Norovirus sucks. It’s been a while since I had it, but it doesn’t conjure up fond memories.  A bunch of Canadian tourists returning from Cuba (and presumably all of the other people on their planes) can also attest to the unpleasant nature of this viral infection.

Norovirus in people is (logically) caused by human norovirus. There are some other types of norovirus that infect other species, but the human version predominantly infects people. Predominantly doesn’t mean always, though, and other hosts of the virus need to be considered. A recent study published in the Journal of Clinical Virology (Summa et al 2012) looked at whether human norovirus could be found in dogs. Researchers collected fecal samples from 92 indoor pet dogs in Finnish households where people had vomiting and diarrhea or where the dog owner had had close contact with someone with norovirus. Fecal samples were tested for the presence of the virus using molecular tests.

Norovirus was detected in feces of 4 of the 92 (4.3%) canine fecal samples. All positive dogs were from households that had more than two sick people. Additionally, kids were present in all households with positive dogs. Two of the dogs had been sick, with nausea and loss of appetite. Illness in the dogs was pretty mild and only present for one day, and it’s not clear whether norovirus was responsible. Fecal samples from owners were only available from one of the positive households, and there the same norovirus strain was found in the owner and the dog.

These results are interesting and indicate that a small percentage of dogs in contact with people with norovirus can shed the virus. The big question is, "what does this mean?" That’s not so clear. Finding norovirus in the dogs’ feces is one thing. Determining that it’s relevant to human (or animal) health is another, and it’s important not to over-interpret the results, because…

  • The testing that was used detects norovirus RNA, i.e. genetic material from the virus. That means that the virus passed through the intestinal tract. It does not necessarily mean that live virus was present, since this type of testing detects both live and dead virus. Dead virus obviously poses no risk to anyone.
  • Even if live virus was being passed in the dogs’ feces, the amount of virus coming out the rear end of the dogs isn’t known. It might be pretty low and therefore of less concern.
  • The relative risk posed by the household dog is a big thing to consider. All dogs that were shedding the virus were from households with multiple sick people, therefore they were already in pretty biohazardous environments with lots of virus being tossed around (in many different ways). This suggests that it may take a lot of exposure for dogs to shed (potentially only a little) virus. Also, it minimizes the relative risk posed by the dog, since if only dogs from severely affected households shed the virus, the dog is only one of many possible sources and probably of lesser risk than exposure to sick people and environmental surfaces they contaminated. Dogs from households with active disease are probably not very likely to encounter lots of other people or dogs (probably less so than the people in the house), therefore limiting their potential role in transmission. There’s no evidence that dogs are long-term carriers of norovirus.

Does this change what you should do if you have norovirus? Not really. It means you should try to limit contamination of the environment, wash your hands frequently, stay away from others as much as possible and avoid puking on your pets. Maybe we should add "keep your dog isolated along with you," just in case.

Another interesting finding was the association between sick kids and norovirus-shedding dogs. It’s further evidence of the "kids are biohazardous" theory. We know that kids are at increased risk of various infectious diseases. At the same time, we have previously shown that contact with kids is a risk factor for dogs shedding Clostridium difficile and MRSA. Whether that’s because kids are more likely to be shedding these bugs, they have closer contact with pets, they pay less attention to hygiene or a combination of these isn’t clear, but this result isn’t particularly surprising. (No, I’m not recommending banning kids from pet-owning households. As parents, we know our kids are effective disease vectors… that’s just part of having kids.)

This study doesn’t tell us whether pets are sources of human norovirus, and we really shouldn’t expect it to. A single study rarely answers all the questions, and good studies sometimes raise more questions than they answer. This is an interesting study and it shows that more work is indicated to clarify the answers to the questions raised above, and to determine whether there is any real concern about dogs and this nasty virus.