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Laboratory Diagnosis

The demonstration of microfilariae in circulating blood (Fig. 26.2) or in urine, hydrocele fluid, or biopsy specimens confirms the diagnosis. The optimal time to obtain a blood specimen is variable and dependent on the species or strain of W. bancrofti, B. malayi, and B. timori. For the infections where there is nocturnal periodicity, the best time for phlebotomy is between 10 p. m. and 4 a. m. The subperiodic forms tend to have the highest microfilaremia in late afternoon, but blood may be positive at any time. Initially, a finger prick blood sample should be examined directly, followed by thin and thick blood smears. All three lymphatic species produce microfilariae that have a sheath, in contrast to other filarial diseases where nonsheathed microfilariae circulate in the blood. Staining the sheath and microfilarial nuclei is important for species differentiation microscopically. Delafield's hematoxylin is preferred to Giemsa as the sheath is stained only by the former while both allow nuclear recognition. Blood concentration techniques are more sensitive than smears. The Knott technique, using formalin sedimentation, or the Nucleopore (polycarbonate) filter method, which traps microfilariae following hemolysis, are both used with success even when there are low circulating levels of microfilariae.


Fig. 26.5A,B. Surgical specimens from patients with W. bancrofti infections. A An abscess within the breast slowing gravid female worms. Note thin cuticle, paired uteri containing microfilariae and single intestine. x75. B Chronic orchitis with gravid female worm in dilated testicular lymphatic. x55. (From Marty and Anderson 1995)

Occasionally, adult worms and larval stages are identified in lymph node and other biopsy specimens (Fig. 26.5). This allows correct diagnosis even when microfilariae are absent in blood or other fluids. Still other methods are available and perhaps preferable to biopsy in symptomatic amicrofilaremic patients. These include a diethylcarbamazine provocation test and several immunological techniques (IFA, ELISA, specific IgE and IgG4 antibody detection) and other methods to detect circulating parasite antigen (monoclonal antibody, polymerase chain reaction).

In patients with tropical pulmonary eosinophilia (see later) there are no circulating microfilariae but there are circulating filarial antigen or antibody, high IgE levels, and high eosinophilia. Markedly increased numbers of inflammatory cells are seen in bronchial washings, of which about 70% are activated eosinophils. Lung biopsy in the rare, complicated case may show eosinophilic precipitate surrounding a degenerating microfilaria and indicate that this acellular material is composed of Splendore-Hoeppli bodies.

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Copyright: Palmer and Reeder