Perianal nodule due to Enterobius vermicularis: Cytomorphological spectrum on fine needle aspiration cytology with a review of literature.

Enterobius vermicularis (EV), an intestinal nematode, causes lesions at ectopic sites also. Although lesions are fully curable with antihelminthic drugs, patients may require surgical intervention in clinically unsuspected cases to arrive at correct diagnosis and appropriate treatment. Accurate diagnosis of these lesions on fine needle aspiration cytology (FNAC) has an advantage of avoiding unwanted surgery. To the best of our literature search, there are few case reports of EV diagnosed on cytology, but none of them describes the various stages of development of eggs and hatching of larvae. Dealing cytopathologist should be cognizant of them as parasite can be present in any of these forms/stages. Recently, we came across an interesting case of young boy who presented with perineal nodule and on FNAC from the lesion showed EV larvae and its eggs in various stages of hatching and development. The patient responded to antihelminthic treatment (AHT). Morphological clues with a review of cytological literature are discussed.

INTRODUCTION

Parasitic lesions presenting only as superficial skin or subcutaneous nodule are rare.[12] Enterobius vermicularis (EV) is an intestinal nematode, causing infections exclusively in human, and can cause nodular lesions at ectopic sites also.[3] Patients are often asymptomatic. A definitive cytological diagnosis in clinically unsuspected cases and at unusual site can lead to complete cure with Antihelminthic therapy (AHT), and thus surgery can be avoided. In many of the instances, the diagnostic adult worm is often not present, and therefore, it is of utmost importance that a cytopathologist is aware of various forms in which eggs/larva can be present in the smear for prompt diagnosis.

We herein report a clinically unsuspected case of parasitic etiology in a young boy who presented with perineal nodule (PN) but showed eggs and larvae of EV on fine needle aspiration cytology (FNAC).

CASE REPORT

A 10-year-old boy presented to the pediatric outpatient department with a painless PN for the past 2 weeks. The patient was otherwise asymptomatic, and the lesion was noted incidentally. On examination, the PN was close to the anal verge, was superficial, firm, nonmobile, and nontender, and was measured 1 cm × 1 cm [Figure 1a]. Overlying skin was unremarkable. He was referred for FNAC from the lesion that yielded scanty blood mixed purulent aspirate. Air-dried Giemsa-stained smears revealed numerous eosinophils in suppurative and hemorrhagic background suggestive of parasitic etiology, and the patient was advised to undergo stool examination for 3 consecutive days along with a repeat FNAC. The repeat FNA yielded a purulent aspirate and smears showed similar findings along with many parasitic eggs which were plano-convex, measured 50–60 μm in length and 18–22 μm in diameter lying in inflammatory background rich in foreign body giant cells (FBGCs), granulomas, and Charcot–Leyden crystals (CLC). The eggs had a distinct translucent shell with larva inside in various stages of development from coiled to uncoiled forms. Many degenerated eggs were seen [Figure 1b–e]. Hatching stages of larvae were also noted [Figure 2]. On careful screening, no adult worm was found. Thus, a cytological diagnosis of parasitic PN caused by EV was rendered. Stool examination was negative. The patient was given a single dose of 400 mg albendazole; after 2 weeks of follow-up, the nodule size was decreased. At this time, another single dose of albendazole was given; on further follow-up after 2 weeks, size of the nodule was almost negligible. After 1 month of the start of treatment, the patient was lost to further follow-up.

Figure 1

(a) Clinical photograph of the patient showing superficial nodule near anal verge (arrow). Fine needle aspiration smear showing (b) diagnostic planoconvex egg of Enterobius vermicularis; (c) early infective stage of egg; (d) late infective stage in the form of coiled larva inside the egg; (e) degenerated egg (a-d: Giemsa ×400)

Figure 2

Fine needle aspiration smear showing (a) vermiform larva in the egg just before hatching (arrow); (b) hatching of egg in early stage (arrow); (c): late stage of hatching of egg (arrow); (d) free vermiform larvae in inflammatory background. Inset showing magnified view of larvae (a-d: Giemsa ×400)

DISCUSSION

EV is a common nematode and affects human beings as its sole host. Infection is transmitted by feco-oral route through ingestion of infective stage of egg, which sticks to palms and soles of affected persons. In ileum, eggs undergo different stages of development and finally hatch and release larvae which then undergo final stage of maturation in adult form in the cecum. These adult worms migrate to the rectum or other ectopic sites, i.e., vagina, fallopian tube, omentum, peritoneum, liver, lung, spleen, and kidney, where they lay eggs.[34]

EV rarely presents as a subcutaneous nodule at ectopic sites, and clinical diagnosis is often difficult in such cases as the patients are asymptomatic and peripheral blood picture seldom reveals eosinophilia. Stool examination gives a positive result only in 5%–15% of cases,[5] and therefore, FNAC remains a sole noninvasive method of diagnosing the lesion with high accuracy.

The patients are frequently subjected to FNAC, but mostly remain under-diagnosed due to lack of familiarity with the morphology of EV eggs or various stages in which larval forms can be present. Adult worms often become degenerated due to their poor preservation in massive suppuration and therefore can become completely unrecognized/absent from the smears.[6]

FNA smears can sometimes yield scanty aspirate with only presence of eosinophils and CLC in smears; a repeat FNAC is warranted in such cases. In the present case also, after an initial inconclusive but suspicious of parasitic pathology on smears, repeat FNAC revealed many classical eggs and larvae, morphologically compatible with EV infection, eliciting prominent granulomatous and FBGC reaction.

Peri-anal region is one of the rare sites of deposition of eggs and <30 cases have been reported in the literature.[78] Intact skin is considered as a barrier for EV infection; however, direct migration of the parasite through healthy anal mucosa/crypts is suggested routes of gaining access to this site like in the present case.[8] As the parasite then completes its entire life cycle here, it can be found at any stage of development; thus, knowledge of these stages and their cytomorphology is important for a cytopathologist.

Literature on cytology is sparse as majority of the EV infection cases were diagnosed on histology, with only few case reports in cytology,[6910] with many mostly as incidental findings on cervicovaginal Pap smear examination. None of these cases highlights the various forms of eggs or hatching stages of larvae that can be encountered in these smears.

CONCLUSION

EV is an obligate parasite known to cause lesions at various ectopic sites. FNAC is a noninvasive procedure and can be rewarding in clinically unsuspected cases for definitive diagnosis. The present case highlights most of the stages of development of eggs into larva and their hatching that a cytopathologist can encounter while dealing with such lesions. Knowledge of this cytomorphological spectrum can aid in the correct cytological diagnosis avoiding unnecessary surgery in these patients who can be effectively treated by AHT.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

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Nil.

Conflicts of interest

There are no conflicts of interest.