Recent advances in the application of molecular biology in filariasis

Monitoring of filarial parasites in the host and vector has traditionally depended on morphological identification. Recently, species-specific DNA probes have been developed for Brugia malayi, Brugia pahangi and Wuchereria bancrofti. Repeated DNA sequences are useful in developing DNA probes because they evolve more rapidly then coding sequences and their high copy number increases the sensitivity of detection. The Hhal repeated DNA family represents 12% of the total B. malayi DNA. This DNA family is present in species of Brugia (B. malayi, B. timori and B. pahangi) but not W. bancrofti. Sequence analysis of the repeated DNA in B. malayi and B. pahangi has allowed construction of two species-specific DNA probes. These probes were used in a double blind field study in Indonesia. Microfilariae (mf) from infected cats and humans were identified by classical morphological methods and DNA probes. Agreement was found in 98.6% of the 642 samples tested by the two different techniques. Besides mf identification DNA probes can be used to determine the species of infective larvae (L3s) in infected mosquitos. This is useful because the L3s have similar morphology. DNA probes for the identification of W. bancrofti have recently been developed and are in the initial stages of testing in China (Piessens, personal communication) and Egypt (Williams, personal communication). An alternative approach for identification of infected individuals is to detect specific parasite antigens in circulation. A WHO initiative to use either an antigen or antibody assay to replace night blood is presently underway. This approach, if successful would not require the presence of microfilariae, but could detect occult infections.