Clinical and Health Affairs
Diseases Pets and People Share
By Jeff B. Bender, D.V.M., M.S., and Larissa Minicucci, D.V.M., M.P.H.
Abstract
Having a companion animal such as a dog, cat, or pocket pet is increasingly popular in the United States. With more human-animal interactions come additional opportunities for disease exchange. This article highlights common zoonoses associated with pets and provides recommendations for preventing disease transmission, including hand hygiene, client or patient education, and appropriate pet care. The article also discusses human diseases in which animals may be mistakenly implicated as the source and the recent identification of human diseases such as methicillin-resistant Staphylococcus aureus in companion animal populations. The article also calls for the strengthening of integrated animal and human surveillance systems and cooperation of medical and veterinary practitioners.
In this and other societies, animals such as dogs and cats are often considered companions because of the unique and special bond that exists between them and their owners. The value of these animals to humans is well-documented. For example, animals have been shown to improve health measures of patients in long-term care facilities and to aid the emotional well-being of people with AIDS.1
Despite these valuable contributions, there is growing concern about diseases that humans can acquire from animals. Although the focus is often on food or wild animals, a number of zoonoses can be acquired from pets. Some that have recently been identified include Monkeypox, Clostridium difficile, and methicillin-resistant Staphylococcus aureus (MRSA).2 For purposes of this discussion, zoonoses are defined as those diseases and infections that are naturally transmitted between vertebrate animals and man, with the animals serving as the reservoir where a pathogen lives or persists and multiplies.
Sixty-three percent of U.S. households have a pet. There are an estimated 73.9 million dogs, 90.5 million cats, 16.6 million birds, 18.2 million pocket pets, and 11 million reptiles living in homes in this country.3 A number of human health problems are associated with common household pets (Table). Those associated with cats include cat scratch disease, toxoplasmosis, bites and bite-associated infections, salmonellosis, campylobacteriosis, dermatophytosis, and rabies. Those frequently associated with dogs include dermatophytosis, roundworm and hookworm infection, campylobacteriosis, rabies, injuries and infections related to bites, and several shared zoonoses such as histoplasmosis or blastomycosis. Zoonoses associated with pocket pets include salmonellosis, campylobacteriosis, leptospirosis, Lymphocytic choriomeningitis virus infection, tularemia, Monkeypox, and Streptobacillus moniloformis infection (rat-bite fever).
In this article, we highlight 3 common illnesses acquired from household pets that physicians and other health care providers may see in clinical practice: salmonellosis, toxoplasmosis, and infections caused by roundworms and hookworms. Furthermore, we hope to dispel concerns about diseases that animals are falsely implicated in transmitting. And finally, we highlight a discouraging trend of human diseases being identified in companion animals.
Salmonellosis
In a case report from New Jersey, a 5-month-old girl was hospitalized with meningitis. Salmonella serotype Rubislaw was cultured from her cerebrospinal fluid.4 The family did not own a reptile, but the child’s babysitter had an iguana. Subsequent fecal cultures from the iguana yielded Salmonella serotype Rubislaw. The infant never had contact with the iguana. And in this case, contamination from human hands was documented in the transfer of infection from the iguana to the child.
Reptiles are popular pets. An estimated 4% of the U.S. population owns a reptile.3 Unfortunately, reptiles can carry Salmonella with no outward clinical signs. Some studies have estimated that more than 90% carry Salmonella but don’t cause disease.5 A recent FoodNet study estimates that 6% of human salmonellosis cases in the United States may be attributed to reptile exposure.6 In interviews of patients with salmonellosis who owned reptiles, we discovered several risky behaviors. These include bathing reptiles in the bathtub, washing reptile housing in the bathtub, or allowing reptiles to wander through the house, including the kitchen area.
As a result of increasing concern about reptiles, the Centers for Disease Control and Prevention (CDC) devised recommendations for reptile contact and ownership.7 Pregnant women, children younger than 5 years of age, and immunocompromised persons (such as individuals with AIDS) are at increased risk for infection or serious complications of salmonellosis and are encouraged not to have contact with reptiles. The CDC recommendations went as far as to say that families expecting a child should remove any pet reptile or amphibian from the home before the infant arrives and that reptiles are not appropriate pets in child-care homes or centers.
