Case guesses:

Philip writes:

Dear TWiP Professors,

Thank you for taking the time to broadcast these TWiP episodes.

They have kept me intellectually charged as I study for board exams.

For the case study on TWiP 152, I would be sure to rule-out an intestinal Entamoeba histolytica infection.

Keep up the good work.  Be nice to Dickson.

– Philip, Long Island, NY

Alexander writes:


I don’t have any background in medicine, but I’d still like to try.

Hi all,

I’m a recent graduate with an Associates of Applied Science in Natural Resources as a forestry technician, however during my education I also took a liking to oceanography and phytoplankton taxonomy. I started listening to this podcast from NPR and grew to like it a lot, so I guess here’s my differential.

With the possibility for water borne pathogens, I got led into looking at Giardia lamblia, Cryptosporidium parvum, Entamoeba histolytica, and Balantidium coli.

For G. lamblia, it is water borne, but the symptoms don’t really add up. For C. parvum, I think I want to rule it out for the weight loss and watery diarrhea, however you did mention on your previous episode that you can still eat more to balance out the weight loss. E. histolytica causes dysentery and can be fatal if left untreated, also most exposed individuals are asymptomatic. B. coli has similar symptoms to E. histolytica, but has also added described as a lack of energy.

Given what I think I know, which is not much, I would say that it could be E. histolytica or B. coli.

I would say to prescribe Metronidazole as it is listed as an effective drug for most of what I mentioned, other than C. parvum.

PS. The books that I used to help identify phytoplankton differed from the Parasitic Diseases (PD) text in amount of photos and information given on specific entries, mine would emphasize shapes and just have a little blurb while Parasitic Diseases would have long blocks of text that really went into a lot of detail, most of which I don’t even comprehend.

Erik writes:

Hi TWiPanosomes,

I don’t write in often, but I have a hunch about the diagnosis on this week’s case. The symptoms described seem pretty straight forward. There aren’t a huge number of pathogens that cause dysentery, as far as I know, so that narrows down my guesses to just a few. If I include bacterial infections in my differential as Dr. Griffin recommended, my list would be the following: Entamoeba histolytica, Shigella, Salmonella enterica, and Trichuris trichiura.

I would rule out the two bacterial pathogens not just because this isn’t an episode of TWiM, but because both Shigellosis and Salmonellosis can cause quite severe symptoms that would be immediately noticeable and distressing. But this patient described not feeling too terribly sick, but just “tired” and with some abdominal pain and blood in stool.

That leaves two potential diagnoses from my list: E. histolytica and Trichuriasis.

E. histolytica is well known to cause bloody, watery diarrhea and can also cause fatigue and weight loss, and symptoms can last anywhere from a few day to years.

Trichuris trichiura, or whipworm, can be asymptomatic if the person is infected with a low worm burden. But heavy worm burden can result in signs and symptoms that include abdominal pain, bloody or “mucusy” diarrhea, and even anemia in severe cases (which may cause tiredness?).

I have to admit, I’m having trouble deciding which of those two pathogens is most likely. Both are present in Central America and can infect adults, causing the symptoms described, and both infest the large intestine. Dr. Griffin said that the rectal exam appeared normal. I’m not sure if that would include looking for worms? Probably the best way to determine which of these two pathogens was the culprit (if it even is one of these two) would be to do a stool O and P and look for distinguishing eggs or larvae or cysts. Would that kind of test be feasible in the rural, low-resource environment described?

My hunch is that the man has trichuriasis rather than E. histolytica, though, just because of the localization of the abdominal pain and the fact that the stools are not described as being excessively watery.

Treatment for Trichuris trichiura would be mebendazole, whereas treatment for E. histolytica would be metronidazole. Maybe both drugs could be given at once? I don’t know if that would be safe or cost-effective.

Hopefully my guesses weren’t way too far off. I like to think I’ve at least learned some things about parasites in the past few years! A book would certainly help, though. 😉

Best wishes,


Marcus writes:

Oh, ova and parasite to confirm if possible.

As for the differential, most bacterial and viral agents can be excluded because of the three week history and lack of a fever.

Might still be campylobacter (though you’d expect a fever), cyclosporiasis or giardiadis (though the diarrhea should be more watery for both of these).

Treat with three days of albendazole, plus a broad spectrum antibiotic for possible bacterial cause or superinfection (common with trichuriasis) if confirmation is unfeasible. Should also consider iron supplements.

Suellen writes:

Hello again, TWIP hosts! This was such a tough case, but I came up with two possible parasitic diagnoses. There is also one bacterial possibility, Shigellosis, but I’m going to set that aside since we do focus on parasitic diseases here.

My two candidates are Balantidium coli and Entamoeba histolytica. Both have several factors in common:

  1. Both can cause dysentery.
  2. Both are common in areas with poor sanitation.
  3. Both are spread by fecal-oral transmission.

While the bloody stools are not by themselves much to go on, I do lean more toward E. histolytica simply because it appears to have few symptoms other than dysentery, while B. coli usually is accompanied by fever, nausea, and vomiting as well as bloody stools. One good thing is that metronidazole is listed as a treatment for both diseases, so absent a microscope to use to tell which parasite has infected our patient, it’s possible he can be cured with metronidazole.

