Case guesses

Hannah writes:

Dear TWiP doctors,

Time to embarrass myself with another case guess! Apologies in advance for the long email.

If it really is an arthropod as you implied (and not e.g. a small mammal or snake), I see three possibilities here: 1. a non-venomous arthropod bite that got infected, 2. a venomous arthropod bite/sting, or 3. a venomous arthropod bite/sting that got infected. In the first case, it could be just about anything, although biting flies seem unlikely given the circumstances of the bite. Getting bitten while putting on pyjamas suggests that the arthropod was hiding inside, and that it was defending itself when crushed against skin. A lot of bugs (Hemiptera) have both the mouthparts and temperament to defend themselves in this manner, as do beetles and many many more.

Since I doubt you’d give us a case where the answer is “one of the several million arthropod species that could break your skin when squashed, letting unknown bacteria into the wound”, let’s move on to the a venomous arthropods. One of the many stinging bees, wasps and ants could conceivably cause these symptoms, at least in conjunction with a bacterial infection. Bees and wasps generally don’t hide in pyjamas, however, and while ants might crawl through clothing on their way somewhere else, it still seems unlikely, especially since there was only one bite/sting.

Centipedes are a real possibility. I don’t know what species are found in her region, but their venom can cause intense pain and swelling, and they could conceivably hide in clothing. I don’t think their venom is likely to cause the other symptoms, so this would once again point to some infection.

This brings us to the arachnids, specifically spiders and scorpions. A minority of species have medically important venom that can, all by itself, cause some or all of the symptoms experienced by the patient. If it’s a scorpion, I can’t speculate further – I know next to nothing about that group, though aside from the pain, the description of symptoms in Parasitic Diseases doesn’t seem to match.

Given that the patient is in Peru, the spider genus Phoneutria immediately comes to mind – Brazilian wandering spiders. Despite the common name, some species are also found in Peru. They are known to hide in clothes, but to the best of my knowledge, they are tropical forest spiders, so I wouldn’t expect to find them in the highlands. Much more likely are the genera Latrodectus (widows) and Loxosceles (recluses). Both widows and recluses are shy, non-aggressive spiders that may occasionally find themselves caught up in clothing. Latrodectus are more commonly encountered in their webs, however, and their bites are not necrotic. Any necrosis seen would be from an infection.

Loxosceles, on the other hand, do not weave webs, love to hide in clothing, and their bites are famous for being necrotic. While bites are typically painless, and necrosis usually takes longer than 2 days to develop, it seems like the most likely culprit. Many Loxosceles species can be found in Peru, but L. laeta is the most well-known, and bites from this species can cause both skin lesions and systemic reactions, including renal failure.

Before I sign out, I just want to share this excellent STAT article on delusional parasitosis (also known as Ekbom syndrome): I’d be very surprised if you didn’t have a few listeners who suffer from this awful condition, and it may benefit them to know that they’re not alone and can get help.

Thank you so much for everything you do!



P.S. Dr. Griffin: I’m the one who brought up some of the issues in the arthropods section via your website’s contact form a while back. I apologise if I came across as overly critical or rude – I really do think you guys are doing amazing work, and I’m thrilled that you got some entomologists on board to make this textbook even better!

Carol writes:

She was bitten by a wandering spider, or “banana” spider.


Wink writes:

Dear TWIP Team,

The case of an apparent bite and multi-organ system failure sounds like a dangerous arthropod to me. My guess this week is spider bite. I found the following excerpted information in Wikipedia: “Loxosceles laeta, commonly known as the Chilean Recluse Spider, is generally considered to be one of the most toxic species. It has a very wide range [including Peru] … and has been documented at elevations.”

Wink Weinberg (Atlanta)

John writes:

Hi TWiPerati,

My initial guess in the case of the Peruvian woman with the inguinal insect bite was that she had disturbed a recluse spider (either Loxoceles laeta  and  L. intermedia). That spider lives in South America; bites when disturbed in clothes; can cause lesions and necrosis.

