Correct Answer: C. Brugia malayi
Brugia malayi microfilariae are uniquely characterized by a sheathed tail with two nuclei positioned at the tail tip, separated from the main column of nuclei by a clear space. This morphological feature is the gold standard for differentiating B. malayi from other filarial parasites in blood smears. The sheath is a remnant of the egg membrane and stains with Romanowsky dyes. The presence of exactly two terminal nuclei (rather than one or none) is pathognomonic for B. malayi. This distinction is critical in endemic areas of South India (Kerala, Tamil Nadu) and Southeast Asia where B. malayi causes lymphatic filariasis. The microfilariae are nocturnal (periodic form) or subperiodic depending on the vector (Mansonia species), and their identification directly guides treatment decisions—albendazole with DEC is the standard regimen per NTEP guidelines. Misidentification can lead to inappropriate therapy or missed diagnosis of lymphedema and elephantiasis complications.
Why the other options are wrong
A. Wuchereria bancrofti — W. bancrofti microfilariae have a sheathed tail with NO nuclei in the tail space (nuclei end before the tail begins). This is the key discriminating feature—while both are sheathed, the tail nuclei pattern differs. W. bancrofti is the most common cause of lymphatic filariasis in India (60% of cases) and causes similar clinical disease, making this a common NBE trap. Students often confuse the two because both cause lymphatic filariasis and have sheaths. B. Loa loa — Loa loa microfilariae are sheathed with nuclei extending to the tip of the tail, not just two at the tail. L. loa is endemic in West Africa and causes Calabar swellings and subconjunctival migration—not typically seen in India. The tail nuclei pattern is continuous rather than the discrete two-nuclei pattern of B. malayi. This option tests whether students confuse tail nuclei patterns across species. D. Onchocerca volvulus — Onchocerca volvulus microfilariae are unsheathed (no egg membrane remnant), making this immediately distinguishable from the question stem which specifies a sheathed microfilaria. O. volvulus causes onchocerciasis (river blindness) with subcutaneous nodules and ocular involvement, not lymphatic disease. The absence of a sheath is the primary discriminator; this option tests whether students recognize the sheath as a defining feature.
High-Yield Facts
- Brugia malayi tail: sheathed with exactly two nuclei at the tail tip separated by a clear space from the main nuclear column—pathognomonic feature.
- W. bancrofti tail: sheathed but no nuclei in the tail space—nuclei end before tail begins; most common filarial infection in India (60%).
- Loa loa tail: sheathed with nuclei extending continuously to the tail tip—causes Calabar swellings, endemic in West Africa, rare in India.
- Onchocerca volvulus: unsheathed microfilariae—causes onchocerciasis with subcutaneous nodules and blindness, not lymphatic disease.
- B. malayi vectors: Mansonia species (periodic and subperiodic forms); endemic in South India, Southeast Asia; causes lymphedema and elephantiasis.
- Microfilariae identification: blood smear with Romanowsky stain (Giemsa/Wright's) is the standard diagnostic method; timing of collection matters for nocturnal forms.
Mnemonics
Tail Nuclei Pattern (Sheathed Filariae) Brugia = Boundary (two nuclei at boundary/tip of tail) | Wuchereria = Without (no nuclei in tail) | Loa = Long (nuclei extend along entire tail). Use when comparing sheathed microfilariae morphology in blood smears. Sheath Status Quick Check SLO = Sheathed (W. bancrofti, L. loa, B. malayi) | O = Onchocerca (Unsheathed). Helps rule out O. volvulus immediately when sheath is mentioned in the question.
NBE Trap
NBE pairs "sheathed microfilariae" with W. bancrofti (the most common Indian filarial infection) to lure students into choosing option A without carefully examining the tail nuclei pattern—the discriminating feature that distinguishes B. malayi from W. bancrofti.
Clinical Pearl
In South Indian endemic zones (Kerala, Tamil Nadu), B. malayi is responsible for 30–40% of lymphatic filariasis cases and presents with lymphedema of lower limbs and genital involvement. Accurate microfilarial identification on blood smear is essential because both B. malayi and W. bancrofti require the same DEC + albendazole regimen, but epidemiological tracking and vector control differ—misidentification can compromise public health surveillance under NTEP.
_Reference: Jawetz, Melnick & Adelberg's Medical Microbiology (Filarial parasites chapter); Park's Textbook of Preventive and Social Medicine (Lymphatic filariasis section); NTEP Guidelines on Lymphatic Filariasis Management_