A mysterious illness is believed to have killed dozens of dogs in Michigan in recent weeks, sending veterinarians, animal-control agencies and state officials scrambling for answers.

The deadly illness is being detected in northern and central Michigan. 

The dogs show symptoms of parvovirus — include vomiting, diarrhea and bloody stool — but tests for the disease have consistently been negative.

The virus is killing mostly dogs under 2 years old within three days of showing symptoms, although some older animals have died as well, according to animal control officials.

Melissa FitzGerald and Rudi Hicks, the animal control directors in Otsego and Clare counties, respectively, have said in the past week they were unaware of any dogs that survived the virus. But one man in Gaylord, which is in Otsego County, told The New York Times his 10-month Labrador retriever, who showed symptoms about three weeks ago, is “bouncing back” after veterinarians performed a battery of inconclusive tests and put the dog on a special diet of chicken and rice.

Hicks told the Clare County Board of Commissioners during an Aug. 17 meeting that more than 30 dogs had died from the illness in the county, the Clare County Cleaver reported. The Otsego County Animal Shelter said the virus has killed over 20 dogs there. Other counties also have seen cases, officials said.

The Michigan Department of Agriculture and Rural Development has partnered with local animal control agencies, the Michigan Association of Animal Control Officers, local veterinarians, the Michigan State University Veterinary Diagnostic Laboratory and the U.S. Agriculture Department to investigate the virus and the source of it. 

“Our work starts with looking for known causes of disease, and if none is found, we’ll explore novel explanations such as new virus variants,” Kim Dodd, director of MSU’s Veterinary Diagnostic Laboratory, said in a statement. “Our team is working hard to find clear answers, and we will provide an update when we know more.”

Dodd’s lab has been performing necropsies on the deceased dogs.

The Otsego County Animal Shelter said in a Facebook post its “best guess” is that the illness is a new strain of parvo.

Canine parvo affects the gastrointestinal tract and is spread by direct dog-to-dog contact or contact with the feces of an infected animal. It cannot be transmitted to people. 

Some of the infected dogs have been vaccinated for parvo, but the Otsego County shelter said it has “not seen any dogs that die that are PROPERLY vaccinated.” FitzGerald noted in an interview with MLive.com that parvo shots must be administered on a specific schedule and the vaccine must be stored at an exact temperature.

Officials are urging dog owners in the area to make sure their pets are up to date on their vaccines. Hicks advised residents in the area: “Keep your dogs home, don’t take them to dog parks, don’t walk them.”

Meanwhile, the state’s Agriculture and Rural Development Department is encouraging animal shelters and kennels to follow the strictest intake, vaccination and isolation protocols for dogs and to clean and disinfect surfaces and equipment.

Owners of dogs showing symptoms should contact their veterinarians.