhibition, or educational purposes.⁷ The reason for the 4-inch rule was that most turtles being sold as pets were hatchlings with a shell size of roughly 1 inch, and at 4 inches it was more difficult for infants and young children to treat turtles as toys and put them in their mouths.^1,7 This ban greatly decreased the incidence of turtle-related salmonellosis.⁷ The ban on small turtles is difficult to enforce, however, partly because of the exceptions to the law. In addition, small turtles are commonly sold at pet stores, flea markets, and swap meets, and from roadside and street vendors. There are also Web sites that advertise small turtles for sale.⁷ 

Salmonella bacteria are excreted in the feces of infected animals,⁷ and transmission is usually via the fecal-oral route.⁸ However, infection can also occur from scratches and bites.⁸ 

Salmonella has been shown to remain infectious in tap water after 89 days and in reptile feces for 30 months.⁸ Using sinks and bathtubs for washing the reptile and its housing increases the risk of human infection.⁵ Prevention strategies include good hand hygiene (i.e., hand washing after handling animals), wearing gloves when cleaning reptile housing, not permitting the reptile to walk freely through the house (particularly where food is prepared), and avoiding the use of household sinks or tubs when cleaning the reptile or its belongings.^1,5,8 Reptiles should not be kept in child care centers.⁸ Pet stores should warn consumers, especially those who are pregnant or have small children, about the dangers of salmonellosis and the risk of contracting it.^5,8 The CDC recommends that pregnant women, children under 5 years of age, and immunocompromised individuals avoid handling reptiles. The CDC also suggests that reptiles be removed from the homes of families expecting babies before the child is born.⁵ 

Human infection is usually self-limited.^1,2 When symptomatic, patients may experience diarrhea, fever, and abdominal cramps within 12 to 72 hours after exposure.^1,4 Infection may become severe, resulting in meningitis, sepsis, carditis, and death.^1,7 Children under 5 years of age, the immunocompromised, and the very elderly are at a higher risk for severe infection.⁷ 

There is no need to treat asymptomatic infection.⁴ When treatment is indicated, a fluoroquinolone (i.e., ciprofloxacin) or ceftriaxone is the drug of choice. Trimethoprim-sulfamethoxazole (TMP-SMX) and high doses of ampicillin (except with resistant strains) are also effective.⁹ 

Toxoplasmosis

Toxoplasmosis (Toxoplasma gondii ) is one of the most common zoonotic infections,⁴ and there are approximately 60 million people infected with it in the United States.¹ Cats are the primary reservoir; however, dogs may be an indirect vector of toxoplasmosis, because of their tendency to roll in feces and animal remains.⁴ 

Infection in cats usually occurs within the first 12 months of their life (if they go outdoors). Sexual reproduction of this parasite occurs in cats, and unsporulated oocytes are excreted for only 2 to 3 weeks.^4,5 The oocytes become infectious within 1 to 5 days, but they may remain infectious for up to 18 months in moist soil.^4,5 The most common ways to contract toxoplasmosis include ingestion of undercooked meat or vegetables from contaminated soil, contact with infected cat feces, and transplacental fetal transmission.^2,4,10 

Prevention consists of keeping cats indoors, wearing gloves while gardening, washing hands after coming in contact with soil, and cleaning the cat’s litter box daily. Pregnant women should avoid cleaning the litter box.⁵ 

In healthy people, infection is usually asymptomatic.⁴ Infection in pregnant women is particularly worrisome, because congenital infection is potentially serious, especially if exposure takes place in the first trimester.^1,4 There are an estimated 3,000 congenital infections in the U.S. annually.¹ Toxoplasmosis can cause chorioretinitis (inflammation of the choroid and retina) in healthy people, and chorioretinitis from congenital infection often develops decades after exposure.^5,10 Infection during pregnancy can cause spontaneous abortion as well as mental retardation, hydrocephalus, convulsions, deafness, blindness, or cerebral palsy in congenitally infected children.^1,2 Even when transplacental infection is asymptomatic in children, they may develop symptoms later in life if at some point they become immunocompromised.² Infection in immunocompromised patients results in symptoms 10% of the time.⁴ Illnesses can become severe in the immunocompromised, including encephalitis and ocular lesions.^1,4,10 

The most common treatment is pyrimethamine plus sulfadiazine plus leucovorin.^4,10 In 40% of cases, treatment must be discontinued due to sulfa toxicity. In these instances, clindamycin may be substituted for sulfadiazine.¹⁰ In pregnant women, spiramycin can be used; however, in the U.S. this is still an investigational drug.⁴ 

