India is home to an endemic case of filariasis, a serious worldwide public health concern [1]. Wuchereria bancrofti and Brugia malayi are the nematode worms that are the cause of the problem. The infection is spread by mosquitoes, depending on the geographic area, but mostly by Culex and Anopheles mosquitoes [2]. The presence of microfilaria in three consecutive nighttime peripheral smears is often used to make the diagnosis [3]. The presence of filarial nodules and their detection by lymph node fine needle aspiration cytology (FNAC) is rare, despite the high frequency in India [4]. Without any clinical signs of filariasis or microfilaremia, the FNAC of the submental lymph node in our example incidentally detected microfilaria.

A 40-year-old male presented with chief complaint of swelling in the submental region of the neck for 10-15 days. It was gradual in onset. The swelling progressively increased in size. He also had a history of right-sided submandibular lymph node swelling five months back. He gave a history of weight loss in the last two to three months. On USG, it was reported as abscess formation. During the local examination, there was a palpable swelling of size 2 cm × 2 cm present over the submental region. It was firm, non-tender, and slightly mobile. Swelling does not move with protrusion of the tongue. There was no local rise in temperature (Figure 1).

Figure 1: Submental swelling of 2 x 2 cm.

Millions of people are thought to carry microfilaria, and a notably large proportion of those individuals experience filaria-related illnesses [5]. Cases of filariasis are found across the tropics and subtropics of Southeast Asia, India, the Pacific Islands, and sub-Saharan Africa. India recorded the largest number of instances. Although the term "filariasis" refers to any infection brought on by a filarial worm, in everyday usage, it refers only to lymphatic filariasis brought on by Wuchereria or Brugia species. The Culex mosquito served as the transmission vector. Microfilaria in peripheral blood shows nocturnal periodicity. In the latter stages of the illness, it manifests as elephantiasis. In blood samples taken at night between 10 p.m. and 4 a.m., peripheral examination reveals the presence of motile microfilariae parasites [6]. These days, the gold standard for demonstrating micro filarial organisms is the identification of circulating filarial antigens. The significance of FNAC as a diagnostic tool for filariasis is not well documented in the literature [7].

The diagnosis of clinically suspected filariasis is not usually made by FNAC. Nonetheless, there have been reports of unintentional fine-needle aspiration cytology smear detections [8, 9]. Peripheral blood film testing may not reveal microfilariae in instances of elephantiasis, occult filariasis, lymphangitis, and early allergy presentations [2, 10].

Rare case reports exist of microfilariae accidentally discovered in cytology smears of breast lesions, axillary lymph nodes, thyroid nodules, and bronchial aspirates [3]. In the absence of any filariasis-related clinical symptoms, our patient exhibited enlargement in the submental lymph nodes.