Prolonged persistence of residual Wuchereria bancrofti infection after cessation of diethylcarbamazine-fortified salt programme


Corresponding Author K. D. Ramaiah, Vector Control Research Centre (ICMR), Medical Complex, Indira Nagar, Pondicherry 605 006, India. Tel.: +91 413 2272841 2272396; Fax: +91 413 2272041; E-mail:


Community-wide administration of diethylcarbamazine (DEC)-fortified salt is a very effective strategy to control lymphatic filariasis (LF). Administration of DEC-fortified salt for a few months has the potential to reduce microfilaria (Mf) prevalence and intensity very appreciably. Despite the advent of mass drug administration (MDA) strategy as the mainstay of the programmes to eliminate LF, DEC-fortified salt – alone or as an adjunct measure – continues to be an important intervention measure to control/eliminate LF.

A 40-month DEC-fortified salt administration programme had been implemented in Karaikal district, Union territory of Pondicherry, India. This programme proved very effective and reduced the Mf prevalence from 4.49% to 0.08%, equivalent to 98% decline. However, subsequent surveys showed that very low levels of microfilaraemia persisted in the intervention area for several years. Prevalence of post-intervention residual and/or low level microfilaraemia is a very common feature in LF. A similar situation is likely to be seen in the coming years in many endemic areas of the world which are under the MDA programme, implemented to eliminate LF. Hence, understanding the duration and epidemiological significance of low level microfilaraemia to transmission and incidence of new infections is very important. Therefore, we undertook a study in Karaikal urban and rural areas to examine (i) the prevalence levels of microfilaraemia during the 20-year period (1987–2006) after the cessation of the 40-month (1982–1986) DEC-fortified salt programme; (ii) the current microfilaraemia levels in the population and (iii) the prevalence of circulating filarial antigenaemia (CFA) in children to understand the incidence of new infections. The data from the study are collated to define the safe level of microfilaraemia at which resurgence and new infections may not occur.

Materials and methods

Study area

The study took place in Karaikal region, which constitutes a district of the Union territory of Pondicherry, India. The region consists of Karaikal town and 29 surrounding villages and is situated about 320 km south of the city of Chennai. The population of the town, consisting of 18 wards, was 74 438 and that of the 29 villages was 96 202 (2001 census). Trade and government service in the urban area and agriculture and fishing in rural areas are the major occupations. Health care is provided by the government-run hospital and private clinics in Karaikal town and a network of primary health centres in rural areas. A National Filaria Control Unit (NFCU) operates in the region, endemic for bancroftian filariasis, transmitted by Culex quinquefasciatus.

Intervention measures (1982–2005)

A DEC-fortified salt programme was implemented by the NFCU to control LF in Karaikal town and all rural areas for 40 months during 1982–1986. Common salt fortified with 0.1% and 0.15% DEC was administered for 3 months each and 0.20% DEC salt was administered for the remaining 34-month period. Traditionally, people buy salt from cooperative stores or grocery shops or street vendors. All these sources were strictly made to procure and sell only DEC-fortified salt from a central location. Testing of 4571 samples of salt during the intervention period showed that none of them were without stipulated percentage of DEC content (Narasimham et al. 1989). More details on making and logistics of distribution of DEC-fortified salt were described elsewhere (Subramanyam Reddy & Venkateswaralu 1996).

Although the Mf rate fell from 4.49% to 0.08% after the DEC-fortified salt intervention, the local NFCU continued detection and treatment of Mf carriers, the core activity of the National Filarial Control Programme (NFCP) to control filariasis in India, during the 1987–2005 period. This was done to further reduce the Mf prevalence and eliminate residual microfilaraemia and because there were no robust guidelines when to stop the intervention.

Also, in 2004 and 2005, a mass single dose DEC administration programme was implemented in the study area, as part of the national programme to eliminate LF, by the health department.

