The negative blood test indicates that “there is not a strong immune response and that there are currently not measurable amounts of antibodies in the blood,” the statement said. However, it continued, “the negative serological test does not suggest that the dog is not infected with the virus.”

That is because antibodies do not always develop in mild or asymptomatic cases of infection in humans, and negative results in early stages of infection are not uncommon, the statement said.

J. Scott Weese, a professor at the University of Guelph’s Ontario Veterinary College who studies zoonotic disease, said he thought the first explanation — that the infection was mild enough to not produce a strong antibody response — is more likely.

“The repeated earlier test results support this being a true infection,” Weese said in an email. “It wouldn’t be surprising for this to be a low-grade infection because dogs are not thought to be very good hosts for this virus, based on its genetic structure and what we know about the original SARS.”

The dog has been quarantined since Feb. 26, after its owner was hospitalized with covid-19. Repeated tests of oral and nasal samples from the dog came back “weak positive,” authorities said.

Initially, the government said the results may have been due to “environmental contamination,” rather than infection. But in an update last week, the government said that it had consulted with local public health experts, as well as the World Organization for Animal Health, to conclude that dog did in fact have a low-level infection in “what is likely to be a case of human-to-animal transmission.”

On Thursday, the government said that DNA sequences of the virus collected from the dog were “very similar” to samples collected from infected humans with whom the animal had come into contact. This indicates that “the virus likely spread from the infected people to the dog,” the statement said.

The dog has shown no symptoms, and further blood tests will be conducted, the government said.

It is unclear what this single case means more broadly. World Health Organization officials said last week that there’s no evidence animals play a large role in the transmission of the coronavirus, but that they are studying the question. This week, the WHO noted on its website that the Hong Kong dog had been “infected,” but emphasized that there remains no evidence that cats, dogs or other pets can transmit the virus to humans.

For now, experts say, healthy people should not treat their pets any differently, nor should they worry about animals contracting the virus. People who have covid-19 or have been exposed, however, should restrict contact with their animals — both to avoid exposing the pets and to prevent getting the virus on their skin or fur, which might be passed on to another person who touches the animal.

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