Pinworm

Background

Pinworm, or Enterobius vermicularis, is prevalent throughout the temperate zones of the world and is the most common helminthic infection in the United States.

E vermicularis is a 2-13 mm, white, slender nematode that dwells in the caecum, the appendix, and the ascending colon of humans (see Image 1). Infection is generally asymptomatic. Some individuals who are infected experience sharp, prickling pains or intense itching in the anal area, especially at night.

Pinworm infection is primarily a condition of children, and parents are typically infected by transmission through their children. Transmission is through direct contact with contaminated furniture, bedclothes, or doorknobs. The parasite can also be transmitted during sexual contact.

Pathophysiology:

Usually, no symptoms occur when pinworms inhabit the caecum and adjacent areas. Diarrhea can occur during acute infection because of inflammation of the

bowel wall.

The primary symptoms, itching or prickling sensation of the perianal area, are produced when a gravid female worm migrates to the anal area and inserts her tail pin into the mucosa for ovideposition.

Although histologic findings of acute appendicitis demonstrated pinworm at the section, this correlation is not significant.

Frequency:

· In the US: Infection is most common in crowded living conditions and in persons who are institutionalized, but the prevalence of infection in some regions may be as high as 12% of the general population.

· Internationally: Egg carrier rates vary by country from 0.1-98.4%.

Mortality/Morbidity:

Pinworm infection does not cause severe morbidity unless ectopic infection occurs. This rare complication occurs in individuals with conditions that compromise the integrity of the bowel wall (eg, inflammatory bowel disease). Parasites migrate through the bowel wall and are found in extracolonic sites. Ectopic enterobiases have been described at the vagina, the salpinx, the inguinal area, the genital area, the pelvic peritoneum, the omentum, the liver, and even the lungs.

Race: No racial predilection is reported.

Sex: No sexual predilection exists.

Age: In adults, the highest rate of infection occurs in parents aged 30-39 years, typically because of transmission from their children aged 5-9 years.
History:

· Prickling pain in the anal area

· Intense anal itching (Although a hospital-based study of children aged 2-12 years indicated that perianal itching was not significantly more common in children who are infected than in children who are uninfected, persons who are Enterobius egg positive visit a physician because of perianal itching.)

· Restless sleep

· Rarely, abdominal discomfort

Physical:

· An adult female worm can be observed in the anal area with the naked eye.

· The worm's appearance is easily confused with bits of cotton thread.

Causes:

· Living with a person who is egg positive

· Eating before washing hands

· Poor personal or group hygiene

Lab Studies:

· Use a perianal cellophane swab or cellophane tape to detect eggs (see Image 2).

o The adult female dies after ovideposition; therefore, a 5-11% false-positive rate for eggs and a 70-95% false-negative rate for eggs exist.

o In one report, a single examination detects 50% of infections, 3 examinations detect 90% of infections, and 5 examinations detect 99% of infections.

o Repeated examinations 1-2 days later produce more accurate results.

Imaging Studies:

· Imaging studies are not necessary.

Other Tests:

· Other tests are not necessary.

Histologic Findings: Microscopic examination shows the elongated ovoid egg distinctly compressed laterally and flattened on one side. The egg measures 50-60 mm X 20-30 mm and contains larva.

Medical Care:

· Anthelmintics are active against E vermicularis.

· Reinfection with E vermicularis immediately after completing drug therapy is common. Additionally, young worms can be resistant to drugs.

o Eggs remain infective in the environment for 2 weeks after deposition.

o Reinfection from other persons who test positive for eggs is common.

o Tests for worms have a high false-negative rate. Therefore, successful eradication requires that individuals take at least 3 doses of medication, separated by 3 weeks. In institutionalized settings, 3 doses separated by 3 weeks have been necessary to achieve successful eradication.

· The recommended treatment includes the following:

o Simultaneously treat all family members and/or classmates who are infected.

o Prescribe drugs at least 3 times at 3-week intervals.

o Personal and group hygiene must be improved. Individuals must wash their hands before eating. Discourage children from activities such as sucking fingers.

· If large numbers of schoolchildren test positive for eggs, the result of treatment is best if all classmates and family members of the children who are infected are treated 3 times at 3-week intervals. If less than 30% of class members test positive for eggs, treating only the children who have positive test results reduces the positive rate for eggs in that class.

The goals of pharmacotherapy are to eradicate the infestation, reduce morbidity, and prevent complications.

Drug Category: Anthelmintics -- Parasite biochemical pathways are sufficiently different from the human host to allow selective interference by chemotherapeutic agents in relatively small doses.

