USE OF PCR FOR DETECTION OF SUBPATENT INFECTIONS OF LIZARD MALARIA - IMPLICATIONS FOR EPIZOOTIOLOGY

The estimated prevalence of a malaria parasite, Plasmodium mexicanum, of western fence lizards, Sceloporus occidentalis, was compared using two techniques: microscopic examination of blood smears, and nested PC R amplification of the 18S small subunit rRNA gene. Two sites in north ern California, USA were investigated, one with known long-term high p revalence of the parasite (30% by blood smear scanning), and one with low prevalence (6%). The nested PCR readily detected very low-level in fections (< 1 parasite per 10 000 erythrocytes); such infections are o ften subpatent by normal microscopic examination. False negatives (sco red as not infected after scanning the blood smear, but found infected via PCR) were rare at both sites (4% at the high-prevalence site, 6% at the low-prevalence site). However, a greater proportion of infectio ns was detected only by PCR at the low-prevalence site (50% vs. 9%). I f 50% of the infections sustain very weak parasitaemia where lizards a re rarely infected, this would accord with hypotheses that predict tha t parasites should reduce infection growth when transmission is uncomm on. The study demonstrates that PCR is a powerful tool to detect very low-level malarial infections in vertebrate hosts, including those wit h nucleated erythrocytes.