'Tomato Flu' Is Likely Just an Unusual Presentation of Hand, Foot, and Mouth Disease

Tomato flu—the "mystery illness" that has been infecting young children in India, and was first identified in May—may actually be a strain of hand, foot, and mouth disease (HFMD) with an atypical presentation.

The virus first grabbed worldwide attention following an August 17 correspondence in The Lancet Respiratory Medicine, when researchers detailed a "new virus"—dubbed tomato flu or tomato fever—in the Indian state of Kerala in children under 5 years old.

The illness reportedly showed symptoms similar to COVID-19 (fever, fatigue, and body aches) as well as a very specific rash—one that erupted with "red and painful blisters throughout the body that gradually [enlarged] to the size of a tomato," the report said.

"They called it a flu, a tomato flu, which is descriptive, but it's not really scientifically accurate," Ian Michelow, MD, head of infectious diseases and immunology at Connecticut Children's Hospital, told Health. "It's really just a description of a rash in children who are ill with a fever, and it's not really describing whether they've got a different type of virus, or a new virus, or an emerging virus, or whether it's just a variant of something that already exists."

At the time of the report, more than 82 children under the age of 5 reportedly had the infection—but after two children based in the United Kingdom developed the same rash, researchers in the U.K. found that the virus to blame was actually a typical strain of HFMD—specifically Coxsackievirus A16 (CA16)—that had been presenting abnormally.

"The 'Kerala tomato flu' in these children were caused by CA16, one of the commonest [enterovirus] causes of HFMD in India," U.K clinicians and researchers wrote in a letter to the editor for The Pediatric Infectious Disease Journal.

The findings were further confirmed by a news report in the BMJ, identifying the so-called tomato flu as a potential variant of the HFMD.

"We have observed these lesions in children as far back as 2001," CK Sasidharan, MD, a pediatric specialist and senior consultant at the Baby Memorial Hospital in Kozhikod, Kerala, told the journal.

Here's what to know about HFMD, how it's likely presenting as tomato flu, and what parents should know about the virus.

A Common Childhood Disease Showing Up Differently

For the letter to the editor in The Pediatric Infectious Disease Journal, the researchers in the U.K. published their sequencing of what appeared to be tomato flu in a 13-month-old girl and her 5-year-old brother, who had recently returned from Kerala.

Both children tested positive for Coxsackievirus A16, a type of enterovirus that frequently causes HFMD, according to lead study author Julian Tang, PhD, clinical virologist and honorary associate professor of respiratory sciences at the University of Leicester.

"Coxsackie[virus] A16 is very common as a cause of HFMD and is well described in India, [but] the presentation is a bit unusual," said Tang. "Normally, you get kind of smaller punctate lesions, which are not as big and fleshy as the ones we saw."

HFMD typically affects children younger than 5 years old, and is not usually serious, but is very contagious. The disease often presents as fever and flu-like symptoms, along with sores in the mouth and a rash on the hands and soles of the feet that are usually small, red, and sometimes slightly raised.

The lesions that Tang and his team saw, in contrast, were raised and almost resembled monkeypox sores (clinicians tested the 13-month-old patient for monkeypox, but they tested negative). These tomato flu cases may be really caused by one or multiple newer evolutions of these coxsackie viruses, he said.

It's this atypical presentation of what is most likely HFMD that caused so much worry, according to Dr. Michelow.

"If you're only used to seeing the typical type of HFMD, and you suddenly see an outbreak of the atypical type of HFMD, maybe it's a different strain of the virus, you'd be concerned because you're not used to seeing that and you think it's a different illness," Dr. Michelow said. "It could be the same virus, same family of viruses, but a different presentation. And that's what we're seeing right now."

More Information on Cases from India Is Still Needed

Even though Tang and his team confirmed that the two tomato flu cases in the U.K. children were caused by Coxsackievirus A16, it doesn't necessarily mean that this is true across the board for all infections in the current Kerala outbreak.

"I don't have any data from the local doctors and children there. So I also want to be careful not to overhype just our little case report," Tang said. "Ours could be atypical, and all the others something else. But I don't think so."

In general, too, there are lots of childhood diseases that cause rashes, everything from impetigo to Ritter's disease, Dr. Michelow explained.

"Rashes in children are very common and they can appear differently even by the same cause of virus or bacteria. They can have different variations in the way they present," Dr. Michelow said. "And so it's difficult to say when, at a certain time of year, something is circulating—is it truly one particular cause, or could it be a variety of different causes that are just happening at the same time?"

According to the Lancet report, dengue fever and chikungunya, common mosquito-borne illnesses in India, had similar symptoms to tomato flu, and that tomato flu may be an after-effect of these illnesses. Though there's no real data right now to confirm or deny this link, Dr. Michelow thinks it could be further evidence that this outbreak could actually be made up of a number of different things—some HFMD, and some others after effects of these mosquito-borne illnesses.

The only way to answer all of the remaining questions about tomato flu is to wait for more information from the Kerala region, according to medical experts.

"Maybe there's some pediatricians there collating all these cases. If they collate like 20 or 30 sequences from viruses from those children," Tang said, "Then they could say with more authority, 'This is really HFMD; we found coxsackie viruses, coxsackie A10, A16, A24, A6, and this is all kind of typical HFMD,' and then that will basically put it to rest."

The good thing for now is that early reports of tomato flu show that it's not severe or life-threatening. Some enteroviruses—EA71, which caused a 2017 outbreak in Thailand, and EVD68, which has popped up mainly in the U.S.—can cause life-threatening complications, like brain swelling or paralysis. But that does not seem to be the case with this HFMD outbreak, Tang.

"Speculation can also make parents more worried unnecessarily. The main thing is that this Kerala tomato flu seems to be self limiting, all the kids recover, no deaths, no complications," Tang said.

He hopes that his findings will alert pediatricians to be on the lookout for HFMD and other rashes that may be presenting differently in children, so that they can get the proper diagnosis. Even in his own practice, Dr. Michelow said he's noticed an increase in the number of children coming in with HFMD rashes that seem to be presenting more abnormally. It's a good thing for doctors to be aware of.

"We're trying to expand the knowledge of pediatricians who see these children everyday," said Tang, "and to include differentials that include these kinds of lesions [and] unusual lesions."