The 5 deadliest zoonotic diseases and pathogens

dvm360dvm360 October 2022
Volume 53
Issue 10
Pages: 24

These organisms frequently endanger veterinary professionals

Melinda Nagy /

Melinda Nagy /

When asked to name the most dangerous occupations, most individuals would probably mention such jobs as a Hollywood stunt double, deckhand on Deadliest Catch, or lion tamer. Most wouldn’t think of veterinary professionals. But it may not be so shocking to discover just how close to the proverbial cliff’s edge many of us walk every day.

Veterinarians and their staff file occupational health claims 2.7 times as often as human health care workers. According to a paper published in 2005, when only severe accidents leading to a loss of more than 3 days of work were analyzed, that figure rose to 9.2 times. Scratches, bites, and kicks made up almost 66% of the accidents reported by veterinary professionals, who filed claims for skin (39%), respiratory (30.5%), and infectious (19.1%) diseases.1

In a 2012 survey of practitioners in Minnesota, 27% reported having been infected with a zoonotic disease during their career.2 Documented zoonotic infections among veterinary personnel include salmonellosis, cryptosporidiosis, sporotrichosis, methicillin-resistant Staphylococcus aureus (MRSA), and psittacosis. Most veterinary personnel (57.4%) are exposed to pathogens via hand contact—we’re touching gross stuff—whereas 21.7% of pathogens exposure occurs orally. Another fun fact? Cats are the most frequently reported source of zoonotic infection among veterinary personnel.1

So which are the most dangerous zoonotic diseases and pathogens? In ascending order of deadliness, they are:

5. Leptospirosis

The mortality rate for individuals with leptospirosis—caused by Leptospira interrogans—is 5.72%.3 In the majority of mammals, its clinical signs are liver or kidney failure. Although most often diagnosed in dogs, leptospirosis can affect cats as well.4 In horses, it typically manifests as ocular disease and in ruminants as a reproductive condition. Again, most of us encounter it in dogs as a simple urinary tract infection or as severe liver or kidney disease. Transmission occurs through contaminated food or, more often, water: when affected raccoons, rodents, feral pigs, and other wildlife urinate in puddles, unsuspecting dogs may become infected on their daily walks. The farm dog is no longer the animal at highest risk5; it is now the city dog weighing less than 15 lb. Fortunately, leptospirosis can be treated with antibiotics and supportive care, based on symptom severity and organ system affected.

4. Tick-borne diseases

Several tick-borne diseases combine for a human mortality rate of 7% to 30%, including Lyme disease, Crimean-Congo hemorrhagic fever (CCHF), and a grab bag of rickettsial diseases that constitute an entire group of uncomfortable possibilities.6-8 Except for CCHF, all these zoonoses require a tick bite for transmission. CCHF can be transmitted via tick bite or direct contact with the blood of an infected mammal. Named for the Connecticut town where it was initially identified, Lyme disease is caused by Borrelia burgdorferi. A handful of states account for 95% of reported cases in the United States. The arachnids of interest are Ixodes scapularis (black-legged or deer tick) and Ixodes pacificus (Western black-legged tick). Although other types, like Dermancentor variabilis (American dog tick), and some insects have been documented as carrying B burgdorferi, transmission of Lyme disease through them has not been proven. Most infected dogs and cats are asymptomatic, but those that manifest symptoms almost always present with arthritis.

CCHF is not a disease many Americans are familiar with, but it merits a mention as it can be found in humans, birds, ticks, domestic animals, rodents—and mosquitoes. Caused by Nairovirus, CCHF is found in Africa, Eastern Europe (particularly the former Soviet Union), Southern Europe, the Mediterranean, the Middle East, northwestern China, central Asia, and the Indian subcontinent. Ixodid ticks of the genus Hyalomma, including Amblyomma variegatum, Boophilus decoloratus, and Rhipicephalus sp are vectors for CCHF, and animals such as cattle, goats, sheep, hares, and hedgehogs serve as amplifying hosts. Although many birds are resistant to infection, ostrich
are susceptible. Clinical presentation in humans begins with nonspecific symptoms like headaches and joint pain, but by the fourth day it has typically progressed to more advanced illness with uncontrolled bleeding.9 Only humans and newborn mice succumb to CCHF.10


Although MRSA does not typically colonize animals (ie, animals are not routine carriers), it does have a 30% mortality rate in humans.11 Because animals can have MRSA infections, contact exposure is possible. But why doesn’t MRSA colonize animals? Because it is a human-adapted, gram-positive bacteria found in our skin and nasal passages. MRSA colonization in cats and dogs normally ranges from 0% to 4%; however, rates in specific populations can be higher (7%-9%). The primary risk factors for MRSA colonization in pets are contact with MRSA-infected humans, repeated courses of antibiotics, going to the veterinarian, having surgery, or being hospitalized for several days.12 Veterinary personnel have a higher risk (4%-18%) of MRSA colonization than the general population (1%-3%).13

