Nematodes: Key Pathogens and Clinical Manifestations Explained

Nematodes like *Toxocara canis* and *Onchocerca volvulus* cause significant health issues, including blindness and skin disorders.

### Understanding Four Key Tissue Nematodes: Toxocara canis, Onchocerca volvulus, Loa loa, Wuchereria bancrofti
* 🐶 The first nematode, *Toxocara canis*, is transmitted fecal-orally, primarily affecting **children who come into contact with infected soil or sandboxes**, leading to symptoms such as **blindness and meningoencephalitis**.
* 👁 Key clinical manifestations include ocular larva migrans, which results in **unilateral vision loss**, and visceral larva migrans, causing infections in organs like the liver and spleen, with **hepatosplenomegaly** being a notable symptom.
* 🌊 Next, *Onchocerca volvulus*, known as African river blindness, is transmitted by black flies and causes **blindness and dermatitis** due to immune reactions against the **adult worms and microfilaria** present in the eye and skin.
* 👁 Symptoms of *Onchocerca volvulus* include **recurrent conjunctivitis**, **progressive visual loss**, and skin conditions such as leopard skin, marked by **pruritis and skin depigmentation**, with treatment involving **Ivermectin**.
* The third nematode, *Loa loa*, or the African eyeworm, is transmitted by deer and mango flies, leading to **painful vision without vision loss**, characterized by **angioedema and pruritus** as the worm migrates through the subcutaneous tissue.
* 👀 The visual symptoms of *Loa loa* arise from the worm's presence in the **subconjunctival tissue**, making it **visually identifiable**, whereas its differentiation from *Onchocerca volvulus* hinges on the **absence of blindness**.
* 🐘 Finally, *Wuchereria bancrofti*, transmitted by mosquitoes, is the **second leading cause of infectious disability globally**, leading to lymphatic filariasis characterized by **lymphedema, adenolymphangitis, and the severe manifestation of elephantiasis**.
* 🏦 The mnemonic for *Wuchereria bancrofti* relates to a "**bank**" and an "**elephant piggy bank**," linking it to the **severe swelling and hyperpigmentation** associated with elephantiasis, with treatment also being **diethylcarbamazine**.

in this video we will continue the conversation about nematodes and specifically focus on the four tissue nematodes so these are the four pathogens we're going to be talking about: *Toxocara canis*, *Onchocerca volvulus*, *Loa loa*, and *Wuchereria bancrofti*. So let's just

go through these one at a time. Again, when it comes to nematodes, you heard me say it in the last video that I published. I want to simplify everything into buzzwords, associations, risk factors, and then throw in pneumonics where appropriate to help you

identify this on test day because after all, on tests I'm going to guess that most of your brain space is going to be occupied by bacteria and viruses, and so when it comes to other types of pathogens such as the nematodes, we really really want to simplify this, make

it as easy as possible so that you don't need to spend hours learning this material. So let's start with *Toxocara canis*. *Toxocara canis* has the word *canis* in the name because the canid animals like dogs, cats, etc. are the ones that get

infected. Now, there's fecal-oral transmission here, and humans are the intermediate host. Risk factors are going to include tropical and subtropical climates, and something that I want you to think about are community sandboxes. So because your canid animals are the

ones that are going to be infected, they have a tendency to interact with soil or perhaps poop on the dirt or the soil in and around sandboxes, and so those who are more likely to get this infection are children because children are more likely to come in contact with infected

soil or in this case infected sandboxes so I haven't really seen this mentioned elsewhere but if you see sand sandboxes in the vignette you really want to think about toxocariasis now the two big symptoms for toxocariasis infection are

one are one blindness and two meningoencephalitis this can be further subdivided into different disease processes which I'll explain in just a second here but big picture visual problems and central nervous system problems equals toxocariasis so now

let's talk specific now the first disease process you want to be on the lookout for is ocular larva migrans and so what happens here is that the pathogen migrates through the posterior eye and that causes unilateral vision loss that's why it's called ocular larva

migrans because the larvae migrates or migrates through the posterior eye causing ocular symptoms so the name tells you exactly what's going on likewise you can have visceral larva migrans because your larvae is migrans or migrating through the viscera so when

it does that it can cause infections in various hollow or solid organs the big one to keep an eye open for on your test is hepatosplenomegaly because of involvement of the liver or the spleen but this is not just limited to the

liver or the spleen you can also see involvement of the heart and other viscera so visceral larva migrans the word larva migrans means that the larvae migrates and then the first word tells you where it's migrating to so again on

