Epidemiological assessment of malaria transmission in an endemic area of East Pakistan and the significance of congenital immunity*

Bull. Org. mond. Sant) 1972, 46, 783-792 Bull. Wid Hithi Org. Epidemiological assessment of malaria transmission in an endemic area of East Pakistan and the significance of congenital immunity * A. Q. KHAN1 & S. A. TALIBI2 A longitudinal epidemiological study was carried out in a hyperendemic malarious area of East Pakistan. Transmission of malaria was observed with two peaks, one in the pre- monsoon period, the other in the pre-winter period. New infections occurred in 51.7% of the infants surveyed who were 2 months old or under when first examined: there were frequent superinfections. The maternal immunity passively transferred to the infants did not play a significant role in restricting the development of the asexual erythrocytic stage or that ofgametocytes, as indicated by the fact that the average parasite density and the gametocyte counts were highest in the 0-2-month age group and declined with increasing age. Mosquitos were collectedfor identification andfor dissection to determine parity, the sporo- zoite rate, the vectorial capacity, and other basic indices. An. minimus was the principal vector, but An. leucosphyrous was also found to be positive. Chittagong Hill Tracts in East Pakistan is consid- ered as a hyperendemic malarious district: Kuraishy & Talibi (1962) recorded a spleen rate of 91 % in children 2-9 years of age in this area. The natural transmission of malaria, in particular mosquito bio- nomics, and the infectivity of the anophelines occur- ring in the area have not been studied very thoroughly. A longitudinal epidemiological study was there- fore carried out to investigate the seasonal transmis- sion of malaria and the significance of congenital immunity in an endemic area of East Pakistan and to determine the various indices for epidemiological assessment. THE STUDY AREA The study area was at Khagrachberi, in a large fertile valley bounded on both sides by chains of hills in the northern part of Chittagong Hill Tracts (Fig. 1). Several drains and natural streams from this valley run into the Chengi river. The area is not flooded during the rainy season and the flat land and most of the hills that have been cleared of forest * Subsequent to the completion of this work the name of East Pakistan was changed to Bangladesh. 1 Director, Malaria Institute of Bangladesh, Dacca, Ban- gladesh. ' Entomologist, Malaria Institute of Bangladesh, Dacca, Bangladesh. are under cultivation. Rice is the main crop but some sugar cane is also grown. The houses are made of split bamboo poles and consist of a central raised platform. The people usually stay indoors at night. The men and children wear very little clothing. The animals are tethered outside, usually in a field near the house. Shelters are sometimes erected for the animals but these shelters are usually without side walls. The area has a tropical monsoon climate, the aver- age annual rainfall being about 250 cm. The maxi- mum and minimum temperatures recorded during the study were 32°C and 10°C, respectively: during the period from February to November, the mean temperature was about 27°C and during the winter months of December and January the mean tempera- ture fell to 18°C. The relative humidity ranged from 70.3% to 93.4% at 07.00 hours and from 41.7% to 85% at 16.00 hours. MATERIALS AND METHODS Villages with high spleen and parasite rates were selected on the basis of a rapid malariometric survey. The basic observations included determinations of the infant parasite rate, and the parasite species, estimates of parasite density, gametocyte counts, and determination of vector bionomics, sporozoite and 2857 -783 A. Q. KHAN & S. A. TALIBI Fig. 1. Chittagong Hill Tracts District. oocyst rates, and abdominal categories. The expec- tation of life, the vectorial capacity, and the basic reproduction rate of the vector species and the inocu- lation rate were also considered. Twelve villages were chosen for routine monthly observations of the occurrence of new infections in infants and for the collection of anopheline mos- quitos for a full year from July 1966 to June 1967. A complete census of houses, population, and par- ticularly infants was made and the census was