Héctor H. García and Robert H. Gilman
Infection with Hymenolepis nana, the dwarf tapeworm, is the most common tapeworm infection in the world. H nana is found in 0.4% of fecal specimens submitted to state laboratories in the United States. Infections occur most frequently in warm countries. It is especially prevalent in the southern part of the former Soviet Union, the Mediterranean, the Indian subcontinent, and South America. Children are more commonly infected than adults, and high prevalence rates have been reported in institutionalized children because of fecal-oral transmission.1
In the usual cycle, H nana is passed between rodents as definitive host and beetles as intermediate hosts. H nana is unique among tapeworms, because humans can serve as both intermediate and definitive hosts and can close the cycle without the need for an animal intermediate host. This leads to human infection being directly acquired (from another human definitive host), thus contributing to its high prevalence.
The adult tapeworm measures 2 to 4 cm in length. It attaches to the mucosa of the small intestine by a scolex that has 4 circular suckers and a retractable structure called a rostellum. Eggs passed in the feces are immediately infectious for another human or for the original host (autoinfection). Ingested eggs hatch in the small intestine. The embryos penetrate the villi and transform into larval cysticercoids. After 4 or 5 days, the new adult tapeworms emerge from the tissue and attach to the intestinal mucosa. Egg production by the new worms begins about 2 to 4 weeks after infection. Eggs released from gravid proglottids in the intestine may hatch and cause internal autoinfection, producing hundreds or thousands of adult tapeworms in a single host.1,2
FIGURE 339-1. H nana egg recovered from feces. Note polar filaments (448×).
CLINICAL MANIFESTATIONS, DIAGNOSIS, AND TREATMENT
Although well-controlled studies of clinical manifestations of H nana infections are scarce, most H nana infections are most probably asymptomatic or unnoticed.3 Symptoms reported from several series of H nana infections are anorexia or increased appetite, nausea, vomiting, pains in the extremities, dizziness, and headache. Other reported symptoms are abdominal pain, diarrhea, restlessness, restless sleep, irritability, and nasal and anal pruritus. There are conflicting reports about correlation between the numbers of parasites and the presence of symptoms. Although a mild eosinophilia is a common finding in H nana infections, it is often absent. Rarely, H diminuta may be diagnosed, mostly in asymptomatic cases in stool surveys.4 The few reported cases do not allow examination of differences in clinical manifestations or response to treatment between H nana and H diminuta.
FIGURE 339-2. H diminuta ovum is larger than H nana, and polar filaments are absent (448×).
Routine fecal examinations using concentration techniques for ova and parasites should reveal eggs of H nana (Fig. 339-1) or more rarely H diminuta (Fig. 339-2). However, a single examination may not be adequate to rule out infection. Proglottids are rarely found in stools since they disintegrate after breaking from the tapeworm. ELISA tests for serum antibodies to H nana have been used in research but are of no clinical use.5
Praziquantel is the drug of choice for the treatment of hymenolepiasis. It is administered in a single dose of 25 mg/kg, although a second dose 10 to 15 days later seems to increase its efficacy. Patients should be informed that the drug is considered investigational by the FDA if used for this purpose. Niclosamide is also effective but is no longer available in the United States. Nitazoxanide can be used as an alternative with slightly lower efficacy.3 Because it is common for several individuals within a household to be infected, fecal examinations should be performed on all household members before initiating treatment. Posttreatment fecal examinations should be done at least 1 month after treatment.