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Components of vectorial capacity (biting density, survival, and host preferences) of a population of Aedes (Finlaya) poicilius, the principal vector of Wuchereria bancrofti in the Philippines, were studied in 1994-1995 in Sorsogon province. Aedes poicilius comprised 20.7% of 3243 mosquitoes of 24 species taken in 42 nights of human-biting collections, and 1.4% of 7586 mosquitoes of 27 species taken in 18 concurrent nights of carabao-trap collections. There were, on average, 16 bites by Ae. poicilius per person per night. As there was no relationship between body size of the female Ae. poicilius and parity status, body size was not a variable affecting survivorship of Ae. poicilius. The probability of daily survival was estimated to be 0.85, and the human blood index was 0.67. None of the 862 Ae. poicilius examined contained W. bancrofti larvae, probably because of distribution of diethylcarbamazine to microfilaraemic people prior to the study. The vectorial capacity of the Ae. poicilius population under study was estimated to be 2.4 new cases per primary case per day. However, the basic reproductive rate (i.e. the number of new cases of W. bancrofti infection generated from a single infective case) was estimated to be very low because of inefficiency in parasite transmission.

Background: WHO has targeted lymphatic filariasis for elimination. Studies of vector-parasite relations of Wuchereria bancrofti suggest that a reduction in the microfilarial reservoir by mass chemotherapy may interrupt transmission and thereby eliminate infection. However, no field data exist on the impact of chemotherapy alone on vector efficiency and transmission intensity of W bancrofti. We compared the impact of an annual community-wide single-dose treatment with diethylcarbamazine alone or with ivermectin on rate and intensity of microfilaraemia, and transmission intensity in an area of Papua New Guinea endemic for intense W bancrofti transmission., Methods: We carried out clinical and parasitological surveys in 14 communities in matched pairs. People aged 5 years or older in seven communities received randomly assigned diethylcarbamazine 6 mg/kg and people in the other seven communities received diethylcarbamazine 6 mg/kg plus ivermectin 400 micrograms/kg. We made physical examinations for hydroceles and leg oedema and investigated microfilarial densities by membrane filtration before and after treatment. We selected five communities for monthly entomological surveys between September, 1993, and September, 1995. Mosquitoes were collected in these communities by the all-night landing catch method and were individually dissected to identify rates of infection and infectiveness., Findings: 2219 (87.6%) of 2534 eligible people received treatment. Microfilarial rate and density had decreased 1 year after treatment in all 14 communities; this decrease was significantly higher in communities given combined therapy than in those given diethylcarbamazine alone (mean decreases 57.5% and 30.6%, respectively; p = 0.0013). Greater decreases were also seen in community-specific microfilarial intensity with combined therapy (mean reductions 91.1% and 69.8%, respectively; p = 0.0047). The rate of leg oedema was not altered, but the frequency of advanced hydroceles decreased by 47% with combined therapy and 56% with diethylcarbamazine alone. 26,641 Anopheles punctulatus mosquitoes were caught during 499 person-nights of landing catches. Exposure to infective third-stage larvae decreased in all monitored five communities. Annual transmission potential decreased by between 75.7% and 98.8% in combined-therapy communities and between 75.6% and 79.4% in communities given diethylcarbamazine alone. Transmission was almost interrupted in two communities treated with combined therapy., Interpretation: Annual single-dose community-wide treatment with diethylcarbamazine alone or with ivermectin is effective for the control of lymphatic filariasis in highly endemic areas, but combination therapy brings about greater decreases in rates and intensity of microfilaraemia.

