Impact of repeated ivermectin treatments against onchocerciasis on the transmission of loiasis: an entomologic evaluation in central Cameroon

Abstract

BackgroundAnnual community-directed treatment with ivermectin (CDTI) have been carried out since 1999 in the Lekie division (central region of Cameroon where most cases of Loa-related post ivermectin severe adverse events were reported) as part of the joined activities of the African Programme for Onchocerciasis Control (APOC) and Mectizan® Donation Program (MDP). As large-scale administration of ivermetine was demonstrated to be an efficient means to control loiasis transmission, it was hypothesized that CDTI would have lowered or halted the transmission of Loa loa in the Lekie division after 13 years of annual drug administration, indicating a possible reduction in the occurrence of Loa-related post-ivermectin severe adverse events.MethodsA 4-month entomologic study was carried out from March to June 2012 in the Lekie division to evaluate the impact of 13 years of CDTI on the transmission of L. loa whose baseline data were recorded in 1999–2000.ResultsThere was a significant reduction in the infection rate for Chrysops silacea and C. dimidiata from 6.8 and 9% in 1999–2000 to 3 and 3.6% in 2012, respectively. The differences in the infective rate (IR) (percentage of flies harboring head L3 larvae), potential infective rate (PIR) (percentage of flies bearing L3 larvae), mean head L3 larvae load (MHL3) (average L3 per infective fly) and mean fly L3 larvae load (MFL3) (average L3 per potentially infective fly) for both C. silacea and C. dimidiata were not significantly different between the two investigation periods. The biting density (BD) was almost three-fold higher in 2012 for C. silacea but not for C. dimidiata. The transmission potential (TP) which is a function of the BD, was higher in the present study than in the baseline investigation for each species.ConclusionThe infection rate remaining high, the high TP and the stability observed in the IR, PIR, MHL3 and MFL3 after 13 years of CDTI suggest that transmission of L. loa is still active. This is an indication that the risk of occurrence of severe adverse events such as fatal encephalopathies is still present, especially for heavily microfilaria-loaded people taken ivermectin for the first time.

Loa loa vectors Chrysops spp.: perspectives on research, distribution, bionomics, and implications for elimination of lymphatic filariasis and onchocerciasis

“…In total, 89 published documents with information on the two main L. loa vectors C. silacea and C dimidiata were collated into a database (see Additional file 1) [19–103]. The number of articles published per decade ranged from 0 to 37, with the highest number published in the 1950s (Fig.…”

“…under a microscope was the only method used for detecting infection, which involved separating the head, thorax and abdomen manually, and identifying the presence (parous) or absence (nulliparous) of L. loa larva [56, 99]. Transmission was related to the frequency of L3 found in the head of the flies and the biting density of vectors with the main measures including (i) parous rates (PR) estimated as the proportion of parous flies to the total number dissected; (ii) potential infection rates (PIR) estimated as proportion of flies with L3s; (iii) infective rates (IR) determined as the proportion of flies with L3s in the head [90, 99, 103]. …”

The Population Biology and Transmission Dynamics of Loa loa

“…Without suitably constructed mathematical models, the impact of these strategies on the transmission dynamics of loiasis will remain unclear and amenable only to informal and less rigorous analysis. For example, it is known that community-directed treatment with ivermectin can effect significant reductions in L. loa prevalence, both in humans and in the Chrysops vectors [94,95]. But the impact of this intervention on loiasis transmission among diverse transmission settings, and the prospects of elimination, remain unclear.…”

The African Chrysops

“…Whether the natural or artificial vegetation, presence of the canopy seems to be the most important criteria for Chrysops to settle, as this offers a resting place from where the host is spotted [2]. The artificial vegetation found as suitable ecological area include cacao farms, crop fields, mixed crop fields and inhabited areas [9,16], regenerated forest [17] and commercial plantations such as rubber tree plantations [2]. Some but few species also live in savanna areas, as is the case for Chrysops flavipes collected in the Sinai in Egypt [12].…”

“…These wounds are potential entry doors for many pathogens. African Chrysops are vectors of L. loa in human and Loa papionis in monkeys [2,9,22]. Due to the biting habit of Chrysops whereby several hosts are often necessary to feed to repletion, they are responsible for the mechanical transmission of diseases [23].…”