Onchocerciasis in the Cameroon-Chad border area after more than twenty years of annual mass ivermectin distribution

doi:10.21203/rs.3.rs-3745872/v1

Download PDF

Research Article

Onchocerciasis in the Cameroon-Chad border area after more than twenty years of annual mass ivermectin distribution

https://doi.org/10.21203/rs.3.rs-3745872/v1

This work is licensed under a CC BY 4.0 License

You are reading this latest preprint version

Background: The main vectors of onchocerciasis in Africa are Simulium damnosum sensu lato, which transmit the causative agent Onchocerca volvulus. The force of transmission is driven by the vector density, hence influencing the disease prevalence and intensity. Onchocerciasis is currently targeted for elimination using Mass Drug Administration (MDA) of ivermectin, a potent microfilaricide. MDA in Cameroon began in 1987 in the Vina valley, an endemic cross-border area with Chad, known for high vector densities and pre-control endemicity. Evaluations in 2008-2010 in this area showed ongoing transmission, meanwhile, border areas in Chad were close to interrupting transmission. This study aimed to evaluate transmission in this area after several rounds of MDA since the last evaluation surveys.

Methods: Blackflies were collected by Human Landing Catches at seven border sites in Cameroon, twice a week, from August 2021 to March 2022. A fraction of the flies was dissected for parity assessment and identification of Onchocerca larval stages. The transmission indices were estimated. Blackfly larvae were also collected from the breeding sites at the fly catching sites and identified to species level by cytotaxonomy.

Results: A total of 14,322 female flies were collected and 6,901 were dissected. Of these 4421 (64.1%) were parous. The total biting rates were high, reaching up to 16,407 bites/man, and transmission potential (3^rd stage larvae (L3) from head/All L3) were 367/702, 146/506, 51/55, 20/32, 0/3, 0/0, and 0/0 infective larvae/man, respectively, for Mbere-Tchad, Babidan, Hajam/V5, Gor, Djeing, Touoro and Koinderi. Infectivity rates (L3 from head) were 16.00, 12.75, 5.15 and 3.00 infective females (L3H)/1000 parous flies for Haijam, Mbere-Tchad, Babidan and Gor, respectively. These values exceed the WHO thresholds of <20 ATP or <1 infective female/1000 parous females. The major vectors identified were Simulium damnosum sensu stricto, S. squamosum, and for the first time in the area, S. yahense.

Conclusions: More than twenty years of MDA has not eliminated onchocerciasis in the study area; hence this area is a potential source of re-introduction of onchocerciasis in Chad and would require alternative treatment strategies. Many factors such as MDA efficiencies, effectiveness of ivermectin and cytospecies composition may be contributing to transmission persistence.

Onchocerciasis

cross-border

Onchocerca volvulus

Simulium damnosum

MDA

Cameroon

Human onchocerciasis also known as river blindness is a debilitating disease with over 220 million people at risk worldwide. The epicentre of the disease is located in Africa [1]. The causative agent of human onchocerciasis is the filarial worm Onchocerca volvulus, which is transmitted by blackflies (genus Simulium), whose larvae develop in fast-running streams and rivers. In Africa, the main vectors which account for transmission of most cases of onchocerciasis belong to the Simulium damnosum complex of sibling species. These are known to disperse an average of 15 to 20km from their breeding site, hence the intensity of the disease is highest in villages 10km or less from a productive breeding site [2].

The burden of human onchocerciasis in Africa has necessitated the formation of large control programs to fight the disease. The Onchocerciasis Control Program (OCP) used large-scale aerial and ground insecticide application against the larval stages of the vectors to control the disease in West Africa. The OCP (1975–2002) successfully controlled onchocerciasis as a public health and socio-economic problem in most of its member countries [3]. The African Programme for Onchocerciasis Control (APOC) (1995–2015), using mainly mass community-directed treatment with ivermectin (CDTI) achieved more than control of onchocerciasis as a public health problem within its participating countries [4]. The initial objective of the APOC saw a paradigm shift from morbidity control to transmission elimination following proof of principle of transmission elimination with ivermectin in some foci in West Africa [5, 6]. The Expanded Special Project for the Elimination of Neglected Tropical Diseases in Africa (ESPEN), was set up in 2016 with responsibility against five preventive chemotherapy NTDs, including onchocerciasis. Amongst ESPEN responsibilities, are to scale up treatment, build health capacity of countries, improve the supply and effective use of the drugs and to increase resource mobilization to accelerate the control and transmission elimination of these diseases [7].

Many foci in former APOC countries have attained interruption/elimination of transmission (including foci in Ethiopia, Sudan, Uganda, Equatorial Guinea (Bioko Island) and Nigeria) using MDA alone or combined with vector control [8]. By 2021, about 2 million people formerly at risk of onchocerciasis in Latin America and Africa had become safe from such risk due to transmission interruption [1, 9]. This figure will likely increase as several countries have stopped MDA and are currently undertaking post-treatment surveillance (PTS) [9].

