Acta Tropiea, 50(1992)295 304 ~7) 1992 Elsevier Science Publishers B.V. All rights reserved 0001-706X/92/$05.00
295
ACTROP 00192
Relationships between circulating S-antigens, naturally acquired antibodies to Plasmodium falciparum exoantigens and malaria attack in a mesoendemic area Bernab6 F.F. Chumpitazi 1, Franc~ois Peyron 1, Christian Boudin 2,*, Stephane Picot 1, Bruno Oury 1'** and Pierre Ambroise-Thomas 1 l D@artement de Parasitologie-Mycologie MOdieale et Mol~culaire, CNRS URA 1344, UniversitO Joseph Fourier-Grenoble 1, La Tronche, France, and 2Antenne ORSTOM, Centre Muraz, Bobo-Dioulasso, Burkina Faso (Received l0 September 1991; accepted 7 November 1991)
A survey involving 77 individuals living in two savannah villages near Bobo Dioulasso (Burkina Faso, West Africa), was performed in June 1987 (before), August-September (during) and January 1988 after the seasonal transmission. The clinical longitudinal study during the seasonal period permitted us to define three different groups in terms of both age and occurrence of malaria attack (MA; > 5000 parasites/mm 3 of blood and axillary fever _>37.8°C). The presence of circulating stable antigen (S-Ag) and the antibody responses against exoantigens (E-Ag) of Plasmodium fah-iparum were also evaluated at three observations periods: beginning, during and after the transmission season. The adult group (III) had the highest rates of IgG Ab to E-Ag although, IgM prevalence to E-Ag was maximal in the group II (individuals with no malaria attack and age _< 15 years old). Group I (persons with < 15 years old and who contracted at least one MA) did not have any S-Ag at the first observation period and showed the lowest rate of antibodies to E-Ag. The probability of occurrence of an MA calculated from these parameters at the beginning of the transmission period were correct in 78.9% of the cases in children (Groups I & II) and in 71.8% of adults during the subsequent transmission period. Therefore these values could be used for evaluating the probability of occurrence of a clinical MA during the transmission period in a mesoendemic area. S-Ag and antibodies to E-Ag could participate positively in the mechanisms involved in the development of the immune status. Key words: Plasmodiumfalciparum; Malaria attack; S-antigens; Exo-antigens; IgM
Introduction M a l a r i a is t h e m a j o r p a r a s i t i c d i s e a s e in t h e w o r l d a n d r e m a i n s p r e v a l e n t t h r o u g h o u t tropical and subtropical areas. Consequently, the cost of the malaria control pro-
*Present address: Dr. C. Boudin, Departement de Parasitologie-Mycologie M+dicale et Mol6culaire (DP3M), CNRS URA 1344, Universit~ Joseph Fourier-Grenoble I, 38706 La Tronche cedex, France. **Present address: Dr. B. Oury, ORSTOM, BP 5045, 34032 Montpellier cedex 1, France. Correspondence address." Dr. B.F.F. Chumpitazi, DP3M, Facult6 de M6decine, Universit~ Joseph-FourierGrenoble I, 38706 La Tronche cedex, France.
296 grams are high and it should be better focused mainly in previous selected population very sensitive to malaria morbidity and mortality. The age and the type of transmission are well known to be good parameters for this purpose. In order to select a malaria sensible population and to obtain a better understanding of the process involved in the acquired immunoprotection, we have evaluated the probability of occurrence of at least one malaria attack (MA) during the seasonal transmission, in terms of the presence of S-antigen (S-Ag; McGregor et al.~ 1968; Wilson and Voller, 1979) and antibodies to exoantigens of P. J~tlciparum (Th6lu et al., 1985; Chumpitazi et al., 1987) in plasma from individuals living in a malaria mesoendemic area of Burkina Faso. The early study of Wilson (1980) showed that S-Ag were stable markers for different parasite strains in dispite of its diversity and its polymorphism in size. S-Ag are released at the schizogony and any function has not yet been found for them. Nevertheless, antibodies to S-Ag were more frequently found in adults than in children. For a review about S-Ag see Anders and Smythe (1989). In previous studies, we have suggested that exoantigens (E-Ag) could be related to S-Ag by their solubility, their partial stability to heat, their presence in the culture fluid of P. falciparum, their high masse for SGE1 strain (Mr: 100 000, 135 000 and 170 000), their low isolectric points, their protein-like nature and their release as soluble antigens at the time of schizont rupture (Th61u et al., 1985: Chumpitazi et al., 1987).
