Abstract The black fly is one of the top three studied arthropod vectors and the second major pest that affects human health in the world, which means it has both ecological and medical importance. Due to geographic distribution, blood-feeding activity, and other properties, black flies transmit causative agents easily, causing animal diseases and human diseases, such as mansonellosis and onchocerciasis. This scientific review focuses on the characteristics of the black fly from its ecology, biology, and pathology. It will also briefly mention the prevention of diseases caused by black flies and future research.
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IntroductionThe family Simuliidae (infraorder: Culicimorpha) is in the order Diptera composed of over 2,200 species. Among pests and vectors that affect humans, 94% of them are members of the family Simuliidae (Adler and McCreadie, 2009).
As a member of the family Simuliidae of nematocerid flies, the black fly is the second major pest that affects human health and welfare in the world and is one of the top three important arthropod vectors (Adler et al.
, 2004). Due to its economic and medical significance, the black fly has attracted great interest in recent years. In this paper, I will briefly introduce more characteristics of black flies from ecology, biology, and pathology. EcologyDue to evolution and life cycle, black flies occupy broad habitats ranging from cold small streams to warm large rivers, from aquatic areas to terrestrial areas. By being preyed on by many invertebrates, such as amphibians and birds, or feeding on vertebrates’ blood, black flies play an important role in both aquatic and terrestrial food chains.
- EvolutionAccording to evolutionary theory and natural genetics of the family Simuliidae, black flies initially evolved in global northern regions. These regions have cool fresh streams and brooks, which are suitable for their survival (Adler et al., 2004). About 255 species of black flies have been found in North America so far, especially in northern regions. For years, due to changes in topographic surface features and climate, black flies gradually migrated from cool small streams to warmer large rivers where human beings also originated. Many river-adapted species, such as Simulium damnosum, emerge in medium-sized to large rivers (Crosskey, 1990; Post et al., 2007). This natural evolution unfortunately brings black flies closer to human beings.
- Life Cycle Figure 1. Typical Life Cycle of Black Flies (egg, larval, pupal, and adult stages). Besides successful evolution, geographic distributions are also determined by the life cycle. Black flies have four stages in a complete life cycle (egg, larva, pupa, and adult stages) with different morphology known as “holometabolous”. Black flies have complete metamorphosis molting from larval stages to winged adults. The first three stages are called aquatic stages. Under aquatic stages, black flies are abundant and important organisms in river ecosystems. The lifespan of black flies is short (only 2-3 weeks). Therefore, the female black fly will lay 150 to 500 eggs every time to maintain fast breeding cycles. Eggs are usually deposited in shallow, fast-running water with submerged objects, such as rocks and vegetation. Normally, the hatching period is most frequently between 4 to 5 days. However, black flies make adjustments on spans of hatching periods depending on different species, climate, and other ecological conditions. After hatching, young larvae attach themselves to submerged objects. The larvae have special body structures to obtain food and filter algae and bacteria (Currie and Craig, 1988). Larvae have certain pathogenicity once infected by various parasites and pathogens. Just like eggs, the period of the larval stage also depends greatly on the food and climate. Then it comes to the last step of aquatic stages: pupation.
In the adult stage, black flies leave the water for food and copulation. Adults feed on nectar; while some species of female adults feed on blood. The reason female adults instead of male adults feed on blood is that they require blood for oviposition. In most species, females fly into male swarms for copulation which often takes place during flight. After copulation, female adults use their mouthparts to cut the host’s skin and suck blood. During this process, they inject anticoagulants of the saliva which may directly cause the host’s symptoms, such as reddening, itching, and swelling. Geographic distributions of larval and adult black flies extend scales of distribution (Colbo and Wotton 1981, Adler and McCreadie 1997). Main living areas of larval stages are ranging from small streams to large rivers, which have small scales, such as meters or less; while winged adults cover entire continents on a scale of kilometers. In this way, black flies have broad dispersal from streams to continents.
- Survival Strategies are crucial for black flies. Large amounts of oviposition and copulation ensure stable reproduction. Other strategies, such as making adjustments under unfavorable conditions, make every stage of the life cycle more resistant and adaptable. These survival strategies are key factors in increasing the survival rate of each generation. Moreover, enormous female adult black flies often emerge at the same time, which increases the success rate of predation. The mass emergence means contemporaneous blood meals which cause more serious blood loss to humans or animals, leading to acute toxemia, Anaphylactic shock, and even death.
- Multiple TransmissionMultiple transmissions are common in black flies. Besides horizontal transmission (the dissemination to hosts), they also have vertical transmission which is known as black flies’ host-symbiote (Anderson and DeFoliart 1962, Ezenwa 1974). Larval black flies can serve as hosts for other parasites or bacteria, such as mermithids, microsporidia, and micromycetes (Ezenwa, 1974). What’s more, larval black flies are symbiotes with various fungi, nematodes, and protists, which is sometimes considered a form of parasitism. Once infected larvae become adults, infected or normal female adults can produce infected eggs via copulation and oviposition. In this way, vertical transmission effects from generation to generation within species (Tarrant, 1984). Since larval stages are key infective stages in both horizontal and vertical transmissions, larvae are more pathogenic than adults.
- Host and Feeding AbilityHost and feeding ability are critical in vectorial capacity. Around 98% (about 2101 species) of black flies feed on avian or mammalian hosts’ blood (Adler et al., 2010). Like mosquitoes, adult females obtain blood from hosts for oviposition. Blood-feeding makes them become worldwide vectors for a wide range of victims, and what’s worse, extending the range of diseases. What’s more, black flies have high host and feeding specificity which means they only feed on blood from one specific host. As pathogens of blood-borne transmission, about 67% of black flies feed on mammals, and about 33% feed on birds. In North America, about 10% of species feed on the blood of humans (Adler, 2009; Adler and Crosskey, 2010; Adler and Crosskey, 2015). This specificity effectively narrows down the scope of pathogenic species, which is beneficial for scientific study.