The 2021–2030 NTD roadmap targets transmission interruption of human onchocerciasis from 12 countries using ivermectin as the main treatment strategy [10]. Ivermectin drastically reduce skin microfilaridermia few days after administration, an effect which lasts for at least ten months, and also temporarily inhibits their release by gravid female worms for several months [11]. This leads to a reduction in intensity and prevalence of infection in the community thereby reducing the number of individuals who could infect the vector [12]. The goal is therefore to administer ivermectin for a period as long as the life span of the gravid female (15 to 17 years), preventing transmission until the natural death of the adult female worm occurs. Elimination of onchocerciasis using MDA requires sustained high annual MDA treatment coverage (> 80% of eligible population) for 15 to 20 years [13, 14]. The North region of Cameroon (precisely the Vina valley -Touboro health district) was one of the first areas in Africa where mass ivermectin distribution trials were carried out by 1987 [15, 16].

Savanna vectors play major role in onchocerciasis transmission in North region of Cameroon; where Simulium damnosum sensu stricto apparently serves as the major vector and S. sirbanum likely plays a secondary role [17]. A small number of larvae of forest vectors -S. squamosum cytotype A and S. mengense - periodically occur in the area [18, 19], possibly contributing to overall transmission. Vector breeding is seasonal in most of the region where the streams and rivers only flow during the rainy season, except for perennial rivers like the Benoue, Vina south and Mbere. Biting rates around the rivers and their tributaries are usually very high [20].

One major challenge with onchocerciasis in West and Central Africa is that the endemic foci are usually large and contiguous cutting across national and international boundaries [21, 22]. This poses a problem when independent projects or country programs on either side of such boundaries show heterogeneity in their progress towards transmission elimination. This is because an area or country can only be certified for transmission elimination when there is minimal risk of re-introduction of onchocerciasis [13]. With the vectors capable of travelling up to 20km, this makes cross-border transmission interruption amongst the main challenges of onchocerciasis elimination in Africa [23].

The North region of Cameroon is characterised by savanna bioecology and consequently the epidemiology of onchocerciasis is mostly manifested by ocular, rather than skin, complications [16]. The region is host to two major hyperendemic foci - Benue basin and Vina-Mbere/Touboro [24]. The Vina-Mbere/Touboro focus is a wide hyperendemic onchocerciasis cross-border focus continuous in Chad and Central African Republic, sustained by the Vina, Mbere and Lim (the three merge to form Logone Occidental) and Pende (called Logone Oriental in Chad) rivers in these countries [25]. Pre-control endemicity in the Vina-Mbere/Touboro focus were extremely high (CMFL of up to 303 mf/s – [26]) compared to levels in all other onchocerciasis endemic areas in the world, with some of such extremely high endemic (holoendemic) villages in Cameroon located close to the Chad border [4, 26].

Mass ivermectin distribution by health personel in the Cameroon side of the Vina-Mbere/Touboro focus began in 1987 [16], however, official records at the Touboro health district for mass drug administration (MDA) with ivermectin are available from 1993 [20]. Communities in the adjacent Rey Bouba and Tcholire health districts have had MDA since 1998. Historically, reported, as well as validated, annual treatment coverages in these districts have been very high, exceeding the 80% threshold required for elimination; validated treatment coverage from 2003 to 2009 ranged between 85.4–94.1% of eligible population [20]. Studies have shown that despite the several rounds, and high coverage, of ivermectin MDA in the Cameroon side of the border, active onchocerciasis transmission is still ongoing [20, 27], while (through MDA) border areas in Chad have either met the stop criteria or are close to doing so [4]. Given that annual MDA has been ongoing since the last evaluations in the area (2008–2010 – [20]), we sought to evaluate the current situation of onchocerciasis transmission in this cross-border area between Cameroon and Chad.

Study site

The study was conducted in seven Chad-Cameroon border sites within three health districts in the North region of Cameroon. Figure 1 and Table 1 show the study sites, their geographical coordinates and population. The choice of study sites was based on previous transmission data, presence of breeding sites and consideration was to select sites across the length of the endemic border between Chad and Cameroon. The area falls within the Sudano-savanna zone with a short unevenly distributed rainy season from July to October and a long dry season from November to June. The main perennial rivers in the study sites are the Mbere and Vina north which meet at the border with Chad to form the West Logone river in Chad. Many seasonal rivers (locally called Mayo) criss-cross the study area, some of which support blackflies breeding when flowing in the rainy season. The main reported vector cytospecies at the different sites are S. damnosum s.s. and small proportion of S. sirbanum, and often with seasonal occurrence of small numbers of S. mengense and S. squamosum (17, 19).

The prevalence and intensity of onchocerciasis in the area generally increases from inland towards the border with Chad [26]. Ivermectin treatment began in villages in the area in 1987 with Babidan amongst the villages that have been treated since 1988 [26]. By 1992, the River Blindness Foundation (RBF) in partnership with the Ministry of Public Health started mass ivermectin distribution program in the North region. Available treatment registers documenting Mass Drug Administration (MDA) in the Touboro Health District were kept from 1993 while those for Tcholire and Rey-Bouba began in 1998 [20]. The African Programme for Onchocerciasis Control (APOC) started funding community-directed treatment with ivermectin (CDTI) projects in the region in 1996.