Subjects and Methods
Study areas This study include the inhabitants of two small villages (Dafinso and Vall6e du Kou) in the region of Bobo-Dioulasso, Burkina Faso. Dafinso is a typical village of the near savannah with a semipermanent stream and Vall6e du Kou is located in the middle of a rice field area. In this region the climate is characterized by a rainy season from June to October, a dry-"cool" season from November to February and a dry-hot season from March to May. Rainfall varies from 800 to 1200 mm/year. Malaria is mesoendemic in both areas. Transmission is mainly due to Anopheles gambiae and to a lesser to A. funestus and A. nili. The density of Anopheline mosquitoes is about four times higher in rice-field area than in savannah. Nevertheless, the number of infected bites for the whole transmission period and per man is 50 in the savannah and about 30 in the second area (Robert et al., 1985). This is mainly explained by the lower longetivity of the vector associated with the use of insecticide and zoophile deviation in the rice field area (Robert, 1989). P. falciparum is by far the most common species but P. malariae and P. ovale are also present.
Investigated population In each area, samples of households of the two villages were selected and visited during the study. In these areas 82 individuals were included in this study but only 77 were longitudinally studied and presented in the three observations periods. Clinical and parasitological data were recorded for each patient twice a week by a physician. An MA was recorded when parasitemia reached at least 500 parasites/ mm 3 and axillary fever >37.8°C. Chloroquine was also handled by a Medical
297 Doctor. In addition, blood specimens were collected by venipuncture and finger prick in heparin tubes at three observation periods: June 1987 (period A), August September 1987 (period B) and January 1988 (period C). The age distribution of the population and the malaria clinical status is shown in Table 1. The groups were formed, according to our clinical data and in agreement with previous information obtained in this area about malaria morbidity (Baudon et al., 1986 and C. Boudin, personal observations).
Microscopic observation Thick film were made for microscopic examination after standard Giemsa staining. Parasite counts were determined from the number of parasites observed per at least 800 leukocytes, assuming a constant leukocyte count of 8000 per gl of blood. Only asexual forms of P. falciparum, the most prevalent species in the area, were counted.
Antibody assay Ab to E-Ag. E-Ag was prepared by SP-Trisacryl chromatography from the culture fluid (CF) of P. falciparum (Th61u et al., 1985) with a simplified isocratic system of elution (Acetate buffer 10 raM, NaCI 300 mM, pH 3.7) and acidification of CF with 1.7 M acetic acid to pH 3.7 (Chumpitazi, 1989). Ab to E-Ag were detected by ELISA (Voller et al., 1978; Chumpitazi et al., 1987). PBS supplemented with 0.05% Tween 20, and 5% skimmed dry milk was used for dilution of plasma and conjugate. Conjugates used were: (a) anti-human IgG heavy chain (from goat) labelled with peroxidase (1:750, Miles Yeda, France); (b) the corresponding anti-human IgM (1:1500, Miles Yeda). The chromogene-substrate was orthophenylenediamine'2HCI-H20 2. The absorbance (A) at 492 nm of the contents of each well was measured using a multiscan photometer (Flow Laboratories). Results were expressed as the difference between absorbance values obtained in coated and uncoated plates. As controls, 50 plasma samples from healthy Europeans (provided by the Blood Transfusion Center of Grenoble, France) diluted at 1 : 100 were tested. The cut-offs were 0.48 for anti-E Ag of IgG type and 0.43 for anti-E-Ag of IgM type. These values were computed from 99th percentile of the ranked absorbance values from the 50 plasma against E-Ag. Circulating S-Ag. The incubation fluid of parasite cultures (IF) was used as a positive control for detection of circulating S-Ag by Enzyme Linked ImmunoElectroDiffusion Assay (ELIEDA; Chumpitazi, 1989). Briefly, parasites