Drug Name


Mebendazole (Vermox) -- Causes worm death by selectively and irreversibly blocking uptake of glucose and other nutrients in susceptible adult intestine where helminths dwell.

Adult Dose


100 mg PO once at least 3 times at 3-wk intervals

Pediatric Dose


<2 years: Not established
>2 years: Administer as in adults

Contraindications


Documented hypersensitivity

Interactions


Carbamazepine and phenytoin may decrease effects; cimetidine may increase levels

Pregnancy


C - Safety for use during pregnancy has not been established.

Precautions


Adjust dose in hepatic impairment; if worm appears repeatedly in anal area, reduce dose to 25 mg for 10 kg of body weight; family members in close contact with patient should be treated



Drug Name


Pyrantel pamoate (Pin-Rid, Pin-X) -- Depolarizing neuromuscular blocking agent. Inhibits cholinesterases, resulting in spastic paralysis of the worm.

Adult Dose


10 mg/kg PO once at least 3 times at 3-wk intervals

Pediatric Dose


<2 years: Not established
>2 years: Administer as in adults

Contraindications


Documented hypersensitivity; hepatic disease; first trimester of pregnancy

Interactions


In ascariasis, pyrantel and piperazine are mutually antagonistic and should not be used concomitantly; theophylline serum levels may increase in pediatric patients following administration

Pregnancy


D - Unsafe in pregnancy

Precautions


Caution in liver impairment, anemia, and malnutrition



Drug Name


Albendazole (Albenza) -- Decreases ATP production in worm causing energy depletion, immobilization, and, finally, death. To avoid inflammatory response in CNS, patient must also be started on anticonvulsants and high-dose glucocorticoids.

Adult Dose


400 mg single PO at least 3 times at 3-wk intervals

Pediatric Dose


10-14 mg/kg PO

Contraindications


Documented hypersensitivity; altered liver function; jaundice after therapy

Interactions


Coadministration with carbamazepine may decrease efficacy; dexamethasone, cimetidine, and praziquantel may increase toxicity

Pregnancy


C - Safety for use during pregnancy has not been established.

Precautions


Reported teratogenic effect to rats and rabbits; discontinue use if LFTs increase significantly (resume when levels decrease to pretest values)



Further Outpatient Care:

· Follow-up examination is necessary for a person who is positive for eggs after chemotherapy to determine whether reinfection exists. Although the first single examination may be negative for Enterobius eggs, a follow-up perianal swab is necessary if the patient has continuous complaints of perianal itching or prickling pain.

Deterrence/Prevention:

· Personal and group hygiene must be improved.

o Individuals must wash their hands before eating.

o Discourage children from activities such as sucking their fingers.

· Treatment is much more effective if the child's family and classmates are treated at the same time.

· Washing sheets, clothes, and towels in a washing machine using regular laundry soap can eliminate eggs. However, eggs become noninfective after 2 weeks. Therefore, an emphasis on washing and environmental cleaning does not significantly improve the effectiveness of therapy.

Complications:

· Pinworm infection does not cause severe morbidity unless ectopic infection occurs. This rare complication occurs in individuals with conditions that compromise the integrity of the bowel wall (eg, inflammatory bowel disease). Parasites migrate through the bowel wall and are found in extracolonic sites. Ectopic enterobiases have been described at the vagina, the salpinx, the inguinal area, the genital area, the pelvic peritoneum, the omentum, liver, and even the lungs.

Prognosis:

· Eradicating pinworm in a group of persons who are institutionalized is difficult. Continuous follow-up examination is necessary.

· Therapy is much more effective if the child's family and classmates are treated at the same time.

Patient Education:

· Focus on handwashing, especially before eating.

· Washing sheets, clothes, and towels in a washing machine using regular laundry soap can eliminate eggs. Eggs become noninfective after 2 weeks. Therefore, an emphasis on washing and environmental cleaning does not significantly improve the effectiveness of therapy.

· For excellent patient education resources, visit eMedicine's Parasites and Worms Center and Esophagus, Stomach, and Intestine Center. Also, see eMedicine's patient education articles, Pinworms and Anal Itching.

Medical/Legal Pitfalls:

· Failure to properly diagnose enterobiasis is a pitfall. The tools used to diagnose enterobiasis are not very sensitive or specific. Perianal swabs only can detect the female worm when her life cycle is terminated (after egg production). Therefore, the false-negative or false-positive results can be a medical and/or legal pitfall.