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Pharmacotherapy of helminth infections: Pinworm and roundworm
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Infection with pinworm and roundworm are likely the most common helminth infections that affect US children. They are transmitted through fecal-oral route by ingestion of contaminated soil and environment, or from other infected children.
Routinely recommended pharmacotherapy options are limited to several agents, and one of these anthelmintics has recently been discontinued from the commercial market. This column will review the pharmacotherapy of these common helminth infections (helminthiases).
Pinworm and roundworm
Infection with pinworm (Enterobius vermicularis) and roundworm (Toxocara species) affect millions of children worldwide and are relatively common in the United States. It is the ingestion of embryonated eggs of the adult female, E. vermicularis, that result in pinworm infection and symptoms. As the eggs hatch, larvae travel through the intestinal tract and mature. Adult female worms migrate out to perianal skin, where they lay eggs (several thousand), resulting in symptoms in the affected child. Diagnosis is made by examination of the perianal area for the presence of eggs (adhesive tape test), and not the stool, as stool contains no eggs or too few for diagnosis. Symptoms, primarily pruritus, result from the presence of the adult pinworm and eggs on the perianal skin. Subsequent secondary infection may also result (cellulitis, urinary tract infection).
Toxocariasis (visceral and ocular larva migrans) result from infection with T. canis and T. cati. These roundworms commonly affect dogs and cats, respectively. By contrast, the only known host of E. vermicularis is human. Infection with Toxocara may be quite common, as evidenced by studies documenting a seroprevalence of 13.9% in the US population. Infection with Toxocara also differs from pinworm in that systemic infection and symptomology occur from infection with Toxocara, resulting from systemic invasion of larvae from the small intestine (unlike pinworm infection).
Edward A. Bell
Distinct syndromes, visceral and ocular, have been described, with clinical manifestations of hepatitis, pneumonitis, laboratory changes (eosinophilia, hypergammaglobulinemia) and potentially central nervous system effects or ocular manifestations, respectively. Some have additionally described a third classification, covert toxocariasis, presenting with some of the symptoms seen with visceral larva migrans, including wheezing (which may be classified as an asthma component), pulmonary infiltrates and eosinophilia. It is the body’s immunologic and hypersensitivity response to the dying or dead systemically traveled Toxocara larvae that primarily results in these symptoms and complications.
Pharmacotherapy
Several pharmacotherapeutics agents are traditionally recommended and effective for treatment of infection with E. vermicularis and Toxocara species. Until recently, two benzimidazole agents were commercially available: mebendazole (formally known as Vermox, Biotech Pharmaceuticals Pty Ltd.) and albendazole (Albenza, GlaxoSmithKline).
Mebendazole was discounted from the commercial market in 2011. As compared with albendazole, mebendazole had decreased oral absorption and penetration into the central nervous system. Pyrantel pamoate, an anthelmintic agent available over-the-counter, may also be used to treat pinworm infection. The AAP Red Book lists albendazole (400 mg once, repeated in 2 weeks for additional treatment of eggs or maturing larvae) or pyrantel pamoate (11 mg/kg once, repeated in 2 weeks) as drugs of choice for treatment of infection with pinworm.
Only albendazole (400 mg twice daily for 5 days) is listed for treatment of toxocariasis in the Red Book. Recommendations from the CDC are similar. No direct comparative published studies of albendazole and pyrantel pamoate are available for these infections.
Although albendazole and pyrantel pamoate are recommended for treatment of the infections described above, neither agent is labeled by the FDA for use in children for these conditions. Fortunately, however, both anthelmintics are believed to be safe for use in children as young as 1 year of age, as described by the CDC. WHO additionally categorizes pyrantel pamoate and albendazole as safe for use in young children (aged 1 year and older).
Although albendazole is an effective anthelmintic, its use has some limitations. It is expensive, with a cash price at a pharmacy of approximately $67 per tablet. A 5-day course may have a cost of $1,200 or more. A generic formulation is not available. Albendazole is not well absorbed with oral administration, and absorption is increased when given with food (thus, caregivers can be told to administer with food). Although considered safe, when given long term, albendazole therapy may result in bone marrow suppression.
Although human pregnancy data are limited, animal data suggest a potential moderate risk for adverse outcomes. Although albendazole is poorly absorbed, administration with food may substantially increase absorption, and subsequent exposure to a metabolite (albendazole sulfoxide) may be teratogenic. It is recommended to avoid albendazole, especially during the first trimester of pregnancy. By contrast, animal data for use of pyrantel pamoate during pregnancy suggest a low risk, although no human pregnancy data are available.
Pyrantel pamoate is available as several different products (Pamix; Pin-X, Quartz Specialty Pharmaceuticals; Reese’s Pinworm Medicine, Reese), and as a suspension, chewable tablet, and tablet. It is relatively inexpensive (approximately $12 for suspension). Pyrantel pamoate dosing is based upon the base (11 mg/kg of base once, repeated in 2 weeks, for treatment of pinworm). If pyrantel pamoate products are used, it is important to clarify dosing based upon the base concentration (eg, 144 mg/mL of product suspension = 50 mg pyrantel base).
References:
Bagheri H. Ann Pharmacother. 2004;38:383-388.
For more information:
Edward A. Bell, PharmD, BCPS, is a professor of clinical sciences at Drake University College of Pharmacy, Blank Children’s Hospital, in Des Moines, Iowa. He is also a member of the Infectious Diseases in Children Editorial Board. He can be reached at: Drake University College of Pharmacy, 2507 University Ave., Des Moines, IA 50311; email: ed.bell@drake.edu.
Disclosure: Bell reports no relevant financial disclosures.