Wuchereria bancrofti infection in human and mosquito populations of a Polynesian village ten years after interruption of mass chemoprophylaxis with diethylcarbamazine

Assessing the presence of Wuchereria bancrofti in vector and human populations from urban communities in Conakry, Guinea

BACKGROUND

The Global Programme to Eliminate Lymphatic Filariasis was launched in 2000 with the goal of interrupting transmission of lymphatic filariasis (LF) through multiple rounds of mass drug administration (MDA). In Guinea, there is evidence of ongoing LF transmission, but little is known about the most densely populated parts of the country, including the capital Conakry. In order to guide the LF control and elimination efforts, serological and entomological surveys were carried out to determine whether or not LF transmission occurs in Conakry.

METHODS

The prevalence of circulating filarial antigen (CFA) of Wuchereria bancrofti was assessed by an immuno-chromatography test (ICT) in people recruited from all five districts of Conakry. Mosquitoes were collected over a 1-year period, in 195 households in 15 communities. A proportion of mosquitoes were analysed for W. bancrofti, using dissection, loop-mediated isothermal amplification (LAMP) assay and conventional polymerase chain reaction (PCR).

RESULTS

CFA test revealed no infection in the 611 individuals examined. A total of 14,334 mosquitoes were collected; 14,135 Culex (98.6 %), 161 Anopheles (1.1 %) and a few other species. Out of 1,312 Culex spp. (9.3 %) and 51 An. gambiae (31.7 %) dissected, none was infected with any stage of the W. bancrofti parasite. However, the LAMP assay revealed that 1.8 % of An. gambiae and 0.31 % of Culex spp. were positive, while PCR determined respective prevalences of 0 % and 0.19 %.

CONCLUSIONS

This study revealed the presence of W. bancrofti DNA in mosquitoes, despite the apparent absence of infection in the human population. Although MDA interventions are not recommended where the prevalence of ICT is below 1 %, the entomological results are suggestive of the circulation of the parasite in the population of Conakry. Therefore, rigorous surveillance is still warranted so that LF transmission in Conakry would be identified rapidly and adequate responses being implemented.

Effect of single to triple dose DEC on microfilaremics up to 5 years

A 5-year study was conducted with 200 selected microfilaremic subjects bearing Mean microfilarial density (MMD) of 11.6 from 16 small foothill villages around Susunia Hill, Bankura, West Bengal during June 2001 to May 2002. All of them were treated with a single dose (6 mg/kg body weight) to triple dose diethylcarbamazine (DEC). On day 180, 365, 730, 1,095, 1,460, and 1,825, recurrence of microfilaremia were 0.0%, 6.0%, 15.5% (6 + 9.5), 27% (15.5 + 11.5), 42% (27 + 15), and 49% (42 + 7), respectively, and corresponding MMD were 0.0, 1.2, 1.4, 2.7, 6.2, and 7.8, respectively. Culex quinquefasciatus was incriminated as the vector and their infection and infectivity rates reduced to 4.2% and 0.81% from the initial values of 6.40% and 1.33%, respectively, after DEC treatment to selected mff carriers. Single-dose DEC treatment was found effective for reduction of microfilaremia and MMD but it was not always sufficient to eliminate all the microfilariae, especially when MMD was high (>40). For microfilaremics with high MMD, a single course (6 mg/kg body weight for 3 days) DEC treatment may be safe for the first instance.

Nature limits filarial transmission

Lymphatic filariasis, caused by Wuchereria bancrofti, Brugia malayi and B. timori is a public health problem of considerable magnitude of the tropics and subtropics. Presently 1.3 billion people are at risk of lymphatic filariasis (LF) infection and about 120 million people are affected in 83 countries. In this context it is worth mentioning that 'nature' itself limits filarial transmission to a great extent in a number of ways such as by reducing vector populations, parasitic load and many other bearings. Possibilities to utilize these bearings of natural control of filariasis should be searched and if manipulations on nature, like indiscriminate urbanization and deforestation, creating sites favourable for the breeding of filarial vectors and unsanitary conditions, water pollution with organic matters etc., are reduced below the threshold level, we will be highly benefited. Understandings of the factors related to natural phenomena of control of filariasis narrated in this article may help to adopt effective control strategies.

