Detection of rare parasite on Pap smear

Identify the parasite

1. Enterobius vermicularis

2. Wuchereria bancrofti

3. Strongyloides stercoralis

4. Synthetic fiber.

Life cycle of Strongyloides

Clinically, it is rare to find S. stercoralis on uterine cervix due to the natural life cycle of the worm. The worm passes its life cycle in one host and unlike other nematodes, a change of the host is not essential. Human is the optimum host for S. stercoralis, although the primary mode of infection is through soil contaminated with larvae. Autoinfection in humans is also known. The female is parthenogenetic and thus can reproduce without male parasite. The filariform larvae penetrate the bare host skin coming in contact with the soil, invade the tissues, and enter the lungs through the right heart via venous blood circulation. From the pulmonary capillaries, larvae enter the alveolar spaces, migrate to bronchi, trachea, larynx, epiglottis and then are swallowed back into the intestinal tract or enter the intestinal tract through the connective tissues where it develops into a full-blown adult worm in small intestine. The females burrow into the mucous membrane and being parthenogenetic, begin to oviposit in the tissues. From the eggs, rhabditiform larvae hatch out and enter the lumen of intestine. Male parasites do not penetrate the mucous membrane and hence are eliminated through feces.

Pathogenicity

The infection with S. stercoralis is known as strongyloidiasis causing skin lesions in the form of urticaria or rash, pulmonary lesions as bronchopneumonia and intractable diarrhea due to intestinal lesions. Marked eosinophilia and moderate leukocytosis may be observed during the invasive stage.

S. stercoralis infection can be a chronic disease of the gut with low parasite load and irregular shedding of larvae. Since majority of patients are asymptomatic or with a vague complaint, it becomes difficult to diagnose and measure the effectiveness of standard treatment imparted and assure complete cure of the disease in the absence of any standard diagnostic tests. The infection may be associated with high or normal eosinophil count during the invasive phase, but this test has limitations.^[2] The definite diagnosis would be achieved by stool examination, but stool examination has its limitation in terms of irregular shedding of the worms.^[3]

S. stercoralis on uterine cervix forms an incidental diagnosis on Pap smear cytology. It is important to differentiate S. stercoralis from other parasites such as Enterobius vermicularis, Entamoeba histolytica, Paragonimus westermani, or microfilaria. S. stercoralis resembles very closely to hookworm in morphology which forms the immediate differential diagnosis on cytology, but the short buccal cavity is unique of E. vermicularis. S. stercoralis can also be distinguished by the prominent genital primordium and the pointed tail end. The larvae are characterized by cuticle, short oral cavity, long slender cylindrical body, and pointed tail with a “V”-shaped notch. Filariform larvae are longer, thinner and have a long oral cavity compared to rhabditiform larvae, which are thick, curved and with a short oral cavity and muscular esophagus. Filariform larvae can be easily mistaken for hookworm larvae.

In the present case, the woman was retrospectively investigated after the result of Pap cytology. Blood count was within normal limits. Serial stool examinations were done on alternative days, as single stool examination fails to detect larvae in 70% of the cases.^[4]

All stool reports were negative for larvae. There was no history of immunodeficiency syndrome. X-ray chest was normal. The possibility of contamination of cytology slides was excluded. The clinical history of walking barefoot on the beach side which may be suggestive of the route of entry of S. stercoralis in the present case was elicited retrospectively. The woman complained of vague abdominal pain which could have been probably related to helminthic infection though sonography of abdomen and pelvis did not reveal any abnormality. The patient was treated with 200 µg/kg ivermectin once daily for 3 consecutive days, considering chronic infection.^[2]

At present, available diagnostic tests for S. stercoralis are cumbersome to perform and have varied sensitivity and specificity reported, making it difficult to reach a diagnostic conclusion based on these laboratory tests.^[5] The present case highlights the gradual increasing number of S. stercoralis cases being reported on Pap smear.^[6-10] Mostly, the cases are asymptomatic and have been reported as an incidental finding on cytology smears among women who presented for routine cervical cancer screening. Recently, a case of S. stercoralis has been reported in cervical biopsy of woman who presented with reproductive tract infection.^[11] With the rising case reports of asymptomatic S. stercoralis being reported from India and in the absence of any consensus on the screening and investigation guidelines for the country, it is difficult to clinically manage the cases for an effective cure. The efficacy of standard drugs recommended has not been widely evaluated which again remains challenge in the absence of a diagnostic test.^[12]