The effect of six rounds of single dose mass treatment with diethylcarbamazine or ivermectin on Wuchereria bancrofti infection and its implications for lymphatic filariasis elimination


K. D. Ramaiah, Vector Control Research Centre, Indian Council of Medical Research, Medical Complex, Indira Nagar, Pondicherry 605 006, India. E-mail:


Annual mass treatment with single-dose diethylcarbamazine (DEC) or ivermectin (IVM) in combination with albendazole (ALB) for 4–6 years is the principal tool of lymphatic filariasis (LF) elimination strategy. This placebo-controlled study examined the potential of six rounds of mass treatment with DEC or IVM to eliminate Wuchereria bancrofti infection in humans in rural areas in south India. Apercentage of 54–75 of the eligible population (≥15 kg body weight) received treatment during different rounds of treatment – 27.4% in the DEC arm and 30.7% in the IVM arm received all six treatments, 4.8% and 5.6% received none, and the remainder received one to five treatments. After six cycles of treatment, the microfilaria (Mf) prevalence in treated communities dropped by 86% in the DEC arm (P < 0.01) (n = 5 villages) and by 72% in the IVM arm (P < 0.01) (n = 5 villages), compared with 37% in the placebo arm (P < 0.05) (n = 5 villages). The geometric mean intensity of Mf fell by 91% (t = 8.11, P < 0.05), 84% (t = 6.91, P < 0.05) and 46% (t = 2.98, P < 0.05) in the DEC, IVM and placebo arms, respectively. The proportion of high-count Mf (>50 Mf per 60 mm3 of blood) carriers was reduced by 94% (P < 0.01) in the DEC arm and by 90% (P < 0.01) in the IVM arm. Among those who received all six treatments, 1.4% in the DEC arm and 2.4% in the IVM arm remained positive for Mf. Two of five villages in the DEC arm and one of five in the IVM arm showed zero Mf prevalence, but continued to have low levels of transmission of infection. The results also indicate that DEC is as effective as or slightly better than IVM against microfilaraemia. Results from this and other recent operational studies proved that single-dose treatment with antifilarials is very effective at community level, feasible, logistically easier and cheap and hence a highly appropriate strategy to control or eliminate LF. Higher treatment coverage than that observed in this study and a few more than six cycles of treatment and more effective treatment tools/strategies may be necessary to reduce microfilaraemia to zero level in all communities, which may lead to elimination of LF.


Lymphatic Filariasis (LF) is a significant health problem in many developing countries. Globally, 1.1 billion people live in known endemic areas (Michael et al. 1996) and about one-fourth of them may be infected (Das et al. 2000). LF is the second leading cause of permanent and long-term disability (WHO 1995) and undermines the social and economic welfare of the affected people and communities (Ramaiah et al. 2000a). The World Health Organization launched a global programme to eliminate LF as a public health problem by the year 2020 (Ottesen 2000).

Annual mass treatment with a single dose of diethylcarbamazine (DEC) or ivermectin (IVM) in combination with albendazole (ALB) is the currently recommended tool of LF elimination programmes (Ottesen 2000). While ALB adds an antifilarial therapeutic effect and ‘beyond filariasis benefits’ (Ottesen et al. 1999), DEC or ivermectin act as thebasic and principal antifilarial drugs. Clinical trials demonstrated that a single dose of DEC or ivermectin has an excellent microfilaricidal effect (Cao et al. 1997). Based on the efficacy of the drugs and biology of the parasite, it was hypothesized that yearly treatment of communities with antifilarial drugs for 4–6 years has the potential to clear microfilaraemia, drastically reduce the transmission, diminish the adult parasite population and prevent new infections. The cumulative effect is expected to be the elimination of LF.

While the effect of single-dose treatment is proved beyond doubt in clinical trials (Cao et al. 1997), its effect at community level has yet to be proved. Such information is needed to understand the prospects of LF elimination and plan the future large-scale elimination programmes. Therefore we initiated a double-blind placebo-controlled study in rural areas in south India endemic for Wuchereria bancrofti transmitted by Culex quinquefasciatus, to examine the impact of repeated annual mass treatment with DEC or ivermectin on microfilaraemia in a human population (Das et al. 2001). The results on the effect of six cycles of treatment, considered to be sufficient to eliminate LF, on microfilaraemia prevalence and intensity are presented in this communication. The implications of the results for LF elimination are discussed.

