Eosinophilia in an Indian Farmer

Menka Doomra, MD; Seema Garg, MBBS; Naveen Kakkar, MD


Lab Med. 2006;37(5):280-282. 

In This Article

Possible Answers

  1. Marked eosinophilia and the presence of a parasite (microfilaria) in the peripheral blood smear ( Table 1 and Figure 1).

  2. Parasitic infection with microfilarial larva of Wuchereria bancrofti, Brugia malayi, Loa loa, Oncocerca volvulus, or Dipetalonema perstans.

  3. The microfilariae are the larval stage of the parasite and appear as colorless, transparent bodies with blunt heads and pointed tails on unstained peripheral blood preparations. When dead and stained with Romanowsky stain, the microfilariae show certain morphological characteristics, including a hyaline sheath, which is a structureless sac and is longer than the larval body; somatic cells (nuclei) which appear as granules in the central axis of the body of the organism and extend from the head to an area just short of the tip of the tail, and a cephalic space at the anterior end of the organism which is devoid of granules. The continuity of the granules is broken at definite places along the sheath that form anatomical landmarks for locating the nerve ring, oblique space, genital cells, anterior V spot, posterior V spot, and the anal pore of the organism. Sheathed microfilariae of Brugia malayi and Loa loa can be distinguished from those of Wuchereria bancrofti on the basis of the morphology of the tail tip. In the case of Brugia malayi, the tail shows 2 discrete nuclei: one at the extreme tip of the tail which is slightly bulbous and the other midway between the tip and posterior column of nuclei with a constriction of the tail in between. The microfilariae of Loa loa contain a column of nuclei extending to the tail tip. The adult worms are long filiform shapes found in lymphatic vessels and lymph nodes of man. The head of the worm terminates in a slightly rounded swelling and female worms are longer than male worms. The tail end of male worms is curved, while that of females is narrow and abruptly pointed.

  4. Most likely diagnosis: asymptomatic microfilarial infection by the larval form of Wuchereria bancrofti. The morphology of the tail in the organism shown in Figure 1 is consistent with the characteristics of the tail found in Wuchereria bancrofti. This parasite belongs to the superfamily, Filariodea and subfamily, Acanthocheilonematinae.

  5. Filariasis is largely confined to the tropics and subtropics, occurring in India (especially along the coast line), West Indies, Southern China, Japan, West and Central Africa, and Central America.

  6. The most common clinical presentation of bancroftian filariasis is asymptomatic microfilaremia (especially with mild infection). In symptomatic patients, the clinical manifestation has 2 phases—an acute inflammatory phase of adenolymphangitis and the phase of chronic lymphatic obstruction. The acute inflammatory phase is characterized by high fever, chills, and malaise. Intense pain, redness, and edema occur along the lymphatic channels and the draining lymph nodes become enlarged and painful. Any lymph node or any body part can be affected but the common sites include upper and lower extremities, groin, and the axilla. Genital lymphatics are commonly involved in males leading to funiculitis, orchitis, epididymitis, and scrotal pain and tenderness. In the chronic obstructive phase, elephantiasis (solid edema) of the affected part occurs in which brawny edema, hyperkeratosis, and fissuring of the skin develops. Genital involvement in males results in the development of hydrocele, scrotal lymphedema, and elephantiasis. In patients with occult filariasis, chyluria, chylothorax, chylous ascites, and chylous diarrhea may also develop. Occult filariasis is a condition characterized by massive eosinophilia, generalized lymphadenopathy, hepatosplenomegaly, pulmonary symptoms, and the absence of microfilaremia.[1]

  7. Man is the definitive host and mosquitoes (Anopheles, Culex, and Aedes species) are the intermediate host of Wuchereria bancrofti. Sheathed microfilariae ingested by the mosquito during its blood meal collect in the anterior end of the stomach where they cast off their sheath and penetrate the gut wall. Then, they migrate to the thoracic muscles where they rest. Further maturation proceeds through 3 stages. In the 3rd stage, larvae enter the proboscis sheath of the mosquito and infect man during the mosquito's blood meal. The larvae enter the skin of man, migrate into the bloodstream, and eventually reach lymphatic channels where they finally settle down in the inguinal, scrotal, and abdominal lymphatics and begin to grow into adult, sexually mature forms. Fertilization occurs and the gravid female gives birth to larvae. A new generation of microfilariae is created which pass either through the thoracic duct or the right lymphatic duct into the venous system and pulmonary capillaries and then into the peripheral circulation where they can be detected on blood smear examination.

  8. The principal lesions that occur during the acute inflammatory phase of Wuchereria bancrofti infection are lymphangitis and lymphadenitis. Both of these occur due to the mechanical irritation caused by motile adult worms residing in afferent lymphatics and the sinuses of lymph nodes and to the toxic metabolites released by growing microfilariae and disintegrating dead worms. In the chronic phase of infection, lymphatic obstruction occurs due to mechanical blocking of the lumina by dead worms, endothelial proliferation, inflammatory thickening of the wall of lymphatic vessels, and excessive fibrosis of the lymphatic vessels due to repeated attacks of lymphangitis.

  9. The laboratory diagnosis of filariasis can be made using direct or indirect methods. The direct methods include visualization of the larval stage (microfilariae) or the adult worm. Microfilariae can be seen in the peripheral blood or other body fluids (eg, urine and ascitic, hydrocele, or lymph fluid). In the peripheral blood, microfilariae can be detected by 3 principal methods: 1) wet smear examination under the microscope where motility can also be seen; 2) examination of a stained peripheral blood smear which is also useful in species identification based on the morphologic features of the organism, especially the tail end; and, 3) thick or concentrated blood smears, if the diagnosis can not be made using the other methods. Concentration smears are especially useful in patients who have low parasitemia. In such cases, membrane filtration techniques can also be used to concentrate organisms and promote their detection and identification. Moreover, microfilariae do not remain in the blood stream all the time. In oriental (Indian and Chinese) individuals with filariasis, the microfilariae appear in the circulation between 10:00 PM and 2:00 AM, the optimal time period for obtaining a blood sample for testing. This phenomenon is thought to be related to the nocturnal feeding habits of the mosquito host. However, a single dose of diethylcarbamazepine can provoke the microfilariae to appear in the circulation during daytime if blood sampling at night is inconvenient.[2] Adult worms can also be demonstrated in a lymph node biopsy, while calcified adult worms can be visualized on an X-ray. Doppler ultrasonography can also be used to detect the presence of motile adult worms in dilated lymphatics.

    The indirect methods include: peripheral blood eosinophilia, elevated serum IgE concentration, and the presence of an antifilarial serum antibody. Intradermal and complement fixation tests have been used in the past; however, these tests are unreliable. Among the better methods available for diagnosing infection with Wuchereria bancrofti, an enzyme linked immunosorbent assay (ELISA) and a rapid format immunochromatographic card test for circulating antigens of Wuchereria bancrofti permit the diagnosis of microfilaremic and amicrofilaremic (cryptic) infection.[3] A polymerase chain reaction (PCR)-based assay for deoxyribonucleic acid (DNA) gene sequences of Wuchereria bancrofti is also available.[4] Lastly, radionuclide lymphoscintigraphic imaging of the limbs can demonstrate wide spread lymphatic abnormalities both in patients with symptomatic microfilaremia and in those with clinical manifestations of lymphatic obstruction.[5]


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