If that is the case, direct treatment options for the woman are limited. Symptoms can be treated to give the body a chance to recover itself.

On the other hand, the family home should be nuked from orbit, it’s the only way to be sure.

However, I am not confident in the diagnosis. The timeframe is extremely short from bite to serious consequences.

The preview of the paper “A Mnemonic Device to Avoid False Diagnoses of Brown Recluse Spider Bites” ( indicates that ulcers would not be expected for a week. Unfortunately, it’s not open access and all I could read was the preview.

I look forward to the answer in the next episode,

Thanks and regards,

John in Limerick, Ireland.

(where it’s 11°C, cloudy with sunny spells)

Nita writes:


   Hello, TWIP team. I have recently tuned into the podcast and find it really fascinating! I am a soon graduating medical student going into neurology, but I really like parasites and had been trying to find a good resource to learn about them! This is my first guess submission, so please be kind! I am going to Tokyo soon for my vacation, and I am excited to visit the Meguro parasitological museum! Hope I’m the lucky 14th emailer!

   For our 24yo female Peruvian patient, my first instinct of a rapidly progressive necrotizing with black central dot is the brown recluse spider, but maybe that’s my affinity to spiders. After doing a little digging, I did find that the Chilean recluse spider is quite venomous in Peru, and recluse spider bites can cause breakdown of muscles that result in rhabdomyolysis and acute renal failure. However, looks like it doesn’t cause increased WBC count necessarily. Womp womp to this guess from a arachnid fan.

   After some digging, such a rapid exacerbation of clinical course as well as the red lesion with a black enter sounds a burrowing sand flea tunga penetrans. The female has a black dot at the read end, and that’s what marks the change. However, I’m not sure of the time course.

   Those are my two silly, sexy answers! Doing this in a rush so I didn’t get to think it through. Just jotted down the two “zebras” that popped into my brain.

John writes:


Greetings from Omaha, the city that hosted the very vibrant meeting of the American Society of Parasitologists (ASP) in 2015.

Check out these exciting and fun playing cards ASP made for our conference goodie bags:

My students used them recently to help prepare for the Parasitology Lab practical. It doesn’t get any more vibrant than that!

I haven’t attended an ASTMH meeting, but ASP includes parasites of veterinary importance and others without medical importance such as gregarines and horsehair worms. ASP also addresses the evolutionary ecology of parasites, which may interest the environmental science major from Colby-Sawyer College. Many attendees enjoy the relatively small size of the meeting, which facilitates collegiality and is undergraduate-friendly. Also, no one tries to sell you anything.

My guess this week for the case study is the brown recluse spider, Loxoceles sp.

Although not a parasite, I look forward to incorporating this case study into my Zoology class.

I concluded my Parasitology Lab with the infamous #109 Case Study. I didn’t tell them about the “twist” in the case and they went nuts when they heard the big reveal. Encouragingly, two students guessed the correct parasite (but not host).


Suellen writes:

This one really has me stumped — which I guess isn’t saying much, considering that I’ve not gotten a diagnosis right yet.

This time, I tried to take more time and jump to fewer diagnostic conclusions, but I can’t find a parasitic disease that would manifest so quickly after a bite from a critter in one’s pajamas. My initial guess was cutaneous leishmaniasis, but that takes weeks or months to develop, and the lesions are not just where one is bitten, they are all over the body — but especially on the face and other exposed areas.

Next, I decided to rule out certain vectors. For example, a mosquito is unlikely to get into someone’s pajamas, and even if it did the person is not likely to be able to catch and bottle the mozzie a day later. Fleas fall into the same category, so I am ruling out mozzie-borne illnesses such as malaria and dengue, as well as anything that fleas might carry.

Sand flies? I don’t know much about them, so it’s hard for me to say whether one is likely to end up in pajama bottoms, or to be able to be captured easily, but the diseases that sand flies and other flies carry just don’t come on that quickly after a bite.

