Top 5 Zoonotic Disease Transmission Routes in Veterinary Medicine

Zoonotic pathogen exposure is a constant risk in veterinary medicine because any patient, including those that appear clinically normal on evaluation, can shed zoonotic pathogens that range from common (eg, Campylobacter spp, Pasteurella spp) to rare (eg, mpox [formerly termed monkeypox]) and have many potential outcomes (eg, ringworm [mild], rabies [fatal]). Although zoonotic disease risks cannot be eliminated, they can be substantially reduced with use of practical measures and recognition of high-risk situations.

Discussions regarding zoonotic disease typically focus on specific diseases, but the field of infectious and zoonotic diseases is dynamic, and concerns about pathogens (recently, severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2], mpox, severe fever with thrombocytopenia syndrome virus, increasing range of multidrug-resistant pathogens) can grow and change.^1-3 Disease prevention measures are often the same for a range of pathogens; therefore, understanding routes of exposure and general prevention measures can help minimize the risk from known entities and emerging threats.  

Following are 5 significant routes of zoonotic pathogen transmission (including important routes, risks, and protective measures) according to the author. There may be overlap, and some pathogens can be transmitted via multiple routes.

1. Direct Contact

Risk for Exposure

Direct contact involves transfer of a pathogen directly from the patient’s skin, mucous membranes, or bodily fluids. Most pathogens require contact with nonintact skin or mucous membranes for infection; however, contact with visibly intact skin can still result in disease (eg, dermatophytosis).  

Risk for zoonotic disease through any route of exposure in humans is typically elevated in very young, elderly, pregnant, and immunocompromised individuals. Those with nonintact skin (including microabrasions, which are not always readily identifiable) or invasive devices (eg, intravenous catheters, peritoneal dialysis catheters, interstitial glucose monitors, insulin pumps, chemotherapy ports) can be at increased risk due to compromise of the normal protective barrier of the skin.

Pathogens

Many pathogens are potentially transmissible via direct contact, as well as through respiratory and fecal–oral routes. Although there is a wide range of possible organisms, risks are primarily from bacteria of the commensal microbiota (eg, staphylococci, Escherichia coli and other Enterobacterales, Pasteurella spp), including multidrug-resistant bacteria (eg, methicillin-resistant staphylococci, extended spectrum beta-lactamase producing–Enterobacterales).^4-6  

Leptospirosis can be transmitted via direct contact with urine from an infected patient. Brucella canis infection is of particular risk through direct contact with reproductive fluids and tissues (especially from aborting dams) but is also possible through contact with other bodily fluids (eg, urine).^7-9 Infections from direct skin contact with oral and GI commensals (eg, Pasteurella spp) are also common.^10-13

Limiting Exposure

Routine use of personal protective equipment (PPE) and proper hand hygiene are foundational protection measures.  

PPE should consist of typical in-clinic clothing (eg, lab coat) that acts as a barrier between human skin and the patient and is easy to remove when soiled. Other items (eg, gloves) should also be worn as needed (ie, based on patient or procedure risk).  

Hand hygiene (eg, handwashing, use of alcohol-based hand sanitizers) is a core component of protection, but compliance and quality are often low in veterinary clinics.¹⁴

2. Respiratory Droplets & Aerosols

Risk for Exposure

Droplets and aerosols generated during breathing, coughing, sneezing, or vocalizing can contain large numbers of pathogens spread over short to moderate distances in the air, possibly resulting in infection via inhalation or contact with mucous membranes.^2,15

Pathogens

A variety of respiratory pathogens and upper airway commensals can pose a risk. Studies are limited, but case studies highlight some relevant pathogens and scenarios.  

Bordetella bronchiseptica can be transmitted through the respiratory route and occasionally causes disease in humans, particularly in immunocompromised individuals.^16-21 Infections may be transmitted from clinically normal dogs and cats, as this bacterium can be found in healthy patients.^22,23 Occupational infection in veterinary staff has not been reported, but some risk is possible. There are also concerns about respiratory (especially ocular) exposure to modified live vaccines because veterinary staff can be exposed to highly concentrated but attenuated bacteria during or shortly after patient vaccination. Risk for infection is low because modified live vaccines are highly attenuated, but a small number of infected pet owners has been reported; high-level exposure of the eyes, mouth, and nose should therefore be avoided during vaccination procedures.^24,25  

Transmission of avian influenza from cats to animal care workers has been reported,^26,27 and although canine influenza strains are believed to pose limited zoonotic risk, transmission cannot be excluded. Streptococcus zooepidemicus causes sporadic disease in humans. Dog–human transmission is likely rare, but respiratory transmission from the sneeze of an affected dog was reported in one case.²⁸ Coxiella burnetii, the cause of Q-fever, is also a potential risk, especially from periparturient cats.^29,30

Plague (ie, Yersinia pestis) is a high-priority zoonotic pathogen in some regions, and zoonotic infections of veterinary staff can occur.³¹ Routes of exposure are often undetermined; however, respiratory transmission from patients with pneumonic plague is a significant concern.

Limiting Exposure

Enhanced PPE should be used when there is concern for droplet, aerosol, or airborne transmission or when splashes might result from a patient or procedure.  

Respiratory precautions include use of a mask, as well as eye (eg, goggles) or face (eg, face shield) protection. Medical masks are most commonly used; however, N95 or equivalent masks provide superior protection and are indicated in some situations (eg, suspected plague). Ideally, N95 masks should be properly fit tested to the individual user, but a nonfit-tested N95 mask is still likely better than a medical mask.  