Collection and dissection of blackflies

Blackflies were collected on two days per week from 7am to 6pm, by two trained volunteers who alternated hourly. Fly collection volunteers sat on a low stool exposing their lower limb. Blood-seeking adult female blackflies were collected using a locally made mouth aspirator, before they begin feeding on the volunteer. Landing rates were therefore considered as biting rates of the blackflies. Collected flies were transferred to the makeshift laboratory at the close of each day’s collection for dissection. A proportion of collected flies were dissected and the rest were preserved in properly labelled bottles containing 90% alcohol for poolscreening. At least 20% of flies for each hour’s collection were dissected and examined for presence of Onchocerca larvae according to procedures described by [28]. Briefly, a drop of physiological solution was placed on each individual flies on a slide, and the abdomen opened to observe the ovaries and Malpighian tubules to determine parity. Nulliparous flies were discarded while parous flies were dissected further in search of larvae of Onchocerca species. Infection with any Onchocerca larval stage (L1, L2 or L3) and the body section (head, thorax or abdomen) where these are observed were recorded on the dissection sheet. When a third stage larva was observed in a fly, the larva was harvested and preserved, alongside parts of the fly’s Malpighian tubules as described by [29] for molecular identification.

Collection and identification of larvae by cytotaxonomy

Collection of blackfly larvae from the breeding sites and identification by cytotaxonomy was performed as described by Ayisi et al. [17]. Briefly, larval developmental (breeding) sites near the sites where adult blackflies were collected were prospected and larvae collected and stored in Carnoy’s solution (3-parts absolute ethanol and 1-part glacial acetic acid) and kept cold until identification by cytotaxonomy. Larvae were dissected, stained with lacto-acetic orcein and the stained polytene chromosomes spread on a glass slide by gently squashing between the glass slide and a cover slip. Species identification was based on chromosome inversions and band characteristics as described by previous authors [18, 30, 31].

Blackfly abundance

In all, 14,322 black flies were collected. The total biting rates (Table 2) were high in most of the communities, with higher rates recorded at Mbere-Tchad (16,407 bites/man/season), Babidan (15,805 bites/man/season) and Gor (9,720 bites/man/season), compared to biting rates at Haijam/V5 (1,554), Touboro (769) and least at Koinderi (186). No Simuliid flies were seen seeking for bloodmeal in the rainy season at the Babidan and Touboro sites. Likewise, none was observed at the Haijam/V5, Djeing and Koinderi sites in the dry season (Additional file 1). The Gor site had flies biting until December but with very few flies in December (81 bites/man/month), whereas flies were caught at Mbere-Tchad throughout the study period with a gradual drop from August to October followed by a slow rise up to January and then a sharp increase to reach a peak in February (9,702 bites/man/month) followed by a drastic drop in March (Fig. 2). Similarly, biting rates in Babidan reached its peak in the month of February and dropped sharply in March. Biting rates in Gor increased from August to September and slightly increased in October before dropping drastically in December. Fluctuation in biting rates for the other productive rainy season sites was slow throughout the rainy season. Only 56 black flies were collected from the Koinderi site in the entire study period.

Transmission potentials and parity rates

6,901 flies were dissected, out of which 4421 (64.1%) were parous. Table 3 presents the entomological parameters, including parity rates, monthly transmission potentials (MTP) and number of infective flies obtained by considering infective flies as those having third stage larvae in the head (L3H). The MTP values were high, with the highest values obtained in January at Babidan (337 infective bites/man/month), followed by February at Mbere-Tchad (238 infective bites/man/month), and then by February at Babidan (169 infective bites/man/month). Except for Mbere-Tchad which recorded MTP values above zero in most of the months in this study (indicating transmission in both seasons), the other sites either had zero MTP values in the dry season (Haijam/V5 and Gor) or in the rainy season (Babidan and Touboro), indicating seasonal transmission. The MTP at Koinderi and Djeing were zero throughout the study period, indicating no infective fly (fly with third stage larva in the head – L3H) was dissected from these sites. The total transmission potential (L3H/All L3) for the study period (Table 2) were, in decreasing order, 367/702, 146/506, 51/55, 20/32, 0/3, 0/0, and 0/0 infective larvae/man/study period, respectively, for Mbere-Tchad, Babidan, Hajam/V5, Gor, Djeing, Touoro and Koinderi. At Mbere-Tchad that had transmission throughout the study period, transmission was higher in the dry season (294 infective bites/man/season) than rainy season (73 infective bites/man/season). Also, the mean seasonal parous rates were high in all the study sites with the highest recorded from rainy season at Koinderi (92.86%) followed by dry season at Mbere-Tchad (86.81%) and then Haijam/V5 rainy season (83.87%). The parity rates of female blackflies at Mbere-Tchad were higher in the dry season (86.81%) than in the rainy season (62.00%).

Effect of recent MDA on transmission

Table 1 shows the study sites and the months during which recent mass ivermectin distribution (MDA) was started in the villages, and the reported therapeutic coverages for some of the sites. Mbere-Tchad had delays between MDAs as it took almost 1 year and 8 months between the MDA in 2020 and that in 2021. Meanwhile, Babidan, Touboro and Koinderi did biannual MDA in 2021. Ivermectin MDA was administered in April 2020 and December 2021 for Mbere-Tchad and in May 2021 and December 2021 for Babidan. Monthly transmission potentials (MTP) for the Mbere-Tchad and Babidan sites increased in January 2022 despite having MDA in December, and it either further increased (Mbere-Tchad) or remained constant (Babidan) in February before dropping rapidly in March (Fig. 3 and Table 3).