Roundworm and Hookworm

Roundworm infection (toxocariasis) is carried by cats and dogs and is caused by Toxocara cati and Toxocara canis, respectively.^4,11 It is a fairly common parasitic infection. In a study done by Won et al, 13.9% of the population 6 years of age or older had antibodies for Toxocara.¹² Eggs are passed through the feces of cats and dogs and remain infectious for many years in soil.¹² 

Infection can occur by the ingestion of contaminated soil.^4,11 Toxocara larvae can also cross the placenta and be excreted in the breast milk of female dogs, so puppies can be born infected.⁴ Prevention includes good hand hygiene and the deworming of pets.⁵ 

Because the larvae do not develop into adult worms in humans, infection is usually asymptomatic.^4,11,12 When symptoms do occur, the condition is usually self-limited.¹¹ The larvae can migrate through the body. When migration is to the liver, lungs, and central nervous system (CNS), it is called visceral larva migrans, and there are an estimated 10,000 cases annually in the U.S.^4,11 The larvae may also migrate to the eyes (ocular larva migrans), and there are an estimated 700 cases annually in the U.S.⁴ 

Symptoms of visceral larva migrans can include eosinophilia, cough, fever, seizures, hepatosplenomegaly, abdominal pain, malaise, anorexia, weight loss, and rashes.^2,4,11,12 Rarely, death can occur due to pneumonia, myocarditis, and severe CNS involvement.¹¹ Symptoms of ocular migrans include en-dophthalmitis, chorioretinitis, and uveitis.^4,11 The patient may experience visual disturbances, strabismus (squinting), ocular pain, and retinal scarring.^11,12 

Visceral larva migrans is treated with albendazole or mebendazole. Allergic reactions caused by disintegration of microfilariae can be controlled with corticosteroids.^2,4,11 Ocular migrans is treated with corticosteroids and albendazole.^4,11 

Hookworm infection is common and is caused by Ancylostoma braziliense. Dogs and cats are the natural hosts, and eggs are passed in the feces. In humans, larvae penetrate the skin, causing cutaneous larva migrans.⁴ Contact with contaminated soil is the cause of infection, and there may be many sites of infection if the person has lain on the ground.¹¹ Papules form at the infection site, and a serpiginous (creeping or trailing) rash occurs as a result of larvae migration.⁴ Larvae can migrate several centimeters daily.¹¹ The condition is highly pruritic, and itching is worse at night.⁴ 

Albendazole or mebendazole may be used to treat symptoms and shorten the duration of infection, with mebendazole being the least expensive option.^4,11 

Rabies

Rabies is caused by viruses of the genus Lyssavirus.¹³ Because of the vaccination of cats and dogs in the U.S., it is rare to contract rabies from a household pet. However, there are still cases in the U.S. from exposure to an infected wild animal,⁴ with most infections occurring via bats.¹⁴ 

In 2009, there were 6,690 animal and four human cases of rabies reported to the CDC. Three of the four human cases resulted in death. Roughly 92% of the rabid animals were wildlife.¹³ Of all the rabid animals reported to the CDC in 2009, 1.2% (81) were dogs and 4.5% (300) were cats. Of all the reported cases of rabies in domestic animals, 59.4% were in cats.¹³ 

The incubation period for rabies is 1 to 3 months. In rare cases, it can be as short as <2 weeks or as long as >1 year.¹⁴ Following the incubation period, the infection progresses through three phases: 1) a prodrome, 2) an acute neurologic phase, and 3) coma and death. The prodrome consists of headache, nausea, vomiting, and sometimes agitation and anxiety. In humans there are two forms of acute neurologic manifestations, encephalitis (80%) and paralysis (20%). Symptoms in the acute neurologic phase include fever, hallucinations, confusion, muscle spasms, hyperactivity, seizures, hypersalivation, hyperhidrosis, gooseflesh, and priapism. The patient then lapses into a coma, which is followed by death in a few days.¹⁴ 

There is no effective treatment after the onset of symptoms; therefore, postexposure prophylaxis (PEP) is administered when there is the possibility of rabies infection. When exposure is from a pet cat, dog, or ferret, the pet can be observed for 10 days. If it remains healthy, then prophylaxis is not necessary. If exposure is from something other than a cat, dog, or ferret, the animal should be euthanized and examined for rabies.¹⁴ 