So, wash your hands thoroughly after touching that super-disgusting, so-ugly-it’s-cute pet. Staphylococcus pseudintermedius is the pet’s answer to MRSA. S pseudintermedius is a host-adapted commensal organism in dogs and cats that can be resistant to methicillin. However, colonization of humans by methicillin-resistant S pseudintermedius (MRSP) is typically only transient, and human MRSP infections are rare.14

2. Parasitic pathogens

At No. 2 on the list are Baylisascaris procyonis (the raccoon roundworm) and Toxoplasma gondii. These bad bugs have a mortality rate of up to 40% in humans.15

The primary definitive host of B procyonis is the raccoon. In the United States, the infection rate for adult raccoons is high in the northeastern, midwestern and West Coast regions, varying from 66% to more than 90%. Thus, it may be prudent to presume that all are infected.16 B procyonis has the typical roundworm life cycle. A couple of things to remember are that roundworm eggs take 2 to 4 weeks to become infected once passed out in raccoon feces17 and that dogs don’t just ingest fecal matter, they roll in it. Because B procyonis eggs are quite sticky, they can get stuck in pet fur, enter the house at night, and jump into bed with you.18

On the farm where I grew up, T gondii was known as the parasite “that pregnant women get from cats.” Today, toxoplasmosis is a leading cause of foodborne-related death in the United States, but is nevertheless considered a “neglected parasitic disease.”19 Cats are the only definitive host for T gondii. Because any felid species can serve as host, if T gondii is present in an area, it means that cats also are, or were, present there.

The parasite’s life cycle is a bit complicated. Initially, large numbers of unsporulated oocysts are shed in the cat’s feces, usually for 1 to 3 weeks. Oocysts take 1 to 5 days to sporulate and become infectious. Intermediate hosts, including birds and rodents, become infected after ingesting soil, water, or plant material contaminated with sporulated oocysts. Once in the intermediate host, oocysts become tachyzoites and localize in neural and muscle tissue, developing into cysts called bradyzoites. Cats typically become infected after consuming sporulated oocysts or intermediate hosts that harbor bradyzoites. Food animals like pigs and cattle may also become infected by ingesting sporulated oocysts.

Humans can become infected by eating under-cooked meat, coming into contact with cat feces (changing the litter box), consuming contaminated food or water, handling contaminated soil, receiving a blood transfusion or organ transplant, and by way of the placenta. In humans, the parasite most commonly forms bradyzoites in skeletal muscle, the myocardium, brain, and eyes, and they may remain present throughout the individual’s life. In the United States, 11% of the population over age 6 years has been infected with T gondii. Many individuals are infected because they let their cats go outside or because the produce they eat hasn’t been properly washed beforehand.19

1. Avian influenze A virusus

Highly pathogenic A (H5N1) avian influenza has a mortality rate of 53% to 56% in humans.20 Dabbling ducks are the origin of all influenzas and thus all influenzas are zoonotic.21 Since the early 2000s when canine flu emerged, veterinarians have joined the frontlines of influenza detection. Although no cases of dog-to-human transmission have been documented, cat-to-human transmission has occurred in close-contact, high-exposure environments like shelters.20 Influenza’s superpower is its ability to mutate because any mutation can increase its transmissibility. In fact, flu mutation merits its own vocabulary. Antigenic drift is the small mutation that causes seasonal flu changes, and antigenic reassortment the larger one that can produce flu of pandemic proportions.20 A flu can drift from bird to bird or season to season and then reassort to cause illness in a new species including humans. Finally, random mutation of a recently reassorted avian virus can make it readily transmissible among the new host species. Many bird species have been confirmed to have avian influenza, including flamingos, ostriches, pelicans, parrots, and macaws.


With such deadly zoonoses potentially sneaking into clinics, how can veterinarians and their staff stay healthy and avoid taking home unwanted bugs? These practical (and potentially obvious) tips have to do with the fact that most transmissions involve putting something disgusting in your mouth. So, to stay safe at work:

  • Close your mouth.
  • Wash your hands, thoroughly and often.
  • Keep food where it should be and not where it shouldn’t.
  • Use an appropriate disinfectant.
  • If you don’t already have one, get an infection control plan for your practice. You can find a template on the website of the National Association of State Public Health Veterinarians.

Jenifer Chatfield, DVM, DACZM, DACVPM, is staff veterinarian at 4J Conservation Center in Dade City, Florida, an instructor for the Federal Emergency Management Association and Department of Homeland Security courses, and a regional commander for the National Disaster Medicine System.


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  2. Fowler HN, Holzbauer SM, Smith KE, Scheftel JM. Survey of occupational hazards in Minnesota veterinary practices in 2012. J Am Vet Med Assoc. 2016;248(2):207-218. doi:10.2460/javma.248.2.207
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