Photos and videos of deer with large fleshy tumors and rabbits with alarming “horns” have been circulating online in the US. While these images are unsettling, scientists explain that these are not new phenomena or caused by some sort of “gene-altering” virus.

When old viruses find new opportunities

Experts confirm that “zombie deer” and “horned rabbits” are actually animals afflicted by viral diseases. The condition in deer is called deer cutaneous fibroma, while in rabbits it’s caused by the Shope papilloma virus. Both belong to the papillomavirus family, a group of viruses known to cause benign tumors.

Dr. Michael Osterholm, an infectious disease researcher at the University of Minnesota, says papillomaviruses have existed in nature for centuries. Their recent spread may be due to two main factors: climate change and increasing interaction between wildlife and humans.

Rising global temperatures, along with longer and hotter summers, have created favorable conditions for disease-carrying insects like mosquitoes and ticks to thrive. Experts at the Centers for Disease Control and Prevention (CDC) say these viruses are primarily spread through blood-sucking insects. As insect populations boom, the risk of transmission from one infected animal to many others increases significantly.

Another factor is urban sprawl. As humans encroach on natural habitats, wildlife is forced closer to residential areas. This increases the chances of contact between animals, as well as between animals and humans, creating an ideal environment for disease transmission.

A deer with tumors. Photo: Reddit

A deer with tumors. Photo: Reddit

Can the viruses infect humans?

This is the question most people are concerned about. The answer from experts is: it's highly unlikely.

“Animal papillomaviruses are species-specific,” explains Dr. Matthew Binnicker, Director of the Clinical Virology Laboratory at Mayo Clinic. “They have adapted to infect and replicate inside the cells of a particular species. The virus's structure prevents it from effectively attaching to and entering human cells.”

This means the virus causing tumors in deer cannot be transmitted to humans, dogs, cats, or other livestock. Similarly, the virus causing “horned rabbits” only infects rabbits.

However, scientists remain cautious. Although the possibility of direct infection is low, they cannot rule out the potential for the virus to evolve and mutate in the future. Experts are closely monitoring viruses in wildlife to detect any signs of mutations that could pose a danger to humans.

Potential threats from other diseases

While the “zombie disease” caused by papillomavirus is not an immediate threat, experts warn that environmental changes are facilitating the spread of other diseases, some of which can infect humans.

Dr. Osterholm cites Lyme disease, a tick-borne illness, as an example. He says Lyme disease is appearing in areas where it was previously absent, such as southern Canada, because climate change has expanded the range of ticks.

A more concerning disease is Chronic Wasting Disease (CWD), also known as “zombie deer disease.”

According to the CDC, CWD is a rare, progressive neurological disorder affecting deer, elk, and other animals. Infected deer are called “zombie deer” because they exhibit weight loss, lack of coordination, stumbling, drooling, and a lack of fear of humans.

Scientists and health officials are concerned that CWD could spread to humans, similar to mad cow disease in the UK in the 1990s. In 2022, Canadian experts published a study on mice suggesting a risk of transmission to humans.

A rabbit with “horns”. Photo: Clinton Forry

A rabbit with “horns”. Photo: Clinton Forry

CWD is classified as a prion disease, characterized by the misfolding of certain “prion proteins,” leading to brain damage and other symptoms. Prion diseases are generally rapidly progressive and always fatal, affecting both humans and animals.

Creutzfeldt-Jakob Disease (CJD) and variant Creutzfeldt-Jakob Disease (vCJD or mad cow disease) are types of prion diseases that occur in humans. Mad cow disease is a prime example of a prion disease that can spread from livestock to humans, which some researchers compare to the spreading “zombie deer disease.”

In mad cow disease, cattle typically incubate the disease for 4 to 6 years before showing symptoms, according to the US Food and Drug Administration (FDA). Deer have an incubation period of up to two years. As a result, many infected animals appear normal for an extended time.

It’s important for the public to be informed to avoid unnecessary panic, experts advise. People should not be alarmed by the tumors on deer and rabbits, which are symptoms of benign diseases. These conditions do not harm humans and often disappear on their own. If you encounter sick wildlife, it's best to keep your distance and avoid approaching or touching them.

Hoang Dung (Forbes, Time, Discover Magazine)