PEP consists of proper wound care, rabies immunoglobulin, and four doses of rabies vaccine.¹³ When the wound is properly cleaned, the risk of rabies can be decreased by as much as 90%. The rabies vaccine and rabies immunoglobulin should not be administered at the same site or from the same syringe. Immunosuppressants, including glucocorticoids, should be discontinued during PEP.¹⁴ 

Tapeworm

Dipylidium caninum is a tapeworm that can be found in cats and dogs. Ingestion of fleas, which are intermediate hosts, is the route of transmission to humans.⁴ However, human infection is rare and occurs mostly in children.² Symptoms include anal itching, abdominal pain, urticaria, and eosinophilia, but this infection is usually asymptomatic.^4,15 Segments of the worms can sometimes be seen in the stool.¹⁵ D caninum is treated with oral praziquantel.^2,4,15 

Tinea Corporis/Tinea Capitis

Commonly known as ringworm, this fungal infection (caused by dermatophytes) can be transmitted by asymptomatic animals. Signs and symptoms include alopecia, erythematous plaques, itching, and rash.⁴ Tinea corporis affects the nearly hairless areas of the skin and can be treated with a topical antifungal, with the exception of nystatin, which is not effective against dermatophytes. Treatment should continue for 1 week after the resolution of symptoms.¹⁶ Tinea capitis is infection of the scalp and can result in hair loss. Topical treatment alone is not effective against tinea capitis, and oral griseofulvin is approved for this indication.¹⁶ 

Methicillin-Resistant Staphylococcus aureus (MRSA)

Approximately one-third of healthy humans are colonized with MRSA, with asymptomatic colonization being more common than infection. MRSA can colonize the skin, mucous membranes, urogenital tract, and alimentary tract.¹⁷ It is thought that pet infection occurs through humans, but once infected, pets (e.g., dogs, cats) can transmit MRSA back to their owners through casual contact.^4,17 This can result in a frustrating cycle of reinfection of both pets and their owners. Good hand hygiene and keeping pets from sleeping in the bed are two measures that may be employed to prevent transmission.¹⁷ 

Symptoms include skin and soft tissue infection, osteomyelitis (inflammation of the bone), endocarditis, pneumonia, and sepsis.^4,17 

The mechanism by which S aureus becomes resistant to methicillin does not involve the production of a beta-lactamase enzyme, but instead is mediated by an altered penicillin-binding protein. For this reason, beta-lactam drugs are absolutely contraindicated for the treatment of MRSA.¹⁷ Treatment should be based on the sensitivity of the strain of MRSA.⁴ Fluoroquinolones are not a good option, because MRSA can rapidly develop resistance to them.¹⁷ Doxycycline and TMP-SMX drugs have been successful in treating susceptible strains, although their use is off label. Clindamycin is also a good option if the strain is susceptible to erythromycins.¹⁷ Linezolid is effective against MRSA, but it should only be used when other medications are failing, as overuse accelerates bacterial resistance to this new agent.¹⁸ 

Lyme Disease

Household pets (i.e., dogs, cats) can transmit Lyme disease indirectly by carrying infectious ticks.⁴ Lyme disease is most commonly seen in the northeastern and upper central states, with approximately 90% of the annually reported cases being from this region.¹⁹ There is a time lag between the initiation of tick feeding and the transmission of the disease; therefore, preventive measures can be taken. The removal of any visible ticks can prevent infection. Likewise, wearing long sleeves and pants, tucking in shirts and pants (into boots or socks), wearing light-colored clothing (to facilitate spotting ticks), using a tick repellant, walking in the middle of a trail (avoiding areas of dense vegetation), and monitoring for the presence of ticks are all strategies that should be employed to prevent infection.¹⁹ 

Within 1 to 36 days after exposure, a rash develops, most often at the site of the tick bite. It expands and clears in the center. The rash will often resolve spontaneously without treatment.¹⁹ At the same time or shortly after the rash develops, flulike symptoms, such as fever, headache, malaise, and muscle and joint pain, occur. If left untreated, the infection can progress to arthritis of the large joints, encephalopathy, and carditis.^4,19 

If caught in the early stages, Lyme disease can be treated with a tetracycline (drug of choice) or amoxicillin.^4,9 With more severe disease, a third-generation cephalosporin or IV penicillin G should be used.⁹ Prophylactic treatment is not indicated.¹⁹ 

Scabies

Infrequently, scabies can be transmitted by dogs to humans; however, the dog mite usually fails to reproduce, resulting in a self-limited infection.⁴ Symptoms include itching and papular rash, and treatment is with topical permethrin or ivermectin.^1,4 