Infectivity rates and parasite load

Thirty-three (33) flies were infective, carrying 73 third stage filarial larvae in the head (L3H) which were morphologically indistinguishable from O. volvulus (Table 3). An additional 56 third stage larvae were found in the thorax (L3-thorax) of dissected flies. The infectivity rates ranged from zero at Koinderi, Djeing and Touboro, to 16 infective females (L3H)/1000 parous flies at Haijam. Mbere-Tchad, Babidan and Gor respectively had infectivity rates of 12.75, 5.15 and 3.00 infective females (L3H)/1000 parous flies (Table 2). The parasite loads were 41.21, 29.75, 11.34 and 3.78 L3H/1000 parous flies in Haijam, Mbere-Tchad, Babidan and Gor, respectively, while it was zero for Koinderi, Djeing and Touboro (Table 2). Two flies at Djeing were infected, one containing second stage larvae in thorax (1L2) and the other infected with one third stage and one second stage larva in the thorax (1L3-thorax and 1L2).

Simulium damnosum sensu lato species identified by cytotaxonomy

Three cytoforms of the Simulium damnosum complex (Simulium damnosum s.s., S. squamosum and S. yahense) were identified by cytotaxonomy from the breeding sites near the blackfly collection points (Table 4). One larva from Djeing (Mayo Lidi) could not be classified either as S. damnosum s.s. or S. sirbanum due to the presence of a complex inversion (IIL-C/C.8.3), hence we designated this as S. damnosum/sirbanum. Simulium damnosum s.s. (savanna cytoform) was the dominant cytoform in the study sites, except for the Mbere-Tchad site which was dominated by forest cytoforms - S. yahense and S. squamosum. This is the first report of S. yahense from northern Cameroon.

The objective of this study was to evaluate the status of transmission of human onchocerciasis in Cameroon’s cross-border area with Chad, and to assess any possible risk this area may pose to the elimination program in Chad. Areas in the North administrative region of Cameroon, including the Vina Valley in the Cameroon-Chad border were known for historically extremely high prevalence of human onchocerciasis and very high blackfly biting rates [4, 20, 32] maintained by breeding sites in the Faro, Vina, Mbere and Benoue rivers and their seasonal tributaries. This study has shown that blackfly vectors are still abundant in this area, with definite seasonality in their human biting activities in the study area. Similar to previous observations [17], blackflies biting in the dry season are exclusively concentrated around the major perennial rivers while in rainy season they mostly occur in the seasonal tributaries of these rivers. Hence, we recommend taking this information into consideration when planning for vector surveillance in the region, as well as when scheduling CDTI (as treatment is recommended to be done during peak transmission, which is generally assumed to coincide with peak fly densities).

Our study found that only the Mbere river had blackfly productivity throughout the study period, which peaks in the dry season. This high vector density contributes to the high force of transmission of onchocerciasis in this area. The parity rates of blackflies were also quite high, implying high survival rates of the flies and hence higher tendency to transmit onchocerciasis in their lifetime. In Mbere-Tchad, where blackflies were collected across the two seasons, we observed higher parity rates (86.81%) in the high blackfly productive dry season than the low blackfly productive rainy season (62.00%). This is contrary to expectation that large blackfly productivity should result in lower parity rates, due to large numbers of newly emerged flies in the population [33]. It is possible that some of the biting blackflies caught around this area may be invading flies coming from seasonal streams and rivers which have dried off, as shown in other studies [34].

It is generally considered, in onchocerciasis control, that infective blackflies are those having third stage larvae in the head. The force of transmission (the Annual Transmission Potential – ATP) estimated from such criteria is said to be unable to sustain onchocerciasis transmission when the value is below 20 [13]. Estimates of transmission potentials using L3H in this study exceed this WHO threshold value in the Mbere-Tchad (367), Babidan (146) and Haijam (51), sites within the Touboro health district. The transmission potential at the Gor site (20), in the Tcholire health district, was borderline to the threshold. These results indicate that there is ongoing transmission in the Chad-Cameroon border areas in Cameroon.

It has been suggested that ATP estimated from all third stage larvae (whether in the head or other parts of the insect - L3All) is likely a better indicator of transmission than using only L3 in the head [35]. The previous evaluation done from 2008–2010 in the area [20] reported annual transmission potentials (estimated from all L3 larvae - from thorax and head - and without distinguishing animal or human origin of L3s) of 543.4 infective bites/man/year for sites in the Touboro health district and 20.7 and 40.0 infective bites/man/year for sites within the Tcholire health district. More than a decade after, the present study has recorded still very high transmission potentials calculated from all L3s from sites in the Touboro health district like Mbere-Tchad (702) and Babidan (506). This is despite a shorter fly collection period in the present study (~ 6 months) compared to the 2008–2010 study (1 year). One explanation for this apparently stable rate of transmission (after more than a decade of additional annual MDA) may be due to differences in choice of fly collection sites, given that only one of the sites in previous study (Babidan) was selected in this present study – highlighting the importance of evaluation sites selection. Nevertheless, there may be other factors contributing to the observed results, than just differences in evaluation sites.

