The esperanza window trap reduces the human biting rate of Simulium ochraceum s.l. in formerly onchocerciasis endemic foci in Southern Mexico

Community-Directed Vector Control to Accelerate Onchocerciasis Elimination

Onchocerciasis, or river blindness, has historically been one of the most important causes of blindness worldwide, and a major cause of socio-economic disruption, particularly in sub-Saharan Africa. Its importance as a cause of morbidity and an impediment to economic development in some of the poorest countries in the world motivated the international community to implement several programs to control or eliminate this scourge. Initially, these involved reducing transmission of the causative agent Onchocerca volvulus through controlling the vector population. When ivermectin was found to be a very effective drug for treating onchocerciasis, the strategy shifted to mass drug administration (MDA) of endemic communities. In some countries, both vector control and ivermectin MDA have been used together. However, traditional vector control methods involve treating rivers in which the black fly vectors breed with insecticides, a process which is expensive, requires trained personnel to administer, and can be ecologically harmful. In this review, we discuss recent research into alternatives to riverine insecticide treatment, which are inexpensive, ecologically less harmful, and can be implemented by the affected communities themselves. These can dramatically reduce vector densities and, when combined with ivermectin MDA, can accelerate the time to elimination when compared to MDA alone.

Trends in black fly density, parity and infection rates from riverside to villages of the Bafia Health District in Cameroon: implication for onchocerciasis vector control

BACKGROUND

Reducing contact between humans and black flies can lead to interruption of onchocerciasis transmission. The Esperanza Window Trap (EWT) has been shown to be an effective tool for reducing black fly densities. Several shape-based improvements to this trapping system have been made to optimise its effectiveness, but optimisation of this trapping system has been based most often on the shape of the trap, collection in areas of high black fly density and the addition of attractants, without considering transmission potentials and parity rates. This study aims to investigate the differences in biting rates and transmission potential between three catch points along a transect to guide the choice of EWT placement.

METHODS

Monthly black fly collection was carried out over a 1-year study period using the human landing method at three catch points along a transect from the riverside toward the centre of two first-line villages (Biatsota and Bayomen), in the Mbam valley in Cameroon. All female black flies caught were counted and dissected, and entomological indicators were computed and compared between the catch points and villages.

RESULTS

A total of 80,732 black flies were caught, of which 57,517 were dissected; of the latter, 2743 (4.8%) were parous and 44 (1.6%) were infective. Regarding the distance to the river, a vector density gradient was observed, with the highest annual biting rates being recorded at the riverside. The highest annual transmission potentials were also recorded at the riverside (165 vs 255 infective larvae/man/year in Bayomen and Biatsota, respectively). Overall, the highest parity rates were recorded at the riverside in Biatsota (5.1%) where various human activities are frequent and at the centre of Bayomen village (6.3%).

CONCLUSION

The results of this study reveal that entomological parameters were the highest at the riverside catch sites and indicate that riverside locations should be prioritised for EWTs or other trapping systems to achieve optimal performance in onchocerciasis control.

Two Nigerian States (Plateau and Nasarawa) Have Eliminated Transmission of Human Onchocerciasis-A Report of Post-Ivermectin Mass Drug Administration Surveillance

Transmission of Onchocerca volvulus (causing "river blindness") was interrupted in two states of Nigeria (Plateau and Nasarawa) in 2017 in accordance with 2016 WHO guidelines. Ivermectin mass drug administration was halted in January 2018, and posttreatment surveillance activities were conducted over a 3-year period. Vector Simulium damnosum s.l. flies were collected during the 2019 (39 sites) and 2020 (42 sites) transmission seasons. Head pools were tested by polymerase chain reaction for the presence of third-stage O. volvulus larvae; 15,585 flies were all negative, demonstrating an infective rate of < 1/2,000 with 95% confidence. In 2021, the Nigerian Federal Ministry of Health declared the two-state area as having eliminated transmission. Plateau and Nasarawa states are the first of 30 endemic states in Nigeria to have met the WHO criteria for onchocerciasis elimination. Post-elimination surveillance will need to continue given the risk of reintroduction of transmission from neighboring states.

