Surveillance of measles in the Sudan using filter paper blood samples

Dried blood spots collected on filter paper are considered potential clinical specimens for measles surveillance because of their ease of collection, storage, and transport. The usefulness of these samples for surveillance of measles was evaluated in a field setting. Blood spots were collected by finger-prick from 316 clinically diagnosed measles patients in suburban Khartoum, mostly within a week after onset of the rash. Samples were collected between October, 2000 and April, 2003, and stored at 4 degrees C. Measles virus-specific IgM antibodies were detected in 200 (63%) of the samples using an "in-house" IgM capture ELISA. For 201 samples reconstitution and IgM measurement was repeated 1 year after initial testing with essentially the same results, showing the stability of IgM in the filter paper under these conditions. In a limited number of samples (n = 38) measles virus-specific IgM was also tested with a commercial indirect IgM ELISA. Although the results of the two assays correlated well, the "in-house" IgM capture ELISA proved slightly more sensitive. Measles virus-specific reverse transcriptase polymerase chain reaction (RT-PCR) amplicons were obtained from 16 of 57 (28%) samples tested. Sequencing of the 3' 456 nucleotides of the nucleoprotein gene showed the continued endemic circulation of genotype B3 viruses identified previously in this region. Although problems related to limited sample quantities were encountered, the present study confirms the usefulness of dried blood spots for measles surveillance. The results also demonstrate that measles continues to be endemic in the Sudan.