The World Health Organization has set a cut-off value for transmission interruption of less than 1 infective blackfly in 1000 infective parous flies (< 1/1000) per transmission zone, assuming a 50% parous rate [13]. The higher infectivity and parous rates recorded in this study indicate that transmission is still ongoing in the area, especially in the Touboro health district. One possible explanation for the delay in progress towards transmission interruption despite decades of ivermectin MDA in the area has been suggested to be due to the initial very high pre-control transmission intensity [4, 20]. This study cannot refute this possibility, although other factors, such as sub-optimal response to ivermectin by the adult worm [36] and inefficiencies in the CDTI process (excluding treatment coverage which historically is higher than the required threshold of 80% – [20]), may equally be contributing to the persistence in transmission. This is supported by the high transmission indices (transmission potential and infectivity rates) recorded in the first two months following ivermectin distribution. These high values suggest three possibilities: (i) the actual transmission rates were very high such that the ivermectin distribution could only successfully reduce the rates to the observed levels; (ii) there were individuals in these communities who still had high skin microfilariae after treatment – either because they missed the recent MDA or they actually participated but were carriers of O. volvulus that responds poorly to ivermectin; (iii) the infective larvae (although morphologically similar to O. volvulus) were coming from animal hosts, hence not affected by treatment of human population. It is equally likely that a combination of these possibilities may be at play in these communities. It is therefore important to carry out further studies to elucidate the factors contributing to the observed high transmission. So far, studies in Cameroon have reported deficiencies in CDTI [37, 38], lower clearance of microfilaridermia and higher repopulation rates [39] as factors contributing to persistent onchocerciasis transmission after several years of MDA.

The parasite loads at Haijam/V5 (41.21) and Babidan (29.75) were relatively high compared to the other sites (11.34 and 3.78 L3H/1000 parous flies for Babidan and Gor). However, the values were closer to those observed by [35] for savanna species (S. damnosum/S. sirbanum), compared to that observed for S. yahense in evergreen forest areas of Liberia. Nonetheless, relatively closer parasite loads (67 and 89) were observed for S. yahense in moist semi-deciduous forest sites in Liberia [35]. The parasite load may therefore not give a good explanation for the cytotaxonomic observation of S. yahense as major vector breeding at the Mbere-Tchad site. However, it may be that the vectorial capacity of this forest vectors is affected by the disease pattern as related to forest and savanna ecological zones, or that both species (forest and savanna) are presently contributing to transmission at these two close-by sites (Haijam/V5 and Mbere-Tchad), reason why their parasite loads are relatively higher. In any case, the observation of S. yahense, a cytospecies that has never been reported in this area, calls for more vigilance in the onchocerciasis control/elimination drive for Cameroon and Chad.

This study has shown that more than twenty years of mass ivermectin distribution around the Chad border areas in Cameroon has not interrupted onchocerciasis transmission. Based on WHO recommendations, the Chad programme cannot be certified to have achieve transmission elimination unless this border transmission situation in Cameroon is resolved. The initial very high force of transmission may be one reason for the persistence in transmission, but we suggest that sub-optimal response of O. volvulus to ivermectin, inefficient CDTI implementation (e.g using several months to complete a single round of MDA in an area) and vectorial factors may also be contributing to the present situation. We therefore recommend that these possible confounding factors be analysed further to identify any role they may be playing in order to mitigate the situation. This study represents the first ever report of Simulium yahense from northern Cameroon, occurring around the Chad border. This forest blackfly cytoform, which has been associated with blinding onchocerciasis, may be playing a significant role in the observed transmission in the area. Meanwhile, we propose that coupling ground larviciding to current interventions may be helpful given the high biting rates and seasonality of transmission in the area.

ATP

Annual Transmission Potential

WHO

World Health Organisation

CDTI

Community-Directed Treatment with ivermectin

MDA

Mass Drug Administration

OCP

Onchocerciasis Control Program

APOC

African Programme for Onchocerciasis Control

ESPEN

Expanded Special Project for the Elimination of Neglected Tropical Diseases

NTDs

Neglected Tropical Diseases

PTS

Post-Treatment Surveillance

CMFL

Communtiy Microfilarial Load

MTP

Monthly Transmission Potential

L3H

Infective larva in the head.

Acknowledgment

We are grateful to the Ministry of Public Health, Cameroon, the Head of the National NTDs Programme – Dr. Nko’Ayissi George, and to the North Regional Delegation of Public Health. We are indebted to the community volunteers and community leaders, and the Directors of the three Health Districts – Touboro, Tcholire and Rey Bouba. We sincerely acknowledge the German Academic Exchange Service (DAAD) for funding the first author’s PhD studies at the University of Ghana.

Author contributions

FA was involved in all aspects of the study and wrote the manuscript. DAB contributed to study design, acquisition of data and reviewing of draft manuscripts. DKD, JT, EBF and BDB contributed in acquisition of data and reviewing of draft manuscripts. All authors read and approved the final manuscript.

Funding

This study was funded through funds received by FA from the END Fund to carry out his PhD thesis research.

Availability of data and materials

The datasets analysed during the current study are available from the corresponding author on reasonable request.

Ethics approval and consent to participate

Administrative Research Authorization was granted by the respective Regional Delegates of Public Health for the larval survey after reading and approving the protocol. The Ethics Committee of the Ministry of Public Health of Cameroon have validated all the activities of the National Onchocerciasis Control Programme (NOCP) within the framework of control and elimination of the disease. In addition, the protocol for human landing collection of adult black flies was read and approved by the Committee for the Control of Health Delivery and Related Activities of the North Regional Delegation of Public Health (Ref. 0550/AR/D/DRSP/BCASS/CN^o1/21). Informed consent was sought from all vector collectors before the commencement of the study. The purpose and schedules for the study was explained to the collectors and those who agreed to serve as collectors signed the informed consent form. Fly collectors were free to opt out any time without prior notification of the research team.

