The recent rise in cases of sudden and severe hepatitis in children has been widely reported around the world. Recently, several news outlets have highlighted a possible link between cases and contacts with pet dogs.
However, the data suggesting this association is very weak – in fact, it is probably much weaker than most of the alternative hypotheses that have been proposed.
The sharp rise in hepatitis cases in children was first observed in the United Kingdom, but has now been reported in Europe, Asia and the Americas. Although the numbers worldwide are still very low, the disease has been severe and some children need a liver transplant.
At least 11 children have died, and there are suggestions that it may continue for some time.
Hepatitis in humans is usually caused either by intoxication, such as alcohol, or by infection with one of several different viruses. However, none of the usual viruses has been identified in these children.
The UK Health Security Agency (UKHSA), the agency responsible for protecting public health in the UK, is working to find the cause of the disease so it can be effectively controlled and treated.
In a recent briefing paper, the agency reported a significant number of “canine exposures” in these cases of childhood acute hepatitis. However, before parents forbid their children to approach the family dog, it is worth considering the results in detail.
The UKHSA found that 70 per cent of patients (64 of 92, where data were available) were from families who owned dogs or were exposed to ‘other dogs’.
However, 33 per cent of households in the UK own dogs, and many children from non-dog households will be exposed to dogs when visiting or playing with their friends. A 70 percent exposure to dogs may be perfectly normal.
To suggest a link, it is important to show not only that exposure to dogs in patients is high, but that it is higher than in uninfected children. Until this is verified in what is known as a case-control study, any link is nothing more than a suggestion.
The second problem with the data is that if you ask enough questions, there is a strong possibility that the answers to one or more questions may appear to be related to the cases.
When we collect very large amounts of data retrospectively, this kind of spurious correlation can easily occur.
In fact, there is a website dedicated to collecting them. Here’s an example: Maine’s divorce rate between 2000 and 2009 appears to be strongly related to per capita consumption of margarine.
The important point about the links identified by retrospective data is that they are hypotheses. They always need to check by collecting more data on new cases. If the link is real, it will continue to appear in the new data. If it is false, it is not.
One of the links on the website for fake links shows another important issue. Between 2000 and 2009, per capita cheese consumption in the United States appears to be associated with deaths from tangled bed sheets.
It’s not actually hard to think that this could happen as a result of the nightmares caused by cowardice. The fact that we can think of a mechanism underlying the link gives us more confidence that it might be true, even if the mechanism is out of reach.
We tend to give more importance to associations where we can think of a cause, even when the evidence is weak.
So what are the possible causes for the high incidence of hepatitis in children, and can any of them be linked to dogs? One virus in particular, an adenovirus, was detected in the blood of 72 percent of patients tested (for comparison, SARS-CoV-2 was detected in only 18 percent).
Where the species can be identified, it has been found to be adenovirus 41 (Ad41), a human species that usually causes diarrhea in children. Although dogs have their own adenoviruses that cause respiratory disease or hepatitis, they are not known to infect humans, and Ad41 has no known association with dogs.
Cases in children do not indicate that the infection is transmitted between children – there are very few and very widely distributed cases of this.
Likewise, the distribution of cases does not suggest that this is a new virus transmitted from dogs to children. Cases have emerged in other countries much faster than the canine virus has spread among dogs.
Are there other possible causes? It has been suggested that the severity of hepatitis is caused by the immune system working improperly – either too aggressively or not strongly enough.
Social distancing during the pandemic has reduced transmission of a whole range of diseases, and a lack of exposure may leave some children unprepared for infections that usually don’t cause a problem.
Similarly, reduced exposure to dirt as a result of hand washing, disinfecting surfaces, and other hygiene measures may have made children susceptible to overreactive immune responses (as suggested for allergic diseases), and hepatitis may be caused by the immune system. response and not a virus.
Finally, and not surprisingly, it has been suggested that previous COVID infections may be susceptible to hepatitis.
All of these are nothing more than theories at the moment, and the available data is insufficient to prioritize any of them or to use them to suggest control measures.
Fortunately, the infection rate is still very low, and until better data is available, parents should probably focus more on monitoring any symptoms in their children rather than reducing their exposure to dogs.
Mick Bailey, Professor of Comparative Immunology, University of Bristol.
This article has been republished from The Conversation under a Creative Commons license. Read the original article.