A zoonotic illness refers to a disease that is shared between animals and humans.9
These illnesses are the primary concern of much of the literature concerning the risks to people posed by animals in public settings. In developed countries, these illnesses are relatively uncommon9
and are even less prevalent in the healthcare setting. However, until more definitive information is available, priority should be placed on patient, visitor, and healthcare provider safety and the use of standard infection prevention measures to prevent animal-to-human transmission of these potential pathogens in healthcare settings.
Contact with animals includes positive or neutral interactions such as petting, touching, kissing, or being in the direct environment of the animal as well as negative interaction such as biting or scratching.10
Diseases that can be transmitted by contact include enteric diseases, internal parasites, skin infections, external ectoparasites, and diseases that are consequences of bites or scratches.10
Enteric bacteria and parasites pose the highest risk of disease transmitted from humans to animals in public settings.11
In the United States in 2012 alone, it was estimated that there were over 400 000 cases of enteric illness attributed to animal contact, almost 5000 hospitalizations, and 76 deaths.12
Enteric diseases can be transmitted through direct contact with an infected or colonized animal or through environmental contamination. The mechanism of transmission of enteric organisms are fecal-oral, but contact with animal fur, hair, feathers, scales, and saliva can transmit these pathogens due to fecal contamination of the animal or environment.10
In addition, there have been exposures associated with contaminated animal bedding, cages, environmental surfaces, clothing, shoes, or other objects.13,14,15,16
Some enteric pathogens can also persist in the environment for long periods of time, documented up to weeks or months in various outbreak investigations.17,18,19
Susceptibility of people to these pathogens is variable but is influenced by human hygiene practices as well as age, gastric acid suppression, and immune status.9
Other infections associated with animal contact include other bacterial illnesses such as tularemia, leptospirosis, and brucellosis; internal parasites such as Toxocara
, or Baylisascaris
; ectoparasites such as Sarcoptes
mites or fleas; skin infections such as poxviruses (monkeypox, orf, cowpox, etc.); Mycobacterium marinum
(from contact with fish tank water) or tinea
fungi (ringworm); biteassociated infections such as rabies, Pasteurella multocida
, and other oral pathogens; or scratch-associated infections such as Bartonella henselae
Droplet and Aerosol
The droplet-associated zoonotic infection of highest global concern is influenza A virus, especially variant strains associated with pigs and birds. There are reports of infections and outbreaks associated with various strains of avian- or swine-associated influenza, though most of these infections reported in the United States are in individuals who have direct contact with pigs.20
Unfortunately, it has been shown that even apparently healthy pigs and birds may shed influenza virus.20
Notable aerosol transmitted zoonotic illness include Q fever due to the bacteria Coxiella burnetii
, especially due to peripartum animals, tularemia due to Francisella tularensis
, and psittacosis from Chlamydia psittaci
acquired from psittacine birds.10
Fortunately, most illnesses associated with animal contact are acquired in nonhospital settings in facilities like zoos, petting zoos, pet stores, fairs, and farms.10
TABLE 25-1 Zoonotic Infection Routes and Infection Classification
Infectious agents (common examples)
STEC, Salmonella, Shigella, Campylobacter, Cryptosporidium, Giardia
Scabies, mites, fleas
Pox viruses, Mycobacterium marinum
Rabies virus, Pasteurella, Capnocytophaga
Influenza, Chlamydia psittaci
Coxiella burnetii, Mycobacterium tuberculosis (in elephants)
STEC, shiga toxin-producing E coli.
In addition to the above zoonotic pathogens, in the healthcare environment, there is also the concern that animals can serve as fomites from acquiring infection or colonization with hospital-acquired multidrug-resistant organisms (MDROs) or opportunistic pathogens (such as Clostridioides difficile
) and transmitting them horizontally between patients and/or the environment. In the household setting, studies have shown an interplay between human and animal colonization with bacteria, including pathogens such as methicillin-resistant Staphylococcus aureus
Animals with histories of medical illness and veterinary care may have been exposed to antibiotics and may harbor medically significant multidrug-resistant pathogens such as carbapenem-resistant Enterobacteriaceae
(CRE) or Pseudomonas
Animal or handler behavior issues can pose a risk to patients, especially those with indwelling devices or disabilities. Finally, there are infection risks associated with animal handlers themselves (similar to any other human visitor to the hospital).