Ambient temperature effects on the extrinsic incubation period of Wuchereria bancrofti in Aedes polynesiensis: implications for filariasis transmission dynamics and distribution in French Polynesia

Temperature effects on development of the human filarial parasite Wuchereria bancrofti (Cobbold) (Filaridea: Onchocercidae) in the main Pacific vector Aedes polynesiensis Marks (Diptera: Culicidae) are analysed in relation to ambient climatic conditions. A statistical model of the extrinsic cycle duration as a function of temperature is described and used to distinguish three patterns of W. bancrofti transmission dynamics: continuous, fluctuating and discontinuous, occurring from north to south geographically among French Polynesian archipelagos. In the northerly Marquesas Islands (8-11 degrees S) filariasis transmission is continuous and very active, facilitated by perennially high temperatures combined with constantly high rates of man-vector contact. In the southerly Australes Islands (21-28 degrees S) filariasis transmission is seasonally discontinuous and, during the cooler months (May-September), the model predicts virtually no transmission because the cycle duration exceeds the life expectancy of the vector. In the Society Islands (16-18 degrees S), between the Marquesas and Australes, transmission is predicted to be intermediate as expected from their latitude, with seasonally fluctuating transmission potential. In the Tuamotu Islands (also geographically intermediate: 14-23 degrees S), with theoretically perennial transmission potential, transmission occurs only intermittently, being limited by other human and environmental factors whereby man-vector contact is confined to seasonal agricultural situations. Generally, among French Polynesian archipelagos where Aedes polynesiensis is the vector, the transmission potential for W. bancrofti and resulting disease manifestations of lymphatic filariasis in humans are correlated with ambient temperature due to the degree of southern latitude.

Long-term effect of mass diethylcarbamazine chemotherapy on bancroftian filariasis, results at four years after start of treatment

The long-term effect of 4 strategies for control of bancroftian filariasis using mass diethylcarbamazine (DEC) chemotherapy was assessed and compared in 4 endemic communities in Tanzania over a period of 4 years. The strategies were the standard 12 d treatment (strategy I), semi-annual single dose treatment (strategy II), monthly low dose treatment (strategy III), and DEC-medicated salt treatment (strategy IV). Treatment was given only during the first year. All strategies resulted in considerable reductions in microfilaraemia, with maximum effects occurring 1-2 years after start of treatment. At 2 years, the greatest reductions were seen for strategies III and IV, followed by strategy II and finally strategy I. The overall performance of the 4 strategies evaluated over the 4 years period followed the same sequence. Between the 2 years and 4 years follow-up surveys, a significant increase in microfilarial (mf) burden occurred in all 4 communities, but the mf geometric mean intensities (GMI) remained low. Thus, in individuals who were microfilaraemic before treatment, the rates of microfilaraemia were 66%, 44%, 34% and 43%, and the mf GMIs were 6.8%, 3.3%, 0.5% and 0.7%, of pre-treatment level, 4 years after start of treatment with strategies I, II, III and IV, respectively. Most individuals who developed microfilaraemia between the 2 years and 4 years follow-up surveys had been microfilaraemic before the start of treatment. Hence, the rate of development of microfilaraemia was much higher (18 times on average in the 4 communities) among those who were microfilaraemic before treatment than among those who were amicrofilaraemic. The long-lasting effect of treatment adds a promising potential to the use of mass DEC chemotherapy for the control of bancroftian filariasis.

Re-assessing the global prevalence and distribution of lymphatic filariasis

This paper estimates the global burden of lymphatic filariasis based on a review of the published literature on infection and disease surveys. A method for aggregating and projecting prevalence data from individual studies to national, regional and global levels, which also facilitates the estimation of gender and age-specific burdens, is presented. The method weights in favour of the larger, and hence presumbably more reliable, studies and relies on estimated empirical relationships between gender, age, infection and disease in order to correct studies with incomplete data. The results presented here suggest that although the overall prevalence of filariasis cases is 2.0% globally (approximately totalling 119 million cases), the disease continues to be of considerable local importance, particularly in India and Sub-Saharan Africa. Estimates by age and gender clearly show that, unlike other helminth infections, filariasis is mainly a disease of the adult and older age-classes and appears to be more prevalent in males. This work suggests that the derivation of more accurate estimates of the burden of filariasis will require a better understanding of both the epidemiology and the spatial aspects of infection and disease. It also suggests that filariasis is preventable based on a geographically targeted strategy for control.

