Anthelmintic medications

Definitions & Key takeaways

Anthelmintic medications are drugs that are used to treat parasitic infections, such as those caused by roundworms and tapeworms. These medications work by killing the parasites or by causing them to be expelled from the body. Some of the more common side effects of anthelmintic drugs include stomach pain, diarrhea, and nausea. In rare cases, these drugs can cause allergic reactions. Common anthelmintic drugs include albendazole, praziquantel, and mebendazole.

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Introduction0:00–0:34

Anthelmintics are a group of antiparasitic antibiotics that treat infections by parasitic worms or helminths. They are roughly divided into two groups: vermifuges, which stun helminths; and vermicides, which kill them.
Now “helminth” is not a term based on taxonomy; instead, it’s a practical term used for many multicellular, worm-like parasites that can infect humans.
These include cestodes, trematodes, and nematodes. Cestodes are tapeworms that can grow to prodigious size, some even reaching over 20 feet, or 7 meters!

Pathology0:34–4:40

Species like Taenia solium, or pork tapeworm, and Taenia saginata, or beef tapeworm, are transmitted via undercooked pork and beef.
Once eaten, they hang out in the small intestine, living off of the nutrient-rich fluid around them. They are also hermaphrodites and can lay over 50,000 eggs in their lifetime, so you’re never lonely when you have tapeworms!
The disease itself, taeniasis, can be asymptomatic or it can cause GI symptoms, like abdominal pain, nausea, diarrhea, and weight loss.
However, if the eggs of the Taenia solium found in human feces is ingested, it could cause cysticercosis. This is where newly hatch larvae burrow into different parts of the body like the eyes, which can cause blindness; and also the brain, which can lead to seizures and death.
Next are trematodes, which are more commonly known as flukes. Common species that infect humans include Schistosoma species, or blood flukes, that cause schistosomiasis, also called snail fever.
These parasites live inside freshwater snails and pop out as free swimming larva that search for unsuspecting swimmers, and penetrate the skin through hair follicles to make their way into capillary beds, where they feed on blood.
Liver flukes like Clonorchis and Opisthorchis species also use snails as a host, but their larvae invade fish, that are then eaten by humans.
Once ingested, the fluke burrows through the intestinal wall and head straight for the liver and bile duct, where they feed on bile.
Nematodes or roundworms are a particularly diverse bunch. Most infections start when a person eats or drinks something contaminated with nematode eggs, which are transmitted through human feces.
This is why it’s important to wash your hands after using the bathroom before eating! The eggs hatch into larvae in the small intestine and cause GI symptoms like malabsorption, diarrhea, and abdominal pain.
The most common species to infect humans are the Ascaris species, which causes ascariasis. Once hatched in the intestine, they bore through the gut to get to the bloodstream, and then travel to the lungs.
There, they travel up the trachea and get coughed up into the oral cavity, where they get swallowed back again so they can mature into adult worms in the intestines.
Severe infections with thousands of these worms could lead to complete obstruction of the intestines. Next are the Enterobius species, or the pinworms, which hatch in the small intestine but migrate to live in the colon.
At night, they come out of the anus and lay their eggs in the nearby skin folds, causing severe anal itching. Next we have whipworms, which hatch in the small intestine and happily burrow deep into the intestinal mucosal and remain there for life.
There’s also the Necator americanus, or hookworms, and Strongyloides stercoralis, or threadworms, which actually hatch in soil contaminated with feces that contain their eggs.
These worms burrow through the skin of unsuspecting humans walking barefoot, and then make their way to the intestines. Finally, we have the filarial worms, that need an insect as an intermediate host.
This includes loa loa which is transmitted by black flies, and invade the skin and eyes; and Wuchereria bancrofti, which is passed on by mosquitoes, and invades the lymphatic vessels, causing elephantiasis.
Okay, now that we’ve taken a trip through the helminth zoo, let’s look at ways to get rid of them. Anthelmintics are roughly divided into antinematodal agents and antiplatyhelmintic agents, which treat flukes and tapeworms.

Praziquantel4:40–5:32

The most commonly used antiplatyhelmintics is praziquantel, and it’s the first line therapy against most tapeworms and flukes.
Praziquantel is well absorbed in the GI tract. It increases the membrane permeability of calcium ions in the parasites, leading to muscle paralysis and death.
This medication is metabolized in the liver by cytochrome p450 enzymes, so inducers of this enzyme can decrease its effectiveness.
Praziquantel have limited effectiveness in treating neurocysticercosis caused by tapeworms, so it’s used as a second line medication.
For ocular cysticercosis, medical treatment is contraindicated as the dead worms could lead to permanent eye damage, so surgical removal is the best choice.
Side effects of praziquantel are usually mild and include headaches and fatigue. Ok, moving on!