SGE1 strain (from Gambia) were cultured at 5% haematocrit (Trager and Jensen, 1976) using a modified medium: RPMI 1640, 10% A + serum, 35 mM HEPES, 25 mM glucose (Osinsaya et al., 1981). When the parasitemia reached 4%, the infected red blood cells were washed once in RPMI 1640 and then incubated for 72 h at the same conditions but without serum. The 1F was collected by centrifugation at 400 x g for 10 min and the supernatant filtered through a 0.45 gm filter and concentrated 20 times (IF x 20). Fifty gl of the plasma to be analysed were added to an Eppendorf tube and heated for 5 min at 100°C before the addition of 50 gl of 50 mM Trisglycine buffer, pH 8.8. From this point, all the following procedure was done at room temperature. After shaking, the prepared samples were centrifuged for 5 min at 10 000 x g. The supernatants were recovered and applied (5 ~tl) to the cellulose
298 acetate membrane for testing S-Ag. The E L I E D A technique (Pinon et al., 1979) was used with some modifications. A pool of human positive sera from healthy African donors with an immunofluorescence titer against schizonts of P. falciparum > 640 was used as Ab to S-Ag. Counter-Immuno-Electrophoresis (CIE) was carried out for 2 h at 110 V on strips (Cellogel TM, 2.5 cm x 17 cm x 190 gin) placed on racks in special tanks (SEBIA TM, S 60 N) with 50 mM Tris-glycine buffer. The samples (5 lab were deposited at each side to the heated and unheated Ag control (8 lal of IF x 20). After electrophoretic migration, the membranes were washed three times for 5 min in PBS-T (0.02% Tween 20). They were coated for 30 rain in PBS-T-5% dry nonfat milk with saking to prevent non-specific binding to the membranes. Subsequently, a peroxidase conjugated affinity-purified goat anti-human |gG heavy chain (Miles) at a 1:750 dilution was added to the membranes and incubated for 2 h with mechanical agitation. The strips were washed three times for 5 min in PBS-T and once in TBS (20 mM Tris-(hydroxymethyl)aminomethane, 500 mM NaCl, pH 7.5). Then, a solution of TBS with 10 ml of 4-chloro-l-naphthol (6 mg) and 30% H202 (6 lal) was added to the membranes for 30 min in the dark and washed twice for 5 min in distilled water.
Statistical analysis The absorbance of Ab and number of arcs of circulating S-Ag were compared by variance analysis, Kruskal-Wallis one-way analysis by ranks and KolmogorovSmirnov two-sample test, P values < 0.05 were considered significant.
Results
In this study, no complicated malaria was observed. Therefore, only three groups of subjects have been formed according to our clinical and parasitological data. Group h children (_< 15 years old; average age 9.1 _+3.1) who had at least one MA ( > 5000 parasites/mm 3 and axillary fever > 37.8°C), during the transmission season. The parasitemia disappeared in one or two days after chloroquine therapy. Group Ih children ( < 15 years old; average age 10.8 ± 2.8) who did not have any MA during the transmission season. This group is similar to the first one in age:sex ratio and socioeconomic-factors. Group IIh adults ( > 15 years old; average age 33.1 + 14.1) who did not have any MA during the transmission season. In this study, all the adults examined did not have any MA and both the Dafinso and Vall6e du Kou groups of subjects were analyzed together as both areas are mesoendemic and the number of infected bites for the whole transmission period and per person are similar.
Parasitological findings (Table 1) The parasitological findings are described in relation to the age, occurrence of at least one MA and the observation periods (passages). In the adult group, no detectable parasitemia was found at the beginning of the transmission season (June). In all groups, the number of subjects showing a parasitemia increased from June to August/September according to the transmission season.
299 TABLE 1 Parasitological findings according to the malaria attacks and passages in patients from a malarial mesoendemic area of Bobo-Dioulasso (Burkina Faso, West Africa) Groups a
Observation periods A (June)
B (Aug./Sep.)
C (Jan.)