The impact of permethrin-impregnated bednets on resting and feeding behaviour of mosquito vectors of Wuchereria bancrofti, causing human lymphatic filariasis was studied in six pairs of villages (treated and untreated) before and after intervention. The study villages were in Kwale District, near the coast of Kenya, where Bancroftian filariasis is highly endemic, transmitted by a combination of both anopheline and culicine mosquito vectors. Mosquitoes were collected weekly in each village, indoors (using pyrethrum spray catches) and outdoors (using pit traps) during 3-4 months following the long rainy season. Of the filariasis vector species of mosquitoes collected in 1994 before intervention. 33.6% were members of the Anopheles gambiae complex, 30% were An. funestus and 36.4% were Culex quinquefasciatus. PCR analysis of the An. gambiae complex species collected in 1995 demonstrated that 98.5% were An. gambiae sensu stricto. 1% An. arabiensis and 0.5% An. merus. Introduction of impregnated bednets in 1995 significantly reduced the number of indoor-resting An. gambiae s.l. by 94.6% and An. funestus by 96.7%, but there was no change in the number of Cx quinquefasciatus collected indoors. The number of outdoor-resting An. gambiae s.l. was significantly reduced, whereas densities of An. funestus and Cx quinquefasciatus remained unaffected outdoors. ELISA analysis of mosquito bloodmeals showed a shift from human to animal feeding after the introduction of treated nets. The human blood index (HBI) for indoor resting Cx quinquefasciatus was reduced from 93.1% to 14.4%. Vector potential based on the HBI and mosquito density was estimated to be reduced by 99% for An. gambiae s.l., 98% for An. funestus and 97% for Cx quinquefasciatus and vectorial capacity would be suppressed even more by the impact on the vector survival rates (not measured). These results suggest that permethrin-impregnated bednets give effective personal protection against transmission of W. bancrofti by An. gambiae, An. funestus and Cx quinquefasciatus in East Africa.

Objective: To study the regular pattern of growth and declination or the transmission potential of filariasis after the disease was basically (the microfilarial rate was lower than 1%) in Henan Province in 1987., Methods: According to the distribution of filaria species and original microfilarial rate, in 7 surveillance sites in 7 counties (cities) the etiology and mosquito vector surveys were carried out continuously during 1988-1995 and no control measurement for pathogen was taken., Results: Ten residual microfilaremias became negative gradually in the first 6 years and no new microfilaremias occurred during 1988-1995. During 1993 to 1995, the microfilaremias rate of population in the sites was 0. The natural infection rates of filarial larvae in vector mosquito were 0.1%-0. During the 8 years, 15 vector mosquitos were positive with a total of 18 filarial larvae which were all of first or second-stage larvae. The C. pipiens pallens was the main vector, the second was A. sinensis, with a small number of C. fatigans and A. anthropophagus. The man-biting rates of mosquitoes for outdoor sleepers fluctuated greatly, the highest one was 360.6 mosquitoes/person per night and the lowest, 7.2. The man-biting rates of mosquitoes for sleepers inside mosquito-net was about 1 mosquito/person per night. The proportion of multiparous mosquitoes also fluctuated more greatly, the highest one was 88.1% and the lowest 27. 2% According to the data described above, the man-biting rate of vector mosquito which contained filarial L3 was 0., Conclusions: The results suggested that after the microfilarial rate was lower than 1%, the residual microfilaremias became negative gradually in 3-6 years, and the transmission of the disease was blocked. Therefore, in the areas where filariasis was basically controlled, elimination of the disease was attainable within sight.

Successful experimental transmission of the human lymphatic dwelling nocturnal periodic strain of Wuchereria bancrofti has been achieved from man to Indian leaf monkey (Presbytis entellus) through a susceptible strain of Aedes aegypti. The prepatent period varied between 195 and 240 days. Microfilaria (mf) levels were in general low and the peak count was attained at 1800 hr. Of the adult worms 5-12.5% were recovered from male langur while females revealed poor (2.54%) recovery. This host--parasite model thus makes available the target parasite material in substantial quantity to work on chemotherapeutic and immunological investigations.

To identify the gaps in people's knowledge of lymphatic filariasis, we collected descriptive and quantitative information in rural areas of Tamil Nadu, South India. People are well aware of filariasis in their communities and recognize its different clinical manifestations. Only 9% of those with and 20% of those without the disease knew that filariasis is caused through mosquito bites; the rest attributed it to many other causes. People's knowledge about transmission and prevention of filariasis is also very poor. The study identifies the rationale for people's misconceptions about the disease. Misconceptions and ignorance could adversely affect personal protection measures against mosquito bites and the use of appropriate treatment. Health education campaigns aimed at highlighting the role of mosquitoes in transmission and the importance of early diagnosis should help people in taking personal protection measures and seeking appropriate treatment.

This study utilizes parallel, longitudinal entomological and parasitological data collected during a 5-year vector control programme in Pondicherry, South India, to quantify Wuchereria bancrofti transmission from the vector to the human population. A simple mathematical model, derived from the standard catalytic model, is used to examine the hypothesis that current infection prevalence in young children is a dynamical function of their cumulative past exposure to infective bites. Maximum likelihood fits of the model to the observed data indicate a constant child infection rate with age, above a threshold representing the pre-patent period, or equivalently, the cumulative biting intensity required to produce patent infections. Extrapolation of the model allows the crude estimation of the equilibrium microfilaria age-prevalence curve due to control. The results suggest that vector control alone may have little impact on the overall age-prevalence of infection even when sustained for long periods. These observations are discussed in terms of the likely impact of density dependent mechanisms, such as acquired immunity, on model predictions.