Tick-borne parasites? Again, everything I researched would take a week or more to become symptomatic. Same with bacterial diseases, they just don’t show up that quickly. And I ruled out Chagas disease because it also takes a while to show symptoms, and also because I don’t think a reduvid bug is likely to show up in someone’s pants leg. (Also, she has no fever.)

So I came to this: Either (1) the bite did not cause the illness, i.e., the patient was getting sick and just happened to get bitten or stung right before she began to feel really bad, or (2) this is a case of myiasis, where she’s actually got a fly larva of some type living in her skin, and that is what is making her sick.

Now, what I’ve read about myiasis seems to indicate that this is more of a tropical or sub-tropical problem, and our Peruvian patient lives in the arid mountainous region of the country, but I suppose it’s still possible. It also appears that usually you get more than one bite, but maybe our patient reacted quickly enough to avoid being bitten multiple times? And, of course, the scalp and neck are the regions where people most often get bitten . . . so, again, this diagnosis is looking a bit thin. A bite that occurs when you are putting on your pants seems much more like a defensive move on the part of the critter than a “oh, look, I think I’ll lay my eggs here while i’m being suffocated in this pants leg” kind of thing. But if I move along this line of reasoning, I need a critter. The only one I could find that seemed to fit the bill was Dermatobia hominis, the human botfly, which is endemic to the highlands of Central and South America. The problem is that the literature seems to be devoid of any symptoms other than the ones related to the skin — itching, etc. No mention of vomiting or other signs of illness. And I don’t see a botfly seeking out a pants leg when a scalp would be much more handy.

So I am left with the possibility that the bite and the illness are not related, but are simply coincidental, or that this is not a parasitic disease after all, but either poison from the bite of a spider or scorpion, or is just something Dr. Griffin thought up to make us all go crazy rooting around in Google. LOL — nah, he wouldn’t do that to us, would he? But I’m afraid that after all my research I really don’t have a diagnosis, but now that I’ve written all this I’m going to send it anyway, so you will know that I tried my best.

Thanks for keeping me guessing!

Carl writes:

Dear TWIPniks,

I was listening to TWIP 130, and as soon as I heard that the center of the unfortunate woman’s lesion had turned black, I hollered “Brown Recluse!”  Fortunately I was by myself and so did not frighten anyone.  Upon later consultation of Parasitic Diseases Sixth Edition, I discovered that I was wrong, as one would expect of an amateur diagnosis.  But I was close– right genus, wrong species.  This is a case of loxoscelism, caused by the bite of a spider in the genus loxosceles.  Given that the case is in Peru, and the severity of the systemic symptoms, the species is most likely loxosceles laeta.   

It’s sunny and a record-setting 87 Fahrenheit here in Lexington Massachusetts.


Anthony writes:

Tracking zoonotic pathogens using blood-sucking flies as ‘flying syringes’

Dave writes:

Dear host. I had a very unusual parasitic experience while shearing sheep in the Kamloops area of BC. I’m used to seeing sheep keds but on this occasion I was shearing ewes and beside her udder was a fully engorged tick (not sure what type). I noticed is as I went through it with the shears. Now generally it takes a lot to creep me but this was at the far edge of weird sh#tometer. Out of the cut in half tick came a pile of baby ticks. These were the size of pinheads but fully formed and crawling. Now I hadn’t been doing drugs or drinking or suffering from any other hallucinations that would explain this so even though I have read and heard that ticks don’t give birth to live young what is the other explanation for this.

Thanks for this in advance and sorry that it isn’t human related but most of my parasitic experiences are of the ovine kind

Dave the shearer in sunny southern AB

ps yes Dickson the fishing is great. about 2 hours west of us is the Elk river famous for float fishing

Andrea writes:

Hi Twipitos!

You have probably already seen this one:

Oh boy! Now paradise is off-limits!

It’s 59°F in Seattle with rain of course!

Love the podcast. Please keep it going even though I may never send in a guess. I do enjoy listening to the cases. Even those that creep me out! I now look at all mangos with suspicion.

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