Microfilaria: not always associated with eosinophilia

Introduction

Lymphatic filariasis is caused by three species of nematodes, namely, Wuchereria bancrofti (WB), Brugia malayi and Brugia timori. A wide variety of mosquitoes including Culex, Anopheles, Aedes and Mansonia can transmit the disease. ¹ Highly sensitive tests such as circulating filarial antigen (CFA) and parasite DNA detection are available but these are expensive. Microscopy, however, is easy, rapid and cost-effective, and remains the cornerstone for diagnosing microfilaria in poor-resource settings.

Case 1

A 60-year-old man, a native of Bihar, India, presented with fever, headache and altered sensorium for 4–5 days. On examination, he had a right-sided hydrocoele. Full blood count revealed anaemia (Hb = 108 g/L) leucocytosis 14.9 × 10⁹/L and platelet count 33 × 10⁹/L with a differential count of 90% neutrophils, 7% lymphocytes, 1% eosinophils and 2% monocytes. Peripheral blood smear (PBS) showed normocytic to microcytic hypochromic red cells, along with presence of microfilaria (Figure 1a). Thus, bicytopenia with neutrophilic leucocytosis and microfilaria was shown. OPEN IN VIEWER

Case 2

A 30-year-old woman, a resident of Delhi, primagravida at 35 weeks gestation, presented with fetal growth retardation and severe pallor. She was severely anaemic (Hb = 65 g/L) with leucocytosis (10.46 × 10⁹/L) and thrombocytopenia (platelets 49 × 10⁹/L) with a differential count of 88% neutrophils, 8% lymphocytes, 2% eosinophils and 2% monocytes. PBS showed macrocytes and macro-ovalocytes as well as the presence of microfilaria (Figure 1b). Thus, bicytopenia with macrocytic anaemia and microfilaria was shown.

Discussion

Filariasis is a major global health problem in tropical and subtropical countries. ² In India, filaria is commonly caused by two nematodes – WB and Brugia malayi – with the former accounting for 98% cases of lymphatic filariasis. The states of Bihar, Kerala and Uttar Pradesh are known endemic zones.

The life cycle of WB occurs in two hosts, man being the definitive and the mosquito the intermediate host. The adult worms enter the lymphatic system after the mosquito takes a blood meal. They reside in lymph nodes where the gravid female releases a large number of microfilariae. These larvae pass through the thoracic duct and pulmonary capillaries and start appearing in the peripheral circulation. ³

Bancroftian filariasis leads to a variety of clinical manifestations. ³ The acute phase is characterised by fever, lymphangitis, lymphadenitis, epididymo-orchitis and funniculitis. Eosinophilia and microfilaremia are common in this phase. The chronic phase manifests as lymphadenopathy, lymphoedema, hydrocoele and elephantiasis. Microfilariae are usually not present in this phase. A significant number of infected individuals, however, remain asymptomatic throughout their lives, even with high microfilaraemia.

Lymphatic filariasis is responsible for secondary eosinophilia and this has been used as a screening tool for the detection of filariasis. Eosinophilia mostly occurs as a hypersensitivity reaction to filarial antigen during migration of the parasite through the tissues. ¹ In any case of eosinophilia (PBE), the PBS must be screened for filariasis in endemic areas. PBS is a simple and inexpensive tool. In the cases described, microfilaria was detected on PBS in the absence of PBE. There have been few case reports of tissue microfilaria with absence of PBE and microfilaremia^2,4,5 with images shown in two.^6,7 This has been variously attributed to a difference in host immune response to parasite from person to person ² , co-infection with malaria,^6,8 concomitant pulmonary tuberculosis with microfilarial infection in asymptomatic phase,^7,9 a possible involvement of oxidative stress during the inflammatory response to the parasite ¹⁰ and pregnancy. ¹¹

Newer tests such as CFA are available for detection and are not dependent upon the presence of circulating microfilaria, but their high cost limit their availability. ¹ Microscopy still remains the cornerstone for diagnosing microfilaria especially by screening the tail end of all PBS at low power magnification as the parasite, being heavier, is preferentially smeared at this end. Our routine practice of doing so may well have detected more parasitaemia than would have normally been detected.

Conclusion

Even in the absence of eosinophilia, the routine screening of the tail end of PBS at a low power magnification may be of paramount importance in detecting microfilaria.