HIGH PREVALENCE OF PLASMODIUM-MALARIAE AND PLASMODIUM-OVALE IN MALARIA PATIENTS ALONG THE THAI-MYANMAR BORDER, AS REVEALED BY ACRIDINE-ORANGE STAINING AND PCR-BASED DIAGNOSES

The prevalence of the four human malaria parasites was investigated am ong malaria patients at northern, central and southern towns in Thaila nd along the border with Myanmar between September 1995 and May 1996. Thin smears obtained from 548 Thai and Burmese patients were reviewed by an acridine orange staining method, and many mixed infections with two to four species, including P. malariae and P. ovale, were detected . These diagnostic results were compared with those by two PCR-based d iagnoses, microtitre plate hybridization (MPH) and a nested PCR method , both of which targets the same, species-specific regions in the 18S rRNA genes. In both PCR diagnoses, many P. malariae and P. ovale infec tions were also detected. Detection sensitivity of P. malariae infecti on was higher in nested PCR than MPH, and a total prevalence of P. mal ariae infection estimated by nested PCR reached 24.3% (133/548). In 16 of them, the size of PCR products amplified by the P. malariae-specif ic primer was about 20-bp shorter than the expected size of 115-bp. Fo ur of 16 possessed two different bands with normal and shorter sizes, suggesting that P. malariae isolates may be separated into two types, and that those with shorter products may be new variant form (s) with a nucleotide deletion in the target region. On the other hand, 21 P. o vale infections (3.8%) were detected by nested PCR, but four of them w ere MPH-negative because of the sequence variation at the probe region . These results indicated that the prevalence of P. malariae and P. ov ale along the Thai-Myanmar border may be substantially higher than pre viously reported.