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Update Aug. 25 3:32 p.m. EDT:

Tests performed by a state lab in Michigan confirmed that an illness that has killed more than 20 dogs is canine parvovirus.

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State officials told The Associated Press that while the dogs had “consistently” tested negative for the virus, the tests the state performed in its lab were more sensitive.

The dogs that were killed by the virus had not been fully vaccinated against the disease, prompting veterinarians to urge dog owners across Michigan to ensure their pets are up to date on their shots.

Original Story:

More than 20 dogs have died after contracting an illness, but doctors are not sure what exactly the sickness is.

The New York Times reported that the dogs died in Michigan and had symptoms including vomiting and diarrhea.

Most of the dogs that died were under 2 years old. Some were elderly. But this illness is not affecting some breeds more than others.

Many of the dogs died within three days, NBC News reported.

Veterinarians are guessing that the illness is a new strain of parvovirus that affects puppies, the Times reported.

There has been some evidence pointing to that disease, which is contagious — passing from dog to dog directly or when an animal comes in contact with contaminated feces or environments — but when tests were performed, they came back negative, Melissa FitzGerald, the director of the Otsego County Animal Shelter, said on Facebook.

But when a necropsy was done on some of the dogs that died, the tests came back positive for parvovirus, USA Today reported.

“We have not spoken to this until now because we really don’t know anything,” she said on Facebook. “The only thing is to make sure your pets are vaccinated and, at the first sign of illness, get to the veterinarian.”

Parvovirus was first discovered in Europe in the mid-1970s but became less of a threat because of vaccines, NBC News reported.