I hope I got this one right!

Alistair writes:

Dear TWIP,

Having missed the chance to send in my diagnosis for the previous episode due to being on holiday in west Africa at the time which was however, extremely interesting, especially as I got the opportunity to visit a diagnostic lab in Gabu, Guinea Bissau and see how diagnosis is carried out in such a resource limited setting, with the hospital lacking even electricity. Having guessed correctly I thought I would try again this time, and hopefully do better than my first attempt.

I’ll keep it relatively brief, as this may be something of a shot in the dark, but I am going to go for Entamoeba histolytica. The symptoms of blood mixed in with partially formed stool immediately made me think of this particular infection, coupled with the fact that the patient is working in the fields, and the fact that Entamoeba are often found in the soil in the tropics. Finally, the lack of any fever, vomiting, etc leads me to preclude any bacterial infections likely to have been the cause of this bloody diarrhoea, furthermore, as far as I am aware Shigella and similar infections usually present as more acute and severe. As for treatment, I would prescribe a course of Nitroimidazole, which google recommends as very effective for the treatment of this condition, and in my professional opinion I am inclined to agree.

I am also attaching a paper that I have been reading at the moment for ideas on multiplexing for LAMP, which I mentioned in my last email which you may find interesting for distinguishing between pathogenic and non-pathogenic Entamoeba species, and which I am planning to adapt the methodology for my own work on DENV which I mentioned in a previous email. I also wanted to add that you will be pleased to know (having said in a previous show when I wrote in before) that I will be transferring over to Parasitology for my PhD, at the University of Glasgow, working on a veterinary helminth point of care LAMP assay, and as such will probably keep trying to push this diagnostic test to you, and with combination with lyophilised reagents and lateral flow dipsticks it effectively eliminates the need for cold chain and any equipment other than a heat block. There are also heat blocks which you can connect to smartphones making this a really effective potential POC test for infectious disease in the resource limited environments. I’ve attached some links below for a few papers on the subject.

I’ve very much enjoyed your show so far, and look forward to the next episode and case, and hopefully the chance to win a book one of these days.

Alistair Antonopoulos

Kevin writes:

Chicago weather: dull grey sky, equivocating drizzle, 17º C – unseasonably cool.

This case calls to mind the old saw: Ars longus, vita brevis- Hippocrates.
TWiP Case 152, with its scanty clinical history resulted in a lot of digging in the ‘Ars’ of medicine, and threatened to brevis my vita.

1.Defining the diarrhea: 3 week duration- we can classify it as persistent. This helps narrow the diagnostic possibilities.
acute<14 days,  persistent=14-29 days, chronic  >30 days

2. Defining the anatomy. The bloody diarrhea focusses attention on the large intestine.
Small intestinal diarrhea has more malabsorptive symptoms with watery and/or bulky stool. Colorectal diarrhea is characterized by the presence of pus, mucus and blood–what is termed dysentery.

3. Etiologies:
infectious–  viral and HIV,AIDS
bacterial and mycobacterial
fungal (rare and often invoked by fringe health people)
noninfectious– inflammatory, toxins, malignant

A persistent large bowel bloody diarrhea effectively rules out the common viral diarrheas such as norovirus and rotavirus. The duration of the illness makes enteric bacterial pathogens such as yersinia, campylobacter, EA and ET E.coli, plesiomonas, aeromonas and Salmonella less likely though not entirely impossible. The clinical scenario does not favor Clostridium difficile as a likely agent. Protozoal diarrhea from Giardia, Isospora, Enterocytozoon, Cyclospora and Cryptosporidium usually do not cause dysentery. Helminthic disease resulting in dysentery is usually ascribed to Trichuris in textbook accounts. Schistosomaisis has been reported to cause bloody diarrhea but is not endemic in Panama.

The differential diagnosis in our patient can be narrowed to the classic infectious causes of dysentery in low resource, tropical countries, to wit: Entamoeba histolytica and Shigella species (dysenteriae, sonnei, flexneri).  Many people who carry E. histolytica remain asymptomatic, which could explain why the patient’s family remain unaffected. –The mind should remain open, however, to other background conditions such as tuberculosis, HIV/AIDS- though the patient’s preserved weight and overall condition make these two less likely. Should also remember to consider folk remedy ingestion-cathartics and purges, some of which contain mercury / toxin exposure-agricultural chemicals, and inflammatory bowel disease, which is reported to be increasing in the tropics.

Patient’s remote location precludes most diagnostic work-up such as nucleic acid amplification and antigen detection in the case of E. histolytica and stool culture for causes of bacillary dysentery. Field microscopy for accurate E. histolytica diagnosis is probably not practicable and may identify non-pathogenic entamoeba species. An empiric trial with metronidazole for E. histolytica and a quinolone or azithromycin for bacillary dysentery, if successful, would give a modicum of diagnostic certainty. If I must choose one diagnosis I will say E. histolytica though my brief look at Central American prevalence data does not inspire confidence.