Giardiasis

Caused by Giardia intestinalis, giardiasis is the most common protozoal infection from all causes in the U.S.¹⁰ It can be carried by any mammal and is contracted through swimming, exposure at day care centers, certain sexual practices, diapers, and travel to places with lower hygienic standards.^1,20 

Infection is most often asymptomatic, but patients may experience diarrhea, abdominal pain, and fatigue.^1,10 Symptoms may be self-limited or chronic.¹⁰ Furazolidone is the only FDA-approved treatment for humans in the U.S.; however, metro-nidazole is often used.¹ Alternative treatment includes tinidazole or nitazoxanide.^4,10 

Cryptosporidiosis

A similar protozoal infection is cryptosporidiosis (Cryptosporidium). Like giardiasis, cryptosporidiosis can be carried by all mammals.¹ Recreational water use is the most common means of contracting this infection, which is usually self-limited.^1,20 Nausea, vomiting, abdominal cramps, low-grade fever, anorexia, and headache may last 2 to 12 weeks.^1,4,10 If treatment is opted for, nitazoxanide is the agent used.^4,10 

Cat Scratch Disease

Cat scratch disease (Bartonella henselae) is most commonly transmitted by cats, but dogs may also transmit this infection.²¹ It is usually self-limited, with symptoms being swelling at the site of the skin lesion and lymphadenopathy, fever, and weight loss.^4,21,22 In the immunocompromised, it can progress to erythema nodosum (tender red nodules), figurate erythema, thrombocytic purpuras, Parinaud’s oculoglandular syndrome, encephalopathy, hepatic granulomas, osteomyelitis, pulmonary disease, and optic neuritis.²¹ In some cases, symptoms can last from weeks to months.²² 

If treatment is indicated, such as with immunocompromised patients, the agent to be used is azithromycin.^4,22 

Psittacosis

Psittacosis (Chlamydia psittaci), also known as parrot fever, is carried by birds and is most commonly found in parrots.²³ Transmission occurs via inhalation of bird droppings or nasal secretions and rarely from the bite of an infected pet bird.^4,23 Prevention includes using disinfectants and wearing gloves, gowns, and masks while cleaning cages.¹ 

Infected birds display subtle signs of the disease, which include ruffled feathers, lethargy, and anorexia; however, the disease can be fatal in humans.²³ The incubation period is 7 to 14 days (in humans), and clinical presentation can vary. Onset may be gradual or may be abrupt, with shaking chills and high fever. Headache is often the chief complaint with the development of a dry, hacking cough. Sore throat and cervical adenopathy are also often seen. Chest pain, pericarditis, and myocarditis are rarely seen. Commonly, the patient will experience photophobia, and in 25% of cases epistaxis (nosebleed) may occur. Symptoms such as generalized myalgia and stiffness and spasm of the neck and back muscles may lead to a misdiagnosis of meningitis. In some cases lethargy, depression, agitation, insomnia, and confusion have been noted, and some severe cases result in stupor and delirium at the end of the first week. Gastrointestinal symptoms include nausea, vomiting, abdominal pain, and diarrhea or constipation.^1,4,23 

Tetracyclines, such as doxycycline, are effective in treating psittacosis and should be continued for 7 to 14 days beyond the absence of fever.^4,23 Erythromycin is an alternative agent that can be used.⁴ 

Myths

Although pets can be a source of disease for their owners, there are some myths that should be dispelled. Animals do not carry lice and do not need to be treated in the case of a family lice outbreak. Furthermore, animals are not a reservoir for pinworms and group A beta-hemolytic streptococci, so they cannot be held responsible for the transmission of these infections.⁵

Conclusion

Although it is relatively rare, household pets can infect their owners with diseases, and the pharmacist should be aware of the more widespread and more serious zoonoses. Pets can play a role in some common conditions, such as rashes, gastroenteritis, and MRSA. Patients are often unaware of zoonotic infections, and the pharmacist can be a valuable resource for them. Please refer to TABLE 2 for additional information. 

