Impact of repeated mass ivermectin administration using a community directed approach on L. loa infection in Chrysops silacea of the rain forest and forest savanna of Cameroon

Abstract

Background: Loiasis is an endemic filarial infection in the rainforest zone of West and Central Africa. Repeated annual community-directed treatment with ivermectin (CDTI) delivered for several years to control onchocerciasis has been shown to reduce the prevalence and intensity of Loiasis in some Loa loa-Onchocerca volvulus co-endemic areas. However, the impact of these multiple rounds of CDTI on entomological indicators of loiasis transmission is not known, and was therefore assessed in this study in areas with contrasting histories of CDTI.

Methods: The study was conducted in the East, North-west and South-west 1 CDTI project sites of Cameroon. Two communities per CDTI project were selected for fly collection and dissection. Ivermectin treatment coverage was documented in these areas, and this was correlated to Chrysops infection and infective rates. A total of 7029 female Chrysops were collected from 6 communities of the 3 CDTI projects (East, North-west, and South-west 1) and from 2 communities in a non-CDTI district (East).

Results: Chrysops biting densities and parous rates were significantly reduced in the North-west and South-west sites post-CDTI, while in the East, biting densities were similar in non-CDTI and CDTI sites, with higher parous rates observed in the non-CDTI site. Infection and infective rates in the East non-CDTI site were 4.4% and 1.8% respectively, as compared to 3.3% and 1.3% in the CDTI site after 10 ivermectin rounds (there were no baseline data for the latter). In the North-west site, significant reductions in Chrysops infection and infective rates from 10.2% and 4.2% respectively, to 3.5% and 1.2 (after 9 rounds of ivermectin treatment), were recorded following CDTI. In the South-west, infection rate significantly increased from 1.74% to 2.8% and infective rate remained statistically unchanged after 14 rounds of CDTI (0.45% - 0.40%). Similar trends in Mean Head L3 were observed except in the East site where this indicator was similar in both CDTI and control sites. Only in the North-west site did monthly transmission potentials decrease significantly.

Conclusion: This study demonstrated that the impact of repeated annual treatment with ivermectin for the control of onchocerciasis using community directed delivery approach on the entomological indicators of loiasis varies with bioecological zones. Community directed treatment with ivermectin induced a significant reduction in the entomological indicators of loiasis in the North-West project site which lies in forest savanna area. A non-significant decrease was observed in the East project site and in contrast, a significant increase was observed in the South-West 1 project site which both lies in the rainforest zones.

Keywords: CDTI; Chrysops; Ivermectin; Loa loa; Microscopy.

Fig. 1

Study sites with fly collection points.

Fig. 2

Chrysops biting densities before and after (A) 10 years of CDTI in the East, (B) 9 years of CDTI in the North-West and (C) 14 years of CDTI in the South-West 1.

Fig. 3

Chrysops infection rate in the East, North-West and South-West 1 CDTI project sites. For the East project, baseline data were not available, but data from a geographically close non-CDTI district are shown. (A) In the East and South-West 1 sites, MHL3 increased after 10 and 14 rounds of CDTI respectively while the contrary occurred in the North-west after 9 rounds of CDTI. (B) Interestingly, the MIBD reduced after varying rounds of CDTI in the East, North-West and South-West 1 sites with a significant difference recorded only in the North-West. (C) The same trend was observed with MTPs in the same study sites with a significant difference still in the North-West only. The Man-Whitney U rank test was used to compare the MBD, MHL3, MIBD and MTP before and after several rounds of CDTI.