Paddy writes:

Hi Twippers,

Paddy here in Ireland where it is 14C, damp and cloudy. Long time listener first time guesser.

I will be as brief as the case clues were, watery diarrhea, salad and Mexico. I suspect Cyclospora cayetanensis, probably determined using microscopy or nucleic acid amplification testing.  

For treatment, P.D.7 suggests trimethoprim-sulfamethoxazole or ciprofloxacin.


Paddy, a lab tech that loves the TWiX audio gallery.

Sara writes:

Dear Drs Racaniello, Despommier, Griffin, and Naula, 

I’ve recently stumbled across your podcast and am absolutely enthralled! 

Here’s my first attempt at a diagnosis for your case (I do hope this makes it in in-time). For this 20-something-year old, HIV-negative man with chronic, non-bloody watery diarrhea after having indulged in all kinds of fruits, vegetables and salads while in Mexico under the unfortunate assumption that hotel-prepared equates to safe from microscopic creatures, I’m guessing Cyclosporidium (an infection caused by Cyclospora species endemic to Mexico commonly found on fresh herbs (like cilantro) as well as in salads. I believe it can be easily missed on routine ova and parasite test, but an acid fast stain should do the trick, or did you send Cyclospora PCR? Is there a reliable stool antigen test? I wish I knew the exact species but I’m going to look it up after sending this email. 

If this person were immune-compromised, I would also consider cryptosporidium which is Cyclospora’s much smaller acid-fast staining friend. 

Thank you for the delightful experience that is your podcast. You’ve brought immense joy (and learning) to my life, and I am truly grateful.  


Sara Rendell, PhD
MD candidate 
Pronouns: She/Her/Hers 
Perelman School of Medicine 
and Department of Anthropology 
University of Pennsylvania

Alexander writes:

Dear TWiPanosomes,

As the differential diagnosis for diarrhoea (acute, going on chronic) is very wide, this gentleman is very lucky to have entered into a case for a podcast about parasitism, which narrows my considerations down considerably. My differential is as follows:

– Helminths: while many helminth infections can cause diarrhoe, there are usually other symptoms and some probable exposure. It’s therefore unlikely to suspect just one of them based on this history alone.

– Protozoans: A variety of protozoans commonly cause diarrhoea. Among them are:

     – Cryptosporidium species: Incubation time is about one week, non-bloody, watery diarrhoe is also typical. Four weeks is not an unreasonable duration for the symptoms, and could even go on much longer in immunocompromised patients. The low inoculum needed for infection would point towards clean looking food as a possible source for infection. A fecal antigen test is quite sensitive and would produce a quick result.

     – Cyclosporidium cayetanensis: The symptoms and history fit well here, as does the country; several outbreaks due to contaminated food in the US have been traced to Middle and South America. The route of infection is not so clear, as cysts need to sporulate in the soil before the become infectious, making direct fecal-oral infection unlikey. Diagnosis can also be challenging, as several stool samples ned to be examined. PCR is also not widely available.

     – Cystoisospora belli: As with Cyclosporidium, route of infection and diagnosis do not align well with the history here. I would also expect the patient to present with more pronounced systemic symptoms.

     – Entamoeba histolytica: I’d expect more systemic symptoms and abdominal pain. Also, the incubation period of one to three weeks would make it unlikey that he got infected while in Mexico (which is, of course, not necessarily the case).

     – Giardia duodenalis: This is the most common intestinal parasitic disease in the US. While some patients with giardiasis complain of copious flatulence, foul-smelling stools and bloating, this is not necessarily the norm. Diagnosis is also possible via antigen test and infection can occur directly via the fecal-oral route. While the diarrhoea in giardiasis rarely lasts four weeks, the infection can leave a temporary lactose intolerance in its wake, which could produce similar symptoms.

In my opinion, Crypto is the most likely diagnosis here. While Giardia is also possible, the presentation is not typical and I wouldn’t expect Dr. Griffin to just order a test for Giardia based on the history.

All the best,

Alexander from Vienna, 19°C / 66 F and cloudy

Luke writes:

In the spirit of Andrew from Pongaroa, I will say ‘Werte!’ from the always sunny Mparntwe, also known as Alice Springs, Australia. Arrernte is the language of the traditional owners of this region, but the town hosts people from all around the region and the world – Walpiri, Anmatyerre and Alyawarr people from the north, Pintubi-Lutitja to the west, and Pitjantjatjara to the south, among others, and then a mix of Australians from the past 250 odd years of immigration. 