Bendazoles5:32–6:19

Since there’s such a wide variety of nematodes, there’s also a lot of antinematodal agents. The medications with the widest spectrum are the members of the benzimidazole family, like albendazole and mebendazole.
These medications work by inhibiting the formation of microtubules, which form the cytoskeleton in the parasite’s cells.
Albendazole is the medication of choice for ascariasis, pinworms, hookworms, and whipworms. Strangely enough, it’s also the first line therapy for treating neurocysticercosis, which is caused by pork tapeworm.
The bendazoles are teratogenic, so they should not be used during pregnancy. Very rarely, they can cause bone marrow suppression and liver toxicity.

Pyrantel Pamoate6:19–6:58

Pyrantel pamoate is another antinematodal agent with a wide spectrum of activity. This medication is taken perorally and it’s poorly absorbed, so it only affects the nematodes in the GI tract.
Its mechanism of action is to stimulate the muscle’s nicotinic receptors at the neuromuscular junction, which causes uncontrolled muscle contractions, leading to paralysis and expulsion through the stool.
It’s an alternative to the bendazoles for many of the same nematode infections, but does not affect whipworms or tapeworms.
Side effects are mild and include headaches and GI symptoms. Another medication that causes paralysis is ivermectin, which interferes with the nervous system of invertebrates by binding to glutamate-gated chloride channels, on neurons and muscle cells.

Ivermectin6:58–7:44

This causes them to open permanently, leading to a rapid influx of chloride ions, which hyperpolarizes the cells. Hyperpolarized neurons can’t fire off action potentials, while hyperpolarized muscles can’t contract, so the nematodes become paralyzed and die.
It’s used as the medication of choice against threadworms, but could also act as a second line medication for ascaris and hookworms.
Side effects of ivermectin might include headache, dizziness, muscle pains, nausea, and diarrhea Now diethylcarbamazine has a narrower spectrum of activity and is the treatment of choice for Wuchereria bancrofti and loa loa filarial worms.

Diethylcarbamazine7:44–8:12

It works by changing the cell membrane of these worms through an unknown mechanism, which makes it easier for our immune system to target and remove them.
Side effects include itching, facial swelling, headaches, and in rare cases vision loss. Okay, for our mnemonic, there’s a farm with a pasture and a garden.

Memory Palace8:12–11:04

In the garden, we’ll put our nematodes, which are divided into 3 groups. The first group includes a scarecrow, for ascaris species, and it’s got a whip for whipworms, to scare away crows.
There are also pins sticking out of it, for pinworms, to make it look like a voodoo doll for extra spookiness. The next group are the soil-transmitted species, so let’s have a fish hook sticking out of the dirt, for hookworms and a fishing line attached to the end for threadworms.
Finally, off to the side, we have the filarial worms, represented by a swollen up elephant, for elephantiasis, caused by Wuchereria bancrofti; and on top of it is a hula dancer celebrating the luau for loa loa.
Now, the bendazoles will be represented by a contortion artist, who’s bending her body in unnatural ways. She’s doing her act between the scarecrow and the hook, since bendazoles can treat both of these groups.
She’s also holding a brain full of holes with tapeworms coming out, to help you remember it’s the first line therapy for neurocysticercosis.
Her assistant is a pirate with a pompadour, for pyrantel pamoate, who can treat most of the same things she can. However, he’s wearing an eyepatch since the whip took out his eye, so he’s not too effective against whipworms.
Ivermectin is a mammoth with large tusks and it’s pulling on the thread since it’s the drug of choice for threadworms. Finally, a carpenter named Ethel, for diethylcarbamazine, is trying to fence off the elephant and the hula dancer.
For some of the drug-specific side effects, let’s put a blindfold on Ethel for vision loss; there’s a spear poking out of the mammoth for muscle pain, and there’s a bent bone and a bent liver by the contortion artist, for bone marrow suppression and liver toxicity.
There’s also a bent pregnancy test to help you remember it’s teratogenic. Phew, now let’s move on to greener pastures, or in this case a pasture full of tapeworms and flukes.
So the livestock in this pasture includes a cow and a pig with tapeworms coming out of them, for beef and pork tapeworms.
Next, is a giant pond snail with a green and a red fluke riding on its back, for the bile-eating liver fluke and the vampiric blood fluke.
The medication used to treat these parasites is praziquantel, represented by a president, let’s use Jimmy Carter since he use to be a peanut farmer.
He’s holding a sign with an eye crossed out to help you remember praziquantel and other medications should not be used for ocular cysticercosis.
All right, as a quick recap. Anthelmintics are used for treatment of parasitic worms, or helminths.

Review11:04–11:24

These medications are divided into antiplatyhelmintic agents like praziquantel, which treats tapeworms and flukes; and antinematodal agents, like the bendazoles and pyrantel pamoate, that treat roundworms.

Mind Map11:24–11:52

But wait, there's more: Here's a mind map with all of the mnemonics from the video. Go ahead and pause the video so you can test yourself to see what you remember.
Stay tuned for the answers at the end.