Onchocerciasis Elimination: Progress and Challenges

Onchocerciasis is a parasitic infection caused by the filarial nematode Onchocerca volvulus and transmitted through the bites of black flies of the genus Similium that breed in rivers and streams. The impact of mass treatment with ivermectin and supplemented by vector control in some countries has changed the global scene of onchocerciasis. There has been reported progress made in elimination of onchocerciasis in central and southern American countries and in some localities in Africa. The target for elimination in the Americas has been set at 2022 while for 12 countries in Africa this is expected in 2030. This review was conducted to examine the current status of onchocerciasis elimination at the global level and report on progress made. Literature searches were made through PubMed, articles in English or English abstracts, reports and any other relevant articles related to the subject. The global burden of onchocerciasis is progressively reducing and is no longer a public health problem in some regions. However, programs are challenged with a range of issues: cross-border transmission, diagnostic tools, Loa loa co-endemicity, limited workforce in entomology and maintaining enthusiasm among community drug distributors. More concerted effort using appropriate tools is required to overcome the challenges.

Identification of Human-Derived Attractants to Simulium damnosum Sensu Stricto in the Madi-Mid North Onchocerciasis Focus of Uganda

Human landing collections (HLCs) have been the standard method for the collection of black flies that serve as vectors for Onchocerca volvulus, the causative agent of onchocerciasis or river blindness. However, HLCs are inefficient and may expose collectors to vector-borne pathogens. The Esperanza window trap (EWT) has been shown to be a potential alternative to HLCs for the collection of Simulium damnosum, the principal vector of O. volvulus in Africa. To improve the performance of the EWT, sweat from individuals highly attractive or less attractive to S. damnosum sensu stricto was examined by gas chromatography and mass spectroscopy. Twelve compounds were identified which were solely present or present in increased amounts in the sweat of the highly attractive individuals. Two of these compounds (naphthalene and tert-hexadecyl mercaptan) were found to be attractive to S. damnosum s.s. in behavioral assays. Traps baited with these compounds outperformed those baited with the current standard bait of worn socks. Using these newly identified compounds as baits will make the EWT more efficient in collecting vector black flies and may enhance the potential utility of the EWT as a local vector control measure.

The Interruption of Transmission of Human Onchocerciasis by an Annual Mass Drug Administration Program in Plateau and Nasarawa States, Nigeria

Plateau and Nasarawa states in central Nigeria were endemic for onchocerciasis. The rural populations of these two states received annual ivermectin mass drug administration (MDA) for a period of 8–26 years (1992–2017). Ivermectin combined with albendazole was given for 8–13 of these years for lymphatic filariasis (LF); the LF MDA program successfully concluded in 2012, but ivermectin MDA continued in areas known to have a baseline meso-/hyperendemic onchocerciasis. In 2017, serological and entomological assessments were undertaken to determine if MDA for onchocerciasis could be stopped in accordance with the current WHO guidelines. Surveys were conducted in 39 sites that included testing 5- to < 10-year-old resident children by using ELISA for OV16 IgG4 antibodies, and Onchocerca volvulus O150 pooled polymerase chain reaction (PCR) testing of Simulium damnosum s.l. vector heads. Only two of 6,262 children were OV16 positive, and none of 19,056 vector heads were positive for parasite DNA. Therefore, both states were able to meet WHO stop-MDA thresholds of an infection rate in children of < 0.1% and a rate of infective blackflies of <1/2,000, with 95% statistical confidence. Transmission of onchocerciasis was declared interrupted in Plateau and Nasarawa states by the Federal Ministry of Health, and 2.2 million ivermectin treatments/year were stopped in 2018. Post-treatment Surveillance was launched focusing on entomological monitoring on borders with neighboring onchocerciasis-endemic states. An apparent positive impact of the LF MDA program on eliminating hypo-endemic onchocerciasis was observed. This is the first stop-MDA decision for onchocerciasis in Nigeria and the largest single stop-MDA decision for onchocerciasis yet reported. This achievement, along with the process used in adapting and implementing the 2016 WHO stop-MDA guidelines, will be important as a potential model for decision makers and national onchocerciasis elimination committees in other African countries that are charged with advancing their programs.

Optimization and evaluation of the Esperanza Window Trap to reduce biting rates of Simulium damnosum sensu lato in Northern Uganda

Background

Onchocerciasis, or river blindness, has historically been an important cause of blindness, skin disease and economic disruption in Africa and the Americas. It is caused by the filarial parasite Onchocerca volvulus, which is transmitted by black flies in the genus Simulium. Over the past decade, several international programs have been formed to control, or more recently eliminate onchocerciasis, using mass drug administration (MDA) of ivermectin. However, in many areas of Africa (particularly those which are endemic for the eyeworm, Loa loa, or where vector densities are very high) ivermectin MDA alone will not be sufficient to achieve elimination. In these situations, additional interventions may be necessary.