1 Median parasite + density % o f + b l o o d slides
9040 (0-54400) b 38% (19 64) c
16480 (0-272900) b 95% (75 100) c
280 (0 4400) b 19% (6 44) c
H Median parasite+density % o f + b l o o d slides
120 (0-200) b 25% (9 49) c
III Median parasite+density % o f + b l o o d slides
3 2 0 0 (0 4 4 0 0 ) 8
4 4 0 (0 7800) b
50% (27 73) ~
68% (59 83y
0 0%
Total % o f + blood slides
160 (0 3400) b
10% (3 24) c
15.6% (8-25) c
4 0 (0
1400) b
14% (6 30) c
56.1% (43-66) ~
28.4% (20-41) c
aGroups: I, 18 patients (_< 15 years old; mean age = 9.1-4-3.1) who had at least one malaria attack (5000 parasites/mm 3 and fever) during the transmission season; II, 20, individuals ( < 15 years old; mean age = 10.8 _+2.8) who did not have any malaria attack during the transmission season; lII, 39 adults ( > 15 years old; mean age = 33.1 +_ 14.1) who did not have any malaria attack during the transmission season. bMedian parasite density. c95% confidence intervals.
Immunological .findings
(a) IgG Ab to E-Ag. IgG Ab to E-Ag detected by ELISA (Fig. l) were more frequently observed in groups II and III than in group I (persons who had at least 100
80
[]
rates of Ab to E-Ag of lgG class.
[]
prevalence of Ab to E Ag of lgM class.
•
prevalence ofchculating S-Ag.
60
•~
40
20
0 Ia
lIa
Jun.
Ilia
Ib
Ilb
lIlb
Aug./Sep.
Ic
IIc
lIlc
Jan.
Fig. 1. Seasonal prevalence and 95% confidence intervals of circulating S-Ag and Ab to P. jalciparum E-Ag in a malaria mesoendemic region of Bobo-Dioulasso (Burkina Faso, West Africa). Groups and observation periods: a, beginning; b, during and c, after the transmission season. See Table 1 for groups.
300 one malaria attack). Between the passages the frequencies were stable in the last two groups, whereas in the first group, the rate increased from June to August/September and was stable from August/September to January. Based upon statistical analysis of the absorbance, the reactivity observed in group III (median=0.52, values from 0.07 to 4.42) was higher (P
295
ACTROP 00192
Relationships between circulating S-antigens, naturally acquired antibodies to Plasmodium falciparum exoantigens and malaria attack in a mesoendemic area Bernab6 F.F. Chumpitazi 1, Franc~ois Peyron 1, Christian Boudin 2,*, Stephane Picot 1, Bruno Oury 1'** and Pierre Ambroise-Thomas 1 l D@artement de Parasitologie-Mycologie MOdieale et Mol~culaire, CNRS URA 1344, UniversitO Joseph Fourier-Grenoble 1, La Tronche, France, and 2Antenne ORSTOM, Centre Muraz, Bobo-Dioulasso, Burkina Faso (Received l0 September 1991; accepted 7 November 1991)
A survey involving 77 individuals living in two savannah villages near Bobo Dioulasso (Burkina Faso, West Africa), was performed in June 1987 (before), August-September (during) and January 1988 after the seasonal transmission. The clinical longitudinal study during the seasonal period permitted us to define three different groups in terms of both age and occurrence of malaria attack (MA; > 5000 parasites/mm 3 of blood and axillary fever _>37.8°C). The presence of circulating stable antigen (S-Ag) and the antibody responses against exoantigens (E-Ag) of Plasmodium fah-iparum were also evaluated at three observations periods: beginning, during and after the transmission season. The adult group (III) had the highest rates of IgG Ab to E-Ag although, IgM prevalence to E-Ag was maximal in the group II (individuals with no malaria attack and age _< 15 years old). Group I (persons with < 15 years old and who contracted at least one MA) did not have any S-Ag at the first observation period and showed the lowest rate of antibodies to E-Ag. The probability of occurrence of an MA calculated from these parameters at the beginning of the transmission period were correct in 78.9% of the cases in children (Groups I & II) and in 71.8% of adults during the subsequent transmission period. Therefore these values could be used for evaluating the probability of occurrence of a clinical MA during the transmission period in a mesoendemic area. S-Ag and antibodies to E-Ag could participate positively in the mechanisms involved in the development of the immune status. Key words: Plasmodiumfalciparum; Malaria attack; S-antigens; Exo-antigens; IgM
Introduction M a l a r i a is t h e m a j o r p a r a s i t i c d i s e a s e in t h e w o r l d a n d r e m a i n s p r e v a l e n t t h r o u g h o u t tropical and subtropical areas. Consequently, the cost of the malaria control pro-
*Present address: Dr. C. Boudin, Departement de Parasitologie-Mycologie M+dicale et Mol6culaire (DP3M), CNRS URA 1344, Universit~ Joseph Fourier-Grenoble I, 38706 La Tronche cedex, France. **Present address: Dr. B. Oury, ORSTOM, BP 5045, 34032 Montpellier cedex 1, France. Correspondence address." Dr. B.F.F. Chumpitazi, DP3M, Facult6 de M6decine, Universit~ Joseph-FourierGrenoble I, 38706 La Tronche cedex, France.
