A patient presents with a worm they found in the toilet.
This is Ascaris lumbricoides, a roundworm. Distinctive morphologic features include tapered ends, mouthparts consisting of three prominent lips (pictured in image 2), and a length of up to 35cm for females. The adults live in the duodenum and proximal jejunum. The eggs have an irregular external mamillated outer shell that gives them a roughened outer surface. Clinically, infection can range from asymptomatic to severe disease, in which the larvae, hatched from ingested eggs, migrate from the small bowel through the circulatory system to the lungs, where they mature in the alveolar capillary bed and cause Ascariasis pneumonitis (Löffler syndrome).
Other diagnostic considerations include Enterobius vermicularis (pinworm), Lumbricus terrestris (earthworm), Trichuris trichiura (whipworm), and the hook worms, Necator americanus and Ancylostoma duodenale.
Enterobius vermicularis, the pinworm, is the most common helminth infection in the United States. Clinically, the classic presentation is a child with pruritus ani. Females measure up to 1.3 cm in length and have a pointed posterior end, and both sexes have lateral alae and a prominent esophageal bulb. The worm in this case is far too large to be a pinworm.
The earthworm, Lumbricus terrestris, is soil-dwelling and non-pathogenic but occasionally encountered in the laboratory for identification purposes. Key morphologic features include a segmented body with no distinctive mouthparts and a clitellum (a mating organ that is a non-segmented portion of the body and often a different color from the rest of the body).
Trichuris trichiura, the whipworm, have a classic whip-like appearance with long, narrow anterior ends that anchor the worm to the large intestine, where they can remain for up to 10 years. Both males and females measure up to 5.0 cm in length, and diagnosis is often made by identification of the eggs, which are football-shaped and have polar plugs at both ends. Clinically, trichuriasis can cause dysentery-type symptoms and, in heavily infected children, can lead to rectal prolapse.
Necator americanus and Ancylostoma duodenale are the hookworms. Adult females measure up to 1.2 cm, and these two species are differentiated by examination of the mouthparts: Necator americanus has cutting plates, while Ancylostoma duodenale has cutting teeth. In addition to the large size difference between hookworms and roundworms, the lamprey-like appearance of these mouthparts is notably different from the “fleshy lips” of Ascaris. Hookworms and roundworms, however, are similar in that their larvae have the ability to migrate through tissue to the blood stream then the lungs, where they can cause Löffler syndrome and are expectorated then swallowed before reaching the small bowel. Unlike Ascaris, the larvae of which hatch from ingested eggs and penetrate the host through the bowel wall to get to the lung capillary beds where they can mature, hookworm larvae hatch outside the body and, on contact with a host (once again, lamprey-style), directly penetrate the skin, enter the circulation, travel to the lungs, then migrate up the bronchial tree to be swallowed. If ingested, Ancylostoma larvae can mature into adults in bowel without needing to migrate through the lungs.
- Centers for Disease Control and Prevention. “Stool Specimens – Intestinal Parasites: Comparative Morphology Tables.” https://www.cdc.gov/dpdx/diagnosticProcedures/stool/morphcomp.html. Last reviewed May 3, 2016. Accessed April 2, 2021.
- “Earthworms.” University of Pennsylvania. https://www.sas.upenn.edu/~rlenet/Earthworms.html. Accessed April 2, 2021.
- McPherson, R, and M Pincus. (2011). Henry’s Clinical Diagnosis and Management By Laboratory Methods (22nd Edition, pp. 1218-1220). Philadelphia, PA: Elsevier Saunders.
-Frederick Eyerer, 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.