Anthelmintics

Anthelmintics

Helminths are parasitic worms, and anthelmintics are the drugs used against them. Helminthiasis (worm infestation) is the most common affliction of humans, affecting more than 2 billion people worldwide. The intestine is a frequent site of infestation. Other sites include the liver, lymphatic system, and blood vessels. Infestation is frequently asymptomatic. However, infestation with some parasites can cause severe complications. Helminthiasis is most prevalent where sanitation is poor. Cleanliness greatly reduces infestation risk.

Treatment of helminthiasis is not always indicated. Most parasitic worms do not reproduce in the human body. Hence, in the absence of reinfestation, many infections simply subside as adult worms die. Accordingly, treatment may be optional. In countries where providers and medication are readily available, drug therapy is definitely indicated. However, in less fortunate locales, several factors—cost of medication, limited medical facilities, and high probability of reinfestation—may render individual treatment impractical. In these places, preventative measures, such as improved hygiene and elimination of carriers, may be the most valuable interventions.

In approaching the anthelmintic drugs, we begin by reviewing classification of the parasitic worms. Next we briefly discuss the characteristics of the more common helminthic infestations. After this, we discuss preferred drugs for treatment.

Classification of parasitic worms

The most common parasitic worms belong to three classes: Nematoda (roundworms), Cestoda (tapeworms), and Trematoda (flukes). Nematodes belong to the phylum Nemathelminthes. Cestodes and trematodes belong to the phylum Platyhelminthes (flat worms).

Nematodes (roundworms)

Parasitic nematodes can be subdivided into two groups: (1) those that infest the intestinal lumen and (2) those that inhabit tissues. There are five major species of intestinal nematodes. Their common names are giant roundworm, pinworm, hookworm, whipworm, and threadworm. Official names (eg, Ascaris lumbricoides) are listed in Table 97–1. Two types of nematodes invade tissues: (1) pork roundworms (responsible for trichinosis) and (2) filariae. The three species of filariae encountered most commonly are listed in Table 97–1.

TABLE 97–1

Drugs of Choice for Parasitic Worms

	Parasitic Organism
Worm Class	Common Name	Official Name	Drugs of Choice
Nematodes (roundworms): intestinal	Giant roundworm	Ascaris lumbricoides	Albendazole or mebendazole or ivermectin
	Pinworm	Enterobius vermicularis	Albendazole or mebendazole or pyrantel pamoate
	Hookworm	Ancylostoma duodenale, Necator americanus
	Whipworm	Trichuris trichiura	Albendazole
	Threadworm	Strongyloides stercoralis	Ivermectin
Nematodes (roundworms) extraintestinal	Pork roundworm	Trichinella spiralis	Albendazole*
	Filariae	Brugia malayi, Loa loa, Wuchereria bancrofti,	Diethylcarbamazine^†
		Onchocerca volvulus	Ivermectin
Cestodes (tapeworms)	Beef tapeworm	Taenia saginata	Praziquantel*
	Pork tapeworm	Taenia solum
	Fish tapeworm	Diphyllobothrium latum
Trematodes (flukes)	Blood fluke	Schistosoma species	Praziquantel
	Intestinal fluke	Fasciolopsis buski
	Lung fluke	Paragonimus westermani
	Liver flukes	Fasciola hepatica (sheep liver fluke)	Triclabendazole^‡
		Clonorchis sinensis (Chinese liver fluke)	Praziquantel or albendazole*

^*Not approved by the Food and Drug Administration for this indication.

^†Available from the Centers for Disease Control and Prevention (CDC).

^‡Not available in the United States.

Cestodes (tapeworms)

Three species of cestodes infest humans. Common names for these parasites are beef tapeworm, pork tapeworm, and fish tapeworm. Their official names appear in Table 97–1.

Trematodes (flukes)

Five species of trematodes infest humans. These organisms fall into four groups having the following common names: blood fluke, liver fluke, intestinal fluke, and lung fluke. Official names of the five species belonging to these groups are given in Table 97–1.

Helminthic infestations

This section describes the major characteristics of infestation by specific helminths. These infestations can differ with respect to anatomic site and danger to the host. Infestations also differ with respect to the drugs employed for treatment (see Table 97–1 for a summary).

The name applied to an infestation is based on the official name of the invading organism. For example, infestation with the giant roundworm, whose official name is Ascaris lumbricoides, is referred to as ascariasis.

In the discussion below, the helminthic infestations are grouped in four categories: (1) nematode infestations of the intestine, (2) nematode infestations of extraintestinal sites, (3) cestode infestations, and (4) trematode infestations.

Nematode infestations (intestinal)

Ascariasis (giant roundworm infestation).

Ascariasis is the most prevalent helminthic infestation. Worldwide, one of every three people is affected. Adult worms inhabit the small intestine. Ascariasis is usually asymptomatic. However, serious complications can result if worms migrate into the pancreatic duct, bile duct, gallbladder, or liver. In addition, if infestation is extremely heavy, intestinal blockage may occur. Because of these potential hazards, ascariasis should always be treated. Drugs of choice are albendazole, mebendazole, and ivermectin.

Enterobiasis (pinworm infestation).

Enterobiasis is the most common helminthic infestation in the United States. Adult pinworms inhabit the ileum and large intestine. Their life span is approximately 2 months. Although usually asymptomatic, some patients experience intense perianal itching. Serious complications are rare. Drugs of choice are albendazole, mebendazole, and pyrantel pamoate. Because enterobiasis is readily transmitted, all family members of an infected individual should be treated simultaneously.

Ancylostomiasis and necatoriasis (hookworm infestation).

Hookworm infestation is most common in rural areas where hygiene is poor and people go barefoot. Adult hookworms attach to the wall of the small intestine and suck blood. As a result, infestation is associated with chronic blood loss and progressive anemia. Symptomatic anemia is most likely in menstruating women and undernourished individuals. Nausea, vomiting, and abdominal pain may accompany the infestation. Albendazole, mebendazole, and pyrantel pamoate are treatments of choice.

Trichuriasis (whipworm infestation).

Trichuriasis is extremely common, affecting about 1 billion people worldwide. Larvae and adult worms inhabit the large intestine. Mature worms may live for 10 years or more. The disease is usually devoid of symptoms. However, when the worm burden is very large, rectal prolapse may occur. Patients with severe infestation require therapy. Albendazole is the treatment of choice.

Strongyloidiasis (threadworm infestation).

Strongyloidiasis is common in the southern United States. Larval and adult threadworms inhabit the small intestine. The disease can be very dangerous, although symptoms are usually absent. Mild infestation may cause abdominal pain and occasional diarrhea. Severe infestation can cause vomiting, massive diarrhea, dehydration, electrolyte imbalance, and secondary bacteremia. Death has occurred. Affected individuals should always be treated. Ivermectin is the treatment of choice.

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Tags: Pharmacology for Nursing Care

Jul 24, 2016 | Posted by admin in NURSING | Comments Off

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