I listened to the last few episodes of TWIP and a few TWIMs on my drive back from holiday in Darwin, a fairly corner deficient 15 hours on the road. 

My guess for your tourist to Oaxaca with persistent watery diarrhoea is Giardiasis, caused by Giardia duodenalis. Daniel likely sent stool for microscopy, with direct fluorescence. Treatment with Tinidazole 2g once or Metronidazole 2g daily for 3/7.

Thanks for all the TWI’s. 


David writes:

Dear honourable TWiP lass and lads,

After skipping an episode because I was too late with the answer, I tried to find out what happened with this otherwise healthy young man.  As usually when there is little to go by, my mind goes blank, I stare into the distance, and use the search function on the pdf of PD – whatever the many advantages of paper may be, a digital search takes under a second. 

Using “watery as a search term”, only 7 parasites came up (distributed over 19 hits).  The first ones to drop off my list were

·         Strongyloides Stercoralis: I assume a larvae penetrating the skin would have been alluded to)

·         Diphyllobothrium latum: raw fish consumption would have been an event that could, should and would have stood out in the patient’s mind when identifying suspicious food sources

Then there is Cyclospora cayetanensis; PD7 mentions transmission through raspberries, basil, snow peas and mesculin lettuce.  With a diet consisting mainly of fruits and vegetables, it seemed like the obvious culprit, and definitely my favourite diagnosis.   However, the same revered book also states, disappointingly, that “in immunocompetent patients, symptoms can last up to 2-3 week”.  The youngster has had diarrhea for four weeks now.  Besides that, the clinical picture does not seem fitting; while a delayed onset of symptoms can be accounted for because of the length of his stay, there are no other frequent symptoms like nausea, vomiting, anorexia or cramps.  So, bye bye CC.

Cryptosporidium parvum paints a similar picture: while the infection may be everything between asymptomatic and profuse diarrhea, there are other common acute features that are not present, and again “C in an immunocompetent host is self limited”.

Cystoisospora belli is also very unlikely, as there are many other symptoms, and is rare to manifest itself in immunocompetent hosts.  As the patient is HIV negative, it really does not fit his presentation.

Two suspects remain: Entamoeba histolytica and Balantidium Coli.  The former can become chronic when untreated, and while heme is detectable, it may not be visible to the naked eye.  Also, the absence of other symptoms does not seem unlikely. The latter presents mainly acute symptoms, and while “Chronic infection has been studied in endemic areas” this seems to suggest that continuous exposure has something to do with it.

With a certain confidence my guess is therefore E histolytica.  As Dr Griffin undoubtedly went through this differential in a flash, I expect he tested for heme in stool and treated with metronidazole.

Having won the book a few episodes ago, I remain a faithful and thankful listener in a dreary Nicaragua,


Andrew writes:

Kia ora from Pongaroa,

No book won yet but I was one away last time. Am I getting closer? Will I win on TWiP 200?

By the time this gets read out I hope New Zealand has returned to having no community cases of Covid19 and that we are reaching a high level of vaccination coverage. Having an outbreak has motivated many to give up their hesitancy and get the ‘jab’ – clouds and silver linings.

Speaking of clouds, there are none here and the temperature is a cool 14°C.

My guess, for the gentleman in his 20s who has recently returned from Mexico and has developed persistent diarrhoea, is Giardia. This is a numbers game as it is the most common cause of diarrhoea that persists and is connected with travel to Mexico. I surmise that the test was a PCR and the successful treatment was metronidazole.



Elise writes:

Dear TWiP Trifecta (Quadfecta?), 

I hope this finds you well in this strangest of seasons when the pandemic is still so very present but there is also a return to school with all of the interest and energy and excitement to rise above it. 