I close with a remedy from Pliny the Elder (died 79 AD), from chapter 19 of the Natural History:Dysentery is removed also, by taking a spotted lizard from beyond seas, boiled down till the skin only is left, the head, feet, and intestines, being first removed. A couple of snails also, and an egg, are beaten up, shells and all, in both cases, and made lukewarm in a new vessel, with some salt, three cyathi of water, and two cyathi of raisin-wine or date-juice, the decoction being taken in drink.

Thanking you all for the ongoing excellent educational experience.


GC Cook (the Hospital for Tropical Diseases) Gut 1994; 35:  582-586 Persisting diarrhoea and malabsorption, Leadling artile -Tropical infection ofthe gastrointestinal tract and liver series.

Infectious causes of chronic diarrhoea. Best Pract Res Clin Gastroenterol. 2012 Oct;26(5):563-71. doi: 10.1016/j.bpg.2012.11.001.
Bacteria are unlikely to cause chronic diarrhoea in immunocompetent individuals with the possible exception of Yersinia, Plesiomonas and Aeromonas. Infectious diarrhoea can trigger other causes of chronic diarrhoea, including inflammatory bowel disease, irritable bowel syndrome and “Brainerd-type” diarrhoea. A thorough evaluation should detect most infections causing chronic diarrhoea.
CDC defines chronic diarrhea as diarrhea that lasts longer than 2-4 wks.
Parasites (e.g., Cryptosporidium, Cyclospora, Entamoeba histolytica, Giardia, microsporidia)
Bacteria (e.g., Aeromonas, Campylobacter, Clostridium difficile, E. coli, Plesiomonas, Salmonella, Shigella)
Viruses (e.g., norovirus, rotavirus) or
Unknown causes thought to be infectious (e.g., Brainerd diarrhea)

Plesiomonas shigelloides Revisited  J. Michael Janda Clin. Microbiol. Rev. April 2016 vol. 29 no. 2 349-374 1 April 2016 :
Plesiomonas enteritis can present in one of three forms: an acute secretory gastroenteritis (most common), bloody or dysenteric colitis, or chronic or persistent diarrhea of >14 days duration

Overdiagnosis of Entamoeba histolytica and Entamoeba dispar in Nicaragua: A Microscopic, Triage Parasite Panel and PCR Study
Byron Leiva,et al May 2006 Volume 37, Issue 4, Pages 529–534 Archives of Medical Research  
PCR results showed that E. histolytica is a rare finding in patients with diarrhea. At the health centers, E. histolytica, E. histolytica/E. dispar were clearly overdiagnosed, with the consequence of overtreatment.
of 134 cases of diarrhea, only 2 had definite E. histolytica.

A survey of intestinal parasites including associated risk factors in humans in Panama Nidia R. Sandoval
Acta Tropica 147 (2015) 54–63
n=1123 fecal samples. 47% of samples contained parasites. Commensal prevalence in samples was 40%, and half of the commensals were amoebas (Entamoeba, Endolimax, Iodamoeba. E.histolytica couldn’t be differentiated from the commensal E. dispar.

Cullen writes:

Hey Twip peeps,

I believe the case of the man with bloody stools is caused by Entamoeba histolytica. The blood in the stool is caused by the infiltration of the parasite into the intestinal wall, and because of the water sanitation, I think this is very plausible. I hope you guys are having a blast at ASM Microbe!


Cullen Lilley

Microbiology Technician

Atlanta, GA

Peter writes:

Bonjour professors TWIP,

Just when you had all mastered the sweet mellifluous language of the Gael, I have migrated from Ireland to Montpellier France to work with my collaborators here.

Before I left I met with lab mate Gwen, visiting researcher Manon and we invited Rosa Hughes, a high school student on work experience from Scotland, to join us as we listened to the last case.

Due to the dysentery we believe the man to be infected with the parasite Entamoeba histolytica. Confirmation by stool examination. We also considered hookworm and strongyloides but believe the persistent bloody diarrhoea is more in keeping with E. histolytica.

As non-eukaryote potential causes were also asked for and I am here in the south of France availing of their 16S metagenomics expertise, an honoury mention to Campylobacter jejuni and Heliobacter pylori as other potential causes of bloody diarrhoea. Although I must admit being very much a novice in this area and not in the possession of the microbial equivalent of Parasitic Diseases 6th edition.

A bientot,

Peter Stuart


David P. writes:

Dear Hosts,

I believe the man from the same Panamanian village featured in the past few episodes of TWiP is suffering from amoebic dysentery caused by Entamoeba histolytica. A listener from last week suggested the woman suffering from giardiasis may have had E. histolytica, and the hosts mentioned that this may have been a reasonable diagnosis if there was blood in the feces – which there was not. This week, however, there is a new patient with bloody diarrhea, and E. histolytica can similarly be spread through rainwater much like Giardia. The lack of external symptoms makes amoebic dysentery an easy culprit, and I am going with my gut.

Thank you once again for the informative and entertaining podcasts.


David P.