The recent increase 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 suggesting such an approximation are extremely limited – in fact probably much more limited than most of the 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 is proving serious and some children have had to undergo liver transplants. At least 11 children have died and it looks like the phenomenon is likely to continue for some time.
Hepatitis in humans is usually caused either by a toxic substance, such as alcohol, or by infection with one of several different types of virus. 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 that it can be effectively controlled and treated.
In a recent briefing, the agency reported a high number of “dog exposures” in these cases of severe childhood hepatitis. But before parents prevent their children from approaching the family dog, the results should be examined in detail.
The UKHSA found that 70% of patients (64 out of 92 where data were available) came from dog-owning families or had been “exposed to other dogs”. However, 33% of British households own a dog, and many more children from non-owners are exposed to them when they visit friends or play with them. A 70% exposure to dogs can be quite normal.
To suggest a link, it is important not only to show that the exposure to dogs in patients is high, but also that it is higher than in unaffected children. Until this has been confirmed in what is called a case-control study, any link is nothing more than a suggestion.
Another problematic point with 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 retrospectively collect very large amounts of data, this kind of false association can easily arise. There is also a website dedicated to collecting these statistics. Here’s an example: Maine’s divorce rate between 2000 and 2009 seems strongly 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 link is genuine, it will continue to appear in new data. If it’s wrong, we’ll not see it again.
One of the associations to the dummy correlation site reveals another important issue. Between 2000 and 2009, cheese consumption per. residents of the United States may be associated with deaths due to entanglement in bedding.
One can easily imagine that this could be the result of cheese-induced nightmares. The fact that we can think of a mechanism underlying the link strengthens our idea that it could be true, even though that mechanism is quite far out. We tend to place more emphasis on associations that we can imagine an explanation for, even if the evidence is weak.
So what are the possible causes of recurrence of cases of hepatitis in children? Could any of these be related to dogs? In particular, one virus, an adenovirus, was detected in the blood of 72% of the patients tested (in comparison, SARS-CoV-2 was detected in only 18% of cases).
In cases where it was possible to identify the virus type, it was the human-like adenovirus serotype 41 (Ad41) that usually causes diarrhea in children. Although dogs can house their own adenoviruses that cause respiratory diseases or hepatitis, these are not known to infect humans. In addition, Ad41 has no known association with dogs.
The cases seen in children do not indicate that the infection is transmitted from one individual to another – the number of cases is too low and the distribution too wide for it. Similarly, the distribution of cases does not indicate that it is a new virus that is transmitted from dogs to children. In other countries, cases have occurred much faster than a canine virus spread between dogs.
Are there other possible causes? It has been suggested that the severity of hepatitis is due to a malfunction of the immune system – either too strong or not strong enough. Social distancing during the pandemic has reduced the transmission of a number of diseases, and lack of exposure to these conditions may have left some children unprepared for infections, which would not normally be a problem.
Similarly, lack of exposure to dirt due to hand washing, sterilization of surfaces and other hygiene measures may have predisposed children to overreactive immune responses (as has been suggested for allergic diseases). Hepatitis may therefore be caused by the immune response rather than by a virus. Finally and not surprisingly, the possibility has been raised that previous Covid-19 infections have predisposed children to hepatitis.
All of these are only theories at present, and there is insufficient data available to prioritize them or use them to propose control measures. Fortunately, the incidence remains extremely low, and until better data are available, parents should probably focus more on observing any symptoms in their children than on reducing their exposure to dogs.