Consent for publication

Not applicable

Competing interests

The authors declare that they have no competing interests

WHO. Onchocerciasis – key facts. World Health Organization; 2023. Site: https://www.who.int/news-room/fact-sheets/detail/onchocerciasis.
Noma M, Nwoke BE, Nutall I, Tambala PA, Enyong P, Namsenmo A, Remme J, Amazigo UV, Kale OO, Seketeli A. Rapid epidemiological mapping of onchocerciasis (REMO): its application by the African Programme for Onchocerciasis Control (APOC). Ann Trop Med Parasitol, 2002;96(Suppl 1):S29-39.
Crump A, Morel CM, Omura S. The Onchocerciasis Chronicle: From the Beginning to the End? Trends Parasitol. 2012;28:9. doi: 10.1016/j.pt.2011.10.001
Tekle AH, Zouré HGM, Noma M, Boussinesq M, Coffeng LE, Stolk WA, Remme JHF. Progress towards onchocerciasis elimination in the participating countries of the African Programme for Onchocerciasis Control: epidemiological evaluation results. Infectious Diseases of Poverty. 2016;5:66. https://doi.org/10.1186/s40249-016-0160-7
WHO/APOC. Conceptual and Operational Framework of Onchocerciasis Elimination with Ivermectin Treatment. WHO/APOC/MG/10.1. World Health Organization; 2010.
Diawara L, Traoré MO, Badji A, Bissan Y, Doumbia K, Goita SF, et al. Feasibility of onchocerciasis elimination with ivermectin treatment in endemic foci in Africa: first evidence from studies in Mali and Senegal. PLoS Negl Trop Dis. 2009;3:e497.
WHO/ESPEN. The Expanded Special Project for Elimination of Neglected Tropical Diseases (ESPEN) – What we do. World Health Organization; 2023. Site: https://espen.afro.who.int/about-espen/what-we-do
Lakwo T, Oguttu D, Ukety T, Post R, Bakajika D. Onchocerciasis Elimination: Progress and Challenges. Res. rep. trop. med. 2020;11:81–95. https://doi.org/10.2147/RRTM.S224364
WHO. Elimination of human onchocerciasis: progress report, 2021. Weekly Epidemiological Record, No 46.World Health Organization; 2022. Site: https://apps.who.int/iris/bitstream/handle/10665/364608/WER9746-591-598-eng-fre.pdf?sequence=1&isAllowed=y.
WHO. Ending the neglect to attain the Sustainable Development Goals – A road map for neglected tropical diseases 2021–2030. Licence: CC BY-NC-SA 3.0 IGO. Geneva: World Health Organization; 2020.
Basanez M-G, Pion SD, Boakes E, Filipe JA, Churcher TS, Boussinesq, M. Effect of single-dose ivermectin on Onchocerca volvulus: a systematic review and meta-analysis. Lancet Infect Dis. 2008; 8:310–322. doi:10.1016/S1473-3099(08)70099-9
Cupp EW, Sauerbrey M, Richards F. Elimination of human onchocerciasis: History of progress and current feasibility using ivermectin (Mectizan®) monotherapy. Acta Tropica, 2011;120(Suppl 1): S100-108.https://doi.org/10.1016/j.actatropica.2010.08.009
WHO. Guidelines for Stopping Mass Drug Administration and Verifying Elimination of Human Onchocerciasis, Criteria and Procedures. WHO/HTM/NTD/PCT/2016.1. World Health Organization; 2016.
Bockarie MJ, Kelly-Hope LA, Rebollo M, Molyneux DH. Preventive chemotherapy as a strategy for elimination of neglected tropical parasitic diseases: endgame challenges. Philos Trans R Soc London B. 2013; 368:20120144.
Meredith SEO, Cross C, Amazigo UV. Empowering communities in combating river blindness and the role of NGOs: case studies from Cameroon, Mali, Nigeria, and Uganda. Health Res. Policy Syst. 2012;10(1); doi:10.1186/1478-4505-10-16.
Prod’hon J, Boussinesq M, Fobi G, Prud’hom JM, Enyong P, Lafleur C, Quillévéré D. Lutte contre l’onchocercose par ivermectine: résultat d’une campagne de masse au Nord-Cameroun. Bulletin de l’Organisation mondiale de la Santé. 1991;69:443–450.
Ayisi F, Sedou N, Dieunang SK, Yaya F, Tchago EF, Ndellejong CE, et al. A cross-sectional study of Simulium damnosum sensu lato breeding sites and species distribution in Sudan savanna, mixed savanna–forest and rainforest regions in Cameroon. Parasit Vectors. 2022; 15:382. https://doi.org/10.1186/s13071-022-05462-w
Traore-Lamizana M, Somiari S, Mafuyai HB, Vajime CG, Post RJ. Sex chromosome variation and cytotaxonomy of the onchocerciasis vector Simulium squamosum in Cameroon and Nigeria. Med Vet Entomol. 2001;15:219–23.
Traoré-Lamizana M. & Lemasson J-J. Participation à une étude de faisabilité d’une campagne de lutte contre l’onchocercose dans la région du bassin du Logone. Répartition des espèces du complexe Simulium damnosum dans la zone camerounaise du projet. Cah. ORSTOM sér. Ent. méd. et Parnsitol. 1987; 25: 171–186.
Katabarwa MN, Eyamba A, Nwane P, Enyong P, Yaya S, Baldiagaï J, et al. Seventeen Years of Annual Distribution of Ivermectin Has Not Interrupted Onchocerciasis Transmission in North Region, Cameroon. Am. J. Trop. Med. Hyg. 2011; 85:1041–1049.
