Department of Biological Sciences, Clark Science Center, Smith College, Laboratory of Parasitic Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Institut Territorial de Recherches Medicales Louis Malarde, Northampton, Massachusetts
To identify Wuchereria bancrofti DNA sequences that could be used as the basis for a simple and rapid parasite detection assay, a genomic library of W. bancrofti was constructed and screened for highly repeated DNA. The repeat found with the highest copy number was 195 basepairs (bps) long, 77% AT, and 300 copies per haploid genome. This sequence was designated the Ssp I repeat because it has a unique recognition site for that restriction endonuclease in all or most of the repeat copies. The Ssp I repeat DNA family is dispersed, genus-specific, and exists in all of the different geographic isolates of W. bancrofti tested. Based on DNA sequence analysis of this repeat, we have developed an assay to detect very small quantities of W. bancrofti DNA using the polymerase chain reaction (PCR). With this PCR assay, the Ssp I repeat was detected in as little as 1 pg of W. bancrofti genomic DNA (about 1% of the DNA in one microfilaria) added to 100 µl of human blood. The PCR assay also amplified Ssp I repeat DNA from geographic isolates of W. bancrofti from around the world but not from other species of filariae or from human or mosquito DNA. Microfilaria-positive human blood samples collected in Mauke, Cook Islands were shown to be Ssp I PCR-positive, while microfilaria-negative samples were PCR-negative. The specificity and sensitivity of the Ssp I PCR assay indicates that this approach has significant potential for improved screening of large human populations for active W. bancrofti infection.