kept up to date by means of the monthly visits. Infants less than 8-10 days old were usually not available for examination. Parasitological observations Monthly parasitological observations were made on all available infants (0-11 months) in the study area. The blood slides were dehaemoglobinized and fixed and were then stained with 3 % Giemsa stain for 35 min. At least 200 fields were examined before a slide was considered negative. The parasite count was made against 400 white blood cells (WBC), 12 000 WBC per cubic millimetre of blood being taken as the standard for the purposes of this study. Entomological observations Adult mosquitos and larvae were collected by several different methods. Adult mosquitos were caught in the early morning from four rooms in each village, using an aspirator for 15 min in each room: after these catches, adults were caught with a spray sheet in one sleeping-room per village. In addition, adult mosquitos were caught in different biotopes in the village and the surrounding areas, using an aspirator: these catches were made inside houses other than those selected for the regular catches, underneath the platforms of houses and in cattle sheds, goat and chicken houses, holes in trees, holes in earth banks, the bases of banana trees, and artificial shelters. At night, mosquitos were caught off human bait in one village. A team of four insect collectors (two indoors and two outdoors) exposed their arms and legs to mosquito bites starting 15 minutes before sunset and continuing throughout the night until 15 minutes after sunrise: collections were made for 30 minutes each hour. Larvae were collected from suitable breeding places, such as streams, springs, ponds, and rice fields. The work of identifying the mosquitos, abdominal grading, dissection for parity-by observing the stretching of the tracheoles of the ovaries (Detinova, 1962)-and the preparation of precipitin papers was carried out in the field laboratory immediately fol- lowing the collections. The salivary glands and gut were usually dissected on the same day. As far as possible the same insect collectors were retained throughout the study and the usual procedure of rotating the duties of insect collectors for indoor and outdoor night collection was maintained. RESULTS Parasitological observations The monthly incidence of new parasite infections in infants has been found to be a sensitive indication of the transmission season. All infants positive for parasites were followed up for determination of the N Intemational boundary . District boundary ........... Sub-division boundary River WHO 10830 784 EPIDEMIOLOGICAL ASSESSMENT OF MALARIA TRANSMISSION 785 other indices, such as parasite density and gametocyte possible on 41 infants from the age of 1 month and count. on 19 from the age of 2 months. The 147 infants available for examination in the Table 1 shows the results of the monthly collec- locality were of different ages when first examined, tions of blood slides from the 60 infants by age but routine monthly surveillance for infection was group. A total of 31 infants were found to be posi- Table 1. Parasite incidence determined at the monthly examinations of blood slides from 60 infants who were 2 months old or under when first examined Age group (months) Infant slides examined 0-2 3-5 6-8 9-11 Total no. 7 0 0 0 7 August no. positive 0 0 0 0 0 no. 10 3 0 0 13 September no. positive 1 0 0 0 1 no. 11 6 0 0 17 October no. positive 1 1 0 0 2 no. 8 17 0 0 25 November no. positive 0 6 0 0 6 no. 10 14 4 0 28 December no. positive a 0 3 (2) 0 0 3 (2) no. 1 1 14 6 0 31 January no. positive 2 0 0 0 2 no. 14 13 14 0 41 February no. positive a 0 1 (1) 0 1 (1) no. 12 14 14 4 44 March no. positive a 3 2 1 (1) 0 6 (1) no. 6 15 14 6 41 Apri no. positivea 1 (1) 3 (1) 2 (1) 0 6 (3) no. 2 10 12 11 35 May no. positive a 0 0 2 1 (1) 3 (1) no. 0 8 11 13 32 June no. positive a 0 0 (1) 1 (2) 1 (3) total no. 91 114 75 34 314 (August-June) no. positive a 8 (1) 16 (3) 5 (4) 2 (3) 31 (11) a Figures in parentheses refer to superinfection. A. Q. KHAN & S. A. TALIBI Table 2. Results of monthly examinations of blood slides from 87 infants who were more than 2 months old when first examined Distribution Total no. of Average Average gametocyte counts/mm"Age group by age group blood slides No.tv