A Malaria Survey of the Island of Jamaica, B. W. I.

XI. Summary and Conclusions The more salient facts presented in the foregoing text may be summarized as follows: a. Endemic malaria in Jamaica is largely if not altogether confined to the coastal plain lowlands. In this region there may be distinguished several foci of high intensity which are surrounded by larger irregular zones where the intensity is much less. The most extensive of the highly endemic foci are the irrigated areas of lower St. Catherine, Vere in Clarendon, the Black River plain and the coastal territory to the west of the Santa Cruz mountains in St. Elizabeth, the vicinity of Sav-la-Mar in Westmoreland, the coastal strip between Annotto Bay in St. Mary and Buff Bay in Portland, the coastal stretch between the Hope and Cane Rivers in St. Andrew and the Plaintain Garden River valley in St. Thomas. In these foci malaria tranmission is frequent and common, and probably does not manifest striking changes in intensity from year to year. In the larger zones of lesser intensity, transmission is probably subject to extensive irregularities. It is in these areas that a series of years with subnormal precipitation produces the most noticeable diminuition in the intensity of transmission, and it is in these that epidemics of malaria are most likely to attract attention. Little if any malaria transmission appears to occur over the interior or greater part of the island. As will be later pointed out, this circumscription of the disease may be ascribed to limitations in the distribution of the probable vector. b. Data are presented to indicate that for approximately the last twenty years the intensity of malaria over the entire island has been declining. Data are available to indicate that for a twenty year period preceding the onset of this decline, the intensity of the disease was far greater than experienced at present. Further data are available to indicate that about 60 years ago the incidence of malaria was probably comparable to that existing at the present day, while the increase in intensity to the high levels noted can be traced. It is shown that this cycle in malaria intensity is paralleled by a cyclical change in the precipitaton received by the island. Further evidence of the parallelism between rainfall and the intensity of malaria is afforded by an examination of the experience of individual years. Malaria is intensified in the years of moderately excessive precipitation, and declines during those with sub-normal rainfall. Owing to irregularities in the season when excessive rains are experienced, the effect of excessive precipitation occurring late in the year may be reflected in an increase of malaria the following year. The effect of excessive rainfall arises from the accumulations of temporary water resulting, increasing the opportunities for the breeding and production of the probable anopheline vector. Transmission is intensified as a consequence of this increase in numbers. c. Considered as an economic unit, the island is enjoying great prosperity. To a considerable extent, this prosperity is due to the successful agricultural exploitation of the fertile lowland soil. The lowland areas where agriculture is most successful coincide with foci of intense malaria. The principal cultivations are of crops that require much water, which if not available from the normal precipitation, is supplemented by irrigation. In these areas, small freeholdings are the exception. The bulk of the land is held in large estates, not uncommonly owned by British or foreign capital. The population residing thereon consists of estate employees, administrators and laborers. Comparatively few of the laborers are fixed residents. The settled rural population of the lowlands is insufficient to supply the labor demand on the estates. A large part of the demand is supplied by the migration of laborers from the territory on the plateau occupied by small freeholders. These are usually susceptible to malaria and suffer severely from the disease when living on the lowlands. If methods of malaria control can be found that are adapted to Jamaican conditions, and are reasonably cheap in their application, the economic value of the greater part of the lowland territory would justify their extensive employment. d. Of the four anopheline species indigenous to Jamaica, three are to be regarded with concern as actual or potential vectors of malaria. Of these A. albimanus is to be regarded with most concern for the following reasons: 1. Its distribution is widespread over the lowlands of the entire island, and coincides with that of malaria. 2. The type of situations it chooses for breeding places indicates that it will avail itself of temporary water. 3. The period of its annual maximum prevalence is during the summer and fall, coinciding with the principal periods of malaria transmission. 4. It is a species which seeks to satisfy its hunger with human blood. 5. It has been proven to be susceptible to infection with the parasites of malaria. A. crucians may be a vector of malaria on the island, but owing to its limited geographical distribution, and relative scarcity, it cannot be of great importance. A. vestitipennis is suspected of being a potential vector owing to its inclination for human blood, but as its distribution is not widespread it probably is not of great importance. We are of the opinion that A. grabhami can probably be exonerated as a carrier of malaria for the following reasons. 1. Its insular distribution is more extensive than that of malaria, being found on the plateau as well as the lowlands. 2. The period of its annual maximum prevalence does not coincide with that of the principal periods of malaria transmission. 3. It does not appear to be a species that particularly endeavors to satisfy its hunger with human blood. For these reasons we conclude that the greater part of the malaria problem of the island is produced by A. albimanus, which can be solved in the areas where the species is brought under a satisfactory degree of control.