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Image from Dr. F.A. Murphy, University of Texas Medical Branch, Galveston, Texas

This negatively stained and colorized micrograph of the rabies virus from an infected cell culture shows the characteristic bullet-shaped virus particles. The magnification is approximately 70,000x.

For more than 60 years, Chicago logged zero cases of rabid dogs. Then, in December, a young dog that had been acting anxious and aggressive was euthanized under suspicion of rabies. Four days later, the Illinois Department of Public Health publicly confirmed the diagnosis.

The dog, which was less than a year old, had been vaccinated against rabies in June. It began showing clinical signs of the disease six months later — an especially long incubation period that has veterinarians and public health experts wondering when and how the pet was infected. Experts may never be able to untangle the case, but new details about the dog's past have emerged after inconsistent facts were given in early reports.

Though cases of rabid pets are rare in the United States, the deadly viral infection seems to be on the rise in wildlife in several parts of the country, including areas of New York, North Carolina, South Carolina, Vermont and Oregon. By late last summer, the U.S. Centers for Disease Control and Prevention was monitoring 15 rabies outbreaks, some in places that had seen very few cases or been rabies-free in previous years.

It may be, however, that the disease isn't more prevalent but that cases are simply better documented. Similarly, whether the Chicago case is an anomaly or a true cause for concern is an open question. In any case, some veterinarians say that the increase in rabies-related news is a valuable reminder that the disease, though historically controlled through strong vaccination and tracking efforts, is still a present and fatal threat.

"Everyone in this country, except the lucky folks in Hawaii, absolutely has rabies spreading in their community," said Dr. Ryan Wallace, a veterinary epidemiologist and head of the CDC's rabies surveillance program. "Every year, about 100,000 Americans have to get vaccinated because of a potential rabies exposure … and we have tens of thousands of dogs euthanized for rabies testing, the majority of which are unvaccinated."

Clinical signs and vectors

Rabies is a communicable disease that spreads through the saliva of infected mammals. Once transmitted — via a bite, scratch or other form of direct contact — the virus travels through the body's central nervous system until it reaches the brain, where it multiplies, attacks neurons and causes inflammation. It's fatal if untreated before clinical signs appear, which usually takes 21 to 80 days in dogs and 28 to 42 days in cats. The incubation period for rabies in humans is typically 60 to 90 days.

Signs of rabies can manifest in three phases: prodromal, furious and paralytic. During the prodromal stage, an animal changes its usual temperament, acting agitated if normally friendly or more affectionate if normally aloof. After a few days, its signs may become furious: The animal is aggressive, restless, extremely reactive and prone to gnawing on and eating foreign objects. It grows more disoriented and, after one to seven days, is likely to die during a seizure.

Afflicted animals can also experience a third, paralytic phase of rabies (or only this phase and never the furious). The animal loses motor control and has difficulty swallowing. In two to four days, full paralysis sets in, and the animal becomes comatose and dies.

The rabies virus is found across North America primarily in bats, raccoons, skunks and foxes, and comes in a number of strains, depending on its vector and geographic location. Bats are the most documented rabies carriers in the U.S. because of their wide geographic range and ability to host and slow the virus during torpor (a hibernation-like state of physiological shutdown). They also tend to be caught by cats, which are the most commonly documented rabid domestic animal in the country. Reports of rabies in feral cats seem to be on the rise, too, with recent cases in areas of Alabama, Florida, South Carolina and Virginia.

"Rabies will infect and kill any mammal, but bats and these other hosts have just the right combination of genetics and ecology that allow them to live just a little bit longer to spread the virus and maintain it in their populations," Wallace said. "What's great, though, is that the vaccine works against all of [the strains]."

Mandated for dogs and cats by most states (Hawaii being a notable exception), the rabies vaccine is extremely effective at preventing the disease in pets, Wallace said. Of the 7,056 rabid dogs and cats documented by the CDC between 2002 and 2022, only 15 (0.2%) had a history of vaccination and are considered "vaccine failures," according to a study he co-authored in the Journal of the American Veterinary Medical Association.

The exact requirements vary from state to state, but most require that dogs and cats (and sometimes domestic ferrets) be vaccinated against rabies at 3 to 4 months of age.

A few states, such as Massachusetts and Delaware, allow owners to wait until their pets are as old as 6 months to be vaccinated. But Dr. Steve Valeika, a small animal practitioner and veterinary epidemiologist in Asheville, North Carolina, said that protecting pets against rabies is always better earlier rather than later.

"All of the vaccines are safe and licensed for 12 weeks or older," he said. "Six months is pretty late. I mean, that's peak socialization time for your puppy. That's after you should be taking it out into the woods for the first time."

A mystery case in Chicago

When the Chicago dog tested positive for rabies six months after it was adequately vaccinated at an estimated 3 months old, it puzzled health officials and put them on high alert for other potential cases.

The dog's past is a bit muddy, according to Dr. Janna Kerins, a veterinarian and medical director of the communicable disease program at the Chicago Department of Public Health. She and her team initially thought the dog was from a litter of puppies in Florida but now believe it was picked up as a stray in Georgia and sent to Chicago via Alabama. The animal rescue PAWS Chicago received the dog on May 26 and vaccinated it the following month against rabies and other diseases. The dog was adopted by a family in Chicago on July 26.

During its five months with the family, the dog displayed resource guarding — menacingly protecting its food and other items — and was regularly seeing a veterinarian for behavioral issues. The Illinois Department of Public Health also reported that on one occasion, the dog bit its owner. But Kerins said that the bite wasn't an act of aggression but a mishap that occurred when the dog launched for a toy.

