Three years ago, human noroviruses were isolated from pet dogs in Europe, but many details of the viruses’ itineraries are still unclear. It is even unclear whether noroviruses can cause clinical disease in dogs. Regardless, there is concern that dogs might be capable of transmitting noroviruses to humans.

This possibility has prompted scientists to take a closer look at whether noroviruses can travel from humans to dogs and back again. Such round-trip voyages would be epidemiologically significant, given that 37–47% of all households in the United States have a dog.

Recent research from the University of Cambridge indicates that some dogs can mount an immune response to human norovirus, which strongly suggests that they have been infected with the virus. This result appeared April 1 in the Journal of Clinical Microbiology, in an article entitled, “Evidence for human norovirus infection of dogs in the UK.”

The article’s first author, Sarah Caddy, Ph.D., added that she and her colleagues also confirmed that human norovirus can bind to the cells of the canine gut, which is the first step required for infection of cells.

“Remarkably, canine seroprevalence to different [human norovirus] genotypes mirrored the seroprevalence in the human population,” wrote the article’s authors. “Though entry and replication within cells has not been demonstrated, the canine serological data indicates that dogs produce an immune response to [human norovirus], implying productive infection.”

To assess the risk of dog-to-human norovirus transmission, Dr. Caddy and colleagues used noninfectious human norovirus particles. In addition, they examined dogs’ stool samples for the presence of antihuman norovirus antibodies.

“Results showed that seven different genotypes of human norovirus [virus-like particles] can bind to canine gastrointestinal tissue, suggesting that infection is at least theoretically possible,” noted the authors.

The investigators found norovirus in serum samples of 43 of 325 dogs tested. Despite dogs' apparent susceptibility, the investigators failed to find norovirus in canine stool samples, including those from dogs with diarrhea. Accordingly, it is still unclear whether dogs can shed the virus in quantities sufficient to infect humans—although clinical investigators have estimated that as few as 18 virus particles can cause human infection.

Moreover, it is yet to be determined whether dogs play a role in the epidemiology of some outbreaks of human norovirus. Some of the biggest outbreaks occur in places from which dogs are absent, such as on cruise ships and in hospitals.

Norovirus, which causes vomiting and diarrhea, is extremely contagious among humans. It infects 19–21 million Americans annually, more than 6% of the U.S. population, according to the CDC. Those infections may result in as many as 71,000 hospitalizations, and 800 deaths.

According to Dr. Caddy, part of the impetus for the current study came from her veterinary practice, and her status as a dog owner.

“As a small animal veterinarian, I am often asked by dog owners if they might be able to pass infections onto their dogs, or if their dogs are contagious to them,” said Dr. Caddy. “There are plenty of anecdotal cases of dogs and humans in the same household, having simultaneous gastroenteritis, but very little rigorous scientific research is conducted in this area.”

“Until more definitive data is available, sensible hygiene precautions should be taken around pets, especially when gastroenteritis in either humans or dogs is present in a household,” Dr. Caddy concluded.