Study area

We selected 15 endemic villages in Villupuram district in Tamil Nadu state, India to implement the study. These villages are well separated from one another and constituted separate transmission zones. The population of the study villages ranged from 517 to 3321, and lives mainly from agriculture and weaving. LF is a major public health problem, as are nutritional disorders and intestinal helminth infection. There had been no organized filarial control activities in the study villages prior to this study. Private practitioners or the National Filaria Control Units in towns or Primary Health Centers (PHCs) or registered medical practitioners are the treatment sources for filariasis patients.

Study design

The study was double-blind, placebo-controlled and consisted of three arms (Das et al. 2001) and six rounds of mass treatment. Of 15 study villages, five villages each were randomly allocated to the three study arms. The three arms were randomly allocated to three treatment groups – placebo, DEC or ivermectin. DEC was administered to the study population at the dose of 6 mg/kg body weight and ivermectin at 400 μg/kg. DEC, ivermectin and placebo (casein) were packaged into identical capsules. During the initial two rounds, neither drug distributors nor recipients knew which one of the three drugs was used in a particular village. However, after the second round of treatment, the drug codes were opened to assess the benefits of treatment, and the study was continued in single-blind form. No placebo tablets were given during the third to sixth rounds of treatment, although we continued to monitor the placebo villages for changes in microfilaria (Mf) prevalence and intensity.

This is the first time ivermectin was used at community level in India. Hence, as a safety measure, children under 15 kg body weight were excluded from treatment in the ivermectin arm. For comparison they were excluded in DEC and placebo arms also. Also excluded from treatment were pregnant women, lactating mothers and terminally ill people.

Materials and methods

Mass treatment

Prior to first treatment, the villages were censused. The census teams visited all households and recorded age, sex and weight of household members. During each round of treatment, the drug distribution teams visited the households and all eligible members (≥ 15 kg body weight) were given the drugs according to their weight. People were persuaded to consume the drug in the presence of drug distributors. Those affected with adverse/side reactions were given supportive treatment by a qualified doctor or referred to the nearest PHC.

Six rounds of mass treatment were administered to the study communities. The initial two rounds were given at a 6-month interval and the subsequent four rounds at 12–15-month intervals. The first cycle of treatment was initiated in November 1994 and the sixth in April 2000.

Microfilaria surveys

The effect of various cycles of mass treatment on infection in the population was evaluated by comparing the prevalence and Geometric Mean Intensity (GMI) of microfilaraemia between pre- and post-treatment periods and DEC and ivermectin and placebo arms. Mf surveys were conducted in all study villages about 1 week prior to each mass treatment and at the end of 1 year after the sixth treatment.

In each village and for each Mf survey, we randomly selected 7% of the households and all members were blood sampled. About 60 mm3 of blood was collected, between 20.00 and 24.00 hours, from each person by finger pricking and prepared into three separate smears of 20 mm3 each on clean glass slides (Sasa 1976). Next day morning, the blood smears were processed – they were dehaemoglobinized in tap water, fixed in methanol and stained in JSB I solution and examined for Mf using a compound microscope. The microfilaraemia status and the number of Mf for positive individuals were recorded. As per the institutional ethical guidelines, all the detected Mf carriers in all study arms were treated with a single dose of DEC.

Statistical analysis

The Mf prevalence is expressed as Mf rate (proportion of examined individuals with Mf) and intensity in terms of GMI. GMI was calculated as antilog [(Σlog (x + 1))/n]– 1, where x is the number of Mf per 60 mm3 of blood. The significance of reduction in Mf prevalence and the proportion of high-count Mf carriers from pre- to post-treatment period was examined using the chi-squared test. The reduction in GMI of microfilaraemia was assessed by Student's t-test. The Z interaction test from a log linear model was used to examine the relative change from pre-treatment to post-treatment period in Mf prevalence and the proportion of high-count Mf carriers between placebo, DEC and the ivermectin arms.