296 grams are high and it should be better focused mainly in previous selected population very sensitive to malaria morbidity and mortality. The age and the type of transmission are well known to be good parameters for this purpose. In order to select a malaria sensible population and to obtain a better understanding of the process involved in the acquired immunoprotection, we have evaluated the probability of occurrence of at least one malaria attack (MA) during the seasonal transmission, in terms of the presence of S-antigen (S-Ag; McGregor et al.~ 1968; Wilson and Voller, 1979) and antibodies to exoantigens of P. J~tlciparum (Th6lu et al., 1985; Chumpitazi et al., 1987) in plasma from individuals living in a malaria mesoendemic area of Burkina Faso. The early study of Wilson (1980) showed that S-Ag were stable markers for different parasite strains in dispite of its diversity and its polymorphism in size. S-Ag are released at the schizogony and any function has not yet been found for them. Nevertheless, antibodies to S-Ag were more frequently found in adults than in children. For a review about S-Ag see Anders and Smythe (1989). In previous studies, we have suggested that exoantigens (E-Ag) could be related to S-Ag by their solubility, their partial stability to heat, their presence in the culture fluid of P. falciparum, their high masse for SGE1 strain (Mr: 100 000, 135 000 and 170 000), their low isolectric points, their protein-like nature and their release as soluble antigens at the time of schizont rupture (Th61u et al., 1985: Chumpitazi et al., 1987).
Subjects and Methods
Study areas This study include the inhabitants of two small villages (Dafinso and Vall6e du Kou) in the region of Bobo-Dioulasso, Burkina Faso. Dafinso is a typical village of the near savannah with a semipermanent stream and Vall6e du Kou is located in the middle of a rice field area. In this region the climate is characterized by a rainy season from June to October, a dry-"cool" season from November to February and a dry-hot season from March to May. Rainfall varies from 800 to 1200 mm/year. Malaria is mesoendemic in both areas. Transmission is mainly due to Anopheles gambiae and to a lesser to A. funestus and A. nili. The density of Anopheline mosquitoes is about four times higher in rice-field area than in savannah. Nevertheless, the number of infected bites for the whole transmission period and per man is 50 in the savannah and about 30 in the second area (Robert et al., 1985). This is mainly explained by the lower longetivity of the vector associated with the use of insecticide and zoophile deviation in the rice field area (Robert, 1989). P. falciparum is by far the most common species but P. malariae and P. ovale are also present.
Investigated population In each area, samples of households of the two villages were selected and visited during the study. In these areas 82 individuals were included in this study but only 77 were longitudinally studied and presented in the three observations periods. Clinical and parasitological data were recorded for each patient twice a week by a physician. An MA was recorded when parasitemia reached at least 500 parasites/ mm 3 and axillary fever >37.8°C. Chloroquine was also handled by a Medical
297 Doctor. In addition, blood specimens were collected by venipuncture and finger prick in heparin tubes at three observation periods: June 1987 (period A), August September 1987 (period B) and January 1988 (period C). The age distribution of the population and the malaria clinical status is shown in Table 1. The groups were formed, according to our clinical data and in agreement with previous information obtained in this area about malaria morbidity (Baudon et al., 1986 and C. Boudin, personal observations).
