Introduction
River blindness, much like the Zika virus or malaria, is a vector borne disease. Currently 25 million people are estimated to be infected by the disease, however River Blindness is often looked over by much of the western world (Anonymous 2013). The vector for this disease is the blackfly of genus Simulium. Simulium spp can be found around the world and are often considered pests, and a select few species of this genus act as the vector for River Blindness, also known as Onchocerciasis. Simulium spp spreads the nematode parasite Onchocerca Volvulus who is the cause of Onchocerciasis (Anonymous 2013). The most notable species that act as a vectors are Simulium damnosum and Simulium ochraceum (Laird, 1981). Onchocerciasis can also be transmitted to cattle by several other species of Simulium (Capinera 2008). Onchocerciasis infects millions in Africa and in Central and South America (Laird 1981). Attempts have been made by different global health organizations to regulate blackfly populations in an effort to eradicate Onchocerciasis. Even though blackflies are not recognized as a problem by a majority of the western world they are still a great pest and a major health concern in Africa and Central America.
Discussion
The blackfly’s physiological makeup is critical to understanding their role as a vector. Blackflies are also known as humpback flies due to their scutum’s hump shape in the adult insects, which is one of the most easily identified characteristics of the black fly (Capinera 2008). Blackfly larvae are often aquatic laying their eggs in fast moving water, the larvae have a distinguished head and a long body (Capinera 2008). Adult blackflies are both blood feeding and plant feeding, however only the female adults are capable of blood feeding (Capinera 2008). The larvae are recognized by either abdominal tubercles or scales on the proleg (Crosskey 1990). A critical factor in the blackfly’s success as a vector is their ability to blood feed. Through this process the female is able to pass on the Onchocerca volvulus parasite to the human or animal host, which then spreads Onchocerciasis. Accordingly, the blackflies capability is to blood feed is also undoubtedly what makes it a nuisance, even when it is not spreading disease.
Blackflies can be found throughout the world, genus Simulium is diverse and widespread. Nevertheless, Simulium is an adaptive group and they have successfully spread to nearly every corner of the globe. Wherever one might go, whether it be Chile or Russia or Australia, amongst many, he is likely to observe a fly of genus Simulium (Crosskey et. al. 2004). Simulium’s large diversity also contributes to its effectiveness as a vector. Largely in part to Simulium’s pervasiveness, Onchocerciasis was able to spread across the Atlantic to Central America. Through way of the slave trade and New World explorers, it is widely accepted that Onchocerciasis was able to cross the Atlantic (Crosskey 1990). Today, thirteen isolated locations of the disease exist in South America, however it is not understood how the disease spread to deep parts of the Amazon where there was little African slave contact with the indigenous Americans (Crosskey). Regardless, without the existence of blackflies in the America’s the disease would not have spread. The spread into the Americas is also a testament to the tenacity of the parasite Onchocerca Volvulus.
The life cycle of Onchocerca Volvulus is fascinating and important to understanding Simulium’s role as vector for Onchocerciasis. Onchocerca spp. is parasitic it’s entire life, with the blackfly and a mammal as it’s two hosts. The filariae will start life in its respective vertebrate host. If the parasite is lucky, it will be sucked up by a blackfly without whom it will die (Crosskey 1990). The blackfly is typically not harmed by the parasite until the number of worms reaches around 160, which will oftentimes cause harm or even death to the fly but the negative effects are largely species dependent (Takaoka 2015). Once the filariae is inside the blackfly it will move to the invertebrate’s flight muscles to begin development, it will eventually molt into the second-stage form larvae or the sausage stage (Laird 1981). The larvae will then molt into its third stage life form and at this point it is infective to vertebrates. At this point it will move away from the wings of the fly and towards the head where it will be transmitted to its next host through the blood feeding of the fly (Laird 1981). Onchocerca volvulus is completely dependent on the blackfly for its survival. Consequently, parasite-host relationship is integral to the spread of River Blindness. Without this relationship between parasite and vector, River Blindness would surely disappear.
