Date Published: March 18, 2015
Publisher: Public Library of Science
Author(s): Maria P. Rebollo, Sana Malang Sambou, Brent Thomas, Nana-Kwadwo Biritwum, Momodou C. Jaye, Louise Kelly-Hope, Alba Gonzalez Escalada, David H. Molyneux, Moses J. Bockarie, Peter Uwe Fischer. http://doi.org/10.1371/journal.pntd.0003642
Abstract: BackgroundThe prevalence of Wuchereria bancrofti, which causes lymphatic filariasis (LF) in The Gambia was among the highest in Africa in the 1950s. However, surveys conducted in 1975 and 1976 revealed a dramatic decline in LF endemicity in the absence of mass drug administration (MDA). The decline in prevalence was partly attributed to a significant reduction in mosquito density through the widespread use of insecticidal nets. Based on findings elsewhere that vector control alone can interrupt LF, we asked the question in 2013 whether the rapid scale up in the use of insecticidal nets in The Gambia had interrupted LF transmission.Methodology/Principal FindingWe present here the results of three independently designed filariasis surveys conducted over a period of 17 years (1997–2013), and involving over 6000 subjects in 21 districts across all administrative divisions in The Gambia. An immunochromatographic (ICT) test was used to detect W. bancrofti antigen during all three surveys. In 2001, tests performed on stored samples collected between 1997 and 2000, in three divisions, failed to show positive individuals from two divisions that were previously highly endemic for LF, suggesting a decline towards extinction in some areas. Results of the second survey conducted in 2003 showed that LF was no longer endemic in 16 of 21 districts surveyed. The 2013 survey used a WHO recommended LF transmission verification tool involving 3180 6–7 year-olds attending 60 schools across the country. We demonstrated that transmission of W. bancrofti has been interrupted in all 21 districts.ConclusionsWe conclude that LF transmission may have been interrupted in The Gambia through the extensive use of insecticidal nets for malaria control for decades. The growing evidence for the impact of malaria vector control activities on parasite transmission has been endorsed by WHO through a position statement in 2011 on integrated vector management to control malaria and LF.
Partial Text: The Gambia is among 73 countries currently considered endemic for lymphatic filariasis (LF) by the World Health Organization . LF, a neglected tropical disease (NTD), is a debilitating mosquito-borne nematode infection that affects 120 million people in low and middle income countries where 1.4 billion people are exposed to the parasites . Wuchereria bancrofti, the causative agent of LF in The Gambia, is responsible for over 90% of the LF infections worldwide; Brugia malayi and Brugia timori account for the remaining infections and have a distribution restricted to the southeast Asian region . The LF parasites are carried by various species of mosquito vectors from the genera Anopheles, Aedes, Culex and Mansonia but in sub-Saharan Africa, Anopheles species are the principal vectors. There is no evidence that Culex species play any significant role in West Africa where the malaria vectors, An. gambiae s.l and An. funestus, are also the vectors of W. bancrofti [3–9]. In 1997, LF was targeted for elimination when the World Health Assembly adopted Resolution WHA 50.29 calling for the elimination of the disease as a public health problem globally . In 2000, WHO, in collaboration with pharmaceutical companies and implementing partners launched the Global Programme to Eliminate LF (GPELF) as a disease specific intervention initiative to interrupt transmission and alleviate morbidity. The GPELF has two strategic objectives for achieving this goal: 1) interruption of parasite transmission through mass drug administration (MDA) using albendazole in combination with either ivermectin or diethylcarbamazine citrate (DEC) and 2) morbidity management and disability prevention (MMDP) by providing access to care for those who suffer clinical manifestations of LF in endemic areas.
The TAS performed in this study verified the absence of transmission in all administrative divisions that were previously highly endemic for LF. The prevalence of W.bancrofti in The Gambia was among the highest in Africa, based on historical data [20, 21, 30–32] and reported in the first global atlas of LF compiled by Michael and Bundy in 1997  and the African LF risk map of Lindsay and Thomas . Village specific MF rates reported in studies carried out in the 1950s, among people 10 years or older, varied between 24.1% and 48.4% [20, 31, 32] and Hawking[4, 5] later reported that the prevalence of LF among adults during the early 1950s was about 50%. Based on our current knowledge of the relationship between MF rates and antigen prevalence such figures suggest that the majority of adults were infected with the parasite in the 1950’s. Night blood surveys conducted in 15 villages in 1975 and 1976 revealed MF rates between 2.9% and 26.9% among adults ≥15 years . The corresponding high prevalence of LF morbidity observed in children and adults in The Gambia in the 1970s confirmed that transmission had continued in the twenty years since the first surveys in the 1950s . In adults older than 15 years, morbidity presented in the form of adenolymphangitis (13.8%–20.0%), lymphoedema (5.3%–10.0%) and hydrocele (11.7%–35.3%) . Late filarial disease in the form of elephantiasis and hydrocoele was also observed in children younger than 15 years  suggesting early exposure and high levels of transmission.