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Cryptosporidiosis On The Rise In UK – Here's All You Need To Know

The parasitic infection causes severe, long-lasting diarrhea.

Laura Simmons - Editor and Staff Writer

Laura Simmons

Laura Simmons - Editor and Staff Writer

Laura Simmons

Editor and Staff Writer

Laura is an editor and staff writer at IFLScience. She obtained her Master's in Experimental Neuroscience from Imperial College London.

Editor and Staff Writer

illustration of Cryptosporidium oocysts releasing sporozoites

The Cryptosporidium parasite typically spreads to humans in contaminated water, especially swimming pools.

Image credit: Kateryna Kon/

The UK is currently in the grip of an “unprecedented” disease outbreak. We’re not talking about COVID this time, or even the usual winter illnesses like the flu. Nope, we’re talking cryptosporidiosis, a parasitic infection that causes – among other nasty symptoms – long-lasting, severe diarrhea. It’s definitely something you want to avoid if you can. So, what is the disease, how do you catch it, and why is there so much more of it around than usual?

What causes cryptosporidiosis?

Cryptosporidiosis, often referred to simply as “Crypto” (and not to be confused with the next Bitcoin) is caused by an infection with parasites in the genus Cryptosporidium


There are several species of this parasite that cause disease in animals, particularly ruminants like cows and sheep, but the most important pathogenic species in humans are Cryptosporidium parvum and C. hominis

Crypto parasites are apicomplexans, the same group of organisms that includes Toxoplasma gondii – scourge of cat owners everywhere – and Plasmodium species, the causative agents of malaria. Unusually for these types of parasites, Cryptosporidium species can complete their entire life cycle in a single host animal, and it only takes about three days. 

In 2022, scientists published a method of imaging the parasite’s development in real-time, uncovering more than ever before about its biology and learning that our previous understanding of its life cycle was not completely accurate. Knowing how the parasite’s life plays out gives us the best possible chance of combating the disease it causes.

How do you catch cryptosporidiosis?

The main route of infection for Crypto is via contaminated water. Structures called oocysts, which have a thick shell surrounding four immature parasites, are excreted by infected hosts and can find their way into the water supply.


Recreational water sources, like public swimming pools and water parks, are a major source of transmission. Crypto oocysts are resistant to common pool-cleaning chemicals like chlorine, so unfortunately even well-maintained facilities can pose a risk.

Parasites can also be found in soil or wild waterways, and experts have also previously estimated that contaminated food – like unwashed fresh produce and unpasteurized milk – is responsible for over 8 million cases of Crypto every year.

Finally, you can also catch Crypto from another infected person via the fecal-oral transmission route, so it’s super important to pay attention to hand hygiene if you live with or are caring for someone suffering from diarrhea.

Young children are the most likely to become infected, and there have been cases of outbreaks affecting daycare centers and other childcare settings. 

What are the symptoms of cryptosporidiosis?

The symptoms of Crypto typically start between two and 10 days after exposure to the parasite.

The most common symptom to look out for is profuse, watery diarrhea. Unlike a lot of other stomach bugs, which can be deeply unpleasant but typically pass quickly, Crypto hangs around for up to two weeks for most people. Even when the symptoms seem to have gone, they can continue to come and go for up to 30 days.

That’s right: up to a month of diarrhea. We may never go swimming again.

Additional symptoms can include stomach cramps, nausea, vomiting, and fever. 


In people with weakened immune systems, such as those taking immune-suppressing medication after a transplant, or people living with HIV, the infection can be more serious and last for much longer. The Centers for Disease Control and Prevention (CDC) advises these individuals to seek medical care if they believe they are infected with Crypto.

Is there any treatment for cryptosporidiosis?

For most people, Crypto will eventually clear up on its own. The most important thing to watch out for is dehydration, and it’s very important to drink plenty of fluids to offset the water loss from diarrhea. For children or adults at increased risk of dehydration, a doctor may recommend the use of specialized rehydration drinks or powders. You should also continue to eat light meals if you feel like it.

In the US, the Food and Drug Administration (FDA) has licensed a specific medication to treat Crypto, called nitazoxanide. The drug can clear up the symptoms within around five days for most people, but the actual parasites can still remain for a couple of weeks, so people can still pass on the infection to others.

Because of how easily Crypto can spread in swimming pools, it’s very important that anyone who is infected stays out of the pool for at least two weeks after their symptoms have gone away.

What’s behind the spike in cryptosporidiosis cases in the UK right now?

A recent paper detailed a preliminary investigation into an ongoing increase in Crypto cases in the UK, beginning in August 2023. The data is based on routine monitoring – Crypto is a notifiable disease, meaning public health authorities need to be alerted when a case is diagnosed.

There is often an increase in cases towards the end of the summer, as people are returning from travel abroad or coming to the end of their summer holidays when they may have been more likely to spend time in pools and water parks. However, the latest report shows that the UK is currently seeing around three times as many cases as would be expected for October. 

graph of cases of cryptosporidiosis in the UK between February and October 2023, with a blue shaded area showing expected range and red line showing actual cases
Cases are way above what we would expect to see for this time of year.
Image credit: Peake et al, Eurosurveillance 2023 (CC BY 4.0)

But what’s behind the rise? 

“Our initial findings would suggest that swimming (either in the UK or abroad), including the use of pools, and foreign travel to a variety of destinations may underlie the current increase,” the paper concludes. “However, at this stage other sources, for example contaminated food, cannot be excluded as contributing to the exceedance.”


The study authors also note that Crypto is underreported, so there could be many other people out there with symptoms that are not captured within this data. Given that over half of participants in the research had symptoms lasting longer than 10 days, it’s another stark reminder of just how unpleasant this parasite is.

Authorities will continue to monitor the situation, but for now there’s no clear answer as to why the UK is having such a bad time of it. But please, for everyone’s sake: if you’ve recently had diarrhea, stay away from the pool.

The study is published in the journal Eurosurveillance.

The content of this article is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of qualified health providers with questions you may have regarding medical conditions.  


All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current. 


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