For the first time in more than 60 years, a dog has tested positive for rabies in Chicago.

The Cook County Department of Animal and Rabies Control in Illinois confirmed the rare case of a rabies-positive dog on Tuesday, Dec. 23. The dog was euthanized on Dec. 19 due to a marked change in behavior and because animals cannot be tested for rabies while they're alive. It's the first rabies-positive dog case in Chicago since 1964 and the first in Illinois since 1994, per the department.

As of Dec. 23, 13 people who were in direct contact with the dog had started post-exposure prophylaxis treatment and had not shown any symptoms, according to a Cook County Department of Animal and Rabies Control news release.

The source of the rabies remains under investigation and the Centers for Disease Control and Prevention has received the virus strain to help determine its origin.

The dog came to the animal shelter PAWS Chicago in May from a rescue partner in Georgia, according to a PAWS Chicago news release. Upon arrival, the dog received his vaccinations, including for rabies, in June. He was then adopted out in late July.

On Dec. 17, the dog was returned to PAWS Chicago because it bit someone at home and displayed "new onset behavioral changes and safety concerns," according to the shelter. After a behavioral assessment, shelter veterinarians decided to euthanize the dog. The positive rabies test came back on Dec. 19.

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What is rabies?

Rabies is a dangerous, but preventable, viral disease that mostly affects the central nervous system, leading to brain disease and death, if medical care is not received before symptoms start, according to the CDC.

The time between rabies exposure and the appearance of symptoms can last for weeks or even months.

What are the symptoms of rabies in a dog?

In dogs, rabies cycles through two main stages. During the first prodromal phase, a dog undergoes a change in temperament, according to VCA Animal Hospitals. For example, a quiet dog may become agitated and an active dog may become nervous.

Next, the dog may experience furious rabies, which is when the animal becomes aggressive and displays depraved appetite, eating things such as stones or trash, according to VCA Animal Hospitals. The dog then dies in a violent seizure.

Or the dog may experience dumb rabies, which is more common. Progressive paralysis affects the dog's limbs and face, making it difficult for the animal to swallow. The dog enters a coma and dies.

How is rabies transmitted from pets to humans?

Rabies can be transmitted through direct contact between broken skin, mucus membranes and infected tissue or fluids, such as saliva, according to the CDC. The virus cannot be transmitted through contaminated objects, including clothes or bedding.

What are the symptoms of rabies in a human?

In a human, the first symptoms of rabies may exhibit like the flu, including fatigue, fever or headache, according to the CDC. Typically, severe disease appears within two weeks of these symptoms, leading to brain dysfunction. Common signs of this include anxiety, confusion, agitation and hallucinations, the CDC states.

How are humans treated for rabies?

There is no established, effective treatment of rabies, according to the Mayo Clinic. Though a small number of infected people have lived, rabies usually results in death.

If you believe you've been exposed to rabies, you undergo a treatment called rabies post-exposure prophylaxis. This treatment includes a dose of human rabies immune globulin, which contains antibodies against rabies, and five doses of the rabies vaccination.

Greta Cross is a national trending reporter at USA TODAY. Story idea? Email her at [email protected].

This article originally appeared on USA TODAY: Chicago reports first rabies-positive dog since 1964