Your exam, if they give you ocular larva migrans, look for ocular symptoms. If they give you visceral larva migrans, look for visceral symptoms. So let's just keep this simple. Lastly, if the infection becomes really, really severe and spreads

to the brain, you get central nervous system involvement, and the big thing to be on the lookout for here is meningoencephalitis. But you can also get secondary neuropsychiatric symptoms and seizures. Treatment for toxocara canis is bendazole. So toxocara canis again has 'canis' in the

name 'canis' because it infects canid or canine animals, dogs, cats, etc. And so really what you need to take away from this is that the two big clinical symptoms that they'll give you in the question if they want to test you on

toxocara canis is blindness and central nervous system involvement. And so my pneumonic here is that canids use their eyes and brains to hunt: eyes for ocular larva migrans or blindness and brains for CNS involvement or meningo

encephalitis. So if you think about a fox for example, using its eyes and its brains to hunt at night or to hunt, find its food, whatever, it really relies on its vision and its brain in order to hunt, and it's a canid. Canid helps you

remember toxocara canis. So that's all I need you to know for toxocara canis. Again, I'm trying to simplify this because you don't need to waste a ton of brain space here. Remember blindness, meningoencephalitis, and remember sandboxes.

That's *Toxocara canis*. Now we'll talk about *Onchocerca volvulus*. This is also known as African river blindness. The reason that this is known as African river blindness is because the black fly is the vector. Now, the black fly, or *Simulium damnosum*, which is very difficult to pronounce, that fly tends to live around rivers. And what happens here is that that fly relies on oxygen-rich, fast-flowing bodies of water like rivers, like rivers in Africa. And so what we tend to see is that because there is a

disproportionate number of black flies around high-flowing, oxygen-rich rivers in Africa, people who live around those rivers tend to make up the pockets of high infectivity. And so if I were to put up a map and show you, which I'm not

going to do, but let's just say if I had a map and I highlighted in red areas of the map where there was a disproportionate number of *Onchocerca volvulus* infections, what you would see is that around rivers there would be a lot of color on the map. Now, *Onchocerca*

*volvulus* gets injected into the human host during the L4 larval stage of growth, and after that it takes approximately one year to fully mature into an adult worm, which is known as a microfilaria. Now, you don't need to understand the stages of larval growth,

but just so you hear it from me in case you see it elsewhere or see it on an exam, when the larvae are growing, they progress through stages of growth, and those stages are numbered L1, then L2, then L3, etc. And so here it's injected during

that L4 larval stage of growth and before the L4 larval stage the pathogen is essentially growing within the black fly through stages L1 through L3. Now it takes approximately one year to mature and then once it matures and it's in the

human host it starts to manifest symptoms. The key symptoms to keep in the back of your mind for Onchocerca volvulus are blindness and dermatitis. And so what's happening here is that there is an immune reaction to the adult worms or there's an immune reaction to the

microfilaria and specifically this occurs in the eye and in the dermis. So we see visual symptoms and we see skin symptoms. The pathophysiology here is that iNOS becomes modified and there is an increased expression of pro-inflammatory interleukins IL-1 and

IL-4. So what we see here is that from the visual perspective you're going to have recurrent conjunctivitis, punctate keratitis, photophobia, and progressive visual loss. So the infection makes its way into the eye because the adult worm is in the eye. There's that immune

reaction and that causes visual symptoms. From the skin perspective you get something called leopard skin. So this is marked by pruritis, papular dermatitis, lichenified dermatitis, and skin depigmentation. So eye symptoms and skin symptoms because the microfilaria are in

the eye and the dermis, and it's not the adult worm itself causing the symptoms, it's the body's immune response to the adult worms that causes the symptoms. And again the ones to look for are blindness and dermatitis, and so visual loss and skin changes.