Treatment coverage

The treatment coverage rate of the eligible population ranged from 49% to 84% in different villages (n = 15) during different rounds (n = 6) of treatment (Table 1). The overall rate for the DEC arm ranged from 54% to 73% and for the ivermectin arm from 57% to 75% (Table 1). Atotal of 7802 people in the DEC arm and 6830 in the ivermectin arm were eligible for treatment during the first round. Follow-up of this cohort of people revealed that 27.4% (2134/7802 × 100) received all six treatments and 4.8%[(378/7802) × 100] did not receive even a single treatment in the DEC arm. The respective figures for the ivermectin arm were 30.7%[(2098/6830) × 100] and 5.6%[(385/6830) × 100]. Others received two to five treatments. Therefore, the average number of treatments received by an eligible person was only 3.4 and 3.5 in DEC and the ivermectin arms, respectively.

Table 1.  Population size and range of treatment coverage during the six rounds of treatment in the study villages in ivermectin, DEC and placebo arms
TreatmentVillageTotal populationRange of treatment coverage (%)
DECMuppili 73955–84
Total/average 988954–73
IvermectinThenputhur 88966–79
Total/average 852657–75
Sendiampakkam 51778–84
Total/average 840573–75

Impact of treatment on microfilaraemia

The changes in Mf rate with each round of mass treatment in various study villages are presented in Figure 1. By the end of 1 year after the sixth mass treatment, the overall Mf rate had declined from pre-treatment level by 86% (13.2–1.9%) (P < 0.01) in the DEC arm and by 72% (14.5–4.1%) (P < 0.01) in the ivermectin arm. The Mf rate fell by 37% (12.9–8.1%) (P < 0.01) in the placebo arm (Figures 1 and 2). While the relative decline in Mf prevalence, from pre-treatment to post-sixth treatment period, between placebo and DEC or the ivermectin arms was significant (DEC vs. placebo: z = 4.50, P < 0.01; ivermectin vs. placebo: z = 3.24, P < 0.01), this was not so between ivermectin and the DEC arms (z = 1.53, P < 0.05). While the decline in Mf rate was gradual with each mass treatment in DEC and the ivermectin arms, it was observed only after the first treatment in placebo group and thereafter remained almost at the same level (Figures 1 and 2). Two of five villages (Muppili and Thenkalavai) in the DEC arm and one (Thenputhur) of five in the ivermectin arm showed zero Mf rate. In both arms, the reduction in Mf rate was least in the villages with the highest pre-treatment Mf rate (Figure 1). The reduction in Mf rate was better in the communities treated with DEC (reduction range: 76–100%) than those treated with ivermectin (reduction range: 56–100%) (Figures 1 and 2).

Figure 1.

Impact of six rounds of treatment with DEC or ivermectin on Mf prevalence in study villages.

Figure 2.

Percentage reduction in prevalence of Mf rate after various rounds of treatment in placebo, ivermectin and DEC aims.

The fall in GMI of microfilaraemia after six rounds of treatment is highly appreciable and was nearly 100% in three (Muppili, Thenkalavai and Padirappuliyur) of five villages in the DEC arm (Table 2). The overall reduction was 91% in the DEC arm, 84% in the ivermectin arm and 46% in the placebo arm (Figure 3). The reduction in GMI also was relatively less in villages with higher pre-treatment microfilaraemia levels (Table 2).

Table 2.  Changes in GMI of microfilaraemia in various villages after various rounds of treatment in the study villages
GMI after treatment round
Average 0.660.510.320.
Average 0.620.
Average 0.580.380.380.450.380.450.31
Figure 3.

Percentage reduction in GMI of microfilaraemia after various rounds of treatment in placebo, ivermectin and DEC arms.

