How to Define a CIV Epizootic

Overhearing some colleagues talking, you learn that a new canine influenza outbreak has occurred. You join the conversation. Questions revolve around updating vaccine protocols, informing pet owners, and performing more respiratory PCR tests. As the discussion develops, you realize that this “outbreak” involved only two positive influenza cases in a nearby county. Should you be concerned?

After all, is this even an outbreak? Or is this an epizootic, an epidemic, a local rise in incidence, or just two unfortunate dogs? How do you interpret a few or even one positive influenza case? In comparison, prevalence maps show hundreds of other infection types in your county regularly. Why is influenza so different?

An epizootic is an epidemic for animals.ⁱ The word “outbreak” works for both. These cases would create a rise in incidence and certainly constitute a couple of unlucky pets. For case(s) to qualify as an epidemic, epizootic, or outbreak, the positive-case number only needs to be more than what we expect for that geography.ⁱⁱ For example, 100 people suffering from the common cold in a county is not an outbreak, but a single Ebola virus infection anywhere in the United States is.

Canine influenza does not characterize as a regular pathogen in any geography; therefore, any detection of this virus represents an outbreak. Of course, some circumstances would likely not qualify. For example, if veterinary professionals identified a traveling dog with influenza and contained the infection before any local pets could be exposed, we would not classify this as an outbreak.

The fear behind the word “epizootic” remains that the true number of exposed and infected animals is unknown and could increase. If these numbers magnify across the globe, the epizootic becomes reclassified as a panzootic.ⁱⁱⁱ Thus, for people and pets, influenza has the potential to become either a pandemic or panzootic, respectively.^iv This ability keeps the virus in the news whenever new cases arise.

Other qualities make the canine influenza virus notable. The virus is enveloped and has no chronic carrier state. Since the virus cannot survive in the environment and may only last within the pet for a short time, the virus must pass to another pet quickly to avoid extinction. Despite these obstacles, the stable H3N2 has infected dogs across the United States for years upon years.

In the 1998 movie Fallen, Denzel Washington portrays a policeman searching for a serial killer.^v The antagonist is a demon who transfers from person to person through simple touch. Influenza A works similarly. The virus moves from pet to pet from a simple touch of the nose or through a cough or sneeze. Many pets remain asymptomatic, whereas a few unfortunate animals suffer fatal consequences. This pathogen is constantly on the move and ever-elusive to discovery.

Like Denzel-Washington-character’s quest, our pursuit of this random killer remains challenging. Any influenza-positive test represents a diagnostic conquest because the virus never stays in an animal for long, is easy to transfer, and elicits varying clinical signs. Moreover, unlike the movie’s killer, the influenza virus can transfer to multiple hosts at once. Thus, when we look at an epidemiologic map of positive tests, any single or small cluster of positive tests represents hundreds to potentially thousands of infected dogs.

When we view any epidemiologic map, we must consider many factors. For example, does the pathogen create long-term infection? How easy, inexpensive, and common is testing in your area? Thus, placing two epidemiologic maps of different pathogens side by side may not always be accurately compared.

Are the maps worth less than we thought? Not at all. All test-positivity maps provide insight. The key remains that the insight is unique to that disease, condition, and subsequent medical response. One rule to remember is that most maps underreport the true prevalence and incidence of any canine disease. The level of underreporting varies from pathogen to pathogen. The nature of the canine influenza virus, asymptomatic patients, the testing costs, and testing use drive the visible numbers of infections much lower than reality.

Knowing this, what do we, as veterinary professionals, do with this knowledge? Digging deeper, “Why do we vaccinate?” We vaccinate to protect against diseases that are common and those that are deadly. For example, Bordetella bronchiseptica represents a common respiratory pathogen that is rarely deadly.^vi By vaccinating, we minimize clinical signs and reduce suffering in many individuals.

Canine influenza virus represents a potentially deadly pathogen that is sporadic and unpredictable in incidence. Both Bordetella and influenza transmit through aerosol and direct transmission. Dog parks, boarding, local neighborhoods, grooming, and even veterinary facilities represent common settings for transmission. With similar risk conditions, Bordetella-risk patients often overlap with canine-influenza-virus-risk patients.^vii

Our profession needs to continue to raise pet-owner awareness of the influenza virus and the available vaccine. Through education, we help promote pet wellness, health, protection, and peace of mind. Vaccines have saved more dog, cat, and human lives outside of clean water than any other advancement in civilization or medicine. Be proud of your recommendations and your ability to save lives through vaccines.^viii