parasite(months) = (%) examined epositive pnsity index P. vivax P. falciparumI P. malarlae All species 3-5 33.3 50 9 VI 500 190 0 397 6-8 46.0 104 24 VI 615 223 0 404 9-11 20.7 138 40 VIl 636 301 510 400 total - 292 ' 73 [ - -_ - I - I - tive, 11 of them showing superinfections. The term superinfection, for the purposes of this study, denotes the detection of a subsequent infection with a hetero- logous species during the course of the study and this has been considered as evidence of fresh trans- mission. The incidence of new infections for the year under study was 51.7%. The results of the parasitological observations on the 87 infants who were more than 2 months old at the time of the first examination are shown in Table 2. Fig. 2 shows the monthly incidence of new infec- tions in the 60 infants. The data in Table 1 show that the incidence of new infections was highest in (A E CA -V c a the period from March to June and in November and December, reflecting two principal transmission seasons, which probably have peaks during the period from February to May (pre-monsoon) and in October and November (pre-winter). The parasite density of a population gives an indication of the collective load of infection and is related to immunity. Among the 31 infants found to be positive, the parasite density was highest in the 0-2-month age group (Class 8) and decreased to Class 5 in the 9-11-month age group. The defini- tion of parasite density used in this study is similar to that used in West Africa by Bruce-Chwatt (1958). SHO 10831 Fig. 2. Occurrence of new infections among 60 infants who were 2 months old or under when first examined. 786 EPIDEMIOLOGICAL ASSESSMENT OF MALARIA TRANSMISSION I LW 1100 1000 900 E 800 & 700 l-9 600 8 500 o 400 300 200 100 0-2 3-5 6.8 Age group (months) 9-11 "O 10832 Fig. 3. Average gametocyte counts, by age group, for the 31 infants found positive out of the 60 infants examined who were 2 months old or less when first examined. The figures in parentheses give the numbers of gametocyte-positive slides in each category. Fig. 3 shows, by age group, the average game- tocyte count per positive examination of 31 infants. For the purposes of these calculations of the aver- age for all species, a slide showing gametocytes of more than one species was considered as two or more separate entities. The highest count was found in the 0-2-month age group for P. vivax. The highest counts of P. falciparum were observed in the 3-5-months age group. The average gametocyte count for all species was highest in the 0-2-month age group and declined with increasing age. The prevalence of parasitaemia in infants of dif- ferent ages is shown in Table 3. If the proportions of infected infants in the different age groups are compared with the curves given by Macdonald Table 3. Prevalence of parasites in infants who were 2 months old or under when first examined, by age group Age group IMedian age I No. No. Pooto(months) (days)a examined positive Proporion 0-2 20 91 9 0.099 3-5 127 114 31 0.272 6-8 218 75 23 0.307 9-11 310 34 11 0.324 (1957) for the estimation of inoculation rate, which is the daily number of bites inflicted on one indi- vidual by mosquitos infected with sporozoites which are actually infective, a daily inoculation rate of 0.0025 is obtained. The continuation of the highly malarious situation in the area during the study period was confirmed by means of two malariometric surveys among chil- dren 2-9 years of age. These were conducted in May 1966, immediately prior to the initiation of this study, and in November 1966 during the expected peak of the malaria season. The results are sum- marized in Table 4. Entomological observations Vector species. Twenty-one species of Anopheles mosquito were collected and identified from the area. Three of the species, An. vagus, An. minimus, and An. aconitus, were common throughout the year, and two species, An. culicifacies and An. jeyporiensis, were less common. The remaining species occurred in very small numbers. An. minimus was usually found inside houses. One specimen was found in a cattle shed and another in an artificial shelter. Other species found resting inside houses were, in order of prevalence, An. vagus, An. aconitus, An. jeyporiensis, An. varuna, An. culici- facies, An. fluviatilis, An. kochi, and An. subpictus. Table 