I am writing with an attempted diagnosis for the gentleman who was visiting Oaxaca. I think he may have experienced a Cyclospora infection, which is unpleasant but not uncommon. There are a few parasites that can trigger the patient’s most obvious symptom: watery diarrhea, but I felt that  Cyclospora matched best. With a Giardia infection, the loose stools tend to be “greasy” and foul smelling and there is accompanying cramps, gas and nausea that were not among the patient’s complaints. Another possibility I considered was a Cryptosporidium infection, but I was less inclined towards crypto as a culprit because in healthy individuals it seems that the infection is pretty self-limiting, even though the symptoms are very similar to the patient’s. 

In addition to a symptom comparison, I was of course, sensitive to the discussion of the patient’s diet in Mexico, and his having eaten a lot of fresh fruit and vegetables and salads. Fresh fruits and vegetables are very common sources for the cyclospora protozoa and it is likely that his healthy eating impulses inadvertently got him infected. 

The diagnosis can be made with an examination of a stool sample and treatment is with  trimethoprimsulfamethoxazole (some brand names for this are Bactrim and Septra).

I do have some questions. Is it just luck that his travel companions didn’t contract the infection if they ate the same food? Is it more likely that they consumed only cooked produce? Is there any way he could have enjoyed fresh fruit and vegetables without risking an infection? (I have heard that if fruit is washed and then peeled the risk can be reduced.) 

As always, thank you, thank you for your work and your energies. 

Best wishes, 

Elise … in LowER Manhattan.

Martha writes:

Dear TWiP team,

Sussing out the parasite in episode 197 felt like solving a good old whodunit.

A young man and his companions had traveled to Mexico. They stayed in a nice hotel where they dined almost exclusively. He could tell it was a good hotel because of all the fresh fruit and vegetables being served. He is now plagued with 4 weeks of watery diarrhea. The infectious disease physician listens to his story, orders one test, and on getting the test result prescribes a medication.

My thought process was to find the parasites that cause prolonged travelers’ diarrhea in an immunocompetent host. Check for those that have a treatment other than supportive care. Cross check with those that have a unique diagnostic test. I ignore viral and bacterial causes of diarrhea because the podcast is This Week in Parasitism. I assume that Dr.Griffin had other reasons for eliminating the virus and bacteria.

Giardia and Entamoeba, both can be diagnosed by ELISA and can be treated by metronidazole, but neither is described as the cause of watery diarrhea. Cryptosporidium is diagnosed by ELISA, but is reportedly treated by supportive care. Microsporidia is allegedly self-limited in the immunocompetent. Which brings me to Cyclospora which although usually self limited, can cause prolonged watery stool. Cyclospora autoflorese under UV light at wavelength 300-380 nm, (based on this alone I want Cyclospora to be the answer because being able to glow blue is just too amazing).  Treatment is with Trimethoprim-Sulfamethoxazole. Cyclospora outbreaks are linked to a variety of fresh produce.

So in the fashion of Clue: it was Cyclospora in the dining room with the salad bar.

Best wishes to you all


Katy-Jane writes:

Hi everyone,

A pleasant 62F / 17C this morning in north central Wisconsin. 

I like to consider myself a fairly well travelled person, and this case struck a chord with me! (I shan’t divulge further!) There are many causes of traveller’s diarrhea, most of which usually resolve fairly quickly on their own. Common culprits are our friends E. coli and Clostrium spp., but according to the CDC, bacterial infections rarely cause persistent infections. (Also, this is a podcast about parasites, not bacteria!) 

Persistent diarrhea occurs in a minority of cases and is usually caused by protozoa: Giardia spp., Cryptosporidium spp., Cyclospora spp., and Entamoeba histolytica. Protozoa like wet environments, so it is highly likely that our patient contracted his disease from eating nicely washed fresh fruit in the hotel. Giardia causes watery diarrhea and is the most common protozoan cause of traveller’s diarrhea, so this is what I am going to go with.

I found the following quote on a Banner Health website, which I thought was good advice: “Cook it, boil it, peel it and, when in doubt, leave it.”

Thanks as always!

Katy-Jane Shanak
Faculty in Veterinary Technology & Dairy Science
Northcentral Technical College
Wausau, WI

Sherrill writes:

Hello again Vincent, Dickson, Daniel and Christina….

The case presented is of prolonged watery diarrhea in a young healthy male traveler with no relevant medical history, recently returned from Oaxaca, Mexico.

Relevant clue given is that patient ate lots of fresh fruits, vegetables and salads, mostly at his nice hotel.