A longitudinal study was conducted to explore transmission dynamics and epidemic trend of bancroftian filariasis in Huayuan Village, Shengli Township, Tancheng County, Shandong Province, a previous highly endemic area of it, from 1984, the fifth year after its preliminary eradication, to 1993, without any control measures being taken. Results showed proportions of residents with microfilaremia decreased to 0.12% in 1993 from 0.56% before the study. Eight of the nine microfilaria carriers before the study became negative spontaneously. Six new microfilaria carriers were detected during the study period, but five of them converted negative in three or four years. Negative conversion rate of blood IgG4 in microfilaremia cases was 88.89% by the end of the study. Natural infection rate in mosquitoes dropped year by year, with 0.21%, 0.19% and 0.06% for 1984, 1985 and 1986, respectively, and no filarial larvae was found in them in 1993 and natural transmission potential dropped from 3.47 to zero. It suggested the threshold for transmission of filariasis was not reached and the transmission was close to the end, with the rates of mosquito biting of 24.1-52.5 and 13.5-21.0 times per person, per night, for those using or non-using mosquito net, respectively.

The impact of unabated population growth and consequent ecological changes is felt in the spread of vector-borne diseases also. Filariasis, once considered to be more associated with urban areas and urbanization, is rapidly emerging as a major problem in rural areas. The need to delimit the endemic areas through a new rapid assessment procedure and the possible control methods of rural filariasis are discussed. Leishmaniasis has re-emerged with a vengeance in some parts of the country and the reasons for this are analyzed. Babesiosis, though zoonotic, has the potential to invade the domain of man in India.

This study examines the contribution of socio-cultural and behavioural factors in mosquito-borne lymphatic filariasis transmission in Southern Thailand. Research was conducted in Nakorn-srithamarat province, which is noted for having the nation's highest Brugia malayi filariasis morbidity rate. Factors examined include traditional knowledge and cultural beliefs concerning etiology, transmission and symptomatology; perceived susceptibility and severity: social stigma: social support in disease prevention and control; and behavioural risk factors and illness behaviours. Data were collected through a multi-method, predominantly qualitative-based approach, including rapid survey and mapping, group interviews, focus group discussions, indepth interviews, and participant observation. Results indicate that poor knowledge and lay, indigenous, traditional belief systems contribute to high risk behaviours, and inappropriate preventive, illness and treatment choice behaviours. Behavioural models for explaining filariasis risk, preventive, illness and treatment choice behaviours are presented. Finally, recommendations for more effective health education programmes are offered.

Bancroftian filariasis is spreading in towns of endemic areas in Recife, northeastern Brazil, where it is a major public health problem. This paper deals with the prevalence of microfilaraemia and filarial disease in two urban areas of Recife, studying their association with individual characteristics and variables related to the exposure to the vectors. The parasitologic survey was performed through a "door-to-door" census and microfilaraemia was examined by the thick-drop technique using 45 microliters of peripheral blood collected between 20:00 and 24:00 o'clock. 2,863 individuals aged between 5 and 65 years were interviewed and submitted to clinical examination. Males aged between 15 and 44 years old presented the greatest risk of being microfilaraemic. Microfilaraemia was also significantly associated with no use of bednet to sleep. The risk of being microfilaraemic was greater among those who had lived in the studied areas for more than 5 years. The overall disease prevalence was 6.3%. Males presented the greatest risk of developing acute disease. The risk of developing chronic manifestations was also greater among males and increased with age. We found no association between time of residence, bednet use, microfilaraemia and acute and chronic disease. We may conclude that in endemic areas there are subgroups of individuals who has a higher risk of being microfilariae carriers due to different behaviours in relation to vector contact.

