The seroepidemiology of malaria in Middle America. V. Antibody responses in an indicator population from an endemic area with attack phase antimalaria activities.

Serologic assessment was carried out in a sample of the population living in an area endemic for malaria in which control measures were being applied. The study area was in El Salvador, Central America, and antimalarial activities consisted of the residual application of the pesticide, propoxur, and drug distribution through a combination of active and passive case detection. Indirect fluorescent antibody (IFA) responses to Plasmodium vivax and P. falciparum antigens were determined on four occasions, at approximately 6-month intervals. The study population consisted of one resident from each of 268 widely distributed houses. Each person was treated with a curative course of chloroquine and primaquine at the beginning of the investigation. An increase in the proportion of positive IFA responses (greater than or equal to 1:20) and an increasing number of serologic conversions (i.e., from negative to positive or a 2-fold increase in titer) during the course of the 18-month study confirmed continued transmission of malaria. The presence of parasites in some individuals without associated serologic changes is discussed.

Observations on two strains of plasmodium falciparum from Haiti in Aotus monkeys.

Two strains of Plasmodium falciparum originating in Haiti were studied in the Aotus monkey. The Haitian I/CDC strain was first adapted to in vitro cultivation and subsequently inoculated into monkeys. The Haitian III/CDC strain was inoculated directly from a human patient into the Aotus monkey. The strains varied in their levels of pathogenicity to the animals. The Haitian I/CDC strain was highly virulent in six splenectomized animals; in one intact animal, the infection could be controlled but not eliminated with periodic doses of quinine and chloroquine. After subsequent splenectomy, the animal developed high parasitemias and died. No gametocytes developed in any of the Haitian I infections. The Haitian III strain was lethal to five of the 14 splenectomized monkeys inoculated, but some were able to control their infections without drug intervention. Gametocytes developed in all infections that persisted for an adequate length of time, and infections of mosquitoes were obtained both during the primary attack and the first recrudescence of the parasitemia. Of the mosquitoes tested, Anopheles freeborni was most susceptible to infection, followed by An. culicifacies, An. dirus, An. maculatus, and An. albimanus. The Haitian III strain was successfully transmitted to four other splenectomized Aotus monkeys via sporozoite inoculation using An. freeborni.

Anopheles pseudopunctipennis: laboratory maintenance and malaria susceptibility of a strain from El Salvador.

The techniques involved in the establishment of a strain of Anopheles pseudopunctipennis pseudopunctipennis from El Salvador, Central America are presented. Results of experimental susceptibility studies using coindigenous strains of Plasmodium falciparum and P. vivax as well as P. vivax from Colombia indicated that this anopheline strain does not possess the biologic capability for malaria transmission. The usefulness of biological susceptibility data in establishing the vector status of anophelines, especially in areas with multiple species as potential vectors, is discussed.

The seroepidemiology of malaria in Middle America. III. Serologic assessment of localized Plasmodium falciparum epidemics.

Epidemiologic studies were done in two localities, one in Panama and one in El Salvador, where outbreaks of Plasmodium falciparum malaria occurred. The indirect fluorescent antibody (IFA) method was helpful in defining the intensity of the outbreaks, the species of parasite involved, and, through parallel surveys of adjacent localities, the localized character of outbreaks. In Escobal, Panama 71 (34%) of 211 persons examined were IFA-positive for P. falciparum, although only 1 positive blood film was seen in this survey. In Jocomontique, El Salvador, 15 (28%) of 53 persons examined were IFA-positive for P. falciparum; 6 persons were parasite-positive. In the Panama study a number of children with positive IFA titers at the time of the epidemic had reverted to negative 6 months later. This lability of titers, in children, probably resulting from very early treatment of their first infection and, thus, a very brief exposure to the parasite antigen, suggests the need for caution in interpreting such negative serologic findings. Serologic studies provide valuable information in malaria epidemics, but must be interpreted in association with other known epidemiologic factors in the area concerned.

The seroepidemiology of malaria in Middle America. II. Studies on the Pacific coast of Costa Rica.

Serologic studies for malaria using the indirect fluorescent antibody technique suggest that active transmission is either absent or very low in 6 villages on the Pacific side of Costa Rica. Positive titers (1:20 or higher) were seen in the under-15-year age group in three of the study localities, but only 5 such responses were encountered among 249 people examined in this age range. In the adults (15 years and over) from the same 3 villages there were 68 positive titers among 161 examined. There were 43 positive responses in 189 adults from the remaining 3 villages where none of 307 persons under 15 years of age showed a titer of 1:20 or higher to any of the 3 malaria antigens tested (Plasmodium falciparum, P. vivax and P. malariae). These data suggest that the positive responses in the latter villages are more likely to be associated with old or imported cases than with current local transmission. Serologic responses of 1:80 or higher to the P. falciparum antigen suggested the continued presence of this parasite in the population in spite of the paucity of positive blood smears with this species in recent years. Positive titers with the P. malariae antigen suggest that this parasite is probably still present in the area. Such serologic studies help to indicate areas where malaria transmission is active and provide information on parasite reservoirs in particular populations.

Application of the indirect fluorescent antibody method in a study of malaria endemicity in Mato Grosso, Brazil.

Four surveys of malaria endemicity were conducted in the Cuiabá Sector of Mato Grosso State, Brazil, at 6-month intervals during the period April 1970 to September 1971; during April 1970 a survey was also conducted in the Campo Grande Sector. Over 4,000 dual specimens--blood films for parasite diagnosis and filter paper blood spots for determination of fluorescent antibody (IFA) response--were collected from the general population, including school populations whenever possible. Parasitologic examinations yielded positivity rates ranging from 0.8 to 2.3%. In the Cuiabá Sector, sero-positives (larger than or equal to 1:20) ranged from 9.3 to 13.6%; in the survey in the Campo Grande Sector only 4.3% of the specimens were positive. There was an expected increase in IFA response with age in both the proportion of positives and mean maximum titers. In the Cuiabá surveys 75 to 91% of the maximum positive responses were to the Plasmodium falciparum antigen, while in Campo Grande only 46% of the maximum titers were for P. falciparum. The wide differences in malaria endemicity observed within the Cuiabá Secctor were attributed to differences in climate, geography, and degree of development of the several regions. A study of surveillance reports from 1966 through the survey dates revealed that the high rates of infection seen often resulted from detection activities in the larger cities and probably represented cases imported from endemic areas to the north and west. In order to interpret accurately the endemicity of malaria in an area and, thus, determine the necessity for introduction or continuation of control measures, thorough epidemiologic studies are necessary. The addition of a serologic method to normal surveillance can increase the accuracy of interpretation.