An unidentified illness has killed dogs in Michigan, officials said, stumping investigators who haven’t determined what is causing the dogs to show symptoms including vomiting, lethargy and loss of appetite.

Local officials said earlier this month that they had been receiving reports of dogs, mostly young ones, throwing up and passing bloody stool. More than 20 dogs had died in Otsego County in northern Michigan, with most of them dying within a few days, according to the Otsego County Animal Shelter.

Dr. Nora Wineland, the state veterinarian, said Monday that an early investigation showed that some of the dogs may have had canine parvovirus, but officials were still investigating the illness.

Canine parvovirus is a highly contagious virus that causes gastrointestinal illness mostly in puppies, according to the American Veterinary Medical Association. It can spread through dog-to-dog contact or contact with contaminated feces or a contaminated area.

AVMA said dogs that are sick with canine parvovirus should be kept warm and hydrated. Dogs that are treated early and properly can survive around 90% of the time.

Most of the cases have been reported in northern Michigan in dogs that were elderly or under the age of two, according to the Otsego County Animal Shelter.

The Michigan Department of Agriculture and Rural Development, which is investigating the illness along with other local and state officials, said Tuesday that it had anecdotal information showing that around 15 to 25 dogs could have been infected. The state didn’t have an official number, however, because veterinarians aren’t required to report canine parvovirus cases to state officials.

The department said Monday that the virus isn’t contagious to humans or other animals.

The Otsego County Animal Shelter said last week that it hadn’t received reports of properly vaccinated dogs that had died from the illness. The American Veterinary Medical Association recommends that dogs receive the canine parvovirus vaccine while they are puppies.

“Dog owners need to ensure their pet is up-to-date on routine vaccinations as it’s the first step in keeping your pet healthy,” Dr. Wineland said.

Write to Alyssa Lukpat at alyssa.lukpat@wsj.com