Growing only more reactive and unpredictable, the dog was returned to PAWS Chicago on Dec. 17. It was euthanized the following day due to "escalating behavior risk and an assessment by both PAWS veterinarians and [its] animal behavior team," the rescue said in a statement. Once the dog was confirmed rabies positive, alerts were sent to the owners of pets and other people with whom the dog had been in contact, including employees of a Chicago pet boarding and day care business where the dog had stayed. No additional rabies cases have been reported so far.

Animals are considered protected against rabies 28 days after being vaccinated, and that's a generous window, according to Dr. Scott Weese, an infectious diseases veterinarian at the University of Guelph's Ontario Veterinary College. He noted that most animals likely have a fairly good amount of antibodies after 10 to 14 days. After one year, pets should receive a booster shot, followed by boosters every one to three years over their lifetimes.

On very rare occasions, an animal's immune system doesn't respond to the vaccine adequately.

"There's always a chance that [the Chicago dog] was exposed to rabies during those 28 days," said Valeika, the epidemiologist in North Carolina. "But if it happened after, it's likely that the dog had what we call a breakthrough infection…. It's a very effective vaccine, but nothing is 100%."

Weese said that it's also possible that the dog was infected prior to vaccination, and long incubation periods can happen. "It's more typical for the canine rabies strain, which isn't in Canada or the U.S.," he said. "But there have occasionally been some cases where pets go six, seven months without showing symptoms."

Veterinarian attacked by rabid raccoon in driveway

Identifying the strain of rabies that infected an animal is one way to trace where it may have been exposed, since some strains exist in only certain parts of the country. The Chicago dog's strain test, however, came back inconclusive, so it's unlikely that health officials will be able to determine where the dog picked up the disease and from what vector.

The only way to test for rabies and its exact strain in an animal is by examining and analyzing its brain tissue after death. If the virus isn't yet overwhelmingly present in the brain, or if the brain has been physically damaged in any way, it can be difficult to effectively sequence the viral RNA and match it with known strains. That's why, when veterinary behaviorist Dr. Christine Calder was attacked by a rabid raccoon outside of her home in southern Maine this month, she instructed her husband to kill the animal and preserve its head.

"The thing ran from the woods and launched itself onto my leg, and I started screaming and kicking at it," Calder said. "Right from the beginning, I thought: rabies."

Pants ripped from the knee down and a large gash in her leg, Calder and her family rushed to the emergency room, leaving the bagged-up raccoon outside the house to be retrieved by the local game warden. An epidemiologist from the CDC rang a few days later to report that the raccoon was indeed rabies positive.

With vast stretches of wildlife habitat, Maine is known for being home to a variety of rabies carriers. It's one of several places included in the U.S. Department of Agriculture's oral rabies vaccination program, which focuses on scattering edible, fish-flavored baits containing a rabies vaccine in rural and wild areas.

Oral vaccinations have been a game-changing method of curbing the spread of rabies among wildlife, the CDC's Wallace said. The tactic was the main driver behind the elimination of a Texas-based coyote-specific strain in 2008, and today it helps keep active strains contained within their current populations.

USDAPhoto_Wildlife-Services

Photo by Anson Eaglin, USDA APHIS

Wildlife Services, a program of the U.S. Department of Agriculture's Animal and Plant Inspection Service, has worked since 1995 to reduce wildlife rabies by distributing oral vaccines in targeted areas. Robert Fey, a Wildlife Services rabies biologist, is shown taking a tissue sample from an anesthetized raccoon in 2006. The sample would have been tested to determine whether the animal ingested enough rabies vaccine to be protected.

Wildlife outbreaks

Why, then, has the CDC been reporting more wildlife rabies outbreaks than usual? Wallace said that it has a lot to do with the organization's new and faster way of collecting information from states about rabies testing.

"Up until 2022, and even some of 2023, we were literally still receiving Excel files and faxes from all of our state health departments once a year about anything rabies-related," he said. "We have officially modernized that system, and now, as soon as most state laboratories conduct a rabies test, we get a message about it and have the results within days."

States are required to review and validate data reports, so public announcements about rabies cases aren't quite as instantaneous. Still, the time required to identify and analyze disease trends and changes has dropped from 18 months to 90 days or fewer, Wallace said.

The CDC's rabies communication upgrade doesn't mean that wildlife rabies cases aren't possibly on the rise. As many urban and suburban communities develop and encroach on wild spaces, human-animal interactions become more common — and so does their documentation. Fast-growing Gaston County, in central North Carolina, for instance, reported 21 rabies cases in 2025, after only seven in 2024.

"We see this kind of thing all the time with increased numbers of cases of Lyme disease, West Nile virus and even plague exposure in the Southwest," said Dr. Radford Davis, a veterinarian and public health professor at Iowa State University College of Veterinary Medicine. "Plus, public health programs everywhere are seeing cuts. Rabies tends to get mediocre funding around the world.… I wouldn't be surprised if some states and local areas are feeling strained."

Despite the disease's prevalence in Maine, Calder said that before her run-in with the rabid raccoon, she didn't think much about rabies. She and her family revel in long walks with their four dogs in the woods, and they regularly encounter all kinds of wildlife. As a veterinarian, she was already vaccinated against rabies and needed only a couple of booster shots after the incident. Her sons and husband needed the full four-shot series and are fully protected now, too.

Still, she can't help but want a little extra protection.

"I go out there at night now like a little kid — with a flashlight," she said. "I might even start carrying a stick."

Editor's note: Drs. Davis, Weese and Valeika are consultants for the Veterinary Information Network, an online community for the profession and the parent of the VIN News Service. Dr. Calder is the director of behavior education content at VIN.