The Centres for Disease Control (CDC) is investigating whether an acute hepatitis virus that has killed five children in the U.S. and sickened dozens of others could be linked to pet dogs.

Health officials in the U.S. have identified 109 cases of the mysterious hepatitis virus, which affects the liver. The cases, which are concentrated in pre-school children, occurred over the past seven months in 24 states and Puerto Rico.

Around 90 percent of the children were hospitalized and 14 percent needed liver transplants, CDC Deputy Director for Infectious Diseases Jay Butler, told a press conference Friday. All were previously healthy.


Similar cases have been reported outside the U.S. The World Health Organization (WHO) said that as of May 1 it had recorded 228 probable cases in 20 countries, the majority of which are in Europe and the U.S.

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A possible link between the unusual number of acute pediatric hepatitis cases and dogs originated with a report Friday from the U.K. Health Security Agency.

The U.K. has reported 163 cases of acute hepatitis in children as of May 3. The agency report said that its leading hypothesis is that the outbreak is linked with adenovirus, a common virus that usually causes mild cold or flu symptoms.


But the report also said it was looking at exposure to other environmental factors, and noted that review of its questionnaires found that it found relatively high numbers of dog-owning families among cases, totalling around 70 percent of cases where data was available. It acknowledged, however, that pet dog ownership is common in the U.K.

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More than half of the U.S. cases also tested positive for adenovirus, Butler said, adding that the CDC is continuing to investigate all possible contributing factors, including exposure to dogs.

"We really are casting a broad net and keeping an open mind in terms of whether the adenovirus data might reflect an innocent bystander or whether there might be co-factors that are making the adenovirus infections manifest in a way that hasn't been commonly seen before," he said.

The risk of hepatitis in children remains low, health officials say. The main symptoms of illness are vomiting, dark urine, light stools and jaundice, in which the skin or the whites of the eyes turn yellow.

There is no indication that the virus is connected to COVID-19 vaccinations, Butler said, adding that as the median age of the children affected is two years, most would not have been eligible for vaccinations, which have only been approved so far for children over five years of age.

"Covid-19 vaccination is not the cause of these illnesses, and we hope that this information helps clarify some of the speculation circulating online," he said.