Geographical distribution of the prevalence of nodules of Onchocerca volvulus in Guatemala over the last four decades.

In Guatemala a national nodulectomy campaign has been carried out since 1935 in three operational areas, namely the Eastern, Central and Western endemic zones. The prevalence of nodules was calculated in each 1 km grid square in these zones in each decade and plotted on mesh maps. A total of nine foci of high endemicity, three in each operational area, was found in the decade 1940-49. Nodule prevalence was concentric in each endemic focus; high in the centre and gradually falling towards periphery. The foci were found mainly on Tertiary or Quaternary-Tertiary volcanic rocks in the Eastern and Central zones, or on non-carbonate sedimentary rocks in the Western zone. Nodule carriers were found at altitudes from 220 to 2200 m above sea level. The altitude of the foci of high endemicity varied, being low in the Eastern zone, medium in the Central zone and high in the Western zone. The nodule prevalence declined from the decade 1940-49 to the decade 1970-79 in most of the grid squares. The Santa Rosa focus in the Eastern zone virtually disappeared before 1970. The extent of endemic area having a nodule prevalence of 1% or higher decreased from 2384 km2 in the decade 1940-49 to 1424 km2 in the decade 1970-79. The area recommended for vector control operation against Simulium ochraceum was estimated at 803 km2.