Noma M, Zouré HGM, Tekle AH, Enyong PAI, Nwoke BEB, Remme JHF. The geographic distribution of onchocerciasis in the 20 participating countries of the African Programme for Onchocerciasis Control: (1) priority areas for ivermectin treatment. Parasit Vectors. 2014;7:325.
Zouré HGM, Noma M, Tekle AH, Amazigo UV, Diggle PJ, Giorgi E, Remme JHF. The geographic distribution of onchocerciasis in the 20 participating countries of the African Programme for Onchocerciasis Control: (2) pre-control endemicity levels and estimated number infected. Parasit Vectors. 2014;7:326.
Gebrezgabiher G, Mekonnen Z, Yewhalaw D, Hailu A. Reaching the last mile: main challenges relating to and recommendations to accelerate onchocerciasis elimination in Africa. Infect. Dis. Poverty. 2019;8:60. DOI: doi.org/10.1186/s40249-019-0567-z.
Macé JM, Boussinesq M, Ngoumou P, Oye JE, Koéranga A, Godin C. Country-wide rapid epidemiological mapping of onchocerciasis (REMO) in Cameroon. Ann. trop. med. parasitol. 1997; 91:379–391.
Boussinesq M. Répartition de l’onchocercose dans les états de l’OCEAC: Revue de la littérature. Bulletin de liaison et de documentation de l'OCEAC. 1991;98:37–55.
Boussinesq M, Prodqhon J, Chippaux J. Onchocerca volvulus: striking decrease in transmission in the Vina valley (Cameroon) after eight annual large scale ivermectin treatments. Trans R Soc Trop Med Hyg. 1997;91:82–6.
Eisenbarth A, Achukwi MD, Renz A. Ongoing Transmission of Onchocerca volvulus after 25 Years of Annual Ivermectin Mass Treatments in the Vina du Nord River Valley, in North Cameroon. PLOS Negl. Trop. Dis. 2016;10:1–16.
Davies JB & Crosskey RW. Simulium - vectors of onchocerciasis. Geneva: World Health Organization, Division of Control of Tropical Diseases. 1991.
Toe L, Back C, Adjami AG, Tang JM, Unnasch TR. Onchocerca volvulus: comparison of field collection methods for the preservation of parasite and vector samples for PCR analysis. Bull World Health Organ. 1997;75:443–7.
Post RJ, Onyenwe E, Somiari SAE, Mafuyai HB, Crainey JL, Ubachukwu PO. A guide to the Simulium damnosum complex (Diptera: Simuliidae) in Nigeria, with a cytotaxonomic key for the identification of the sibling species. Ann Trop Med Parasitol. 2011;105:277–97.
Boakye DA. A pictorial guide to the chromosomal identification of members of the Simulium damnosum Theobald complex in West Africa with particular reference to the Onchocerciasis Control Programme Area. Trop Med Parasitol. 1993; 44:223–44.
Hougard JM, Escaffre H, Lochouam L, Le Goff G, Prud'hom JM, Quillévéré D. Lutte antivectorielle contre l'onchocercose de savane dans le bassin de la Bénoué (région de Poli -Nord Cameroun). Bull. liais. dac. OCEAC. 1988; 86:79–81.
Toé LD, Koala L, Burkett-Cadena ND, Traoré BM, Sanfo M, Kambiré SR, et al. Optimization of the Esperanza window trap for the collection of the African Onchocerciasis vector Simulium damnosum sensu lato. Acta Trop. 2014;137: 39–43.
Koala K, Nikièma AS, Paré AB, Drabo F, Toé LD, Belem AMG, et al. Entomological assessment of the transmission following recrudescence of onchocerciasis in the Comoé Valley, Burkina Faso. Parasit Vectors, 12:34; doi.org/10.1186/s13071-019-3290-5.
Cheke RA, Garms R. Indices of onchocerciasis transmission by different members of the Simulium damnosum complex conflict with the paradigm of forest and savanna parasite strains. Acta Tropica. 2013;125:43–52.
Osei-Atweneboana MY, Awadzi K, Attah SK, Boakye DA, Gyapong JO, Prichard RK. Phenotypic Evidence of Emerging Ivermectin Resistance in Onchocerca volvulus. PLoS Negl Trop Dis. 2011;5(3): e998. https://doi.org/10.1371/journal.pntd.0000998
Kamga GR, Dissak-Delon FN, Nana-Djeunga HC, Biholong BD, Ghogomu SM, Souopgui J, et al. Audit of the community-directed treatment with ivermectin (CDTI) for onchocerciasis and factors associated with adherence in three regions of Cameroon. Parasites Vectors. 2018;11:356 https://doi.org/10.1186/s13071-018-2944-z
Senyonjo L, Oye J, Bakajika D, Biholong B, Tekle A, Boakye D, Schmidt E, Elhassan E. Factors Associated with Ivermectin Non-Compliance and Its Potential Role in Sustaining Onchocerca volvulus Transmission in the West Region of Cameroon. PLoS Negl. Trop. Dis. 2016;10, e0004905. https://doi.org/10.1371/journal.pntd.0004905
Abong RA, Amambo GN, Ndongmo PWC, Njouendou AJ, Ritter M, Beng AA, et al. Differential susceptibility of Onchocerca volvulus microfilaria to ivermectin in two areas of contrasting history of mass drug administration in Cameroon: relevance of microscopy and molecular techniques for the monitoring of skin microfilarial repopulation within six months of direct observed treatment. BMC Infect Dis. 2020; 20:726. https://doi.org/10.1186/s12879-020-05444-2