The current Salmonella concern is not only limited to reptiles. A variety of sporadic cases and outbreaks of disease have been linked to hamsters and mice, chicks and ducks, veterinary clinics, animal adoption facilities, and pet treats.8-11 Some of these have involved multidrug-resistant strains, which has an impact on treatment options for clinicians.
Prevention involves appropriate hand hygiene, especially among children in contact with any type of animal. Pet stores should make consumers, especially those with infants, pregnant women, and the immunocompromised, aware of the Salmonella risk associated with reptiles. Reptile owners also should refrain from bathing reptiles in sinks or bath tubs used by people.
Toxoplasmosis
Another common zoonotic agent of concern, especially to severely immunocompromised patients and pregnant women, is Toxoplasma gondii. This coccidian parasite of cats and other felids can spread to humans. Humans likely are exposed through several routes, including consumption of oocysts from contaminated soil, vegetables, or water or through consumption of tissue cysts in contaminated meat. The prevalence of toxoplasmosis is high in countries where undercooked meat is consumed. Toxoplasmosis is the third leading cause of food-related deaths in the United States, with half believed to be associated with eating contaminated meat. It is estimated that 400 to 4,000 cases of congenital toxoplasmosis occur each year in the United States, causing 750 deaths annually.12 Toxoplasma is also a leading cause of chorioretinitis in persons with normal immune systems. As many as 85% of congenitally infected children may develop ocular disease by age 20.13
Most feral cats and domestic outdoor cats become infected during their first 12 months of life. Infected cats shed oocysts over a brief period (2 to 3 weeks); during this time, they can shed up to 20 million oocysts per day.14 These oocysts can remain viable for 18 months in moist soil. It takes 1 to 5 days after excretion for sporulation or development of the infectious form. To reduce the chance of exposure to humans, veterinarians recommend the daily cleaning of litter boxes. Serological testing of cats is of limited value except that a seropositive cat has protective antibodies and, therefore, poses less risk to humans than a seronegative cat. For women who are hoping to become pregnant, it is more important that they consult with their health care provider and check their serologic status prior to pregnancy. Those with positive titers have protective antibodies. Those without evidence of prior exposure should take heightened precautions to minimize exposure during pregnancy. A recent Minnesota survey found a seroprevalence of 13% among women ages 20 to 30 years.15
Precautions should include avoiding consumption of undercooked meat, using irradiated or frozen meat, washing or peeling fruits and vegetables, and cleaning all cooking surfaces and utensils after contact with raw meat or produce. Wearing gloves while gardening and thoroughly washing hands after contact with soil or sand is also recommended. Other precautions include keeping cats inside, only using commercial cat food (ie, not feeding raw or undercooked meat to cats), and changing the cat litter daily. Someone who is not pregnant should clean the litter box; if a woman is pregnant, she should wear gloves and wash her hands thoroughly afterward.
Infections Caused by Roundworms and Hookworms
Toxocara infections are common. The estimated seroprevalence in U.S. children is 4.6% to 7.3%; it is higher in children of lower socioeconomic status.16 An individual female Toxocara roundworm can produce up to 200,000 eggs in a day. With some puppies and kittens having intestinal burdens of several hundred roundworms, daily environmental contamination may reach millions. Toxocara eggs have a prolonged extrinsic incubation period of 2 to 4 weeks and can readily survive in the environment, especially in warmer climates where they aren’t exposed to killing freeze-thaw cycles. Because of this prolonged incubation period, human infection directly from dogs or cats is unlikely. Risk factors for toxocariasis include the presence of pups in the household and children with a history of pica.
Illness in people includes visceral larval migrans, where the infective-stage larvae migrate through the lungs, liver, or other organs, and ocular larval migrans, involving larvae migration through the eye. Clinical reports are infrequent; however, it is estimated that there are 750 cases of ocular larval migrans each year in the United States.
Cutaneous larval migrans is a common skin problem of Western travelers to tropical countries. This progressive, eruptive skin disease is caused by the migration of Ancylostoma sp. or other related animal hookworms in the skin. Dogs and cats can serve as a reservoir host, contaminating soil through defecation.