Microscopic observation Thick film were made for microscopic examination after standard Giemsa staining. Parasite counts were determined from the number of parasites observed per at least 800 leukocytes, assuming a constant leukocyte count of 8000 per gl of blood. Only asexual forms of P. falciparum, the most prevalent species in the area, were counted.
Antibody assay Ab to E-Ag. E-Ag was prepared by SP-Trisacryl chromatography from the culture fluid (CF) of P. falciparum (Th61u et al., 1985) with a simplified isocratic system of elution (Acetate buffer 10 raM, NaCI 300 mM, pH 3.7) and acidification of CF with 1.7 M acetic acid to pH 3.7 (Chumpitazi, 1989). Ab to E-Ag were detected by ELISA (Voller et al., 1978; Chumpitazi et al., 1987). PBS supplemented with 0.05% Tween 20, and 5% skimmed dry milk was used for dilution of plasma and conjugate. Conjugates used were: (a) anti-human IgG heavy chain (from goat) labelled with peroxidase (1:750, Miles Yeda, France); (b) the corresponding anti-human IgM (1:1500, Miles Yeda). The chromogene-substrate was orthophenylenediamine'2HCI-H20 2. The absorbance (A) at 492 nm of the contents of each well was measured using a multiscan photometer (Flow Laboratories). Results were expressed as the difference between absorbance values obtained in coated and uncoated plates. As controls, 50 plasma samples from healthy Europeans (provided by the Blood Transfusion Center of Grenoble, France) diluted at 1 : 100 were tested. The cut-offs were 0.48 for anti-E Ag of IgG type and 0.43 for anti-E-Ag of IgM type. These values were computed from 99th percentile of the ranked absorbance values from the 50 plasma against E-Ag. Circulating S-Ag. The incubation fluid of parasite cultures (IF) was used as a positive control for detection of circulating S-Ag by Enzyme Linked ImmunoElectroDiffusion Assay (ELIEDA; Chumpitazi, 1989). Briefly, parasites SGE1 strain (from Gambia) were cultured at 5% haematocrit (Trager and Jensen, 1976) using a modified medium: RPMI 1640, 10% A + serum, 35 mM HEPES, 25 mM glucose (Osinsaya et al., 1981). When the parasitemia reached 4%, the infected red blood cells were washed once in RPMI 1640 and then incubated for 72 h at the same conditions but without serum. The 1F was collected by centrifugation at 400 x g for 10 min and the supernatant filtered through a 0.45 gm filter and concentrated 20 times (IF x 20). Fifty gl of the plasma to be analysed were added to an Eppendorf tube and heated for 5 min at 100°C before the addition of 50 gl of 50 mM Trisglycine buffer, pH 8.8. From this point, all the following procedure was done at room temperature. After shaking, the prepared samples were centrifuged for 5 min at 10 000 x g. The supernatants were recovered and applied (5 ~tl) to the cellulose
298 acetate membrane for testing S-Ag. The E L I E D A technique (Pinon et al., 1979) was used with some modifications. A pool of human positive sera from healthy African donors with an immunofluorescence titer against schizonts of P. falciparum > 640 was used as Ab to S-Ag. Counter-Immuno-Electrophoresis (CIE) was carried out for 2 h at 110 V on strips (Cellogel TM, 2.5 cm x 17 cm x 190 gin) placed on racks in special tanks (SEBIA TM, S 60 N) with 50 mM Tris-glycine buffer. The samples (5 lab were deposited at each side to the heated and unheated Ag control (8 lal of IF x 20). After electrophoretic migration, the membranes were washed three times for 5 min in PBS-T (0.02% Tween 20). They were coated for 30 rain in PBS-T-5% dry nonfat milk with saking to prevent non-specific binding to the membranes. Subsequently, a peroxidase conjugated affinity-purified goat anti-human |gG heavy chain (Miles) at a 1:750 dilution was added to the membranes and incubated for 2 h with mechanical agitation. The strips were washed three times for 5 min in PBS-T and once in TBS (20 mM Tris-(hydroxymethyl)aminomethane, 500 mM NaCl, pH 7.5). Then, a solution of TBS with 10 ml of 4-chloro-l-naphthol (6 mg) and 30% H202 (6 lal) was added to the membranes for 30 min in the dark and washed twice for 5 min in distilled water.