treatment is going to be Ivermectin now the way that I think about this is ancho Circa sounds like ancho circus and I think about the circus and specifically I think about people performing in the circus who are the blind man and the

skin guy and I know this might sound out there but every circus has a blind man and every circus has that skin guy that has really stretchy skin that can pull his skin and it kind of looks like a fish in one way or another so I think

the ancho circus blind man and skin guy and what this helps me remember is that blind man there's going to be visual loss and blindness and visual changes and the stretchy skin guy in the circus helps me remember that there's going to be skin changes dermatitis etc so ancho

circus your circus performer is the blind man and the skin guy that's it now let's talk about Loa loa this is also known as the African eyeworm and you'll understand on the next slide why it's called the African eyeworm the vectors

for Loa loa are your deerfly and your mango fly the key symptoms this is really the big part of what they test you on on USMLE and COMLEX is you you have to remember that Loa loa causes painful vision but no vision loss and dermatological

manifestation so if you're sitting there thinking wait a minute you're telling me stuff in the eye and stuff in the skin how am I supposed to differentiate Loa loa from onchocerciasis hang on because almost 100% of the time they're going to

give you a picture on your exam now the changes that you have to look for the clinical symptoms you're going to have angioedema pruritus urticaria and painful vision without vision loss and that last part kind of differentiates it from Onchocerca

because you're not going to get blindness there's no visual loss it's just pain in the eye so the angioedema is because the Loa loa worm migrates through human subcutaneous tissue as it does that if you can imagine a worm

moving through the subcutaneous tissue what might happen in response to that well you'd get swelling you'd get angioedema you'd get itching you'd get pruritus you'd get urticaria and then when that worm makes its way to the eye it

causes pain with vision or painful vision and that's because it's in the subconjunctival tissue the treatment for Loa loa is diethylcarbamazine now I want you to relax because like I said you're probably sitting there starting to worry a little

bit like all of these different uh nematodes are in the eye how do I keep this straight if the exam writer wants you to pick Loa loa they're likely going to give you a picture Loa loa in the eye looks like this you literally

see it in the eye and that's why it causes painful vision because the worm is in the subconjunctival tissue and it moves around in the eye because of that there's sort of like a space-occupying lesion although in this case it's a worm

in the eye, so it's going to be painful, it's going to feel full, and you're literally going to see it if you inspect the eye. So if you see a picture like this on your exam, the answer is *Loa loa*. You cannot get this confused with

onchocerciasis or volvulus because although that does have visual symptoms, those visual symptoms are due to an immune reaction to the microfilaria. Whereas in *Loa loa*, literally the worm is in your eye, and that is the cause of the symptoms. So the way that you can memorize this is

that *Loa* and *I* each have three letters. So if you're sitting there on your exam and you're like, okay, I have a picture, I see a worm in the eye. It has three letters. Which nematode has three letters? *Loa*. The answer is *Loa*. *Loa* and you're

done. So that's all you need to memorize, that's it. Let's conclude by talking about *Wuchereria bancrofti*. This is my favorite name because it sounds wild. So *Wuchereria bancrofti* is spread by mosquitoes, and interestingly, this is the

second leading cause of infectious disability worldwide, and you'll understand why that is in just a moment. Adult filarial worms reside in the lymphatics, the *epididymis*, the *epididymis*, and the *iliac* lymphatics, and so the symptoms that you're going to see clinically are going

to have to do with problems with lymphatic drainage and flow. So the big one is lymphatic filariasis, and what you can see within that spectrum of disease is one, lymphedema, because we have lymph anasia blockage and inflammation, which is triggered by those dying worms in the

lymphatics, so they reside in the lymphatics and then they die. They block the lymphatic ducts and they cause lymphangiectasia. And so collectively these things are going to cause lymph edema. Additionally, you can have the presence of something called