Clinical and parasitological surveys were carried out concurrently during 1986 in Pondicherry. The analyses showed that there was no significant micro-spatial variation in prevalence of total diseases (acute and chronic) and the manifestations such as hydrocele and lymphedema in the different zones and stations of Pondicherry urban area, a stable endemic area. Analyses on different filariometric indices in different stations showed a significant correlation between disease and mf prevalence (r = 0.4106; p = 0.037). The prevalence of disease and hydrocele in microfilaremic individuals (9.4% and 20.0% respectively) was higher compared to that observed in amicrofilaremic persons (6.4% and 11.2% respectively). The relative risk (RR) of parasite carriers developing disease (any manifestations) was marginally higher compared to amicrofilaremic persons (1.18). However, the RR of developing hydrocele manifestation due to microfilaremia was much greater (1.5) compared to amicrofilaremic persons. The attributable risk (AR) due to microfilaremia for developing hydrocele was 0.05. This suggests that although the risk is high in mf carriers, there might be alternate ways of developing disease without the infected person becoming microfilaremic. The limitations of point prevalence data on understanding complex dynamics of infection and disease are discussed.

In mid-1987 a baseline microfilarial prevalence survey was conducted among five villages in the Mt Bosavi region of the Southern Highlands Province of Papua New Guinea. Through use of the Nucleopore filtration technique, it was determined that 48% of villagers had detectable microfilaraemia. The highest prevalence was documented in Fogomaiyu, where the microfilaraemia rate was 92%. On the basis of this initial survey and the expressed interest of the community, the Division of Health in the Southern Highlands Province undertook an integrated community-based pilot control program. The project used two principal control methods: (a) drug treatment with low-dose diethylcarbamazine citrate (DEC) distributed to the community weekly and (b) vector control with permethrin-impregnated bednets. Results six months after the intervention indicate that the program was successful in reducing microfilaraemia at Fogomaiyu village from 92% to 6%. The reduction is principally related to the effects of DEC, although the bednets, by limiting vector-person contact, are expected to reduce the incidence of both filariasis and malaria.

Two quantitative trait loci (QTL) affecting susceptibility of the mosquito Aedes aegypti to the filarial worm parasite Brugia malayi were identified using restriction fragment length polymorphism (RFLP) markers. The first locus, fsb[1,LF178], resides within a 10 cM interval on chromosome 1 and exhibits a recessive effect with respect to susceptibility. The second locus, fsb[2,LF98], resides within a 9 cM interval on chromosome 2 and exhibits an additive effect on susceptibility. Significant epistasis was detected between these loci, although the effect of fsb[2,LF98] was dependent on the genetic background of the mosquito strains. Suggestions for a standard QTL nomenclature are included.

The impact of a 5-year vector control programme on transmission of bancroftian filariasis and microfilaraemia levels in the human population was examined in Pondicherry, South India. The programme reduced and maintained the transmission of Wuchereria bancrofti at appreciably low levels in many localities of Pondicherry town, but its impact on overall microfilaraemia prevalence was only limited. However, prevalence of infection was reduced drastically in children born after establishment of the programme and only 18% of the number expected without vector control were found infected. No new infections were found in localities with less transmission intensity. It is proposed that a Transmission Intensity Index of 0.50 and Annual Transmission Potential of less than the 96-105 range are permissible levels of transmission, and below these levels no new infections may occur. The required duration of vector control to eliminate Culex-borne filariasis foci is estimated to be 11-12 years, slightly longer than that required to eradicate anopheline-borne filariasis.

Twenty-one villages from ten provinces and one autonomous region with different mf rates in population and different levels of mean mf density per 60 microliters blood in positive cases and representative species of filaria and vector after DEC treatment were selected as study sites. In these villages a longitudinal investigation for a period of 3-5 years was carried out without adopting any control measure. The main results were: 1. Among 279 residual microfilaremia cases, 235 (84.2%) turned negative spontaneously and successively. The negative conversion rates were 86.6% (46/53) in malayan cases, 60.8% (28/46) in bancroftian cases after 3 years' follow-up, and 100% (29/29) in malayan cases, 87.4% (132/151) in bancroftian cases after 4-5 year's follow-up. Among 110 bancroftian cases followed-up to seven years, 109 (99.1%) turned negative. 2. Twenty-two new microfilaremia cases were detected, of which, 18 might be missed in previous blood examination, 4 might be new infection. 3. Mf rate in the population in twenty villages has been trending down, whereas in one village fluctuated around the original level. Among the 20 villages, no microfilaremia case was found in 7 villages. 4. The natural infection rates of filarial larvae and infective rates in mosquito vectors were in descending trend in most of these villages, and no filarial larva was found in 12 villages.(ABSTRACT TRUNCATED AT 250 WORDS)

