PREVALENCE OF A MALARIAL PARASITE OVER TIME AND SPACE - PLASMODIUM-MEXICANUM IN ITS VERTEBRATE HOST, THE WESTERN FENCE LIZARD SCELOPORUS-OCCIDENTALIS

1. We studied patterns of abundance of the malarial parasite Plasmodiu m mexicanum in its vertebrate host, the western fence lizard Sceloporu s occidentalis, over a 13-year period at 51 study sites in northern Ca lifornia, USA. Abundance of the vectors, sandflies in the genus Lutzom yia, was also studied among sites during a single warm season, and amo ng nights at one site during two seasons. 2. The parasite differed in prevalence (per cent of lizards infected) in males and females, males were more often infected. Prevalence increased with body size ( = olde r lizards were more often infected). 3. Malaria prevalence varied amon g sites: 0-50% of lizards were infected. Topography explains part of t his variation because malaria was rare or absent at sites > 500 m elev ation. However, sites at lower elevations, even when within short dist ances of one another, varied in malaria prevalence. 4. Abundance of ve ctors was not related to prevalence of malaria at a site; sandflies we re found at some sites where malaria was rare or absent, including tho se at high elevations. Wind speed, relative humidity and sky brightnes s did not affect numbers of sandflies active, but vectors were active only when air temperature was > 16 degrees C. 5. Parasite prevalence v aried among years. Environmental conditions (temperature, rainfall and plant biomass produced at the study region) were not correlated with parasite prevalence. The changes in prevalence over time resembled a c ycle of long duration (10 years). 6. Classical models developed for st udy of malaria in humans suggest explanations for variation in prevale nce of P. mexicanum among sites, and a more recent theory suggests an explanation for the possible long-duration cycle observed in this stud y.