Permethrin-impregnated bednets are more effective than DDT house-spraying to control malaria in Solomon Islands.

A field trial compared DDT house-spraying with permethrin-impregnated bednets for malaria control in Solomon Islands from 1987 to 1991. Mortality-rates of malaria vector Anopheles farauti in exit window traps were 11.6% from an untreated hut, 10.1% from a hut sprayed with DDT 2 g/m2, and 98% of those from a hut in which the occupants used bednets treated with permethrin 0.5 g/m2. Since bioassays of the DDT-sprayed walls (15 min exposure in W.H.O. standard test cones) gave 77% mortality of An.farauti, it was concluded that the insignificant impact of DDT could be explained by the exophilic behaviour of endophagic vectors, whereas the greater impact of permethrin was attributed to the more effective exposure of An.farauti females to the impregnated bednets-attracted by the occupants. The parous rate was higher indoors, except in the area with permethrin-impregnated bednets. It was therefore concluded that permethrin-impregnated bednets reduced the mean longevity of An.farauti and hence its vectorial capacity. The circumsporozoite (CS) antigen positivity rate of An.farauti in the DDT area was 0.18% outdoors, significantly less than 1.42% indoors. In the comparison area CS rates were 0.65% outdoors and 0.75% indoors. CS antigen was not detected in An.farauti from the bednet area, indicating the apparent prevention of malaria transmission. As DDT spraying was so much less effective, it was discontinued in 1993 and permethrin-impregnated bednets are now the principal malaria control method in Solomon Islands.

Filarial vector studies in a diethylcarbamazine-treated and in untreated villages in Papua New Guinea.

Entomological studies were undertaken in three villages in the East Sepik Province of Papua New Guinea. The inhabitants of one village, Nanaha, had been treated with diethylcarbamazine (DEC) to reduce the prevalence and density of microfilaraemia of Wuchereria bancrofti. No intervention was undertaken in the other two villages, Yauatong and Musenau, in which bancroftian filariasis was present but with markedly different human prevalence rates and mean parasite densities. In Yauatong, infection rates in anopheline vectors (Anopheles punctulatus and An. koliensis) varied from 20.5 to 46.6% with infectivity rates of 0-1.4% while these rates were 10.9-14.3% and 0-1.1% respectively in Culex quinquefasciatus. In Nanaha after DEC treatment, infection rates were as high as 16.3% in An. koliensis and infectivity rates reached 7.0% for An. punctulatus despite a 45% reduction in the number of people with detectable microfilariae (mf) and a 94% reduction in mf density in those who remained positive.

The economic impact of Plasmodium falciparum malaria on education investment: a Pacific Island case study.

A small but intensive study was carried out adopting a simple method which attempts to quantify the economic consequences Plasmodium falciparum malaria on education investment through school pupil absenteeism in a community in Solomon Islands. In a randomized sample of 4,920 cases of P. falciparum malaria in a community, 2,886 occurred in children of primary school age group of 7 to 13 years. On average a case gave rise to a mean school absenteeism of 5.3 days. In the final analysis a total of 11,028 pupil days schooling were lost due to the sampled cases of malaria caused by the species. This is equivalent to 55.14 school pupil years as a child is expected to attend school for 200 days in a year. A primary school teacher with an average annual salary of US$3,990.00 is expected to teach 6,500 school pupil days a year. When this is adopted as an economic indicator for investment in education the loss is calculated to be US$6,769.57 or equivalent to US$1.38 per case. When this is applied nationally, assuming that rates are similar for 79,203 cases of P. falciparum reported in 1990, the total consequence for investment in education is US$108,966.00, which is equivalent to 27.31 teachers being paid for not teaching, an unacceptable impact for a small nation like the Solomon Islands.

Egg viability in urinary schistosomiasis. III. Repeatability and reproducibility of new methods.

Two new field methods for the detection and quantification of egg viability/hatchability in urinary schistosomiasis due to Schistosoma haematobium have been described recently, and compared with reference methods; the new methods are cheap and simple to use, and may be used singly or together. Before recommending their widespread use in control programmes, repeatability (r) and reproducibility (R) were studied using the guide for determination prepared by the British Standards Institution. Both parameters were found to be low in relation to the mean proportions of eggs viable/hatchable except at very low egg counts, and the new methods can safely be used in PHC-based control programmes.

Egg viability in urinary schistosomiasis. II. Simplifying modifications and standardization of new methods.

