Animal samples tested for parvovirus at the Michigan State University Veterinary Diagnostic Laboratory have come back positive, said Kim Dodd, the lab’s director.

“We know we’re looking at a canine parvovirus,” she said Tuesday.

The Michigan Department of Agriculture and Rural Development confirmed Wednesday that the illness afflicting dogs was parvovirus and said the affected canines did not have a history of complete vaccination.

The concern was caused by reports this month that dogs were dying within three days of developing parvovirus symptoms, including vomiting and bloody stool, but testing negative for the condition. Melissa FitzGerald, director of Otsego County Animal Shelter in northern Michigan, said most dogs with the illness seen by the shelter were younger than 2 or were elderly, and cases have been found in the northern and central parts of the state.

As of Tuesday, the Michigan agriculture department had received anecdotal reports of 15 to 25 infections but did not have an exact count because residents are not required to report the disease to the state. FitzGerald said she was aware of at least 20 deaths caused by the virus in her county.

The discrepancy between dogs testing negative at veterinary clinics or animal shelters but positive at the university lab may be due to the kind of test used, Dodd said. The lab uses highly accurate PCR tests, while shelters and clinics tend to use less-sensitive rapid tests.

Rapid tests are particularly prone to false negatives late in an infection, which is when many of the animals were tested, Dodd said. But she said veterinary officials would continue to test the samples to learn if anything specific about this strain of the virus could be making the rapid tests less effective.

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Scott Weese, a professor at Ontario Veterinary College, said some negative tests may also happen because not every dog has the same illness. But he said he suspects that the outbreak as a whole is due to a known parvovirus strain.

“Most of the time when we see situations like this, it’s just our normal suspects doing their normal things or doing something a little different,” he said. “But it’s not usually something completely new.”

Canine parvovirus is highly contagious among dogs and can be deadly. But there are several effective vaccines that veterinarians typically administer, beginning when a puppy is a few weeks old. Dodd said the dogs that have tested positive at the lab so far have lacked a clear parvovirus vaccination history, including regular boosters.

State officials urged dog owners to keep up with vaccinations and make sure puppies are fully immunized before they interact with other animals. Dogs’ hair and feet can transmit the virus, according to the American Veterinary Medical Association.

Owners should also clean up after their pets on walks — contaminated feces can spread illness — and keep dogs home and contact their veterinarian if the canines show any signs of illness, officials said. Canine parvovirus is not contagious to other animals or to people.

Whether they live in or outside of Michigan, Dodd said, dog owners should take this outbreak as a reminder to keep their dogs up to date on their immunizations.

“We do not want people to panic over this,” she said. “Parvovirus is something that we see on a regular basis in unvaccinated dogs.”