The recent rise in cases of sudden and severe hepatitis in children has been widely reported worldwide. Recently, several media outlets have highlighted a possible link between these cases and contact with pet dogs. However, the data indicating such an approximation is very limited – in fact, it is probably much more limited than most other hypotheses that have been proposed.
Outbreaks of hepatitis in children were first seen in the UK, but have now been reported in Europe, Asia and the Americas. Although the numbers are still very low globally, the disease has proven to be serious, and some children have had to have liver transplants. At least 11 children have died, and this phenomenon appears likely to continue for some time.
Hepatitis in humans is usually caused either by a toxin, such as alcohol, or by infection with one of several different types of viruses. However, none of the usual viruses were detected in these children.
The UK Health Security Agency (UKHSA), the body 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 brief, the agency reported a significant number of “dog exposures” in these cases of childhood acute hepatitis. However, before parents forbid their children to approach the family dog, the results should be examined in detail.
The UKHSA found that 70% of patients (64 of 92, where data were available) came from families who owned dogs or were ‘exposed to other dogs’. However, 33% of households in the UK own a dog, and many children from non-owning households are exposed to it when visiting friends or playing with them. A dog exposure of 70% can be completely normal.
To suggest an association, it is important to show not only that exposure to dogs in sick children is high, but also that it is higher than in uninfected children. Until this is verified in so-called case-control studies, any link is only a suggestion.
The second problematic point in this data is that by asking enough questions, there is a high probability that the answers to one or more of these questions appear to be related to the cases.
When we collect very large amounts of data retrospectively, this kind of spurious correlation can easily occur. There is also a website dedicated to collecting these stats. Here’s an example: Maine’s divorce rate between 2000 and 2009 appears to be closely related to per capita margarine consumption.
The point to remember about links identified by historical data is that they are assumptions. They should always be verified by gathering additional information on new cases. If the correlation is real, it will continue to appear in the new data. If it’s a mistake, we’ll never see it again.
One of the fake links site links reveals another important problem. Between 2000 and 2009, per capita cheese consumption in the United States may be associated with deaths from tangles in bed sheets.
One could easily imagine that this might be the result of nightmares caused by cowardice. The fact that we can think of a mechanism underlying the link bolsters our idea that it could be true, even if said mechanism is a long way off. We tend to give more weight to associations for which we can imagine an explanation, even if the evidence is weak.
So what are the possible reasons for the recurrence of hepatitis cases in children? Could any of these be related to dogs? One virus in particular, an adenovirus, was detected in the blood of 72% of the patients tested (for comparison, SARS-CoV-2 was only detected in 18% of cases).
In cases where it was possible to determine the type of virus, it was serotype of human-like adenovirus 41 (Ad41), which normally causes diarrhea in children. Although dogs can harbor their own adenoviruses that cause respiratory disease or hepatitis, they are not known to infect humans. Additionally, Ad41 has no known relationship to dogs.
Cases seen in children do not indicate that the infection is transmitted from one individual to another – the number of cases is very low and the distribution is therefore very wide. Likewise, the distribution of cases does not suggest that this is a new virus transmitted from dogs to children. In other countries, cases have emerged much faster than canine virus spread among dogs.
Are there other possible causes? It has been said that the severity of hepatitis is caused by a malfunction of the immune system – either too strong or not strong enough. Social distancing during a pandemic has reduced transmission of a range of diseases, and the lack of exposure to these conditions may have left some children unprepared for infections that would normally not be a problem.
Likewise, lack of exposure to dirt from hand washing, disinfecting surfaces, and other hygiene measures may have made children susceptible to overactive immune responses (as suggested for allergic diseases). So hepatitis can be caused by the immune response and not by the virus. Finally, and unsurprisingly, the possibility was raised that previous Covid-19 infections had led to children contracting hepatitis.
These are all just theories at the moment, and there is not enough data available to prioritize them or use them to suggest control measures. Fortunately, the infection rate is still very low, and until better data is available, perhaps parents should focus more on noticing any symptoms in their children rather than reducing their exposure to dogs.
The original version of this article was published on The Conversation, a non-profit news site dedicated to exchanging ideas between academic experts and the general public.