Two new methods were recently introduced for the determination and quantification of egg viability in Schistosoma haematobium infections by staining eggs filtered from urine on Nytrel filters with mercurochrome and iodine. Both methods showed reasonable agreement with the best currently available method, centrifugation hatching, but it appeared that improved agreement could be obtained by modifications of timing and technique. The present paper describes the standardization of staining times and microscopic counting techniques; the modifications made gave substantially improved agreement with the reference method and achieved the most important objective, not to have zero estimates of viability when the centrifugation method detected hatching.

Egg viability in urinary schistosomiasis. I. New methods compared with available methods.

A new method of quantifying viability and hatchability of Schistosoma haematobium eggs in urine has been developed. It consists of filtering urine samples through Nytrel filters followed by mercurochrome staining to assist in the detection of viable (motile) miracidia in egg shells, and iodine staining to show hatched miracidia. The new method is compared with two existing techniques for viability/hatchability determination, centrifugation hatching and Nuclepore filtration trypan blue staining. Seventy-three egg positive urine samples are compared; the overall proportions of viable eggs are very close for centrifugation hatching and the new method, but 33% of eggs are lost in centrifugation and 19% of eggs are unclassifiable by Nuclepore filtration trypan blue staining. The main advantages and disadvantages of the new and existing methods are described and discussed; the cheapness, simplicity and ease of performance of the new method make it suitable for large-scale field use. Suggestions for improvements and standardization of the new method are made.

Repeatability and reproducibility of egg counts of Schistosoma haematobium in urine.

Egg output in urinary schistosomiasis can be influenced by several factors, such as time of collection of urine, day to day variations, seasonal variations and environmental conditions. Additionally there are unavoidable random errors inherent in every test procedure. To assess the precision of Schistosoma haematobium egg counts, carried out after paper filtration, two measures are determined, namely repeatability (r) and reproducibility (R). 10 aliquots of 10 urine samples are examined by 5 microscopists and values for r and R are calculated following the methods of the British Standards Institution (BSI, 1987). Results show unexpectedly high values for the 2 measures in relation to the overall means in all 10 urine samples, and analyses of variance confirm that there are large variations between aliquots of equal volume drawn from the same urine specimen, highly significant in all 10 urine specimens; variations between microscopists are considerably lower and only highly significant in one urine specimen. The Poisson heterogeneity test for all urine samples refutes in 9 out of 10 cases the hypothesis that aliquots of equal volume taken from a well mixed urine sample could reasonably be drawn from a Poisson distribution. It is therefore concluded that egg counts are very valuable for community studies and should always be performed, but results must be interpreted with more caution than before and great care taken in analyzing counts (especially in longitudinal studies), in classifying them and in relating them to morbidity.(ABSTRACT TRUNCATED AT 250 WORDS)

Factors affecting transmission of Wuchereria bancrofti by anopheline mosquitoes. 4. Facilitation, limitation, proportionality and their epidemiological significance.

Quantitative understanding of the transmission dynamics of lymphatic filarial parasites is essential for the rational planning of control strategies. One of the most important determinants of transmission dynamics is the relationship between parasite yield, the success rate of ingested microfilariae (mf) becoming infective larvae in a mosquito vector, and mf density in the source of the human blood meal. Three types of relationship have been recognized in human filaria/mosquito couples--limitation, facilitation and proportionality; facilitation has hitherto been observed only in the couple Wuchereria bancrofti/Anopheles gambiae in Burkina Faso, in experimental studies on a high density mf carrier. The present paper demonstrates facilitation in W. bancrofti/An. gambiae and W. bancrofti/An. arabiensis in lower mf density carriers in The Gambia and Tanzania, and in W. bancrofti/An. funestus in Tanzania. Facilitation was not found in An. melas in The Gambia nor in An. merus in Tanzania. Analysis of published data shows limitation at low level mf densities in W. bancrofti/Culex quinquefasciatus in Sri Lanka, and in the same couple in India. Limitation also occurs in Brugia malayi/Aedes togoi in experimental cats; proportionality occurs in B. malayi/Mansonia bonneae in Malaysia. The epidemiological significance of these host/parasite relationships is discussed, and supporting evidence for its validity is presented from the published results of large-scale control programmes.

The significance of low density microfilaraemia in the transmission of lymphatic filarial parasites.