Variation in the vector competence of Aedes polynesiensis for Wuchereria bancrofti

The vector competences of 6 geographic strains of Aedes polynesiensis for Wuchereria bancrofti were studied using two types of experimental infections. Experimental infection of laboratory-bred mosquitoes fed on the carriers' forearms with different levels of microfilaraemia showed that microfilariae (mf) uptake was directly proportional to the carrier's mf density and, as mf densities decreased, concentration capacity of Ae. polynesiensis increased. It was also shown that infection has an important effect on mosquito mortality, and that the mortality rate differed among mosquito strains. In infections using artificial feeders, the mf uptake was closely regulated, thus showing differences in the vectorial efficiency of Ae. polynesiensis related to the geographic origin of the mosquito strain. The mosquitoes from the Society archipelago were more efficient intermediate hosts than geographically distant strains when infected with W. bancrofti from an island within the archipelago (Tahiti). Mosquito strains from the Society archipelago developed the highest proportion of infective-stage larvae and exhibited the lowest mortality rate when infected with sympatric Tahitian W. bancrofti.

Epidemiology of subperiodic Wuchereria bancrofti infection in the Nicobar Islands, India

Diurnally subperiodic filariasis due to Wuchereria bancrofti has been reported from tribal populations in the Nancowry group of islands in the Andaman and Nicobar Islands, India. This was confirmed in a survey carried out during the monsoon season in 1993. Microfilariae were detected in the peripheral blood throughout a 24 h period with a peak at 18:00. The microfilaria (mf) rate ranged from 1.2% to 18.7%, with a low disease rate (mean = 1.9%). Incidence of mf was low in children less than 10 years old (3.5%), but increased with increasing age. Culex (Culex) quinquefasciatus was present at a very low density. Aedes (Finlaya) niveus and Aedes (Stegomyia) malayensis were the only species commonly biting man. The former was incriminated as a vector, and found naturally infected with W. bancrofti (infection and infectivity rates 1.1% and 0.9% respectively). After experimental feeding on donors, W. bancrofti developed to L3 stage larvae in A. niveus but failed to develop in A. malayensis and A. aegypti.

Wuchereria bancrofti filariasis in French Polynesia: age-specific patterns of microfilaremia, circulating antigen, and specific IgG and IgG4 responses according to transmission level

The age-specific patterns of microfilaremia, Og4C3 antigenemia, anti-Brugia malayi IgG and IgG4 were assessed in 3 villages of low, medium and high transmission level for Wuchereria bancrofti filariasis. The prevalence rates for each of the 4 markers were clearly age dependent and their patterns strongly associated with the transmission level. The antigenemia prevalence rate was consistently higher than the microfilaremia prevalence rate, in all age groups. The prevalences of anti-B. malayi IgG and IgG4 responses were very similar and much higher than those of microfilaremia or antigenemia. Antibody responses reached the plateau at an earlier age and at a higher prevalence with increased intensity of transmission. For all the markers, the prevalence rates were significantly higher in males than in females.

Advantages of ivermectin at a single dose of 400 micrograms/kg compared with 100 micrograms/kg for community treatment of lymphatic filariasis in Polynesia

In April and October in 1991-1993, 5 supervised single doses of ivermectin were given to inhabitants aged > or = 3 years in a Polynesian district: the first 3 treatments were with 100 micrograms/kg and the 2 latter with 400 micrograms/kg. At each treatment, about 97% of the eligible population (899) were treated and blood samples were collected just before treatment from 96% of the 613 inhabitants aged > or = 15 years. Following the 5 successive treatments, adverse reactions were observed in, respectively, 23.8, 13, 6.2, 13.6 and 7.9% of the microfilariae (mf) carriers, and in less than 1% of amicrofilaraemic subjects. Neither the frequency nor the intensity of adverse reactions was significantly different between single doses of 100 micrograms/kg and 400 micrograms/kg. Although the geometric mean microfilaraemia (GMM) was reduced, the mf carrier prevalence remained unchanged before and after 3 mass treatments with 100 micrograms/kg (21.4 and 20.7% respectively), and the mf recurrence rate 6 months after each dose of 100 micrograms/kg was roughly stable (respectively, 34.3%, 21.6% and 31.2% of the initial GMM). In contrast, after one dose round of 400 micrograms/kg, the mf carrier prevalence decreased significantly to 14.9% (P < 10(-6)), and the mf recurrence rate dropped to 9.9% (P < 10(-3)) of the initial GMM. These results confirm the safety and the effectiveness of 400 micrograms/kg of ivermectin for lymphatic filariasis control.