Precautions for preventing both hookworm and roundworm infections include good hand hygiene, supervision of children in potentially contaminated areas, and elimination of intestinal parasites from pets through routine deworming.
Diseases in Which Animals Are Not the Source
Patients with refractory diseases may present to their physicians or other health care providers concerned that the family pet is serving as the source for an ongoing infection. Examples of such infections or infestations include head lice, pinworms, and Group A beta-hemolytic streptococci infections. Health care providers may encourage patients to contact their veterinarian for treatment of the pet. However, the ecological niche where these pathogens survive and multiply is humans, not animals.
Head lice are common and often problematic especially in families with school-aged children. Children spread the infestation by sharing combs, brushes, hats, coats, etc. Recently, permethrin resistance has been identified, complicating treatment options and frustrating parents with refractory and reoccurring infestations. Animals are not part of the life cycle of head lice and do not need to be treated in households with human infection. Nymphal stage lice can survive up to 48 hours in the absence of their human host. Viable nits, however, can survive several weeks on clothing, stuffed animals, bedding, or pillows.
Another human infection in which animals are commonly implicated is enterobiasis or human pinworm infection. This is the most common helminthic infection in humans, with 42 million cases estimated annually in the United States. It is common in children, institutionalized groups, and family households. Again, pinworms are species-specific; therefore, human pinworms are not found in pets. There are animal-specific pinworms, however.
Streptococcal sore throat is another common ailment in families with young children. As a common bacterial infection of children, it peaks in incidence during the first few weeks of school and is readily spread by person-to-person contact. Occasional anecdotal case reports have cited treatment of household pets as a means to control ongoing household transmission. However, a longitudinal study examining children with acute pharyngitis, and in which concurrent oropharyngeal cultures were taken from their pets, showed no evidence of group A beta-hemolytic streptococci in 230 animal samples.17 Health care providers may be confronted with reoccurring cases in a single family. If the family pet is referred to the local veterinarian for screening and treatment, it should be noted that dogs will need to be sedated to adequately obtain tonsillar swabs. Rare detection of group A streptococci in dogs has been described, but they carry the infection for only 2 to 3 weeks. The general consensus is that dogs are not important in the ongoing transmission of group A streptococci infection in humans.
Some infections acquired from common environmental sources may affect both humans and pets. Such diseases include Lyme disease and Rocky Mountain spotted fever or systemic fungal infections (ie, Blastomyces dermatitidis, Histoplasma capsulatum, Coccidiodes immitis). These infections are not spread directly from the animal to the human but are acquired from contaminated soils (saprozoonoses) or through a common arthropod such as the deer tick (Ixodes scapularis) for Lyme disease.
Another area where animals have been wrongly implicated is “delusional parasitosis.” This illness is defined as mistakenly believing that one is infested with ectoparasites or infected with internal parasites. Afflicted individuals tend to visit or contact a variety of health care professionals, including veterinarians. They often believe that there is widespread infestation of their environment or co-infection of family members and/or pets. This belief often translates into excessive cleaning of the environment and treatment of household pets.
Diseases Humans Transmit to Animals
We infrequently hear about diseases that animals can acquire from humans. There are concerns related to polio and measles in endangered primates. A recent case of Mycobacterium tuberculosis infection in a 3.5-year-old Yorkshire Terrier highlights that these concerns are relevant for pets as well.18 This particular dog was living with a human with active tuberculosis. It is believed that transmission occurred through close proximity and direct exposure to aerosolized droplets from the infected owner. In a study of dogs diagnosed with M. tuberculosis from 1962 to 1979, researchers documented that all the dogs had a history of contact with clinically ill humans.19
Another concern is methicillin-resistant Staphylococcus aureus (MRSA) infections in animals. This important pathogen, often associated with human health care facilities, accounts for as many as 50% of nosocomial infections at some hospitals. It is a leading cause of pneumonia, and surgical wound and bloodstream infections in hospitalized patients or residents of long-term care facilities. Cases in which there is no history of recent hospitalization have been increasing.