Six rounds of treatment resulted in appreciable reduction in the number of Mf carriers with a higher count of Mf. For example, 6.7% of the Mf carriers in the DEC arm and 5.2% in the ivermectin arm and 5.9% in placebo arm harboured more than 50 Mf per 60 mm3 of blood. These proportions declined, following six rounds of treatment, to 0.4% (94%) (P < 0.01) and 0.5% (90%) (P < 0.01) in the DEC and the ivermectin arms, respectively, compared with 2.8% (53%) (P < 0.01) in the placebo arm (Table 3). These reductions were relatively less in people with low count (1–10 and 11–50 Mf per 60 mm3 of blood) of Mf (Table 3). The relative change between the treatment and placebo arms in high count Mf carriers from pre-treatment to post-treatment period was significant (ivermectin vs. placebo: z = 2.50, P < 0.01; DEC vs. placebo z = 3.09, P < 0.01). This was, however, not the case between ivermectin and DEC arms (z = 0.40, P > 0.05).

Table 3.  Changes in the proportion of Mf carriers with various counts of Mf from pre- to post-treatment period
Mf count/
60 mm3
of Mf carriers
Post-sixth treatment
of Mf carriers
% Reduction
DEC1–1020/2.8 3/0.486
11–5029/4.1 8/1.173
>5045/6.4 3/0.494
11–5030/4.9 9/1.471
>5032/5.2 3/0.590

Microfilaraemia continued to persist in a proportion of treated population, particularly in those who underwent fewer (1–4) rounds of treatment. Note that microfilaraemia persisted in 1.4 and 2.4% of the people who received even five to six rounds of treatment with DEC or ivermectin, respectively (Table 4).

Table 4.  Prevalence of microfilaraemia in relation to number of treatments received by people
No. of
rate %
GMI of micro


After the demonstration in clinical trials of the excellent microfilaricidal effect of DEC or ivermectin in single dose, the focus has shifted to their effect at community level. DEC or ivermectin, alone or in combination with ALB, are the mainstay of LF control/elimination programmes in India and Africa, respectively, which together account for 72% of the world's infected population (Michael et al. 1996). Therefore, the community level impact of these drugs is being studied widely. In Papua New Guinea, one cycle of mass treatment with a single dose of DEC (6 mg/kg) reduced the Mf rate and intensity by 31% and 70%, respectively (Bockarie et al. 1998), two cycles by 34% and 59% in Tanzania (Meyrowitsch et al. 1996) and four cycles by 65% and 74% (Balakrishnan et al. 1992) and 48% and 60% in India (Das et al. 2001). Two cycles of ivermectin (400 μg/kg) in Ghana reduced the Mf rate and intensity by 25% and 40%, respectively (Gyapong 2000) and four cycles by 65% and 80% in an Indian study (Das et al. 2001). These studies suggest that one or two treatments exert only limited effect and more than four spaced treatments are necessary to reduce the Mf prevalence to zero. We report the impact of six rounds of mass treatment, considered to be adequate to eliminate LF, and hence having significant implications for LF elimination programmes.

In this study six cycles of mass treatment with DEC or ivermectin reduced the Mf prevalence by 86% and 72%, Mf intensity by 91% and 84% and the proportion of high count Mf carriers by 94% and 90%, respectively. These reductions appear highly significant if we consider that (i) children < 15 kg body weight, 4.6% of who harboured Mf and account for 9% of the total population, were excluded from treatment, (ii) only 54–73% of the eligible (≥ 15 kg body weight) population received treatment, (iii) about 5% did not undergo even one treatment and (iv) the average number of treatments per person was only about 3.5, instead of the ideal number of 6. Lower treatment coverage and exclusion of children from treatment might have diluted the effect of treatment. However, currently, the treatment coverage rates in large-scale elimination programmes are also in the range of 51–75% (Ramaiah et al. 2000b). Hence our results may also reflect the likely outcome of the ongoing large-scale programmes. Apart from treatment coverage, the persistence of Mf in some people (1.4% in The DEC arm and 2.4% in the ivermectin arm) (Table 4), even after five to six treatments, may be a limiting factor to LF elimination efforts. Persistent microfilaraemia in some treated individuals has been reported earlier (Kimura et al. 1985; Ottesen 1985; Pani et al. 1991; Esterre et al. 2001) and may be due to lower susceptibility to drugs of a proportion of parasite population in some people. However, clinical trials showed that coadministration of DEC or ivermectin with ALB achieves better clearance of microfilaraemia than DEC or ivermectin alone (Ottesen et al. 1999). Therefore, problems such as residual microfilaraemia and prevalence of low levels of infection even after six rounds of treatment, observed with DEC or ivermectin alone (Table 4), may be overcome by coadministration of DEC or ivermectin with ALB. This may help to achieve total interruption of transmission of infection and accelerate the process of control/elimination of LF.