Table 1: Study sites, population, geographical coordinates and month of the recent MDA activity prior or during the period of this study

Health District	Village	*Population	Geographical coordinates	Recent MDA activity dates
Touboro	Haijam/V5	3,764	7.57717^oN, 15.45432^oE	June 2021
	Mbere-Tchad	1,389	7.56532^oN, 15.53692^oE	April 2020 & December 2021
	Babidan	532^§	7.78244^oN, 15.52055^oE	May & December 2021
	Touboro (Sodecoton)	2,124	7.74654^oN, 15.36103^oE	May & December 2021
Tcholire	Gor (Goindassou 1)	1,086	8.66768^oN, 14.98278^oE	April 2021
Tcholire	Djeing	1,137	8.42090^oN, 15.13168^oE	April 2021
Rey Bouba	Koinderi	1,284	8.97749^oN, 14.47348^oE	March & November 2021

*Population figures are from 2016 population census done by Community Drug Distributors (CDD) in preparation for MDA (Mass Drug Administration). ^§Census figures for 2015 - because 2016 census figures were far lower. In Touboro and Gor, which are very large villages, the population mentioned here are those of the quarter/neighbourhood closest to the black fly breeding river.

Table 2: Simulium damnosum complex vector species identified by cytotaxonomy

Site	Biting rate (bites/man/period)	Parity (%)	Infective females (L3H)/1000 parous flies	Infective larvae (L3H)/1000 parous flies	TP – L3H(L3All) (infective larvae/man/period)
Haijam/V5	1,554	83.87	16 12.75 5.15 0 3 0 0	41 29.75 11.34 0 3 0 0	51(55) 367(702) 146(506) 0(0) 20(32) 0(3) 0(0)
Mbere-Tchad	16,407	75.67
Babidan	15,805	66.26
Touboro	769	56.36
Gor	9,720	55.85
Djeing	2,148	39.43
Koinderi	186	92.86

Note: L3All= All 3rd stage larvae retrieved from both thorax and head of the fly; L3H= 3rd stage larvae retrieved from the head of the fly. Period = the entire period of the study. Mean values do not include months that no fly was caught. Transmission Potential (TP) was calculated by adding the individual monthly transmission potentials of the months covered in this study (Table 3). Biting rate (BR) is calculated as the sum of the individual monthly biting rates of the months covered in this study.

Table 3: Transmission indices of the study sites

Monthly Transmission Potential (MTP)/N^o of infective flies

N^o dissected

Rainy season

Dry season

Site

Parous rate %

Aug-21

Sept-21

Oct-21

Total

Parous rate %

Dec-21

Jan-22

Feb-22

Mar-22

Total

Haijam/

434

83.87

27.1/3

24/3

51.1/6

0/0

Mbere-Tchad

1,866

62.00

3.9/1

42/5

27.1/2

73/8

86.81

0/0

32.3/2

238.8/7

22.9/1

294.0/10

Babidan

1,464

0/0

66.26

0/0

337.1/3

169.7/2

0/0

506.8/5

Touboro

110

0/0

56.36

0/0

Gor

2,370

41.69

7.8/0

8.0/2

17.0/2

32.7/4

70.00

0/0

Djeing

601

39.43

0/0

Koinderi

92.86

0/0

Total

6,901

156.8/18

800.8/15

MTP values were calculated from L3 larvae from the head (L3H) which were morphologically indistinguishable from Onchocerca volvulus.

Table 4: Cytospecies of Simulium damnosum sensu lato in the study sites

Site/River	Species (Number of larvae)
Haijam(V5)/Ebe	S. damn s.s (06)
Mbere-Tchad/Mbere	S. yahense (12), S. squam (02), S. damn s.s (01)
Babidan/ Vina south	S. damn s.s (11), S. sirb (02), S. damn/sirb (02)
Touboro/ Vina south	S. damn s.s (10)
Gor/Gelcoma	S. damn s.s (17)
Djeing/Lidi	S. damn s.s (10), S. damn/sirb (01)

Note: S. damn s.s = Simulium damnosum sensu stricto; S. squam = S. squamosum; S. damn/sirb = Simulium damnosum/sirbanum. No S. damnosum larvae were found at the Koinderi site.

Supplementary File 1 is not available with this version

No competing interests reported.

Download PDF

Editorial decision: Revision requested
12 Feb, 2024
Reviewers agreed at journal
25 Dec, 2023
Reviews received at journal
20 Dec, 2023
Reviewers agreed at journal
15 Dec, 2023
Reviewers invited by journal
15 Dec, 2023
Editor assigned by journal
13 Dec, 2023
Submission checks completed at journal
13 Dec, 2023
First submitted to journal
12 Dec, 2023

You are reading this latest preprint version

Onchocerciasis in the Cameroon-Chad border area after more than twenty years of annual mass ivermectin distribution

Status:

Version 1

Abstract

Figures

Background

Methods

Study site

Collection and dissection of blackflies

Collection and identification of larvae by cytotaxonomy

Results

Blackfly abundance

Transmission potentials and parity rates

Effect of recent MDA on transmission

Infectivity rates and parasite load

Discussion

Conclusions

Abbreviations

Declarations

References

Tables

Additional Files

Additional Declarations

Status:

Version 1