4. Results of malariometric surveys in children 2-9 years of age Parasite Relative prevalence (%) Gameto- surveyo children rate rate cyte ra/ceDateyof |examined a (%) (%) P. fa/cl- P. vivax P. malariae (P.arumc) May 1966 344 (283) 58.8 69.6 50.1 39.0 10.8 33.3 Nov. 1966 188 (140) [ 54.0 | 56.4 59.3 31.5 | 9.3 32.1 a Figures in parentheses relate to the parasite survey. (6) P. faiciparum . P. vivax . ........... P. malariae All species (9) (1) u, 787 788 A. Q. KHAN & S. A. TALIBI Man-vector contact. A total of 363 anopheline mosquitos were caught biting at night off human bait and these belonged to 14 species. Of these, 248 (68%) were collected biting indoors and 115 (32%) biting outdoors. Of the total night catch, 57.3% was An. minimus. The average man-biting rate for An. minimus was found to be 5.7 for the year and man-biting activity was most pronounced in the months of October and November. Of the total of 208 An. minimus collected, 182 (87.5%O) were caught biting indoors and 26 (12.5%) were caught biting outdoors. The period of man-biting activity for An. minimus is shown in Fig. 4 and it may be observed that the maximum biting activity occurred 7 hours after sunset, i.e., at midnight. 50 45 40- _35- 30- 2 5- 20- 15- 10- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Hours after sunset Fig. 4. Night-biting activity of An. minimus. Abdominal categories. During abdominal grading, gravid An. vagus, An. minimus, An. aconitus, An. culi- cifacies, An. jeyporiensis, and An. varuna were found in the day catches from human dwellings. The proportions of the different abdominal cate- gories for An. minimus were as follows: unfed, 5.3 %; freshly fed, 49.0%; half-gravid, 34.6%; and gravid, 11.1%. The low proportion of gravid females in the day catch suggests some degree of exophily and the proportions of fully fed females that were half-gravid and gravid, 0.52 and 0.48, respectively, indicate a 2-day gonotrophic cycle. To avoid any tendency for specimens between late-fed stage and subgravid to be classified as half-gravid the two-stage classification of fed and gravid was adopted.1 Serological identification ofthe blood meal. A total of 1 235 blood smears were made from Anopheles mosquitos collected from resting places indoors and outdoors. Precipitin tests were carried out at the Lister Institute of Preventive Medicine, London, and showed that 93 % of the An. minimus fed on human blood. Dissection for determination of parity and sporo- zoite rate. All the freshly fed and unfed mosquitos with ovaries in Christopher's stage II that were col- lected biting at night, or by other collection methods, were dissected to determine parity and the probability of survival. The probability of survival was highest (90%) for An. minimus; that for An. vagus was 86%, for An. jeyporienisis 81 %, and for An. philippinensis 72%. A total of 2 432 mosquitos of 16 species were dis- sected to detect salivary gland infections. Of the total of2 060 An. minimus dissected, 27 were infected, giving a sporozoite rate of 1.31 %. Only 4 An. keuco- sphyrus were collected throughout the year and 1, from a night catch, was infected with sporozoites. An. minimus is considered to be the principal vector in the area. Estimation of vectorial capacity and other indices. Using the table of Oganov-Rayevski, as reproduced by Pampana (1963), the average duration of sporo- gony for P. vivax at the temperatures found in this area is 10 days. For P. falciparum the duration of sporogony is 12 days. Using the probability of sur- vival based on age grading, read with the table given by Macdonald (1957), the average expectation of infective life (longevity factor) of female An. minimus was determined for P. vivax as 3.3 days and for P. falciparum as 2.7 days. The expected rate of malaria inoculations in man per infective case per day, i.e., the vectorial capacity (Garrett-Jones, 1964), which is the product of the man-biting rate, the longevity factor, and the index of man-biting, for An. minimus was 4.008 for P. vivax and 3.24 for P. falciparum. The number of new infections that would be expected in the surrounding population from a totally nonimmune and untreated case, if that population had been untouched by malaria until the case arrived in their midst, i.e., the basic reproduction rate (Macdonald, 1957), was calculated, and was 320.7 for P. vivax and 259.1 for 1 This classification was proposed in: Practical entomology in malaria eradication, Part I, p. 54 (unpublished WHO document MHO/PA/62.63) EPIDEMIOLOGICAL ASSESSMENT OF MALARIA TRANSMISSION 789 100 Vectorial capacity Man biting rate .. Sporozoite rate % ........... .No. of new infections in infants 10 CL)~~~~~~~~~~~~~~~~~~~~~~~~. 