(NewsNation) — An illness that killed dozens of dogs in Michigan’s northern Lower Peninsula was found to be canine parvovirus, according to state officials.

NewsNation local affiliate WOOD reported that 60 dogs have died because of the illness — a number that doubled in just days. Dogs who got sick were not fully vaccinated, WOOD said.

At first, the exact sickness affecting the dogs went unidentified. Officials said the symptoms were akin to those seen in parvovirus, such as bloody diarrhea, vomiting, lethargy and loss of appetite, but when the canines were taken to a veterinarian, their parvo test came back negative.

“Screening tests for parvo are done to help guide immediate isolation, disinfection, and treatment protocols,” Kim Dodd, Michigan State University Veterinary Diagnostic Laboratory director said in a news release, adding that the situation is “complex.” “While those tests are valuable in the clinical setting, they are not as sensitive as the diagnostic tests we can perform here in the laboratory. We continue to further characterize the virus in hopes of better understanding why those animals were testing negative on screening tests.”

Testing that confirmed the canine parvovirus infections was done at Michigan State University’s lab in Lansing and facilitated by the Michigan Department of Agriculture and Rural Development.

Canine parvovirus is a highly contagious viral disease. It causes acute gastrointestinal illness in dogs, according to the Baker Institute for Animal Health. Although it most often affects puppies between six and 20 weeks old, elderly animals are also sometimes affected.

The Michigan Department of Agriculture and Rural Development said pet owners should make sure their dogs are fully vaccinated.

“We have a highly effective vaccine available to help protect dogs from the virus. Dogs that are not fully vaccinated against this virus are the most at risk,” state veterinarian Nora Wineland, DVM, said in the release.

Health experts said dogs and puppies should be kept away from other animals until they are fully vaccinated or show any signs of illness.

“When you’re taking your dog for a walk, maybe don’t go pet everybody’s pet at this point,” Dr. Erin Schroeder of the show “Heartland Docs, DVM” on National Geographic told NewsNation. “Because you don’t want to be a vector, you don’t want to be an inadvertent contaminant.”

The Associated Press contributed to this report.