Low density microfilaraemia (mf) is a density of circulating mf which is often undetected by standard survey techniques; it occurs naturally, after anti-filarial drug administration and after vector control. Its occurrence in human populations is closely related to the observed mf frequency distributions in them, and it is an important cause of underestimation of mf prevalence rates in epidemiological surveys. In the present paper it is defined quantitatively as a count of less than 4 mf 20 microliters-1 of capillary blood or less than 30 mf ml-1 of venous blood. Detection of low intensity transmission of parasites is difficult; detection by clinical, entomological or immunological methods may be more sensitive than the usually employed parasitological techniques, due to the extreme inefficiency of the transmission process. Mosquito vectors of filariasis ingest and develop low density mf readily; since they exhibit limitation or proportionality, Aedes, Culex and Mansonia spp. vectors do this more efficiently than Anopheles spp. which exhibit facilitation. Field studies indicate that low level microfilaraemia can initiate a resumption of transmission after very efficient control programmes where Aedes spp. are vectors, whereas eradication has been achieved in areas of Anopheles transmission by levels of vector control which fall far short of eradicating malaria. The situation in the extensive endemic areas where Culex spp. are vectors is less clear, and should be a research priority.

Intensity and efficiency of transmission and the development of microfilaraemia and disease: their relationship in lymphatic filariasis.

Currently available methods for measuring the intensity of transmission in the lymphatic filariases are all to some extent unsatisfactory. Clinical methods for defining the presence of filarial disease are also less than perfect, and many infected persons remain free of symptoms and signs for very long periods. Incidence rates of microfilaraemia calculated from prevalence rate data are combined with 'entomological inoculation rates' obtained by direct observations of vector landing rates, infective larval rates and infective larval densities in field studies to determine efficiencies of transmission, and to relate transmission parameters to observed microfilarial and disease prevalence rates. Published studies from various endemic areas of the world which provide sufficient data have been analysed, using reversible catalytic models. In general, it seems that much less intense levels of transmission are needed in sub-Saharan Africa to produce given rates of microfilaraemia and disease than in Asia and Oceania, in the case of Wuchereria bancrofti infections; similarly, the genus Anopheles appears to produce infection and disease much more efficiently than the genera Culex and Aedes when transmitting W. bancrofti. The only example of Brugia malayi transmitted by Mansonia spp. analysed provided the highest level of transmission efficiency found. Tolerable levels of transmission analogous to those used in onchocerciasis cannot at present be defined for the lymphatic filariases; it seems that the intensity of transmission required to produce new cases of disease is probably below that required to produce new cases or episodes of readily detectable microfilaraemia.

Factors affecting transmission of Wuchereria bancrofti by anopheline mosquitoes. 1. Uptake of microfilariae.

Ingestion of Wuchereria bancrofti microfilariae (mf) from humans by 639 Anopheles gambiae, 557 An. arabiensis, 117 An. melas and 9 An. funestus was investigated. The mf densities in blood fell into 3 groups; 0-32mf/ml, 107-122 mf/ml and 421-1140 mf/ml. In An. gambiae and An. arabiensis percentage of mosquitoes ingesting mf was strongly associated with mf density in host blood; in An. melas the association was much weaker. Mean number of mf ingested per mosquito was also strongly correlated to mf density in An. gambiae and An. arabiensis but not in An. melas. At low mf densities both An. gambiae and An. arabiensis concentrated mf, with concentration decreasing as density increased. From regression analysis, observed and expected uptake of mf would be equal at 622.9 mf/ml in An. gambiae and 391.6 mf/ml in An. arabiensis.

Factors affecting transmission of Wuchereria bancrofti by anopheline mosquitoes. 2. Damage to ingested microfilariae by mosquito foregut armatures and development of filarial larvae in mosquitoes.

Microfilariae (mf) of Wuchereria bancrofti from the midgut of 639 Anopheles gambiae, 557 An. arabiensis, 117 An. melas and 9 An. funestus were examined immediately after the mosquitoes had fed on carriers with different densities of mf. The percentages of mf damaged during ingestion were 57.1-60.0 in An. gambiae, 33.3-50.6 in An. arabiensis and 38.7-55.7 in An. melas. In each species the percentage of mf damaged was independent of mf density in the human host. A further 3657 An. gambiae, 2875 An. arabiensis, 347 An. melas and 32 An. funestus were examined 7 d or more after feeding on mf carriers. In An. gambiae and An. arabiensis, mean numbers of larvae per mosquito were strongly correlated to mf blood density, with similar regression slopes to those obtained from the regression of mf blood density on mean uptake of mf/mosquito. The ratio of mean numbers of larvae per mosquito to mean numbers of intact mf ingested per mosquito increased as the density of mf in the human host increased in An. gambiae and An. arabiensis, but decreased in An. melas as host mf density increased.

The epidemiology of schistosomiasis in the later stages of a control programme based on chemotherapy: the Basrah study. 1. Descriptive epidemiology and parasitological results.