The risk of infection (RI) with Wuchereria bancrofti by vector mosquitoes has been conventionally estimated from the biting population of all age classes. This estimate of RI had no correlation with microfilaria (mf) prevalence and intensity of infection in humans. A new method for estimating RI from a resting population of Culex quinquefasciatus is proposed based on a longitudinal study in seventeen sites in Pondicherry, India. Unlike previous estimates, RI calculated by the present method considering the density of parous mosquitoes collected per man-hour and the mean number of infective stage larvae per parous mosquito significantly correlated with mf prevalence in humans during both early (r = 0.559; P = 0.038) and late in the control operation (r = 0.572; P = 0.033). RI varied significantly between the early and late in the control operation and different sites. On average, RI was reduced significantly during the control operation. Further epidemiological implications are discussed.

Entomological and filariasis surveys were conducted simultaneously in a rural and an urban environment in West Bengal from September 1986 to August 1987. The aim of the surveys was to see if any correlation existed between the preferred biting site of Culex quinquefasciatus, the filarial vector, and the region of the body affected by clinical filariasis. Significantly more mosquitoes (> 55%) attacked the thighs, legs and feet of human volunteers than other sites on their bodies, whether the volunteers were indoors or outdoors, in rural or urban environments. Significantly more patients had clinical filariasis of these lower body parts than of the upper parts. The vector's preferred biting sites and the anatomical sites most affected by clinical filariasis were therefore the same.

The prevalence of transplacental transfer of microfilariae and structural lesions in the placentas of amicrofilaraemic (n = 10) and Wuchereria bancrofti microfilaraemic (n = 10) pregnant women from the prenatal care unit of the Hospital das Clinicas, Recife, Brazil, was investigated. Study participants were selected by filtration (3 microns/13 mm) of nocturnal (2300-0100 h) venous blood. A detailed histopathological examination was performed to detect abnormalities in the placenta, the cord and the foetal membranes. Both study groups had similar obstetric profiles. Parasitaemia was present during labour in nine of ten microfilaraemic women, but no microfilariae were found by filtration of 5-10 ml cord blood. No macroscopic abnormalities were seen in placentas from microfilaraemic mothers; two placentas from the amicrofilaraemic cases contained, respectively, infarcted areas and cysts at the cut surface. Microscopy failed to detect microfilariae in the intervillous spaces, chorionic and decidual vessels, or the umbilical cord. Thus, transplacental transfer of Wuchereria bancrofti microfilariae seems to be a rare phenomenon, and microfilaraemia does not appear to be a risk factor for placental pathology.

Formal control programmes do not exist for lymphatic filariasis in much of the endemic world. The literature on the social, economic and clinical impacts of the disease is so sparse as to provide virtually no guidance on whether the disease should be accorded more importance in national or local public health programmes. This type of research is a major priority. Putting together what little is known about the socioeconomic determinants of filariasis with the fairly extensive experience in control leads to a finding that control programmes must be undertaken at the community level to be effective. Diethylcarbamazine (DEC) is a readily available and apparently safe drug that can be deployed successfully for community control. While research currently is exploring the potential for individuals to protect themselves with DEC or a newer drug, ivermectin, community-wide control is unlikely to be achieved in that way. Under some special circumstances, controlling the mosquito vectors may be sufficient to control the disease, and in other cases, it may complement chemotherapy, but in general, it cannot be relied upon as a primary measure. DEC may be used in a variety of regimens which vary in their cost, duration, incidence of side effects and degree of community participation. Some, including DEC-medicated salt, are particularly attractive alternatives for many filariasis-endemic areas. The search for less expensive, yet effective, control options must continue, and this requires research not only into the costs of the various options, but also into the determinants of community acceptance, compliance and participation.

To document the occurrence of transplacental transmission of microfilariae and to determine how frequently it occurred, umbilical cord blood samples and placental tissues were collected from 22 microfilaria-positive women in an area with endemic Wuchereria bancrofti. Microfilaria (mf) counts in the women ranged from 1 to 3,820 mf/ml. Microfilariae were detected in 2 placenta samples and a single cord blood sample. The positive cord blood sample and 1 of the positive placenta samples came from the same woman; no microfilariae were found in a finger prick sample taken from the infant 3 wk after delivery. Our results suggest that microfilariae cross the placenta in less than 10% of pregnancies of microfilaria-positive mothers. Furthermore, the microfilaria count of the mother does not seem to influence directly whether microfilariae are present in the placental blood pool. Although actual transfer of microfilariae to the fetus may occur infrequently, exposure to parasite antigens occurs with much greater frequency. The effect of in utero exposure to either microfilariae or parasite antigens may render newborns tolerant and explain why children born to infected mothers are almost 3 times more likely to become infected than are children born to uninfected women.

