Following Research from Tails to Taps – the hunt for zoonotic parasites in drinking water

A review of current research by PhD student Claire Paton

Parasites – a global problem

Parasites are tricky little things, aren’t they? They’re often small and hard to find, can be difficult to treat, and by their very nature, they thrive by making their hosts miserable. Many people hear the word and think of tapeworms, ticks, or that zombie wasp fungus that made it into National Geographic. But today we’re talking about something smaller, more subtle, and often much closer to home.

Cryptosporidium is a protozoan parasite – basically, a single-celled organism which can survive alone in the environment, but which requires a host digestive tract in order to reproduce. That’s right – they multiply in your intestines. For a healthy host, a quick dose of Cryptosporidium is nothing to worry about – the parasite might not affect you at all, or it might give you an upset stomach for a couple of days before life returns to normal. The problem comes when Cryptosporidium affects a vulnerable host such as a young calf or an immunocompromised human – then we see crippling abdominal pain, severe diarrhoea, weight loss, and in some cases even death.

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Cryptosporidium sporozoites emerging from an oocyst. Credit: cdc.gov

But how would an average person contract this unpleasant little parasite? Unfortunately there are many routes of transmission. Infection occurs when a susceptible host eats or drinks Cryptosporidium oocysts – the egg-like stage of the developmental cycle. Oocysts can be picked up directly from infected cattle or humans (it isn’t picky and has no problems crossing species barriers) or, more worryingly, from swimming pools, tap water, and fresh vegetables. The robust outer coating of the oocyst allows it to survive for extended periods of time in the environment and makes it resistant to many treatment processes.

Why am I telling you all of this? Is it to terrify you into never swimming again and boiling all tap water before it touches your lips? Not quite. I’m merely setting the scene as I introduce you to a saga that’s been going on for decades. There are lots of different research groups working on Cryptosporidium – looking into the national and global economic impacts, searching for a vaccine or cure, trying to better understand how the parasite behaves and what we can do to manage it. And this is where my research comes in. I want to know how oocysts behave in the environment, how they mobilise and migrate from their source in faeces to receiving waters, and what management strategies we can create to reduce the risk of Cryptosporidium outbreak.

 

Tiny needles and enormous haystacks

The obvious issue with this is that drinking water catchments are, in general, very big, whilst Cryptosporidium oocysts are very, very small. You can guarantee that crypto-positive faecal deposits will leach oocysts into the surrounding soil, but when confronted with a three-thousand-acre hillside and free-roaming livestock, the phrase needle in a haystack springs to mind. So how do we go about determining the contribution that each subset of catchment inhabitants might contribute to the environmental load of parasites?

The short answer is test everything. For my current campaign, I’m looking at two drinking water catchments on the Orkney mainland – an island plagued by an enormous resident and migrant goose population. As well as answering general questions on oocyst movement, I want to know if the geese (who are already accepted as a general nuisance to crops) are posing a threat to public and livestock health.

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Greylag geese on the Orkney mainland. Credit: The Orcadian

Several farmers based across both study catchments allowed us access to their herds; thus we have ample faecal samples from both adult and juvenile cattle. The abundant geese are often found grazing alongside cattle, and thus fresh goose samples are readily available from our farm sites; we also collected fresh goose samples from a large flock (several thousand geese) nesting on a stretch of public moorland. If there’s Cryptosporidium present in either of these populations, we’ll find it.

For now, we’ve decided to forgo soil testing. Again – needle, haystack, headache. But if the animals grazing the land are indeed contributing to the environmental load of oocysts found in the water, then the soil is just an intermediate step between the physical animals and the receiving waters. For this reason, we’re cutting out the middle man and testing the water directly. Water is easier to test because we can filter large volumes of it – up to 1000L per sample – through a fine mesh filter, allowing us to concentrate the oocysts passing through.

Finally, I’m testing the sediment from the bottoms of both reservoir lochs in my catchments. We know nothing about the interactions of oocysts in sediment – do they sink? Are they re-mobilised when the sediment is disturbed, ready to infect a new host? Or are they dragged down and destroyed in the murky, oxygen-deprived depths? We’ll soon find out – I’ve collected a lot of mud.

 

From Field to Lab – The science never stops

Of course, lots of samples equates to lots of analysis. My field campaign took place in April, and we’re only now starting to see initial results – throughout the summer, it was all hands on deck to perform the oocyst extraction process on all of the samples before the positive ones became badly degraded. There are already published methods for testing water, calf, and adult cattle samples, and established methods within our lab group for use on goose and other wildlife samples. The sediment samples presented something entirely new to us, but a pilot study indicates that artificially spiked sediment samples behave a lot like adult cattle faeces under extraction conditions.

This happy scientist will therefore be spending the foreseeable future squirrelled away in the Cat-2 pathogen lab, doing lots of mysterious things with saturated salt solutions and centrifuges. Well… not entirely mysterious, since I’ve already provided a link to the established methods.

I’m excited to see the results of this study. Any information on the interactions between wildlife, livestock and the environment informs our big picture of how Cryptosporidium behaves – and takes us one step closer to eliminating the spread of its disease.

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Healthy cows are happy cows. Credit: imgflip.com

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