Methodology/Principal findings

The Esperanza Window trap (EWT), a simple trap originally developed to replace human landing collections for entomological surveillance of O. volvulus transmission was optimized, resulting in a 17-fold improvement in trap performance. The optimized trap was tested in trials in schools and in agricultural fields to determine if it could reduce vector biting locally. The traps resulted in a 90% reduction in biting in the school setting. In the field setting, results varied. In one location, the traps reduced biting by roughly 50%, while in a separate trial, the traps did not significantly reduce the biting rate. Examination of the two settings suggested that trap placement may be critical to their success.

Conclusions/Significance

These results suggest that the optimized EWT might be capable of reducing local vector black fly biting in areas commonly frequented by residents. Together with other recently developed methods of community directed vector control, the traps may augment ivermectin MDA, bringing the goal of onchocerciasis elimination within reach in much of Africa.

Onchocerciasis or river blindness is historically one of the most important causes of blindness and skin disease in the developing world. It is caused by infection with the filarial parasite Onchocerca volvulus. The finding that ivermectin was an effective and safe treatment for onchocerciasis and the decision by its manufacturer to donate it to treat this infection spawned the development of programs to eliminate river blindness through mass drug administration of ivermectin to the afflicted populations. This has dramatically reduced the prevalence of onchocerciasis worldwide and has resulted in its near elimination in the Americas. But ivermectin alone will not eliminate river blindness in much of Africa; additional interventions are necessary. Here we report the optimization of a simple trap for the black fly vector of O. volvulus and show that these traps can dramatically reduce vector biting in some settings. Together with other recently developed community directed methods of vector control, these traps may augment the effect of the ivermectin distribution programs, bringing the goal of elimination within reach in much of Africa.

The Search for an Efficient Black Fly Trap for Xenomonitoring of Onchocerciasis

Onchocerciasis is a neglected tropical disease that has plagued mankind for decades with pathologies that involve the eyes and the skin. The WHO and the global health community have earmarked the disease for global elimination by 2045. However, as control programmes shift focus from reduction of the burden of the disease to elimination, new tools and strategies may be needed to meet targets. Monitoring Onchocerca volvulus larvae in the black fly vectors is an important tool needed to monitor disease dynamics and certify elimination. For decades, human landing collections have been the sole means of acquiring vectors for monitoring of the disease. This procedure has been plagued with ethical concerns and sometimes the inability to harvest enough black flies needed to carry out effective monitoring. Since the 1960s, the WHO recognized the need to replace human landing collections but relatively few field studies have designed and tested alternative traps. This review article systematically discusses some of the key traps tested, their successes, and their challenges. It is the aim of the review to direct research and development focus to the most successful and promising vector traps which could potentially replace the human landing collections.

Alternative treatment strategies to accelerate the elimination of onchocerciasis

The use of alternative (or complementary) treatment strategies (ATSs) i.e. differing from annual community-directed treatment with ivermectin (CDTI) is required in some African foci to eliminate onchocerciasis by 2025. ATSs include vector control, biannual or pluriannual CDTI, better timing of CDTI, community-directed treatment with combinations of currently available anthelminthics or new drugs, and 'test-and-treat' (TNT) strategies requiring diagnosis of infection and/or contraindications to treatment for decisions on who to treat with what regimen. Two TNT strategies can be considered. Loa-first TNT, designed for loiasis-endemic areas and currently being evaluated using a rapid test (LoaScope), consists of identifying individuals with levels of Loa microfilaremia associated with a risk of post-ivermectin severe adverse events to exclude them from ivermectin treatment and in treating the rest (usually >97%) of the population safely. Oncho-first TNT consists of testing community members for onchocerciasis before giving treatment (currently ivermectin or doxycycline) to those who are infected. The choice of the ATS depends on the prevalences and intensities of infection with Onchocerca volvulus and Loa loa and on the relative cost-effectiveness of the strategies for the given epidemiological situation. Modelling can help select the optimal strategies, but field evaluations to determine the relative cost-effectiveness are urgently needed.