Until 2005, there were relatively few case reports of MRSA in animals. MRSA has now been identified in cats, dogs, horses, and pigs.20-23 Staphylococcus intermedius is the most common skin flora isolated from dogs and cats. However, even now the resistance gene associated with MRSA is being detected in multidrug-resistant Staphylococcus intermedius isolates.
With detection of MRSA in companion animals, there is a recognition that pets could act as an ongoing source of infection for people. A recent case report documented human-to-dog transmission.24 MRSA was cultured from the nose of a healthy dog whose owner was colonized while working in a Dutch nursing home. DNA subtyping and virulence factor characterization revealed that both human and canine MRSA strains were indistinguishable. The nurse had been treated with antibiotics twice following an outbreak in the nursing home where she worked. With evidence of recolonization, officials screened her family and home environment. MRSA colonization of the nose and skin was documented in her 1-year-old daughter. The healthy family dog also had nasal colonization at the time. It was postulated that the nurse was recolonized through contact with the baby, the dog, or another unknown source. Treatment of the mother, baby, and dog eventually eliminated MRSA colonization. A similar case was documented in a patient with diabetes who had recurrent infections with mupricin-resistant MRSA.25 MRSA was cultured from nasal swabs from the family dog, and treatment of the dog prevented further recurrence.
We have observed similar cases in our veterinary teaching hospital. Between October 2003 and December 2006, we documented 14 MRSA infections in companion animals at the University of Minnesota Veterinary Medical Center. The animals often present with nonhealing skin lesions or postoperative infections. Interestingly, the pets’ owners were human health care providers (n=4) or had been recently hospitalized or were caring for a recently hospitalized family member (n=5). Most of the isolates from animals have been indistinguishable from the strains associated with human health care facilities (clonal group USA 100).
Not all MRSA cases in animals have been related to humans. A recent outbreak in horses occurred at the Ontario Veterinary College in 2002 following several cases of postoperative or catheter-site infections.22 The investigation identified 27 infected horses seen at the veterinary college as well as 17 infected personnel. Most of the human patients reported direct contact with an MRSA-positive horse. Widespread environmental contamination was observed, with 62% of horse stalls previously housing an MRSA-positive horse testing positive. Contaminated equipment such as stethoscopes, muzzles, and twitches were also found. This outbreak was attributed to the emergence of a novel MRSA strain in horses and subsequent spillover to their human caretakers. As a result, several infection-control measures including routine equine surveillance and periodic human and environmental sampling were instituted to control the outbreak and prevent further spread. Glove use was required for people handling any horse, and full barrier precautions and isolation were applied to all MRSA-positive horses.
The isolation of MRSA from a household pet, whether colonized or ill, raises concern about the potential for nosocomial and human infection. Should such animals be treated or monitored? What recommendations are appropriate yet not alarming to the animal’s owner? With our recent cases, owners were instructed to consult with their health care provider, review available educational materials, and practice good hand hygiene. Similarly, should recommendations be given to humans with MRSA infections who have pets at home to prevent transmission to the animals? These cases emphasize the importance of monitoring for multidrug-resistant pathogens in both animals and people and encouraging collaboration and communication between human health care providers and veterinarians.
Summary
We can acquire some diseases from pets; but thankfully, the risk of transmission and serious infection is low. Common sense measures such as hand washing after contact with pets and their environment and before eating are key to prevention. With the emergence of multidrug-resistant organisms, it is even more critical to encourage good hand hygiene. This practice will prevent human illness and possibly prevent the transfer of resistant organisms from humans to pets. Pet owners should be encouraged to seek appropriate veterinary care for their animals, including routine and timely deworming, vaccination against diseases such as rabies, and care to prevent ectoparasites such as fleas and ticks.
Health care providers and veterinarians should be watchful for infections with unusual clinical presentations or those that are unresponsive to treatment. Practitioners need to take a thorough history and obtain appropriate cultures if needed. The development of integrated human/animal surveillance systems and continued communication between human and veterinary professionals can help to identify new infections and treat those infections that cross species. MM
Jeff Bender is an associate professor of veterinary public health and Larissa Minicucci is the director of the Veterinary Public Health Program at the University of Minnesota College of Veterinary Medicine.
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