Fort Collins' "Tentacle Rabbits" Explained

In early August 2025, residents of Fort Collins began reporting an unsettling sight: wild rabbits with what appeared to be black tentacles or horns sprouting from their heads. The phenomenon quickly went viral, sparking both fascination and concern as images of these seemingly "monstrous" creatures spread across social media. While the growths may look like something from a horror movie, they're actually caused by a well-documented virus that poses no threat to humans or pets.

The culprit behind these dramatic transformations is the Shope papilloma virus (SPV), also known as cottontail rabbit papillomavirus (CRPV). First discovered in 1933 by researcher Richard E. Shope, this virus holds the distinction of being the first mammalian virus proven to cause cancer. The virus creates keratinous growths—essentially hardened masses of skin tissue—that can appear as black, horn-like protrusions extending from a rabbit's head, face, and ears. These growths are made of the same material as fingernails and hair, which explains their firm, horn-like appearance.

The Fort Collins outbreak unfolds

The current outbreak in Fort Collins gained widespread attention when local resident Susan Mansfield reported observing an infected rabbit in southeast Fort Collins for two consecutive years. "It looks like it was black quills or black toothpicks sticking out all around his or her mouth," Mansfield told 9NEWS reporter Amanda Gilbert. She noted with surprise that the rabbit survived the winter and returned with even larger growths the following year.

The story broke into mainstream media on August 9, 2025, when 9NEWS published their initial report. By August 10, their tweet describing "bunnies with what appear to be tentacles growing from their faces" had gone viral, prompting national coverage from outlets including the Denver Post, Fox Weather, CNN, and Gizmodo. Colorado Parks and Wildlife spokesperson Kara Van Hoose confirmed that the agency received "a handful of reports" following the media coverage, with most sightings concentrated within a 10-mile radius of Fort Collins, particularly in the southeastern areas near the Cache la Poudre River corridor.

Understanding the science behind the "horns"

The Shope papilloma virus belongs to the Papillomaviridae family, specifically classified as Kappapapillomavirus 2. When the virus infects a rabbit, it enters through small breaks in the skin, typically facilitated by biting insects like mosquitoes, ticks, and fleas. The virus hijacks the normal cell cycle, causing infected epithelial cells to proliferate uncontrollably and produce excessive amounts of keratin. This disrupted keratinization process creates the characteristic horn-like growths that can reach 0.5 to 2 centimeters in length.

The growths develop their distinctive black or greyish color due to abundant melanin pigmentation. Under microscopic examination, they reveal a complex architecture of elongated tissue projections surrounding central cores of connective tissue. Multiple growth centers can develop simultaneously, causing the surrounding tissue to bulge outward and create the tentacle-like appearance that has captured public attention. While alarming in appearance, the growths themselves are generally painless for the rabbits unless they interfere with essential functions like eating or vision.

Health impacts on infected rabbits vary widely

For most wild cottontail rabbits, the infection follows a relatively benign course. Approximately 95% of infected cottontails experience only benign papillomas that often regress naturally within months to a year. The rabbit's immune system, particularly CD8+ T-cells, can mount an effective response that leads to complete resolution of the growths. This natural recovery process explains why many infected rabbits, like the one Mansfield observed, can survive multiple seasons despite their dramatic appearance.

However, the virus can have more serious consequences in some cases. About 25% of infections in wild cottontail rabbits progress to squamous cell carcinoma, a form of skin cancer that can metastasize to internal organs. Domestic rabbits face even greater risk, with malignancy rates reaching 75% in experimental studies. Large growths around the mouth or eyes can physically prevent rabbits from eating or seeing, leading to starvation or predation. Colorado State University veterinarian Dr. Emily Carter described the Fort Collins situation as "truly unprecedented" in terms of public attention, though the biological phenomenon itself is well-documented.

No danger to humans or pets

Despite the alarming appearance of infected rabbits, the Shope papilloma virus poses absolutely no risk to humans. The virus is strictly species-specific to rabbits and other leporids (members of the rabbit family). Nearly a century of research since Shope's initial discovery has produced zero documented cases of human transmission. This host specificity is characteristic of papillomaviruses—each species has its own distinct strains that cannot cross species barriers.

Dogs and cats are similarly safe from infection. Colorado Parks and Wildlife's Kara Van Hoose emphasized this point to concerned Fort Collins residents: "While it looks alarming, the virus does not harm people or pets." The only domestic animals at risk are pet rabbits, which can contract the virus through the same insect vectors that infect wild rabbits. Pet rabbit owners should keep their animals indoors during summer and fall months when mosquito and tick populations peak, and seek veterinary care if growths appear.

