Microbiology Case Study: A 19 Year Old Woman Passes a Worm in Her Stool

Case History

A 19 year old woman with no significant past medical history presented to an outside clinic with psychological distress after passing a worm per rectum. She had no other complaints or symptoms.

Image 1. Eggs are spherical, 31 to 43 μm in diameter, with a thick yellow-brown (bile-stained) shell having radial striations and contain a hexacanth oncosphere (Image courtesy of: Fred Patterson, Parasitologist).
Image 2. Adult proglottids are longer (up to 20 mm) than they are wide (up to 7 mm) and have a genital pore (*) at the lateral margin.
Image 3. Formalin-fixed paraffin-embedded H&E stained section of a proglottid’s lateral uterine branches on each side of a central uterine stem (20x).

Discussion

Adult cestodes (tapeworms) have long, ribbon-like bodies made up of proglottids (egg-producing segments) that develop at the posterior of a scolex (specialized structure for attachment to the small intestine of a host). Taenia has 32 species, 2 of which are medically important for causing taeniasis: Taenia saginata and Taenia solium (beef and pork tapeworm infection, respectively). These parasites are distributed worldwide, with T. saginata being more common than T. solium.

The lifecycle of Taenia involves adult, egg, and larval stages. Adults release gravid proglottids and eggs that are passed in feces. The eggs reach pasture land via soil or water and are ingested by an intermediate host. For T. saginata, the intermediate host is a herbivore (cow), as eggs of T. saginata do not infect humans, and for T. solium, the intermediate host is a pig, human, or other animal. Ingested eggs hatch and release the hexacanth oncosphere (6-hooked embryo) that can penetrate tissues. Over 2 to 3 months, infective cysticerci (0.5 to 2.0 mm in diameter larvae) develop in muscles. Of note, only T. solium can cause cysticercosis (extra-intestinal larval forms within human tissues, ie. the human becomes the intermediate host) and this can be life-threatening if cysticerci invade the brain. When humans consume raw or undercooked beef/pork meat that is infected, cysticerci will attach to the small intestinal mucosa and, over 3 to 5 months, mature into the adult form. The adult T. saginata can reach 4 to 12 meters in length and the adult T. solium can reach 1.5 to 8 meters in length. Adult tapeworms can live within intestines for over 25 years while gravid proglottids and eggs are passed in stool.

Infections are usually asymptomatic or cause mild indigestion, anorexia, and abdominal discomfort. The eggs can be identified by ova and parasite examination (Figure 1) or a cellulose tape preparation of perianal skin. The oncosphere must be visualized to avoid misidentifying a pollen grain. However, eggs of Taenia species and Echinococcus species are indistinguishable. Diagnosis is also made by recovering gravid proglottids from the anal opening or passed in feces (Figures 2 and 3). Distinguishing the two species can be done by examining gravid proglottids for the number of lateral uterine branches present on one side of a central uterine stem. T. saginata have 15 to 30 lateral uterine branches while T. solium have 7 to 13 branches. Both species have a small anterior scolex (measuring 1 to 2 mm in diameter for T. saginata and 1 mm in diameter for T. solium) with 4 suckers. Definitive identification is possible since T. solium’s scolex has a rostellum (crown) with 2 rows of hooks whereas T. saginata’s scolex bears no rostellum or hooks. Treatment is a single dose of praziquantel and successful treatment is defined as passing zero proglottids over 4 consecutive months.

-Adina Bodolan, MD is a 3rd year anatomic and clinical pathology resident at the University of Vermont Medical Center.

-Christi Wojewoda, MD, is the Director of Clinical Microbiology at the University of Vermont Medical Center and an Associate Professor at the University of Vermont.

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