Eye protection is often underused but typically needed in addition to a mask if there is concern for respiratory transmission or risk from splashes, as ocular mucous membranes are at high risk for pathogen entry. In the first documented cat–human transmission of SARS-CoV-2, a clinician wearing a mask but no eye protection was infected by the sneeze of an infected cat.²

3. Needlesticks

Risk for Exposure

Sharps injuries are common in the veterinary clinic and can be associated with soft tissue trauma, adverse reactions to drugs, and risk for infection.³² Bloodborne pathogen risks from veterinary patients are minor compared with human medicine, resulting in markedly different approaches to reduce and address needlestick injuries. In human medicine, a needlestick injury requires an immediate and detailed response, whereas in veterinary medicine, these injuries are often considered a relatively normal and benign occupational inevitability. There are, however, potentially serious adverse outcomes.

Pathogens

Although the incidence of disease after a needlestick is likely very low, data are limited, and complications (including transmission of infectious diseases) can occur. Needlestick injuries can lead to infection from human or patient skin microbiota (eg, Staphylococcus aureus), as well as infection (eg, blastomycosis and leishmaniosis from fine-needle aspiration needlesticks, Bartonella spp infection from venipuncture needlesticks) from pathogens on the needle.^33-36

Limiting Exposure

Safe sharps handling practices are highly effective at reducing risk for exposure (see Common Methods to Reduce Risk for Needlestick Injuries); however, poor sharps handling is common in veterinary clinics, resulting in avoidable risks and liability. In 2 survey-based studies, >75% of clinicians and veterinary nurses reported having had a needlestick injury in the preceding year.^37,38 A video observation study identified common deficiencies (eg, lack of a sharps container in examination rooms, use of temporary unapproved containers for sharps, failing to immediately and properly dispose of sharps, recapping needles). Similar issues were identified in a direct observation study that noted clinicians were more likely to use poor needle-handling practices than veterinary nurses.³⁹

4. Bites & Scratches

Risk for Exposure

Direct trauma and secondary infections from bites and scratches likely cause the greatest overall disease burden for veterinary staff.

Pathogens

Rabies is always a consideration in areas where it is present in wildlife or domestic mammals. Although risks from domestic mammals are low in areas where the canine rabies virus variant is not present, rabies cannot be ignored (even when rare) because of its severity. Reporting bites and ensuring postexposure prophylaxis is administered, when indicated, are key. 

Myriad bacteria, predominantly S aureus and Pasteurella spp, from a patient’s mouth or human skin can cause opportunistic infections.^11,40-43 Capnocytophaga canimorsus, a common canine commensal, can cause fatal septicemia, predominantly in splenectomized or immunocompromised patients.⁴⁴ Infections caused by these widespread commensals highlight the risk posed by any bite, not just bites from patients with clinical disease.  

Less common but regionally relevant risks include sporotrichosis and other fungi (eg, Blastomyces spp).^45-48 Bites from companion animal species other than dogs and cats carry similar overall risks as from dogs and cats, with some substitution of pathogens (eg, Streptobacillus moniliformis, which causes rat-bite fever).⁴⁹

Limiting Exposure

Bites and scratches may be considered unavoidable occupational risks, but avoidance and proper first aid can help prevent bites and scratches and resulting infections. Antimicrobial prophylaxis after a bite may be indicated, primarily for bites over a joint, tendon sheath, or nerve; bites to the face; crush injuries; puncture wounds; and bites to individuals without a functional spleen or who are immunocompromised (ie, those who are very young, elderly, pregnant, or immunocompromised by disease or treatments); and bites near a prosthetic device. Prompt first aid, including thorough flushing of the wound, should be performed when bite or scratch injuries occur.

5. Fecal–Oral Exposure

Risk for Exposure

Numerous zoonotic pathogens can be shed in feces of healthy and diseased patients, but risks of enteropathogen shedding are presumed highest in both diarrheic and young patients.

Pathogens

Greatest risks are from enteropathogenic bacteria (eg, Campylobacter spp, Salmonella spp), with increasing concerns for multidrug-resistant strains.^50-52  

Although commonly found in some dog populations, parasites pose lower zoonotic disease risk in the clinic than in general community settings. Some parasites (eg, Toxocara spp) must be present in the environment for days before posing a risk. These parasites pose lesser risk but can be a concern in debilitated patients with fecal staining of the haircoat. Giardia spp are potentially zoonotic, but most dogs are infected with nonzoonotic assemblages (ie, strains).⁵³ There are some risks from parasites such as Echinococcus multilocularis. 

Fecal–oral exposure can also result in colonization with multidrug-resistant bacteria (eg, extended spectrum beta-lactamase producing–Enterobacterales) that can cause extraintestinal infections.

Limiting Exposure

Prevention of fecal–oral exposure focuses on reducing the incidence of fecal contamination on clothing, hands, and surfaces through use of PPE, hand hygiene, environmental cleaning and disinfection, and isolation of patients with suspected infectious diarrhea, as well as prevention of contamination of food (eg, avoiding storing specimens and human food in the same refrigerator) and/or items (eg, pens) that may come into contact with the mouth.

Conclusion

Zoonotic pathogen exposure is an inherent risk when in contact with any patient but can vary based on species, individual characteristics (eg, health status, age), and type of contact. Although risks cannot be eliminated, they can be reduced with knowledge and use of basic preventive measures.