An antihelminthic is an antiparasitic drug that treats infections caused by parasitic worms, known as helminths. These medications work by either killing the worms or stunning them so the body can expel them. Helminths are multicellular organisms that live in and feed on a host, causing a condition called helminthiasis. These drugs are a common method for managing such infections in both humans and animals.
Types of Helminth Infections
Helminth infections are caused by parasitic worms classified into three main groups: Nematodes (roundworms), Cestodes (tapeworms), and Trematodes (flukes). Each group contains species that cause distinct infections, which are prevalent in areas with warm climates and inadequate sanitation. The transmission and location of the infection within the body differ between these worm types.
Nematodes, or roundworms, have cylindrical bodies and cause some of the most common worm infections. For example, Ascaris lumbricoides causes ascariasis, where adult worms live in the intestines. Other significant nematode infections include enterobiasis (pinworms) and hookworm infections, where larvae can penetrate the skin. These worms reside in the intestinal tract and absorb nutrients from the host.
Cestodes, known as tapeworms, are long, segmented flatworms that attach to the intestinal wall. Humans become infected by consuming undercooked meat containing tapeworm cysts, such as Taenia solium from pork or Taenia saginata from beef. Adult tapeworms remain within the intestines, using suckers or hooks on their head to anchor themselves.
Trematodes, or flukes, are leaf-shaped flatworms with suckers that allow them to attach to host tissues. Depending on the species, flukes can infect organs like the liver, lungs, and blood vessels. Schistosomiasis is a significant disease caused by blood flukes of the genus Schistosoma, which reside in the bloodstream. Other flukes can cause infections in the bile ducts or intestines.
How Antihelminthics Work
Antihelminthic drugs are selectively toxic, targeting the parasite’s biology while causing minimal harm to the host by exploiting cellular differences. The main strategies involve disrupting the worm’s neuromuscular coordination, interfering with its energy metabolism, or compromising its cellular integrity. These actions lead to the parasite’s death or expulsion.
One mechanism is inducing paralysis in the worm. Some drugs, like pyrantel pamoate, act on receptors in the parasite’s muscle cells, triggering sustained contractions and a spastic paralysis. This prevents the worm from maintaining its position in the host’s gut. Other medications, such as piperazine, cause a flaccid paralysis, which also results in the worm being expelled.
Another strategy is to disrupt the worm’s ability to absorb nutrients, effectively starving it. Benzimidazoles, a prominent class of antihelminthics, work by binding to a protein called beta-tubulin in the parasite’s cells. This binding prevents the formation of microtubules, structures needed for glucose uptake. Without glucose, the worm cannot produce energy, leading to immobilization and death.
A third mechanism involves damaging the protective outer layer, or tegument, of certain worms. Praziquantel, for example, is effective against flukes and tapeworms because it increases their cell membrane permeability to calcium. This influx causes severe muscle contractions and damages the tegument, exposing the parasite to the host’s immune system.
Common Antihelminthic Medications
Antihelminthic medications treat various parasitic worm infections and are available over-the-counter or by prescription. These drugs are categorized into chemical classes, like benzimidazoles, which share similar mechanisms of action. The choice of medication depends on the specific helminth causing the infection.
The benzimidazoles, including albendazole and mebendazole, are a prominent class of antihelminthics. They are effective against a broad spectrum of nematodes like roundworms, hookworms, and whipworms. Albendazole is used for systemic infections, while mebendazole is better for intestinal worms due to its poor absorption. Both require a prescription.
Ivermectin is a prescription medication effective against certain roundworm infections like onchocerciasis (river blindness) and strongyloidiasis. It belongs to the macrocyclic lactone class and works by paralyzing the parasite. Ivermectin is administered as an oral tablet.
Over-the-counter options are available for some infections. Pyrantel pamoate is a common non-prescription treatment for pinworms that paralyzes the worms, allowing them to be passed through stool. Praziquantel is the prescription drug for treating infections caused by flukes and tapeworms, including schistosomiasis.
Administration and Side Effects
Antihelminthic medications are administered orally as a chewable tablet, standard tablet, or liquid suspension. The treatment course is short, sometimes a single dose, but can extend to several days depending on the infection and drug. For instance, a single dose of albendazole is sufficient for ascariasis, while other infections may require a three-day course.
Follow the dosage instructions from a healthcare professional or on the product packaging. Some medications are best taken on an empty stomach to target intestinal worms, while others are taken with a fatty meal to increase absorption for tissue-based infections. In households with pinworms, it is recommended that all members be treated simultaneously to prevent reinfection.
Side effects from antihelminthic drugs are mild and temporary. Common adverse effects are gastrointestinal issues, including:
- Nausea
- Stomach pain
- Vomiting
- Diarrhea
Other possible side effects are headache, dizziness, and hair loss with prolonged use.
Severe side effects are uncommon but can occur. Allergic reactions are possible, and prolonged therapy may affect liver function or bone marrow. With infections like filariasis, the rapid death of parasites can trigger an immune reaction, leading to fever and other systemic symptoms. Patients should consult a healthcare provider for the correct diagnosis and treatment plan.