After six treatments, the Mf rate fell to zero in two villages (Muppili and Thenkalavai) in the DEC arm and one village (Thenputhur) in the ivermectin arm. However, 0.3–1.0% of mosquitoes from these villages were found infected, 0.06–0.1% with infective stage larvae (L3) (Vector Control Research Centre, unpublished data). This indicates the persistence of low level microfilaraemia undetectable by the conventional blood smear technique, used in this study. Application of a more sensitive technique like membrane filtration may reveal some low density Mf carriers in these treated communities (Desowitz & Southgate 1973). McGreevy et al. (1982) reported that a low-density Mf carrier could contribute to 0–15 L3 per year. This report and our finding of mosquitoes with L3 suggest the possibility of low level transmission in zero prevalence villages also. At this juncture it is difficult for us to predict how many more treatments are required to reduce Mf rate and mosquito infection rate to zero level.

One village (Alagramam) in the DEC arm and two villages (Nedi and Peramandur) in the ivermectin arm had the maximum Mf rate of > 4.0% after six treatments (Figure 1) and these villages had higher pre-treatment Mf rate and intensity. Mf carriers with high intensity microfilaraemias are less likely to become amicrofilaraemic (Ottesen 1984), possibly because of which considerable number of people continued to be microfilaraemic in these villages. This calls for more vigorous and prolonged treatment in high endemicity villages.

The conclusion from clinical trials is that DEC is as effective as or slightly ‘more favourable’ than ivermectin (Cao et al. 1997). In this community-based study also, DEC caused 14% higher reduction in Mf prevalence and 7% in GMI than ivermectin (Figure 2; Table 2), although the relative change from pre- to post-treatment periods in Mf rate between the DEC and the ivermectin arms was not statistically significant (z = 1.53, P > 0.05). These results suggest that DEC is as good as or slightly better than Ivermectin. DEC is also cheap and safe and has been extensively used in some countries including India. Therefore, in these countries, DEC may remain, unless contra-indicated, the drug of choice for LF control/elimination.

This study suggests that repeated single-dose mass treatment with DEC or ivermectin is an excellent tool to control/eliminate LF because it is able to achieve, even with modest treatment coverage, 72–86% reduction in prevalence and 84–91% in GMI of microfilaraemia. Also, it is a feasible, cheap and logistically easier strategy (Ramaiah et al. 2000b, 2001) and hence, perhaps, very much suitable to large countries like India, where other options such as vector control and selective treatment have several limitations. However, while excellent control is evident in this study, elimination of LF may require still better control of microfilaraemia. This may be possible through high treatment coverage, a few more than six rounds of treatment and better treatment tools such as combination of DEC or ivermectin with ALB.


The study received financial support from UNDP/World Bank/WHO Special Programme for Research and Training in Tropical Diseases (grant no. 920702). The authors gratefully acknowledge the fieldwork carried out by the staff of the Applied Filed Research and Community Health divisions of the Vector Control Research Centre (VCRC). They thank Dr Ravi Rengachari, VCRC, for his constant support and suggestions. Our thanks are due to Drs P. Krishnamoorthy and D.J. Augustin, Directorate of Public Health and Preventive Medicine, Chennai, Government of Tamil Nadu, India. They thankfully acknowledge the support extended to the study by Drs Hans Remme, Eric Ottesen and V. Kumaraswamy as members of the task force at WHO.