Research animals add an additional complication, as these animals will not be housebroken or behaviorally controlled, and may harbor unique pathogens, based on species or usage in research. Spread of infections from research animals to laboratory workers has been reported, such as ratbite fever (Streptobacillus moniliformis
or Spirillum minus
) from rats and B virus (herpes B, monkey B virus, herpesvirus simiae
) encephalitis from macaque monkeys.46,47
The healthcare setting offers unique challenges for preventing spread of infection from animals to people. Hospitalized patients may have altered host defenses that could increase susceptibility to zoonotic infections and/or increase the severity of clinical disease, and the necessity of sterility and hygiene in the hospital increases potential opportunities for infection. Different populations in the hospital will have different degrees of susceptibility to zoonotic illnesses (pregnant patients, pediatrics, neutropenic patients, oncology patients, etc.) and will potentially interact with animals in different ways. In addition, there is greater regulatory and legal oversight and scrutiny of hospitals than other settings where animals might encounter people. While there will be overlap with other uses of animals in nonhospital public settings, some of the use of animals in hospitals will be unique, such as certain animalassisted activities or research animals.
Despite this increased risk, there are few reports of disease outbreaks directly associated with animals in hospitals, and most fail to identify a causal link (Table 25-2
). In general, there is no evidence that animals carry any more risk of transmitting infection than do people.32
While there are isolated case reports such as MRSA colonization of the
paws of a cat in a geriatric unit during an MRSA epidemic24
or a therapy dog with MRSA and C difficile
formal studies evaluating the potential risks of zoonotic illness in hospitals are lacking.32
TABLE 25-2 Studies of Pathogens and Outbreaks Associated With Animals in Healthcare (AHC)
Author, year (ref. no)
Lefebvre et al., 200623
Healthy visitation dogs (n = 102) assessed for presence of zoonotic pathogens
Zoonotic agents isolated from 80% of animals including toxigenic C difficile (40.1%), Salmonella spp. (3%), ESBL-producing E coli (4%), Pasteurella spp. (29%), Malassezia pachydermatis (8%), Toxocara canis (2%), and Ancylostoma caninum (2%).
Scott et al., 198824
Outbreak of methicillin-resistant Staphylococcus aureus (MRSA) on a geriatric ward
Paws and fur of a cat that roamed the ward were heavily colonized by MRSA; the cat was a possible vector for the MRSA transmission.
Lyons et al., 198025
Outbreak of Salmonella Heidelberg in a hospital nursery
Outbreak traced to infected calves on a dairy farm, where the mother of the index patient lived.
Richet et al., 199126
Outbreak of Rhodococcus (Gordona) bronchialis sternal surgical site infections after coronary-artery bypass surgery
Outbreak linked to a nurse whose hands, scalp, and vagina were colonized with the pathogen. Skin cultures from two of her three dogs were also positive; however, it was unclear if the dogs were the source for colonizing the nurse or whether the dogs and nurse were colonized from an environmental reservoir.
Chang et al., 199827
Outbreak of Malassezia pachydermatis in an intensive care nursery
Isolates all 15 case patients, 9 colonized infants, 1 healthcare worker (HCP), and 3 pet dogs owned by HCP were identical. The authors believed it likely that M pachydermatis was introduced into the intensive care nursery from the HCW’s hands after being colonized from pet dogs at home and then persisted in the nursery through patient-topatient transmission. Patient infections included 8 bloodstream infections, 2 urinary tract infections, and 1 meningitis and 4 others with asymptomatic colonization.
Mossovitch et al., 199628
Snider et al., 199329
Multiple nosocomial outbreaks of Microsporum canis (ringworm) in newborn nurseries or neonatal ICUs
Person-to-person transmission described in a neonatal ICU outbreak; the source of infection was a nurse likely infected from her pet cat.
Enoch et al., 200530
Therapy dog screened for MRSA and isolate compared to clinical isolates of MRSA in the hospital
An 11-year-old border collie acquired MRSA in a UK general hospital after visiting care-of-the-elderly wards. All isolates from seven patients with known MRSA on the wards visited by the dog were characterized phenotypically and genotypically. Seven isolates displayed five different PFGE patterns, and all differed from the dog isolate.
Boyle et al., 201931
Two-part study investigating the risk of zoonotic pathogen animal colonization in a university-based AAA program as well as the handlers’ understanding of the risks of zoonoses and their adherence to infection control practices
17 positive screening results from 118 performed, 14 of which were potential pathogens. In part two of the study, 70% of respondents expressed no concerns regarding infectious disease transmission in AAA settings. Despite handler education and guidelines, adherence to infection control practices was lacking.
ESBL, extended-spectrum beta-lactamase; HCP, healthcare provider; RFLP, restriction fragment length polymorphisms; ICU, intensive care unit.