Assessment of microfilaria prevalence (1982–2005)

To establish the base-line data for the DEC-fortified salt intervention (1982), to evaluate its impact (1983–1986) and to detect and treat Mf carriers (1987–2005), varying numbers of people were surveyed and examined for Mf in different years. These surveys were not very systematic – a proportion of the population in ward after ward in urban areas and village after village in rural areas was sampled. After completion of all the wards and villages, the surveys were repeated. The surveys were carried out by the NFCU following the standard guidelines of the NFCP – one blood smear of 20 ml of blood was collected from each of the sampled individuals. The blood smears were processed and stained using Jaswant Singh and Bhattacharya stain I in the laboratory next day morning. The stained blood smears were examined under compound microscope and the number of Mf present in the positive blood smears was recorded. All detected Mf carriers were treated with a 12-day course of DEC (12 × 6 mg/kg body weight/day).

Re-assessment of microfilaria prevalence (2006)

We conducted a more systematic and comprehensive Mf survey in 2006 to precisely assess the Mf prevalence and pattern of distribution of Mf carriers. Such an assessment is necessary to understand the epidemiological implications of residual microfilaraemia levels over the 20-year period from 1987 to 2006 (post-DEC-fortified salt programme) and the need and scope of further intervention measures in such situations. Fifteen of 18 urban wards and 17 of 29 villages were randomly selected to assess the Mf prevalence. In urban wards, we used purposive sampling – areas characterized by thatched roof dwellings and higher mosquito density were chosen. This is in accordance with WHO guidelines to evaluate the impact of intervention measures in highly endemic areas (World Health Organization 2005). In each selected urban ward and village, four clusters of five to six households were randomly selected. In each selected household, all family members were blood sampled, between 20.00 and 23.00 hours, for the assessment of Mf prevalence. From each person, approximately 60 ml of finger prick blood was collected using disposable needles and made into three thick smears of 20 ml each. Processing of thick blood smears and treatment of detected Mf carriers was done as described above. The geometric intensity of Mf per 60 ml is calculated as described elsewhere (Ramaiah et al. 2007a). Written consent was obtained from each of the sampled adult person. For children <10 years, consent of the parents was obtained.

Antigenaemia survey (2006)

The prevalence of CFA was assessed, using ICT card test (BinaxNow; Binax Inc., Scarborough, ME, USA) in children aged 2–6 years to estimate the incidence of new infections. This was done in 14 of 15 urban wards and 16 of 17 villages included for Mf surveys. Ten children from each ward and village were selected and a total of 300 children were assessed. In each ward/village, four streets were randomly selected and in each street one house was selected randomly. Two to three children present in the selected and/or adjoining household were included for assessment. The ICT card test was carried out following the instructions provided by the manufacturer. Only disposable needles were used to prick the finger and collect the blood. Written consent of the parents was obtained.

Ethical approval and conflict of interest

The study was approved by the institutional ethical committee. The authors have no conflict of interest with anybody or any organization.


Microfilaria prevalence (1982–2005)

The DEC-fortified salt intervention reduced the Mf prevalence from 4.49% (1982) to 0.08% (1986), equivalent to a 98% decline. During 1986–2005 period, the number of people assessed for microfilaraemia ranged from 3091 (1989) to 32 655 (2003). The Mf rate during this period varied from 0.03% (n = 32 655 and 21 764 respectively) in 2003 and 2005 to 0.42% (n = 4744) in 1994 (Table 1).

Table 1.   Number of people examined and microfilaria rate in Karaikal urban and rural areas
InterventionYearNumber of people examined Mf rate (%)
  1. The surveys were carried out by the National Filaria Control Unit as part of its activities. Quantity of blood examined in the surveys was 20 ml per person.