Statistical analysis The absorbance of Ab and number of arcs of circulating S-Ag were compared by variance analysis, Kruskal-Wallis one-way analysis by ranks and KolmogorovSmirnov two-sample test, P values < 0.05 were considered significant.
Results
In this study, no complicated malaria was observed. Therefore, only three groups of subjects have been formed according to our clinical and parasitological data. Group h children (_< 15 years old; average age 9.1 _+3.1) who had at least one MA ( > 5000 parasites/mm 3 and axillary fever > 37.8°C), during the transmission season. The parasitemia disappeared in one or two days after chloroquine therapy. Group Ih children ( < 15 years old; average age 10.8 ± 2.8) who did not have any MA during the transmission season. This group is similar to the first one in age:sex ratio and socioeconomic-factors. Group IIh adults ( > 15 years old; average age 33.1 + 14.1) who did not have any MA during the transmission season. In this study, all the adults examined did not have any MA and both the Dafinso and Vall6e du Kou groups of subjects were analyzed together as both areas are mesoendemic and the number of infected bites for the whole transmission period and per person are similar.
Parasitological findings (Table 1) The parasitological findings are described in relation to the age, occurrence of at least one MA and the observation periods (passages). In the adult group, no detectable parasitemia was found at the beginning of the transmission season (June). In all groups, the number of subjects showing a parasitemia increased from June to August/September according to the transmission season.
299 TABLE 1 Parasitological findings according to the malaria attacks and passages in patients from a malarial mesoendemic area of Bobo-Dioulasso (Burkina Faso, West Africa) Groups a
Observation periods A (June)
B (Aug./Sep.)
C (Jan.)
1 Median parasite + density % o f + b l o o d slides
9040 (0-54400) b 38% (19 64) c
16480 (0-272900) b 95% (75 100) c
280 (0 4400) b 19% (6 44) c
H Median parasite+density % o f + b l o o d slides
120 (0-200) b 25% (9 49) c
III Median parasite+density % o f + b l o o d slides
3 2 0 0 (0 4 4 0 0 ) 8
4 4 0 (0 7800) b
50% (27 73) ~
68% (59 83y
0 0%
Total % o f + blood slides
160 (0 3400) b
10% (3 24) c
15.6% (8-25) c
4 0 (0
1400) b
14% (6 30) c
56.1% (43-66) ~
28.4% (20-41) c
aGroups: I, 18 patients (_< 15 years old; mean age = 9.1-4-3.1) who had at least one malaria attack (5000 parasites/mm 3 and fever) during the transmission season; II, 20, individuals ( < 15 years old; mean age = 10.8 _+2.8) who did not have any malaria attack during the transmission season; lII, 39 adults ( > 15 years old; mean age = 33.1 +_ 14.1) who did not have any malaria attack during the transmission season. bMedian parasite density. c95% confidence intervals.
Immunological .findings
(a) IgG Ab to E-Ag. IgG Ab to E-Ag detected by ELISA (Fig. l) were more frequently observed in groups II and III than in group I (persons who had at least 100
80
[]
rates of Ab to E-Ag of lgG class.
[]
prevalence of Ab to E Ag of lgM class.
•
prevalence ofchculating S-Ag.
60
•~
40
20
0 Ia
lIa
Jun.
Ilia
Ib
Ilb
lIlb
Aug./Sep.
Ic
IIc
lIlc
Jan.
Fig. 1. Seasonal prevalence and 95% confidence intervals of circulating S-Ag and Ab to P. jalciparum E-Ag in a malaria mesoendemic region of Bobo-Dioulasso (Burkina Faso, West Africa). Groups and observation periods: a, beginning; b, during and c, after the transmission season. See Table 1 for groups.
300 one malaria attack). Between the passages the frequencies were stable in the last two groups, whereas in the first group, the rate increased from June to August/September and was stable from August/September to January. Based upon statistical analysis of the absorbance, the reactivity observed in group III (median=0.52, values from 0.07 to 4.42) was higher (P