0.1 Juy Ag July Aug. Sep. Oct. Nov. Dec. Jan. Feb. March April May June Fig. 5. Trends of vectorial capacity, man-biting rate, sporozoite rate, and the occurrence of new infections in infants. P. falciparum. The actual reproduction rate as deter- mined for the study area was 1.03 for P. vivax and 1.05 for P. falciparum. The stability index of the study area was 4.4. The mean daily number of bites inflicted on one individual by mosquitos infected with sporozoites that were actually infective, or the inoculation rate (Macdonald, 1957), was determined as 0.034. The malaria transmission trend as derived from various indices is presented in Fig. 5. DISCUSSION Congenital transmission of malaria is an exceed- ingly rare phenomenon (Covel, 1950). Bruce-Chwatt (1952) found not a single case of congenital malaria in 332 African newborn babies in Lagos. Similar negative results have been quoted by Cannon (1958) and Spitz (1959) from Nigeria and by Jilly 1 from Ghana. A study by Jelliffe 2 in Kampala, Uganda, showed that although 5.6% of 570 pregnant women had infected peripheral blood and 16.1 % of them 1 Jilly, P. (1966) Anaemia in parturient women with special reference to malarial infection of the placenta and sickle-cell trait (Unpublished document WHO/Mal/66.563). 2 Jelliffe, E. F. (1966) Low birth weight and malaria infection of the placenta (Unpublished document WHO/ Mal/66.558). had a malaria-infected placenta, only 1 case of con- genital malaria was seen among the neonates. Thus the incidence of new infections in infants can be regarded as a sensitive index of transmission. In this study, transmission of malaria occurred throughout the year with the exception of the months of July and August. It was possible to continue routine monthly surveillance of 41 infants from the age of 1 month and of a further 19 from the age of 2 months. Out of the total of 42 episodes of trans- mission that occurred in 31 infants, 24 (57%) were detected during the period from March to June and 11 (26%) in November and December, reflecting two main transmission seasons; one in the pre- monsoon period in February-May and one in the pre-winter period in October and November. The pre-winter transmission period was shorter than the pre-monsoon period, possibly because of the onset of winter. The infrequent occurrence of new infec- tions in infants for the first 2 months of study might have been the result of the small number of infants examined, rather than of the absence of transmission. The occurrence of new infections in 51.7% of the infants examined and the occurrence of 11 super- infections with heterologous species in 10 positive infants (out of a total of 31 positives) indicate a fairly high level of transmission in the area. P. vivax Q. A. KHAN & S. A. TALIBI cases were well distributed throughout the months of the study except July and August, but P. falci- parum cases were found only in November, Decem- ber, and January and again in April and June. The largest number of new P. vivax infections was ob- served in March whereas the largest number of new P. falciparum infections was in April. Congenital transmission of immunity in malaria was the subject of a comprehensive review by Bruce- Chwatt (1963). Davidson & Draper (1953), com- menting on the discrepancy between the potential and actual malaria inoculation rates in Tanganyika, state that " The cause appears to be within the infant, taking the form of an immunity which sup- presses the majority of infections but as we have seen exerting little restraint on those which break through." Gilles (1957) and McGregor et al. (1956) have reported low incidence and low parasite den- sities in very young infants. Four possible causes for this have been considered by McGregor (1960) and by Bruce-Chwatt (1963): (1) the relative aver- sion of Anopheles to feeding on infants, (2) the defi- ciency of p-aminobenzoic acid in the exclusive milk diet during early