Simulium also spreads Onchocerca spp. parasites to vertebrates other than humans. Blackflies infect deer, cattle and moose with Onchocerca spp. in nearly every continent including the Americas, Europe and Africa (Crosskey 1990). Not all Onchocerca spp. has a Simulium host, many of the parasites still require further research to know what their intermediate vector is and how they are spread to vertebrates (Crosskey 1990). The drive force behind blackfly research and Onchocerciasis research has been the negative impacts on humans and human society. As result, Onchocerciasis that arises is mammals, other than humans, has been minimally researched and the negative consequences of blackflies on these animals is not well understood. Onchocerciasis in cattle is by far the most heavily researched because it can be used as a model for understanding Onchocerciasis in humans. Simulium damnosum, the main human vector, is also the vector of Onchocerciasis in cattle, however cattle are infected with the Onchocerca ochengi nematode (Allen et. al. 2008). Cattle make good models for Onchocerciasis because the life cycle O. ochengi is similar to that of O. volvulus, so the information learned from the disease in cattle can be easily applied to the disease in humans. The disease in deer and moose have little relevance to humans and as a result, there is a huge gap in knowledge. More work should be done on all aspects of the disease by the scientific community, with this would come a more rounded knowledge of Onchocerciasis and its effect on all species.
The control of Onchocerciasis is a high priority to world health organizations and has been for decades. Different methods have been implemented for the control of the blackfly population and certain methods have proven effective. The most recent and one of the highly effective campaigns against the blackfly is the African Programme for Onchocerciasis Control, also known as APOC. APOC was launched in 1995 and continues to be on the forefront against the fight of Onchocerciasis (Tekle et. al. 2016). The use of ivermectin is the primary mode that APOC employees against the disease, which kills the filarial worms in the skin of its vertebrate host (Tekle et. al. 2016). In the countries where APOC is active, the elimination of Onchocerciasis is feasible in the near future (Tekle et. al. 2016). Other, more direct approaches, have been taken to eliminate Simulium as well. While a direct approach has been feasible in the Americas, it is shown to be too expensive and inefficient to actively eliminate the vector in Africa (Anonymous 2016). The use of environmentally safe insecticides against the blackfly is the current method of control utilized in the Americas, which is distributed in hotspots for blackfly mating (Anonymous 2016). In Equatorial Guinea the use of the insecticide, matched with ivermectin has eradicated the endemic vector, Simulium yahense, and is expected to soon eliminate onchocerciasis at the specific loci (Anonymous 2016).
The blackfly vector and onchocerciasis can be eliminated with the proper techniques. The use of medicines, such as ivermectin, should be the top method for the disease control over the direct elimination of blackfly populations. Direct elimination of the blackfly is problematic, because of the possible impacts on the surrounding environment. Additionally, the use of insecticides to eradicate Simulium could have unwarranted effects on other invertebrate populations in the same habitat. Simulium typically mates in fast flowing waters, as a result the application of insecticides could damage water quality and river habitats unnecessarily. Even though the locations of Onchocerciasis in the America’s are much smaller and easier to manage than those in Africa, it is still possible to control the disease and the blackfly population across the globe.
Conclusion:
The disease Onchocerciasis plagues several million members of humanity. To understand this disease, Simulium spp. must also be understood. The fly’s life cycle, anatomy and biological control must be known to ever eradicate Onchocerciasis. Onchocerciasis also affects animals other than humans. Veterinary entomology would be furthered by an understanding of the disease in nonhuman vertebrates. Nevertheless, Onchocerciasis should be acknowledged and eradicated. Fortunately, eradication of the disease should come in the coming decades thanks to modern medicine and the ability of science to control blackfly populations in South and Central America. Once programs are implemented throughout the Americas and Africa to fight Onchocerciasis, elimination of the disease would happen readily. With the elimination of the disease would also come great loss to certain blackfly populations, specifically in the Americas. However, Simulium spp. has many damaging effects on human societies due to their vector capabilities, so control of their populations must be of utmost importance to both the scientific and the world community.
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