  2. MDA, mass drug administration; Mf, microfilaria.

DEC-fortified salt198222 2324.49
198620 4540.08
Detection and treatment of Mf carriers198711 9300.32
199122 0680.25
199214 5950.27
199916 3390.13
200019 8520.15
200123 0950.20
200232 6420.09
200332 6550.03
MDA200430 6320.05
200521 7640.03

Microfilaria prevalence (2006)

In each selected ward and village 100–103 people were blood tested for Mf. In the 15 wards of Karaikal town a total of 1511 people and in 17 villages a total of 1700 people were sampled for blood Mf. The results of the survey are summarized in Table 2. No Mf carrier was detected in 11 of 15 urban wards and 15 of 17 villages. The Mf rate in the other four wards ranged from 0.98% to 3.0%; it was 1.0% and 2.0% in the two villages. In villages, the highest Mf rate of 2.0% was observed in a large village with a population of 6181. The overall Mf rate for the region was 0.32% (10/3211), 0.46% (7/1511) for Karaikal town and 0.18% (3/1700) for villages. Most of the detected Mf carriers were natives of the study area. All the detected 10 Mf carriers were in the 21–50 years age group and seven of 10 were males (Table 3). The age of the youngest Mf carrier was 30 years; the mean age of the seven Mf carriers was 36.14 ± 6.47 years in Karaikal town. The respective figures for rural areas were 21 years and 33 ± 12 years. The number of Mf per 60 ml of blood among the seven Mf carriers in Karaikal town was 1, 1, 4, 7, 16, 16 and 137, their geometric mean intensity (GMI) of Mf being 7.18. The three microfilaraemic individuals in rural areas showed the Mf counts of 18, 21 and 101 and their GMI of Mf was 33.67.

Table 2.   Summary of the work carried out during the year 2006
 Urban areasRural areas
  1. Quantity of blood examined in the surveys was 60 ml per person.

  2. CFA, circulating filarial antigenaemia; Mf, microfilaria.

No. of areas sampled1517
No. of areas with Mf+ve individuals (%)4 (27)2 (12)
No. of people sampled for Mf15111700
Mf rate (range)0.46% (1.0–3.0%)0.18% (1.0–2.0%)
Geometric mean intensity of Mf of +ve individuals7.1833.67
Mean age (SD) of the Mf carriers (years)36.14 ± 6.4733 ± 12
Age of the youngest Mf carrier3021
No. of areas sampled for CFA1416
No. of children for CFA140160
No. of CFA+ve children00
Table 3.   Prevalence of microfilaraemia (Mf) in different age groups of the sampled population in 2006
Age group (years)No. of sampledNo. of +ve for MfMf rate (%)
  1. Mf, microfilaria.


Circulating filarial antigenaemia prevalence (2006)

The total number of children assessed for CFA was 300; 140 in 14 wards of Karaikal town and 160 in 16 villages; 152 girls and 148 boys. Of the children sampled 110 were 2–3 years old, 67 were 3.1–4 years old, 66 were 4.1–5 years old and 57 were 5.1–6.0 years old. All tested negative for CFA.


Administration of DEC-fortified salt for 40 months (1982–1986) to the study population reduceed the community Mf rate by 98% from 4.49% to 0.08% (Narasimham et al. 1989; Subramanyam Reddy & Venkateswaralu 1996). The reduction was similar to that observed in other studies (Fan et al. 1975a,b,c; Rao et al. 1980, 1981; Fan 1990). While a 98% reduction is appreciable, the presence of microfilaraemia in a small proportion of the population (0.08%) may be due to the ability of the parasite to tolerate DEC treatment (Eberhard et al. 1991). Over the following 19-year period (1987–2005), the Mf rate persisted at a very low level of 0.03–0.42% (Table 1), although a proportion of the population was surveyed for Mf every year, Mf carriers were treated and two rounds of MDA administered (2004–2005). Thus, in very low endemic situations, yearly blood screening of only a proportion of the population and treatment of Mf carriers from such screening or fewer rounds of MDA may not eliminate residual microfilaraemia.