infancy, (3) the inability of the fetal and other haemoglobins to support plasmodial development, and (4) passive immunity inherited from the mother. In a hyperendemic situation, such as that in the study area, where transmission is continuous and intense, the adult population, including the expec- tant mothers, enjoy a reasonably high degree of acquired immunity, which may be passively trans- ferred to the infants. If this passively transferred immunity is of some significance, and if it lasts for the first 3 months, the highest average parasite den- sity and the highest average gametocyte count should not occur in the 0-2-month age group. In the present study the average parasite density and the average gametocyte count were highest in the 0-2-month age group and decreased with increas- ing age: there were 7 positives in the 0-2-month age group (one of whom was 1 month old) out of the total of 31 positive infants. Thus it seems that in hyperendemic areas, where transmission is con- tinuous and intense, the maternal immunity pas- sively transferred to the infants may not play a significant role in restricting the development of the asexual erythrocytic stage and of the gametocytes. Macdonald (1957) has said that " the earliest immune response is a reduction of gametocyte out- put, and in intensely malarious places it is visible by three months of age in the form of greatly reduced densities, without at first marked reduction of rate ". Thus, the gradual decline of the average parasite density indices and that of the average gametocyte counts with increasing age in infants that were observed in this study are indicative of the building up of active immunity as a result of acute infection rather than the occurrence of infec- tion in an already immune population. The large number of anopheline species collected from the area points to the richness of the mosquito fauna. Only two species, however, An. minimus and An. leucosphyrus, were infected with sporozoites. An. minimus was the main vector in the area and from the estimates of the basic reproduction rates (320.7 for P. vivax and 259.1 for P. falciparum), the actual reproduction rates (1.03 for P. vivax and 1.05 for P. falciparum), and the stability index (4.4), it is clear that this species was responsible for main- taining highly stable malaria in the area. The occurrence of new infections in infants fol- lowed broadly the seasonal prevalence ofAn. minimus. The two peak months of new infections were pre- ceded by increased densities of An. minimus and a fall in density was followed by a decrease in the occurrence of new infections. As regards other pos- sible vectors of malaria in the area, the density of An. jeyporiensis in relation to man was 12.2% and its human blood index was 34.5%; thus it appears that this species could be a vector ofsome importance. Although An. philippinensis does not seem to play an important part in malaria transmission at present, its presence in night catches (2 indoors and 20 out- doors) is indicative of exophilic and exophagic behav- iour in the area. No specimens were found in day collections from indoor resting places. If this species were to become a vector in the area, the spraying of insecticides inside houses might not be effective. The vectorial capacity of An. minimus was as high as 12.2-16.2 during March, September, October, and November. During the remaining months it varied from 2.0 to 4.2 except in January and February. During these cold months it was not possible to estimate the vectorial capacity as no mosquitos were caught at night. This may be explained by the fact that the mosquito collectors were located in cold sites. It is, however, probable that mosquitos were biting the local inhabitants in the warmer indoor rooms. The inoculation rate for this area calculated from entomological data is 0.034, and this value was 14 times that calculated on the basis of the propor- tion of infants infected, which was 0.0025. In areas 790 EPIDEMIOLOGICAL ASSESSMENT OF MALARIA TRANSMISSION 791 of high endemicity, a discrepancy of this kind is quite common. Discussing possible reasons for these discrepancies, Macdonald (1957) stated that the ento- mological approach estimates the total number of inoculations whereas the parasitological approach estimates only the successful