I found this case quite tricky to pin down because there are so very many infections that include watery “travelers diarrhea”.  Such diseases are abundant in Mexico and are sometimes referred to as “Montezuma’s Revenge”.

Here is my working list-by no means exhaustive:

1) Entamoeba histolytica:  This is a worldwide protozoan transmitted by fecal matter in water or soil.  Cysts can lie dormant in the environment, contaminating fresh water and food.

2) Norovirus: Common “stomach” virus transmitted in uncooked raw food, water & on contaminated surfaces. 

3) Cryptosporidia: Another protozoan involving fecal contamination of water or uncooked food. Resistant to chlorination and thus can be acquired from pools.

4) Giardia: One more intestinal protozoan parasite, flagellated, that is transmitted via fecal matter in fresh water or uncooked food. 

5) E. Coli: Rod shaped bacteria transmitted through water, uncooked food or soil. 

I better shorten up these notes or this email will be too long……..

6) Shigella: Bacteria

7) Salmonella: Bacteria

8) Campypylobacter: Bacteria

9) Vibro: Bacteria

10) Rotavirus 

11) Enterovirus 

12) Cyclospora cayetenensis

My best guess is Cyclospora cayetenensis  because my google research shows this one to most closely fit the facts.  Weeks of watery diarrhea and no other symptoms reported after eating a lot of fresh produce in Mexico.

The Cyclospora parasite is coccidean and oocytes are ingested on fresh produce.  

The other contender was giardia….which might be more common-but the clue here of persistent diarrhea seemed to fit Cyclospora better.  Giardia symptoms would probably include, foul smelling gas and bloating. And giardia seems to be acquired more commonly from contaminated fresh water than say salads. 

The treatment for Cyclospora cayetenensis is oral TMP-SMX aka Bactrim.   Same drug I have given my horses to treat infections many times. 

I learned so much from this quest …but admit I am still struggling to master the correct spelling of diarrhea…..I take it the h is silent? 

Many thanks.  Here is the link I found convincing on this one:


The University of Central Lancashire Parasitology Club writes:<=winner

Dear TWIP Professors,  

Hello again from the Parasitology Club at the University of Central Lancashire in the beautiful Northwest of England.   

We recently enjoyed our first Parasitology Summer School. With socially distanced microscopes we studied a variety of enteric protozoa and helminths. We looked at dung from horses to discover which had been successfully wormed and played a game called ‘Which parasite would you most rather avoid?’.  

Our collective investigations in the case of the young man with persistent watery diarrhoea who recently visited Mexico led us to a couple of coccidian candidates in Cryptosporidium or Cyclospora.  

Cryptosporidium is a cause of acute diarrhoea which normally resolves without treatment within a week or so in the immunocompetent individual.  This can be transmitted via contaminated produce and water. We considered the possibility that the subject may have ingested oocysts while swimming in the hotel pool after a faecal accident, as we know that Crypto is highly resistant to chlorination and there have been outbreaks associated with swimming pool water (Suppes et al, 2016).  

However, because of the prolonged episode of diarrhoea, we believe that this is a case of Cyclosporiasis caused by parasite Cyclospora cayetanensis. Cyclospora is also highly resistant to chlorine disinfection, therefore, it could be transmitted by ingesting contaminated produce or water even when vegetables and fruits are washed in potable water (Ortega et al., 2010). Outbreaks of cyclosporiasis have been reported worldwide associated with soft fruits and fresh produce including a few outbreaks associated with coriander leaves (Cilantro) from Mexico (CDC, 2021a). 

When most healthy individuals become infected symptoms may persist for several weeks without treatment. 

Stool examination by either differential interference contrast (DIC) microscopy or fluorescence microscopy at 330-360nm should reveal 8-10um round oocysts with a distinct oocyst wall and internal sporozoite structures or a beautiful blue autofluorescence respectively (CDC, 2021b). 

Trimethoprim/sulfamethoxazole is the usual therapy for Cyclospora infection.   

Thank you as always for your wonderful podcasts and stimulating case studies. 

We are keeping our fingers crossed for a signed copy of your wonderful book to make the long voyage across the Atlantic to Lancashire. 

What do you call a magical poop? 


On behalf of the University of Central Lancashire Parasitology Club