A study of 325,000 residents of 314 villages in six governorates of the Nile delta area of Egypt revealed that the prevalence of lymphatic filariasis increased from < 1% in 1965 to > 20% in 1991, especially in the governorates of Qalyubiya, Monufiya, Dakhaliya, and Giza. The distribution of the communites with endemic filariasis is focal. Clusters of villages with high prevalences are surrounded by others in which the disease is absent, although their environmental, social, and agricultural features appear similar. The article analyses why the significant decline in filariasis between 1945 and 1965 in Egypt has been followed by a resurgence of the disease.

This paper reports the susceptibility of Anopheles anthropophagus (Xu et Feng, 1975) to experimental infection with Wuchereria bancrofti (Cobbold, 1877), comparing with that of An. sinensis Wiedemann, 1828, Culex pipiens quinque fasciatus Say, 1823, and Aedes togoi (Theobald, 1907). Of 188 An. anthropophagus, 202 An. sinensis, 280 Cx. pipiens quinque fasciatus and 129 Ae. togoi infected by the blood of a bancroftian microfilaremia with mff density of 190 mff/20cmm, the infective rates were 35.64%, 9.41%, 30.00% and 65.89%, respectively; while of 188 An. lesteri anthropophagus, 134 An. sinensis, 289 Cx, pipiens quinque fasciatus and 176 Ae. togoi infected by the blood of another bancroftian microfilaremia with mff density of 83 mff/20 cmm, the infective rates were 18.09%, 3.73%, 13.84% and 39.77%, respectively. It is concluded that the susceptibility of An. anthropophagus to experimental infection with W. bancrofti is significantly higher than that of An. sinensis, significantly lower than that of Ae. togoi and at the same level with that of Cx. pipiens quinquefasciatus (Fig. 1-7).

The possible involvement of nulliparous females of Culex quinquefasciatus in the transmission of bancroftian filariasis under field conditions was examined in Pondicherry, South India. Biting nulliparous females that had previously ingested partial blood meals were found infected with microfilariae/L1 stage larvae. None of them harboured infective-stage larvae. These findings suggest that nulliparous females are not involved in the transmission of filariasis. Therefore, their inclusion for estimating the transmission parameters is questionable. However, nulliparous females should also be dissected in order to determine the vector infection rates accurately. Infective larvae were encountered in females of all other age groups.

Observations on Culex quinquefasciatus Say in Samoa during a study of the epidemiology and control of subperiodic bancroftian filariasis are reported. The man-biting rate of C. quinquefasciatus was comparable with that of Aedes (Finlaya) samoanus Gruenberg in one indicator village and lower in another. The house-frequenting behaviour of the two species was similar. Culex quinquefasciatus was active throughout the night with peak biting during midnight hours, and its survivorship, estimated by two methods, was higher than those of the main vectors, A. (Stegomyia) polynesiensis Marks and A. samoanus. Culex quinquefasciatus showed low susceptibility to subperiodic Wuchereria bancrofti, compared with the two main vectors, in laboratory feeding experiments on volunteers with three levels of microfilaraemia. This agrees with the observed very low natural infection rate, suggesting that C. quinquefasciatus is an inefficient vector of subperiodic W. bancrofti in Samoa.

An Integrated Vector Management strategy, implemented as an alternative to the conventional control operations that include mainly chemical control in Pondicherry, South India, reduced very substantially the population density of Culex quinquefasciatus. This resulted in drastic decrease in the intensity of transmission of bancroftian filariasis transmitted by Culex quinquefasciatus and consequently the incidence of new infections in children of 0-5 age group was minimized. When the IVM strategy was withdrawn after five years of implementation and conventional control measures were re-adopted, resilience of Culex quinquefasciatus population was observed and human exposure to the risk of infection increased. The results suggest that maintenance of vector density at reduced levels for prolonged periods, is necessary to control infectious diseases like filariasis, which is difficult in the present day urban situations in developing countries. Hence the emphasis should be on chemotherapy to achieve control of lymphatic filariasis.

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.