adenolymphangitis, which is a very fancy way of saying fever and painful lymph adenopathy. And then the most severe manifestation of a Wuchereria bancrofti infection is elephantitis. Elephantitis is the name that's given to very severe swelling, usually of the

limbs, although not always, associated with hyperpigmentation and hyperkeratosis. And I'm going to show you an image of what this looks like. You'll never forget it once you see it. And the treatment here is diethylcarbamazine. So here's a

picture of elephantitis. Again, what you're seeing here in this image is the most severe manifestation of the disease where you have really horrible, severe swelling of the limbs with hyperpigmentation and hyperkeratosis. Excuse me for going back and forth on

the slide. The way that I remember this is Wuchereria bancrofti sounds like "bank", right? "Bank" cofty. It's a bank, and that makes me think of a piggy bank, but in this case, the piggy bank is actually an elephant piggy bank. So your elephant

piggy bank, elephantiasis, associated with bancrofti. That's it. Four nematodes, not a lot that you need to know, but each of them have a very distinct and a very high-yield clinical manifestation. Don't waste a lot of brain space here because again

These are not as high-yield as your bacteria and your viruses, but you absolutely need to get these free points on test day if they test you on it. So for that reason, I would encourage you to re-watch this video if you need to.

Nematodes (TISSUE parasites)

Dirty Medicine

Introduction to Tissue Nematodes and Learning Strategy

Focus on four tissue nematodes: *Toxocara canis*, *Onchocerca volvulus*, *Loa loa*, and *Wuchereria bancrofti*.

Emphasis on simplification using buzzwords, associations, and pneumonics for efficient learning.

Prioritization due to the greater emphasis on bacteria and viruses in exams.

Introduction of *Toxocara canis*, its association with canid animals, and fecal-oral transmission.

Toxocara canis: Transmission, Risk Factors, and Symptoms

Risk factors include tropical/subtropical climates and community sandboxes.

Children are at higher risk due to contact with infected soil or sandboxes.

Main symptoms: blindness and meningoencephalitis.

Disease processes: ocular larva migrans (unilateral vision loss), visceral larva migrans (hepatosplenomegaly, heart, other organs involvement), and central nervous system involvement (meningoencephalitis, neuropsychiatric symptoms, seizures).

Treatment: bendazole.  Mnemonic: Canids use eyes and brains to hunt.

Onchocerca volvulus: Transmission, Symptoms, and Treatment

Also known as African river blindness; black fly (*Simulium damnosum*) as vector, prevalent near rivers.

Key symptoms: blindness and dermatitis (immune reaction to adult worms/microfilaria).

Pathophysiology: iNOS modification, increased pro-inflammatory interleukins (IL-1, IL-4).

Visual symptoms: conjunctivitis, keratitis, photophobia, visual loss. Skin symptoms: "leopard skin" (pruritis, papular/lichenified dermatitis, depigmentation).

Treatment: Ivermectin. Mnemonic: "Ancho circus" (blind man and skin guy).

Loa loa: Transmission, Symptoms, and Treatment

Also known as African eyeworm; vectors: deerfly and mango fly.

Key symptoms: painful vision (no vision loss) and dermatological manifestations (angioedema, pruritus, urticaria).

Angioedema, pruritus, and urticaria due to worm migration through subcutaneous tissue; painful vision due to worm presence in subconjunctival tissue.

Treatment: diethylcarbamazine.  Often diagnosed visually.

Wuchereria bancrofti: Transmission, Symptoms, and Treatment

Spread by mosquitoes; second leading cause of infectious disability worldwide.

Adult worms reside in lymphatics; symptoms related to lymphatic drainage problems.

Lymphatic filariasis: lymphedema (lymph anasia, inflammation, dying worms), adenolymphangitis (fever, painful lymph adenopathy), and elephantitis (severe swelling, hyperpigmentation, hyperkeratosis).

Treatment: diethylcarbamazine. Mnemonic: "Bank" (elephant piggy bank).