No Evidence for Lymphatic Filariasis Transmission in Big Cities Affected by Conflict Related Rural-Urban Migration in Sierra Leone and Liberia

Abstract

BackgroundIn West Africa, the principal vectors of lymphatic filariasis (LF) are Anopheles species with Culex species playing only a minor role in transmission, if any. Being a predominantly rural disease, the question remains whether conflict-related migration of rural populations into urban areas would be sufficient for active transmission of the parasite.Methodology/Principal FindingsWe examined LF transmission in urban areas in post-conflict Sierra Leone and Liberia that experienced significant rural-urban migration. Mosquitoes from Freetown and Monrovia, were analyzed for infection with Wuchereria bancrofti. We also undertook a transmission assessment survey (TAS) in Bo and Pujehun districts in Sierra Leone. The majority of the mosquitoes collected were Culex species, while Anopheles species were present in low numbers. The mosquitoes were analyzed in pools, with a maximum of 20 mosquitoes per pool. In both countries, a total of 1731 An. gambiae and 14342 Culex were analyzed for W. bancrofti, using the PCR. Two pools of Culex mosquitoes and 1 pool of An. gambiae were found infected from one community in Freetown. Pool screening analysis indicated a maximum likelihood of infection of 0.004 (95% CI of 0.00012–0.021) and 0.015 (95% CI of 0.0018–0.052) for the An. gambiae and Culex respectively. The results indicate that An. gambiae is present in low numbers, with a microfilaria prevalence breaking threshold value not sufficient to maintain transmission. The results of the TAS in Bo and Pujehun also indicated an antigen prevalence of 0.19% and 0.67% in children, respectively. This is well below the recommended 2% level for stopping MDA in Anopheles transmission areas, according to WHO guidelines.ConclusionsWe found no evidence for active transmission of LF in cities, where internally displaced persons from rural areas lived for many years during the more than 10 years conflict in Sierra Leone and Liberia.

Increased Biting Rate of Insecticide-Resistant Culex Mosquitoes and Community Adherence to IRS for Malaria Control in Urban Malabo, Bioko Island, Equatorial Guinea

“…Culex mosquitoes are widely distributed in the African continent, and are said to be the predominant mosquitoes in the cities and urban areas in West Africa where they breed in high levels polluted water bodies (Barr 1967, Subra 1981,de Souza et al 2014. Furthermore, they are known vectors of lymphatic filariasis and a variety of arboviruses in East Africa (Simonsen et al 2010, Braack et al 2018.…”

Operation United Assistance: Infectious Disease Threats to Deployed Military Personnel

“…134 The Loa loci vector (Chrysops fly) is also present, but there is a low burden of the disease. 135 In addition, Senegal has reported Borrelia crocidurae in Senlocal egal ticks.136 Lastly, myiasis is associated with the infection caused by the African Tumbu fly (Cordylobia anthropophaga).…”

Testing a method of sampling for entomological determination of transmission of Wuchereria bancrofti to inform lymphatic filariasis treatment strategy in urban settings

“…Currently, urban, rural or peri-urban areas inside implementation units (IUs) are treated based on the IU Transmission Assessment Survey (TAS) result. MDA in urban settings presents significant challenges and costs, and often coverage is inadequate [3][4][5][6][7].…”

“…While presence of LF in urban areas has been demonstrated, transmission of W. bancrofti itself has been a subject of debate [5][6][7]. Where mapping results have led to treatment in urban areas, elimination was achieved with three or fewer yearly rounds of treatment or without recourse to MDA [7] implying the TAS results may not reflect on-going local transmission [6,7].…”

“…Where mapping results have led to treatment in urban areas, elimination was achieved with three or fewer yearly rounds of treatment or without recourse to MDA [7] implying the TAS results may not reflect on-going local transmission [6,7]. In addition, the findings that West African Culex, the most abundant urban mosquitoes appears to be less susceptible to W. bancrofti, have led to the hypothesis that urban cases of LF could be due to transient inward migration from rural areas rather than be the result of local transmission [4,5,7]. Therefore, the existence of significant transmission of W. bancrofti in urban areas needs to be ascertained beyond the currently recommended TAS.…”