How to put pet-related zoonotic risk into perspective

Assessing risk requires a combination of caution, science and common sense

December 6, 2018 (published)
By Andrew Hennenfent

Dr. Andrew Hennenfent

When we were taught the "zebras" in veterinary school, our professors often stressed to us that common things happen commonly. In other words, they guarded us against arriving at an exotic medical diagnosis when commonplace explanations were more likely.

What does this mean when interpreting zoonotic disease risks?

Given that many of the pathogens found in domestic animals can play a role in human infection, the media often focuses on extreme cases with the most severe outcomes. This was displayed in recent reports of Capnocytophaga canimorsus infections, which have resulted in severe disfigurements of humans, or even death. When these extreme cases make headlines, it sounds like owning a dog can mean substantially risking your health.

However, the chances of contracting a severe infection from pets are quite small for most owners.

Consider the pathogen best-known for causing cat scratch disease, the bacterium Bartonella henselae. People typically develop cat scratch disease after a cat scratch or bite. The condition is characterized by swelling of the lymph nodes, along with fever, headache, fatigue and/or poor appetite. 

Photo courtesy of Dr. Andrew Hennenfent

The U.S. Centers for Disease Control and Prevention states that 40 percent of all cats carry B. henselae in their bloodstreams during some point in their lives. Does this mean that four out of every 10 cats will transmit B. henselae to the people they live with? No, because determining the risk of infection is more complex than that. To illustrate this, consider the following points and the cats depicted in the image to the right: 

  • Cats younger than one year of age are more likely to carry B. henselae than adult cats. Are any of the cats in the image kittens? No, so the risk has decreased.
  • Cats pick up B. henselae from fleas and fighting with other cats that carry the bacterium. Let's assume all the cats in the photo live indoors and are on flea preventatives. This also decreases their risk of carrying the bacteria.
  • B. henselae typically is spread from cats to people when an infected cat bites or scratches them. Does this mean every person bitten or scratched by a cat is exposed? No, because not all cats carry B. henselae. Does this mean every person exposed to the bacterium will develop cat scratch disease? Also, no. Not everyone exposed to a disease-causing agent will become sick. Different infectious agents have different levels of infectivity. A person's immune status also is an important determinant; healthy people are less likely to become ill than those with immune-system deficiencies.
  • Furthermore, while the CDC warns that B. henselae can be spread when infected cats lick a person's scab or wound, that only means transmission is possible — not probable. It does not mean it is as effective a means of transmission as a bite or scratch.

All these things factor in the epidemiological triad, the standard, triangular model used by scientists to study infectious diseases and understand how they spread. At one corner of the epidemiological triad is the host — in this case, the human or cat. In another corner is the agent — in this case, the pathogen B. henselae. The environment, or where the cat lives, is at the triad's third corner. 

Each of the three corners interacts with the others. 

Consider the photo of cats in relation to the epidemiological triad. If all the cats are kittens, not on flea and tick prevention, and live outdoors, then it is far more likely that four of the 10 cats (or even more) currently are carrying B. henselae. But when we factor variable age, environment and health status, the odds that they're carrying the bacterium are much, much lower.  

So, if 40 percent of cats carry B. henselae at some point in their lives, do four of the cats depicted carry the bacterium? Maybe. Do four out of 10 of them currently carry it? Most likely, no.

When thinking of risk and evaluating patients, the simplest explanation is usually the correct one. To phrase that message another way, common things happen commonly. 

Most people who are bitten or scratched by a cat do not develop cat scratch disease; it is an uncommon condition. This doesn't mean you shouldn't advise clients to follow zoonotic disease prevention recommendations around cats, or suggest immunocompromised clients take extra precautions.

But that risk should always be put into perspective and not used as a reason to entirely avoid animals.

About the author: Andrew K. Hennenfent, DVM, MPH, earned his veterinary degree in 2012 from the University of Illinois at Urbana-Champaign and a masters of public health from the University of Illinois at Chicago in 2014. He completed a CDC/CSTE Applied Epidemiology Fellowship at the District of Columbia Department of Health, where he also served as the state public health veterinarian until November. While there, he led the zoonotic disease and investigation program and published manuscripts on veterinary disease surveillance, Ebola and rabies. Dr. Hennenfent has served as a zoonoses and public health consultant on the Veterinary Information Network since February 2017.

DISCLAIMER: Any views and opinions expressed are solely those of the author's and do not in any way reflect or represent any policy or position of any company, organization, or other entity. 

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