inoculations. Elaborat- ing further, Pampana (1963) stated that many infec- tions are distributed in infants who are already infected and who have already developed some immunity; in addition, the inoculations probably contain so few sporozoites that only a few subjects develop parasitaemia. ACKNOWLEDGEMENTS The authors express their gratitude to Lt. Col. M. K. Q. Hashmi, Project Director, Malaria Eradication, Pakistan, for supporting the study and to Major A. Q. B. Rahman, Provincial Chief, Malaria Eradication, East Paldstan, for his cooperation and for providing the necessary transport. This study was conducted with partial financial assistance from the World Health Organization, Geneva, Switzerland. RtSUMt EVALUATION EPIDEMIOLOGIQUE DE LA TRANSMISSION DU PALUDISME DANS UNE REGION D'ENDEMICITE DU PAKISTAN ORIENTAL; ROLE DE L'IMMUNITE CONGENITALE On a mend une enquete dans une region d'endemicite paludeenne du Pakistan oriental afin d'etudier les aspects saisonniers de la transmission du paludisme, d'apprecier l'influence de l'immunite congenitale et d'dtablir les indices indispensables a une evaluation epidemiologique. Apres une rapide enquete paludom6trique, on a choisi des villages oiu les indices plasmodiques et spleniques etaient eleves. L'examen de tous les enfants de 0 a 11 mois a ete r6pet6 mensuellement. Des captures de jour et de nuit ont permis de recolter des moustiques adultes; on les a identifies, 6valu6 le stade de la digestion san- guine, differenci6 les femelles pares des nullipares et recueilli des dchantillons de repas de sang pour les epreuves de seroprecipitation. Sur un total de 147 enfants etudies, 60 (41 ages de 1 mois et 19 ages de 2 mois) ont fait l'objet d'examens parasitologiques mensuels; 31 d'entre eux ont et6 trouves positifs, ce qui correspond i une incidence parasitaire annuelle chez les nourrissons de 51,7%. On a decele 11 cas de surinfection par une espece heterologue, indice d'une transmission recente. La frequence des nouvelles infections a ete particulierement elevee de mars a juin et en novembre-decembre, avec deux clochers de transmis- sion maximale de fevrier a mai (avant la mousson) et en octobre-novembre (avant l'hiver). Le nombre quotidien de piquires infectantes, par individu, a ete estime a 0,0025. L'indice de densit6 parasitaire et l'indice gametocytaire, tres eleves chez les enfants de 0 'a 2 mois, s'abaissaient chez les enfants plus ag6s. On en conclut que l'immunite neonatale ne joue aucun role determinant. Deux especes d'anopheles, Anopheles minimus et A. keucosphyrus, ont ete trouvees infectees par des sporo- zoltes, la premiere etant consideree comme le principal vecteur dans la region. Le taux d'agressivit6 d'A. minimus est maximal vers minuit, 7 heures apres le coucher du soleil. La duree de son cycle gonotrophique est de 2 jours. Les epreuves de seropr6cipitation indiquent qu'il se nourrit essentiellement sur l'homme (93 %). Sur 2060 femelles d'A. minimus dissequees, 27 etaient infectieuses (indice sporozoltique: 1,31 %). La dure'e probable de vie infectante est de 3,3 jours pour Plasmodium vivax et de 2,7 jours pour P. falciparum. L'aptitude vectorielle d'A. minimus est estim6e a 4,008 pour P. vivax et a 3,24 pour P. falciparum. REFERENCES Bruce-Chwatt, L. J. (1952) Ann. trop. Med. Parasit., 46, 172 Bruce-Chwatt, L. J. (1958) Trans. roy. Soc. trop. Med. Hyg., 52, 389 Bruce-Chwatt, L. J. (1963) In: Garnham, P. C. C., Pierce, A. E. & Roitt, L., ed., Immunity to protozoa, London, Oxford University Press Cannon, D. S. H. (1958) Brit. med. J., 2, 877 Covel, G. (1950) Trop. Dis. Bull., 47, 1147 Davidson, G. & Draper, C. C. (1953) Trans. roy. Soc. trop. Med. Hyg., 47, 522 Detinova, T. S. (1962) Age-grouping methods in Diptera of medical importance, Geneva, World Health Organ- ization 792 A. Q. KHAN & S. A. TALIBI Garrett-Jones, C. (1964) Bull. Wld Hlth Org., 30, 241-261 Gilles, H. M. (1957) Ann. trop. Med. Parasit., 51, 58 Kuraishy, N. A. & Talibi, S. A. (1962) Pak. J. med. Res., 11, 74-79 Macdonald, G. (1957) The epidemiology and control of malaria, London, Oxford University Press McGregor, I. A. et al. (1956) Brit. med. J., 2, 686 McGregor, I. A. (1960) W. afr. med. J., 9, 260 Pampana, E. (1963) A text book of malaria eradication, London, Oxford University Press Spitz, A. J. W. (1959) Bull. Wld Hlth Org., 21, 242