The association between infection with Schistosoma haematobium and various factors including water contact pattern, past history of haematuria and treatment for schistosomiasis and cercarial dermatitis was investigated in Southern Iraq. The study covered mainly primary schoolchildren in different areas of Basrah and in Al-maadan locality where the majority of households with infected schoolchildren occurred. Urine examination was by the sedimentation and Nuclepore filtration methods. S. haematobium infection was very focal in distribution. There was evidence of local transmission in the Al-maadan locality, while in Abu-al-Jawzi schistosomiasis might have been acquired from endemic areas outside Basrah governorate. Swimming, history of haematuria, and history of treatment of schistosomiasis were significantly associated with the level of infection determined by egg count. Cercarial dermatitis, and bathing, washing clothes and fetching water were not significantly associated. The prevalence of infection among male members of households of infected children was significantly higher than that among male members of the remaining households in Al-maadan. There was no evidence of clustering of cases at household level. The implications of these findings for control and for future research on schistosomiasis in Basrah are discussed.

The epidemiology of schistosomiasis in the later stages of a control programme based on chemotherapy: the Basrah study. 2. The serological profile and the validity of the ELISA in seroepidemiological studies.

An epidemiological study of Schistosoma haematobium infection in Al-maadan locality, considered to be the main remaining endemic focus in Basrah, southern Iraq was carried out. The association between the serological profile of the population, measured by the enzyme linked immunosorbent assay (ELISA), and various factors including current infection by urine examination, water contact pattern, past history of, and treatment for, schistosomiasis, and cercarial dermatitis was investigated. Further study of the serological data by the relative operating characteristic (ROC) analysis to assess the validity of the ELISA in detecting schistosomal infection, showed that current infection, past history of infection, age, cercarial dermatitis and category of household were significantly associated with the serological profile of the population. The analysis allowed quantification of the effects of past history of infection and cercarial dermatitis on the validity of the ELISA in detecting schistosomiasis.

The epidemiology of schistosomiasis in the later stages of a control programme based on chemotherapy: the Basrah study. 3. Antibody distributions and the use of age catalytic models and log-probit analysis in seroepidemiology.

A comparative seroepidemiological survey of Schistosoma haematobium infection among primary schoolchildren in areas of different levels and patterns of endemicity in Basrah governorate, southern Iraq, also covered all inhabitants of Al-maadan locality, a known endemic focus in Basrah. Blood samples were tested by the enzyme linked immunosorbent assay. Serological data were analysed to describe the pattern of distribution of antischistosomal antibodies, to relate the level of infection to age and sex, and to estimate the force of infection per year by applying age catalytic models. The distributions of antibodies among the study populations were positively skewed; 3 patterns were revealed by relating serological data to age and sex, depending on the level of infection. The simple catalytic model was the best to estimate the force of infection among the younger age groups.

Some unsolved problems in the epidemiology of onchocerciasis.

The descriptive epidemiology of onchocerciasis is well understood in the major foci of infection; a great stimulus to epidemiological research has been the implementation of the first control programme, which led to the eradication of infection in Kenya and, more recently, the vast Onchocerciasis Control Programme in the Volta River Basin of West Africa. However, there are still significant gaps in epidemiological knowledge which hamper the planning of future control programmes and the evaluation of current programmes. The four most important unsolved problems are: the refinement of field diagnostic techniques to identify skin microfilariae at the ultra-low densities that will become common in the late stages of vector-control campaigns; the definitive identification of Onchocerca volvulus infective larvae in Simulium species; strain analysis in the field of microfilariae from humans and of developing stages from Simulium, to determine their potential for ocular pathogenicity; determination of the lifespan, or maximum fecundity span, of adult female O. volvulus after the interruption of transmission. Three other unsolved problems are of enormous interest epidemiologically, although less urgent in practical importance. They are the identification of factors causing severe disease as opposed to heavy infection; the effects of seasonal as opposed to perennial transmission; and the importance of transplacental transmission of microfilariae or soluble antigens.

The maximum density of anopheline mosquitoes that can be permitted in the absence of continuing transmission of filariasis.

Using two years of mosquito collections from two fixed stations on Guadalcanal it was possible to compare the average man-biting rate with the prevalence of microfilaraemia. One area was found to be free of infection while the other still had one positive case. The DDT residual spraying had caused a change in biting habits of A. farauti No. 1 allowing a greater density of mosquitoes to bite man at times when they had lower microfilaria densities. This difference was compared with the pre-spray density so that the maximum permissible number of mosquitoes biting man without transmission of filariasis taking place, could be calculated. A theoretical value for m of 19.84 had been worked out previously; this study gave a corrected observed value of m of approximately 20, very close to the calculated theoretical value. The situation studied here probably showed incomplete interruption of transmission, so it was considered preferrable to recommend a figure of 0.66 average vector man-biting rate per hour indoors for the critical value.