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Anthelmintic medications

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Anthelmintic medications

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USMLE® Step 2 style questions USMLE

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A 14-year-old girl comes to the office because of fatigue, anxiety, and balding for a month. Her parents are missionaries and note that her appetite has been odd roughly since they returned to the United States from a trip abroad. Among her odd behaviors, she has a new found craving for all things made of ice and the other day they say they caught her eating dirt. When questioned about her behavior the girl expresses shame but states that she does these things "almost compulsively." Examination shows a non-tender abdomen with areas of diffuse fullness, and palpebral pallor. Peripheral smear shows anisocytic, microcytic, hypochromic red blood cells. Eosinophilia and lymphocytosis are also shown. Iron studies show decreased transferrin, and decreased ferritin levels. A trichobezoar is retrieved on upper endoscopy. Which of the following is the most appropriate next step in treatment?

Transcript

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!

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 hatched 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.

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.

Sources
  1. "Katzung & Trevor's Pharmacology Examination and Board Review,12th Edition" McGraw-Hill Education / Medical (2018)
  2. "Rang and Dale's Pharmacology" Elsevier (2019)
  3. "Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th Edition" McGraw-Hill Education / Medical (2017)
  4. "Selective mass treatment with ivermectin to control intestinal helminthiases and parasitic skin diseases in a severely affected population" Bull World Health Organ (2004)
  5. "Treatment of hydatid disease with high oral doses of mebendazole" European Journal of Clinical Pharmacology (1986)
  6. "Management of intestinal obstruction caused by ascariasis" Journal of Pediatric Surgery (1997)
  7. "Resolution of Praziquantel" PLoS Neglected Tropical Diseases (2011)