Transmission occurs primarily through insects

The virus spreads between rabbits primarily through arthropod vectors rather than direct contact. Mosquitoes serve as the most common transmission vector, followed by ticks (especially continental rabbit ticks), fleas, and other biting insects. These insects pick up viral particles when feeding on infected rabbits and transmit them to healthy animals through subsequent bites. This explains the seasonal pattern of infections, with peak transmission occurring during warmer months when insect populations are most active.

The virus can also spread through direct contact between rabbits, particularly through abraded skin or existing wounds. Environmental contamination plays a minor role, as the virus demonstrates remarkable resistance to desiccation and can survive on bedding, food bowls, and other surfaces. However, the arthropod-mediated route remains the primary concern for transmission, which is why indoor housing provides effective protection for domestic rabbits.

Historical significance and the jackalope connection

The phenomenon of "horned rabbits" has deep historical roots extending far beyond modern scientific understanding. The virus is widely believed to be the real-world inspiration for the mythical jackalope, the antlered rabbit that has become an icon of American West folklore. In 1932, just one year before Shope's scientific discovery, Douglas and Ralph Herrick of Wyoming created the first taxidermied "jackalope" by mounting deer antlers on a jackrabbit carcass, inadvertently commercializing what nature had already created through viral infection.

Historical documentation reveals even older connections. The University of Kansas Natural History Museum confirmed a case from 1915, predating Shope's discovery by 18 years. Medieval texts from 13th century Persia described horned rabbits, and European Renaissance paintings depict rabbits with horn-like protrusions, suggesting humans have observed this phenomenon for centuries. The virus's scientific importance extends beyond folklore—Shope's research established the field of viral oncology and directly contributed to the development of human papillomavirus vaccines that prevent cervical cancer.

Expert responses emphasize calm observation

Wildlife officials have responded to the Fort Collins outbreak with measured reassurance rather than alarm. Colorado Parks and Wildlife has adopted a non-intervention approach, recognizing that the virus represents a natural phenomenon that typically resolves without human interference. "We don't recommend euthanizing rabbits with papillomas unless they are interfering with the rabbit's ability to eat and drink," the agency stated. This hands-off management strategy reflects decades of experience showing that wild rabbit populations can effectively manage the virus through natural immune responses.

Dr. Anna Meredith, a veterinary expert writing in Veterinary Clinics of North America, notes that manual removal of growths results in healing for wild rabbits, though the higher malignancy rates in domestic rabbits require more aggressive veterinary intervention. Julie Lindstrom, Animal Control Supervisor in Sioux Falls, provides additional perspective: "We receive a few reports of this every year. We haven't seen any drastic increases or trends in the number of rabbits with the disease." This suggests the Fort Collins outbreak, while garnering unusual media attention, falls within normal patterns of viral activity.

Geographic distribution reveals widespread presence

While Fort Collins has captured recent headlines, the Shope papilloma virus exists throughout much of North America. The virus is most common in the Midwestern United States, with documented cases from Minnesota and North Dakota south to Texas and west to Colorado. Historical records show infections in Kansas, Iowa, Nebraska, Nevada, New Mexico, Washington State, and even Jalisco, Mexico. Museum studies indicate approximately 1.8% of wild Sylvilagus (cottontail) specimens show evidence of infection, with Eastern cottontails (Sylvilagus floridanus) displaying the highest infection rate at 2.8%.

The Fort Collins outbreak appears concentrated in the southeastern part of the city, particularly near the Cache la Poudre River corridor. This localized pattern is typical of SPV outbreaks, which often cluster in areas with suitable habitat and high rabbit populations. The virus's presence in Colorado is not new—the state falls within the virus's established range—but the concentration of visible cases in an urban area has brought unprecedented public attention to a phenomenon wildlife biologists have long monitored.

What residents should do when encountering affected rabbits

If you encounter a rabbit with horn-like growths in Fort Collins or elsewhere, the primary recommendation from wildlife officials is simple: leave it alone. Colorado Parks and Wildlife advises maintaining the same distance you would from any wild animal, not because of disease risk but as a matter of general wildlife safety. The rabbits, despite their unusual appearance, are simply dealing with a viral infection and should be allowed to continue their normal activities without human interference.

Residents who observe infected rabbits are encouraged to report sightings to Colorado Parks and Wildlife, though this is for monitoring purposes rather than emergency response. The agency uses these reports to track the geographic spread and prevalence of the virus, contributing to broader understanding of wildlife disease patterns. Photography from a safe distance is acceptable and can help document the outbreak, but residents should never attempt to capture, handle, or "rescue" affected rabbits. If you find a rabbit that appears unable to eat or move due to growths, contact local wildlife rehabilitation centers rather than attempting intervention yourself. Most importantly, understand that these rabbits, while visually striking, represent a natural phenomenon that has occurred for centuries and will likely resolve as cooler weather reduces insect vector populations in the coming months.