In this study, purposive sampling in relatively poorer urban localities, where the vector density and risk of LF transmission are known to be high, revealed an overall Mf prevalence of 0.46%. The prevalence is expected to be much lower in the middle and high income localities, which did have a lower Mf rate than the poorer localities (Vijay Kumar & Ramaiah 2008). Absence of CFA in children (2–6 years) and microfilaraemia in 1–20 years age group in high transmission urban localities and also the rural areas indicates prolonged interruption of transmission and zero incidence of microfilaraemia during the 20-year period (1987–2006). Over this period, however, 0.03% (2003) to 0.32% (in the present study, 2006) of the population showed blood microfilaraemia. Persistence of infection, although meagre, was not due to inward migration of infected people as most of the detected Mf carriers were natives of the study area. It may be due to prolonged fecundic lifespan of the adult parasite in some infected individuals, in whom DEC-fortified salt intervention perhaps failed to clear the adult worm infection completely. We are, however, not certain if extremely low levels of transmission and incidence of new infections have been taking place, resulting in the persistence of low levels of microfilaraemia which could not be detected by the methods or population sampling frame used in the present study.

While we have no more robust evidence for the suspected prolonged lifespan of the parasite, a recent study (Dreyer et al. 2005) showed that transmission intensity is inversely related to adult worm survival and in low transmission situations the adult worms survived longer than those in high transmission situations. In the absence of new incoming larvae, the fewer adult worms that survived the DEC-fortified salt treatment might have lived longer than the currently believed fecundic lifespan of 5 years (Vanamail et al. 1996). A similar low prevalence of microfilaraemia combined with the incidence of infection in a 2-year-old child was seen, despite 34 years of DEC treatment, in Maupiti, where Aedes polynesiensis is the vector (Esterre et al. 2001).

Studies in China revealed that post-intervention residual microfilaraemia will disappear over a period of 7 years (Shi 1994). While, we also observed almost total disappearance of Wuchereria bancrofti after 6–10 rounds of annual mass administration of DEC in smaller villages (Ramaiah et al. 2007a), and no evidence for resurgence during next 4 years (K.D. Ramaiah, unpublished data), it was not the case in the present study population. Residual microfilaraemia persisted for 20 years (1987–2006) and its expected spontaneous disappearance remained elusive. It is difficult to predict the lifespan of this residual microfilaraemia. The persistent residual microfilaraemia is epidemiologically more significant and may act as a source of transmission in areas with more efficient vectors such as Aedes species.

Notably, the microfilaraemia was stable without any recrudescence for 20-year period (1987–2006) at ≤0.42%, mostly observed in 20 ml of blood (Table 1). This suggests that an Mf rate of about <0.5% in 20 ml of blood, which is equivalent to 0.60–0.70% in 60 ml of blood (Sasa 1976), achieved by effective intervention (DEC-fortified salt in the present case), is a very safe level and can be considered to stop intervention measures such as MDA, in areas where C. quinquefasciatus is the vector. Other workers projected a value of 0.5% to <1.0% as safe level (Xu et al. 1997; Michael et al. 2006). Stability of microfilaraemia and absence of recrudescence may not be due to detection and treatment of Mf carriers, as during certain years (1988–1990, 1993–1998) only a very small proportion of the population was screened (Table 1). However, gradual socio-economic and housing improvement and increasing use of personal protection measures, common phenomenon in many developing countries, might have contributed to some extent for stable microfilaraemia with no resurgence.

Evaluation of the impact of MDA in highly endemic areas (sentinel sites) and an Mf rate of <1% in such areas is one of the criteria to stop intervention (World Health Organization 2005). In this study area, with an Mf rate of 0.46% in urban areas and 0.18% in rural areas, three of 15 of the former and two of 17 of the latter were found with ≥1% Mf rate. Thus, with the progress of intervention and reduction in microfilaraemia levels, there is a probability of a few scattered areas showing ≥1% Mf rate. It is not easy to identify such areas in large intervention programmes such as the ongoing LF elimination programmes and decision making on stopping intervention becomes a challenging task. This makes innovative sampling and monitoring and evaluation strategies a necessity for LF elimination programmes.