REFERENCES

1. Souza MJ. Bacterial and parasitic zoonoses of exotic pets. Vet Clin North Am Exot Anim Pract. 2009;12:401-415.
2. Meredith JT. Zoonotic infections. In: Tintinalli JE, Kelen GD, Strapczynski JS, et al, eds. Tintinalli’s Emergency Medicine: A Comprehensive Study Guide. 6th ed. New York, NY: The McGraw-Hill Companies; 2004.
3. Pickering LK, Marano N, Bocchini JA, Angulo FJ; Committee on Infectious Diseases. Exposure to nontraditional pets at home and to animals in public settings: risks to children. Pediatrics. 2008:122:876-886.
4. Rabinowitz PM, Gordon Z, Odofin L. Pet-related infections. Am Fam Physician. 2007;76:1314-1322.
5. Bender JB, Minicucci L. Diseases pets and people share. Minn Med. 2007;90:43-47.
6. Sanchez S, Hofacre CL, Lee MD, et al. Animal sources of salmonellosis in humans. J Am Vet Med Assoc. 2002;221:492-497.
7. Harris JR, Neil KP, Behravesh CB, et al. Recent multistate outbreaks of human salmonella infections acquired from turtles: a continuing public health challenge. Clin Infect Dis. 2010;50:554-559.
8. Warwick C, Lambiris AJ, Westwood D, Steadman C. Reptile-related salmonellosis. J R Soc Med. 2001;94:125-126.
9. Petri WA. Penicillins, cephalosporins, and other beta-lactam antibiotics. In: Brunton LL, Lazo JS, Parker KL, eds. Goodman & Gilman’s The Pharmacological Basis of Therapeutics. 11th ed. New York, NY: The McGraw-Hill Companies; 2006.
10. Phillips MA, Stanley SL. Chemotherapy of protozoal infections: amebiasis, giardiasis, trichomoniasis, trypanosomiasis, leishmaniasis, and other protozoal infections. In: Brunton LL, Lazo JS, Parker KL, eds. Goodman & Gilman’s The Pharmacological Basis of Therapeutics. 11th ed. New York, NY: The McGraw-Hill Companies; 2006.
11. Weller PF. Trichinella and other tissue nematodes. In: Fauci AS, Braunwald E, Kasper DL, et al, eds. Harrison’s Principles of Internal Medicine. 17th ed. New York, NY: The McGraw-Hill Companies; 2008.
12. Won KY, Kruszon-Moran D, Schantz PM, Jones JL. National seroprevalence and risk factors for Zoonotic Toxocara spp. infection. Am J Trop Med Hyg. 2008;79:552-557.
13. Blanton JD, Palmer D, Rupprecht C. Rabies surveillance in the United States during 2009. J Am Vet Med Assoc. 2010;237:646-657.
14. Jackson AC, Johannsen EC. Rabies and other rhabdovirus infections. In: Fauci AS, Braunwald E, Kasper DL, et al, eds. Harrison’s Principles of Internal Medicine. 17th ed. New York, NY: The McGraw-Hill Companies; 2008.
15. Weller PF, White AC. Cestodes. In: Fauci AS, Braunwald E, Kasper DL, et al, eds. Harrison’s Principles of Internal Medicine. 17th ed. New York, NY: The McGraw-Hill Companies; 2008.
16. McCall CO, Lawley TJ. Eczema, psoriasis, cutaneous infections, acne, and other common skin disorders. In: Fauci AS, Braunwald E, Kasper DL, et al, eds. Harrison’s Principles of Internal Medicine. 17th ed. New York, NY: The McGraw-Hill Companies; 2008.
17. Cohn LA, Middleton JR. A veterinary perspective on methicillin-resistant staphylococci. J Vet Emerg Crit Care (San Antonio). 2010;20:31-45.
18. Chambers HF. Protein synthesis inhibitors and miscellaneous antibacterial agents. In: Brunton LL, Lazo JS, Parker KL, eds. Goodman & Gilman’s The Pharmacological Basis of Therapeutics. 11th ed. New York, NY: The McGraw-Hill Companies; 2006.
19. Fritz CL, Kjemtrup AM. Lyme borreliosis. J Am Vet Med Assoc. 2003;223:1261-1270.
20. Bowman DD, Lucio-Forster A. Cryptosporidiosis and giardiasis in dogs and cats: veterinary and public health importance. Exp Parasitol. 2010;124:121-127.
21. Skerget M, Wenisch C, Daxboeck F, et al. Cat or dog ownership and seroprevalence of ehrlichiosis, Q fever, and cat scratch disease. Emerg Infect Dis. 2003;9:1337-1340.
22. Spach DH, Darby E. Bartonella infections, including cat scratch disease. In: Fauci AS, Braunwald E, Kasper DL, et al, eds. Harrison’s Principles of Internal Medicine. 17th ed. New York, NY: The McGraw-Hill Companies; 2008.
23. Stamm WE. Chlamydial infections. In: Fauci AS, Braunwald E, Kasper DL, et al, eds. Harrison’s Principles of Internal Medicine. 17th ed. New York, NY: The McGraw-Hill Companies; 2008. 

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