The relative importance and distribution of Aedes polynesiensis and Ae. aegypti larval habitats in Samoa

In preparation for a Filariasis Control programme in Samoa, during 1978 monthly larval surveys of the vector mosquito Aedes polynesiensis were carried out in four study villages in the main island of Upolu. A more extensive survey of larval habitat distribution was then made in twenty-two villages of Upolu and eighteen of Savai'i island, to determine the importance of habitat types according to their abundance, volume of water and whether their productivity was permanent or seasonal. Ae.aegypti larval densities and habitat distribution were also monitored and the occurrence of predatory Toxorhynchites amboinensis larvae in northern Upolu was recorded from forty-one collections. Aedes Breteau and container indices fluctuated with the pattern of rainfall in two coastal villages and an inland bush village, but not in a coconut plantation community. The five main Aedes larval habitat types encountered were: 200 litre water-storage drums, discarded tins and bottles, coconut shells, automobile tyres and treeholes. Aedes immatures occurred perennially in drums and tree holes, but breeding discontinued in tins, bottles and coconut shells during the driest month of July. For Ae. polynesiensis in Upolu the Breteau and container indices of 104.5 +/- SD 80.9 and 35.3 +/- 12.4 respectively were significantly higher than those in Savai'i: 33.1 +/- 25.0 and 24.3 +/- 20.0 respectively. Likewise for Ae.aegypti the Breteau and container indices of 50.8 +/- 32.5 and 23.9 +/- 15.6, respectively, were also significantly higher than those in Savai'i: 12.7 +/- 17.1 and 9.4 +/- 13.2 respectively. Habitat types greater or lesser importance were determined by plotting the percentage of each type of cotnainer utilized for Aedes breeding against the percentage of ech type amongst all larva-positive containers. Ae.polynesiensis preferred tree-holes but not water-storage drums. Ae.aegypti preferred drums and tyres; mixed populations of larvae of both species were commonest in these two types of habitat. Ae.polynesiensis occurred in every village. Ae.aegypti was encountered in all twenty-two villages surveyed in Upolu and nine of eighteen villages in Savai'i. Total larval surveys revealed that drums and tree-holes contained the highest numbers of Aedes larvae. The study provided criteria for planning a control programme.

Mass chemoprophylaxis of lymphatic filariasis with a single dose of ivermectin in a Polynesian community with a high Wuchereria bancrofti infection rate

In April 1991 supervised mass prophylaxis of lymphatic filariasis with a single dose of ivermectin, 100 micrograms/kg, was carried out in a Polynesian village with a high infection rate of Wuchereria bancrofti in humans and active transmission by the vector mosquito, Aedes polynesiensis. Of 876 inhabitants aged 3 years or more (pregnant women excluded), 864 (98.6%) were treated. Simultaneously, venous blood samples were collected from 577 (97.5%) of the 595 inhabitants aged 15 years or more, of whom 122 (21.4%) were found to be microfilaria (mf) carriers (86 males and 36 females). The geometric mean microfilariae (GMM) count was 358.7 mf/ml for the whole group, 387 mf/ml for males (range 1-8160 mf/ml) and 280 mf/ml for females (range 1-7769 mf/ml). Following treatment, 33 (3.8%) of the 864 persons treated experienced some adverse reactions (21 with grade 1 and 12 with grade 2). Of the 33 with reactions, 29 were among the 122 (23.8%) mf carriers and 4 among the 831 (0.5%) non-microfilaraemic persons. Six months later, 123 (21.1%) of 584 inhabitants sampled were microfilaraemic: the GMM count for the whole group was 106 mf/ml (1-8177), with 29 mf/ml (1-3740) in 35 female and 177 mf/ml (1-8177) in 88 male carriers. Of these 123, 15 (whose GMM count was 4.5 mf/ml; range 1-204) were amicrofilaraemic 6 months before, and 19 had a microfilaraemia level higher than that 6 months earlier, before treatment. 117 of the 122 carriers identified in April were resampled: comparison of their GMM counts before and 6 months after mass treatment indicated that treatment with a single dose of 100 micrograms/kg ivermectin resulted in a reduction of microfilaraemia by 69%.