A reconnaissance survey for the presence of lymphatic filariasis is made in 41 chiefdoms of north east Ghana. Four disease levels are identified culminating in hyperendemic disease foci associated with two Government-introduced rice irrigation projects. Attention is also drawn to the disease effects of small village dams. Multiple concurrent infections are noted. Within the most stricken irrigation villages, aspects of concealment, stigma and marriage are considered. Failure to control lymphatic filariasis has led to hospital avoidance and neglect of the disease jointly by patients, physicians and nurses. Culpability rests with the irrigation authority and Government health services. An outline is given of possible measures for disease control. A multisectoral policy of 'prevention before development' is strongly advocated.

The melanization response of two mosquito against Wuchereria bancrofti was studied under different conditions of temperatures, adult age and feeding regimes. The two mosquito species were Culex pipiens, a susceptible species for W. bancrofti transmission and Aedes Caspius, a refractory one.

Laboratory-derived females of Culex quinquefasciatus and Aedes aegypti were experimentally infected with Wuchereria bancrofti by allowing the insects to feed on infected volunteers of known microfilariae density between 24.00 and 2.00 hours. Of the 240 mosquitoes used for the experiments only 67 (55.8%) of C. quinquefasciatus and 83 (69.2%) of the Ae.aegypti actually fed on the volunteers' blood. After 16 days post-infection period, 50 (74.62%) of the engorged C. quinquefasciatus harboured infective L3 larval stages of W. bancrofti, while 16 (23.9%) and 1 (1.49%) harboured L2 and L1 pre-infective larval stages, respectively. On the other hand, no development to the L3 infective stage was observed in the engorged Ae.aegypti though 34 (40.96%) harboured L1 pre-infective stages of the parasite. Our results suggest that C.quinquefasciatus could be actively involved in the transmission of urban bancroftian filariasis in Nigeria.

Filaria surveys conducted in some select slum clusters namely Hari Nagar, Yamuna pusht near Vijaya Ghat along the Ring Road and Timarpur in Delhi during 1989, 1991 and 1992 respectively, covering a population of approximately 5000 slum dwellers revealed the presence of bancroftian microfilaria (mf) carriers and disease cases. The mf and disease rates (per cent) in these three slum areas were in the order of 6.3, 2.2, 3.7 and 1.4, 0.5 and 0.1 respectively. The mf density varied from 3.1 to 12.3 per 20 cumm. blood. High ten man hour densities of Culex quinquefasciatus (581) in Yamuna pusht followed by (355) in Timarpur were recorded during entomological investigations. Hari Nagar accounted for least ten man hour density of Cx. quinquefasciatus (160), because collection was made during winter months (November-December). The dissection of Cx. quinquefasciatus did not reveal any human filarial infection except in Yamuna pusht where out of 139 only one Cx. quinquefasciatus was found infective.

The quantitative uptake of Wuchereria bancrofti microfilariae by mosquito species of different susceptibility to filarial infection was studied. There was no relationship between the degree of susceptibility and the number of ingested microfilariae. However, in all tested mosquito strains the females ingested as a mean 1.2-1.9 times more microfilariae of W. bancrofti than expected.

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.

Seasonal variations in biting Culex quinquefasciatus and transmission of bancroftian filariasis were investigated in Pondicherry, South India. The biting density of C. quinquefasciatus, the principal vector species, was lowest in the summer months and higher during the monsoon and winter months. The survival of the vectors as indicated by the proportion of parous mosquitoes was found to be less in the summer season. Biting mosquitoes with infective stage larvae were not encountered during the hottest months of May, June and July and the early monsoon month of August indicating seasonality of transmission. Maximum transmission took place between November and March. These findings suggest that vector control measures according to the season of transmission may produce more cost-effective results than year round control operations.

Susceptibilities of natural populations of sibling species A, B and C of the Anopheles quadrimaculatus complex and the colonized strain A to subperiodic Brugia malayi and Brugia pahangi were compared. All 3 sibling species showed varying degrees of susceptibility to both B. pahangi and B. malayi, and they were considerably more susceptible to B. pahangi than to B. malayi. The rate and intensity of infection to B. pahangi were highest for species A (66.2% and 7.4 L3/female, respectively) and lowest for species B (21.3% and 1.7 L3/female). For B. malayi these values were higher for species A (29.7% and 1.84 L3/female) than for species B (13.3% and 0.86 L3/female) and C (12.6% and 0.75 L3/female). The colonized strain A of An. quadrimaculatus was significantly more susceptible to both Brugia species than the natural populations of sibling species A, B and C.