The data from this and other recent studies (Ramaiah et al. 2007a,b; Vijay Kumar and Ramaiah (2008)) suggest that the residual infection persists longer in highly endemic areas i.e. socio-economically poorer urban areas and historically more endemic or large endemic rural communities, and, highly infected population groups, i.e. the adult age-classes. This is because, post-intervention, persistence of infection is higher in high intensity Mf carriers (Meyrowitsch et al. 2004; Ramaiah et al. 2007a,b), who are more common in highly endemic areas and adult population groups. Epidemiological situations similar to the one in this study are close to achieve the criteria to stop intervention, i.e. <1% Mf rate in all areas and enter the post-intervention surveillance phase. However, as there are a few areas with >1% Mf rate, it may not be safe to stop intervention (MDA) in such situations. As most of the remaining infections are concentrated in adult age groups and in poorer urban and large rural areas, further interventions, such as a few more rounds of MDA targeting treatment of at least 65–80% of the adult population in socio-economically underdeveloped urban areas and historically highly endemic and large endemic rural areas, may achieve a <1% Mf rate. Such a targeted intervention during the ‘end game’ of the LF control/elimination programmes may be more economical, logistically easier and effective. More probability of persistence of infection and higher vector density in underdeveloped urban areas and historically highly endemic or large endemic rural areas make them appropriate sites for monitoring and epidemiological assessment of MDA and post-MDA surveillance.


We thank Dr P. Jambulingam, Director, VCRC, Pondicherry, for very helpful comments on the manuscript and valuable suggestions; and we express our gratitude to the Director, Department of Health and Family Welfare, Government of Pondicherry, Pondicherry, for extending support to the study. Our thanks are due to the technical staff members of the VCRC – G. Sridharan, N. Pachaiappan, S. B. Chakravarthy, G. Meganathan, S. Balakrishnan, A. Dayaram and R. Harikrishnan – for their participation in the field work. We also gratefully acknowledge the support extended to the field work by technical staff of the Filaria Control Unit, Karaikal.



A diethylcarbamazine (DEC)-fortified salt intervention programme was implemented between 1982 and 1986 in Karaikal district, Union territory of Pondicherry, south India, to control Culex transmitted bancroftian filariasis. The intervention reduced the microfilaria (Mf) rate from 4.49% to 0.08%. To eliminate the residual microfilaraemia, the health department detected and treated Mf carriers from 1987 to 2005 and mass-administered drugs in 2004 and 2005. Surveillance from 1987 to 2005 revealed persistent microfilaraemia in 0.03–0.42% of the population. In 2006, we conducted a more detailed Mf survey and a child antigenaemia (Ag) survey in 15 urban wards and 17 rural villages. These surveys showed an overall Mf rate of 0.46% in the high-risk urban areas and 0.18% in the rural areas; none of the sampled children was positive for Ag. All detected Mf carriers were >20 years old. The age of the youngest Mf carrier was 30 years in urban and 21 years in rural areas, which suggests that transmission was interrupted and there was no incidence of new Mf case after cessation of DEC salt programme. Eleven of 15 urban and 15 of 17 villages were totally free from microfilaraemia. Nevertheless, three of 15 surveyed urban localities and two of 17 villages showed >1% Mf rate. Thus, it seems that (i) post-intervention very low levels of microfilaraemia can continue as long as 20 years; (ii) 0.60–0.70% Mf rate is a safe level and at this level recrudescence of infection may not occur; (iii) there can be isolated localities with >1% Mf rate and their detection for further intervention measures could be challenging in larger control/elimination programmes and (iv) the residual infection mostly gets concentrated in the adult population, in underdeveloped urban areas and in historically highly endemic or large endemic rural areas. These groups and areas should be targeted with rigorous intervention measures such as mass drug administration to eliminate the residual infection.