Brugia malayi is a parasitic nematode that causes lymphatic filariasis in humans. The monoclonal antibody MF1, which mediates clearance of peripheral microfilaremia in a gerbil infection model, recognizes two stage-specific proteins, p70 and p75, in B. malayi microfilariae. cDNA coding for the MF1 antigen was sequenced, and the predicted protein sequence shows significant similarities to chitinases from bacteria and yeast. When microfilarial extracts and purified preparations of the MF1 antigen were tested for chitinase activity, strong bands of chitin-degrading activity comigrated in SDS/PAGE with p70 and p75 and showed a reduction-dependent mobility shift characteristic of the MF1 antigen. Thus, the MF1 antigen is microfilarial chitinase, which may function to degrade chitin-containing structures in the microfilaria or in its mosquito vector during parasite development and transmission.

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.

A single-dose of diethylcarbamazine citrate (DEC, 6 mg per kg body weight) was administered in three mass treatment campaigns to > 80% of the estimated total Samoan population (160,000) in 1982, 1983, and 1986. The effect of the drug was evaluated before and after each campaign by conducting four blood surveys covering 9600 to 13,700 people from 26-34 villages on each occasion. The drug reduced the prevalence of Wuchereria bancrofti microfilariae from 5.6% to 2.5% (a 55% reduction), while the transmission potential (the estimated mosquito infection rate if everyone is equally exposed to mosquito bites) dropped from 2.18 to 0.67 (a 70% reduction). The total number of microfilariae in the Samoan population is estimated to have been reduced by 80%. A spaced, single-dose treatment with DEC at a 1-2-year interval therefore seems to be an effective control measure against diurnally subperiodic W. bancrofti.

A programme was launched in 1986 for the control of brugian filariasis in an area of high endemicity in southern India. Sustained community involvement was achieved by integrating economic measures to improve the lot of the people with the creation of widespread awareness about the cause of the disease and about the possibilities of combating it.

Third-stage larvae (L3) of Wuchereria bancrofti, Brugia malayi and B. pahangi are known to be recovered not only from the head and thorax but also from the abdomen of dissected vector mosquitoes. For epidemiological reasons, was of interest to determine whether L3 larvae from the abdomen of the vector would be infectious for the final host. Early abdominal L3 larvae of B. pahangi isolated as early as on day 7 post-infection (p.i.) from Aedes aegypti were injected s.c. into five male Meriones unguiculatus. Four of the five jirds were microfilaria-positive after 67 days.

During 1982-1990, a longitudinal observation on prevalence trend of malayan filariasis has been made in endemic areas with An. anthropophagus as the main vector. A total of 22,795 person-times of blood examination were made, and 30,439 An. anthropophagus and 10,061 An. sinensis were dissected respectively. The microfilaraemia rate dropped from 1.0% to 0.14%, and the infection rate of An. anthropophagus decreased from 0.74% to 0.09% in 3 endemic villages, while no positive case or infected vector occurred in 5 villages where microfilaraemia cases were absent since the beginning of the study. In a cross-sectional survey, 855 villages of 17 counties has been monitored for 10 years. Out of 213,934 person-times of blood examination, only 56 were positive, the average microfilaraemia rate being 0.0262%, and 94.64% of the positive had already been detected before 1986. Based on these data, it has been suggested that in endemic areas with An. anthropophagus as main vector, when the microfilaraemia rate dropped to less than 1% after control, there was no indication that the rate would upgrade during the survey period. A decline trend of the transmission of malayan filariasis, therefore, has been exhibited.

Rabbit antisera were raised to the malaria vector Anopheles tessellatus, head-thorax, abdomen, and midgut preparations. Reactivity of the antisera with An. tessellatus and the filariasis vector, Culex quinquefasciatus, were examined by enzyme-linked immunosorbent assay (ELISA) and Western blots. Although many antigens were shared between the two species of mosquitoes, tissue- and species-specific antigens also were identified.

Brugia sp. microfilariae were observed in more than 60% of wild rabbits collected on Nantucket Island, Massachusetts. The microfilariae measured 294-344 microns in length and had the characteristic subterminal and terminal nuclei observed in other Brugia microfilariae. The microfilaria is similar to those described for Brugia leporis in rabbits in Louisiana. This may be the Brugia species responsible for 21 documented cases of human infection in the northeastern United States.