As the second-most studied pests of arthropods, black flies affect animals’ health and have medical importance for human beings.
- HistoryDue to blood-feeding activity, black flies are considered ideal disease vectors to transmit causative agents (Crosskey, 1990). In Arizona in 1992, black flies were found to transmit vesicular stomatitis virus-New Jersey serotype (VSNJ) to domesticated animals (Cupp, 1996). Since then, more and more evidence proved the important roles that black flies play in the transmission of arthropod-borne viruses. For example, black flies cause bovine onchocerciasis, mansonellosis, and leucocytozoonosis in domestic animals. Throughout history, black flies have lowered the quality of human life and reduced livestock production by transmitting human and animal diseases, causing economic and medical disasters.
- OnchocerciasisAmong all these diseases, onchocerciasis (river blindness) is one of the most serious diseases. Onchocerca volvulus is the causative agent of river blindness. In an infected human body, adult worms of onchocerca volvulus produce embryonic larvae (microfilariae) that migrate to other organs, such as skin and eyes. When a female black fly bites that infected human, it also ingests microfilariae which develops further. Experiencing two times of molting, microfilariae reach the third-stage larvae which are infective and stay in the black fly’s proboscis, ready for being transmitted to the next human host. During blood meals, the third-stage larvae of onchocerca volvulus penetrate the bite wound and migrate to subcutaneous tissues where they reach maturity. After maturing, adult female worms produce microfilariae which migrate to the skin to cause lesions. These lesions ultimately lead to severe symptoms, such as itching and disfiguring skin conditions. Irreversible blindness also occurs when microfilariae migrate to the eyes. Because the main habitat of black flies is a river, onchocerciasis is also called river blindness. As the second leading infectious cause of blindness in the world, onchocerciasis causes 0.8 million cases of blindness per year and about 15.5 million people are infected every year (Hay et.al, 2017; Centers for Disease Control and Prevention, 2013). In some regions with poor sanitation such as sub-Saharan Africa, a large number of infected patients lose the ability to work. Thus, local economic developments are hindered. In recent years, lots of onchocerciasis control programs spent millions to reduce the impact of the disease on humans and the economy, such as the African Programme for Onchocerciasis Control (APOC) and the Onchocerciasis Elimination Program in the Americas (OEPA). Under efforts made by these control programs, this epidemic transmitted by black flies was put under control.
- PreventionMany diseases caused by black flies can be controlled by using medications to kill causative agents. For example, many countries had greatly decreased the morbidity by using the medicine ivermectin(CDTI) in some programs for onchocerciasis control. CDTI is an antiparasitic agent to effectively kills major filarial parasites in humans. However, merely depending on medications is not enough. On the one hand, CDTI for onchocerciasis treatment may cause the potential drug resistance to Onchocerca volvulus (Osei-Atweneboana et al., 2007); on the other hand, CDTI may cause potential adverse effects on humans (Edwards, 2003; Woodward, 2012).
Eliminating black flies which serve as vectors to transmit causative agents is another prevention, such as using chemical insecticides. However, biocontrol agents and large-scale elimination negatively affect the biosphere. Firstly, persistent and strong biocontrol agents not only cause environmental pollution but also aggravate resistance and variation in some species. Secondly, nonselective insecticides reduce biodiversity since not all species of black flies transmit pathogens. Take onchocerciasis as an example, S. damnosum and S. sirbanum are major species responsible for transmitting pathogens. In recent years, scientists adapted preventive strategies according to long-term effects: narrowing down the scope of specific pathogenic species and finding other effective control agents. Confirming specific vectors can effectively measure vectors’ geographic distribution and population size. After moving focus upon the isolation of pathogenic species habitats, the next step is sequential regulation of black flies’ population density. Microbial control agents of black flies show potential developments (Jamnback, 1973; Chapman, 1974; Lacey and Undeen, 1986). Microbial control agents include fungi, viruses, bacteria, protozoa, and nematodes. Due to their low toxicity and high stability, they are valuable to be exploited and applied, especially to the broader spectrum of Bacillus thuringiens (H-14) (Lacey and Undeen, 1986; Becker, 2000).
Activated B.thuringiens protoxin can be absorbed by black flies larvae midgut cells through special filter devices, leading to larvae death (Gaugler and Finney, 1982; Becker, 2000). However, for species with non-susceptibility and undetected habitats, microbial control agents also have limitations. Based on what I mentioned above, the best way to achieve long-term control is conventional elimination of causative agents and black flies, finding pathogenic species and more effective control agents to work together. Future ResearchThe study direction is usually proportional to their corresponding economic or medical importance. Among all species in black flies, scientists start to research well-characterized species which have medical importance, such as onchocerciasis vectors from both Africa and Latin America, including S. damnosum and S. sirbanum.
However, scientists have trouble researching well-characterized species because black flies have low levels of differentiation in Isoenzyme variation within species and species-specific molecular variation (Snyder, 1982; Feraday and Leonhardt, 1989; Scarpassa and Hamada, 2003). Variation between sibling species of black flies is lower than between closely related species of Drosophila (Ayala and Powell, 1972; Meredith and Townson, 1981). In other words, they lack representative genetic markers and other information about dynamic factors of the population. Thus, it is hard to distinguish well-characterized species that are originally defined from non-characterized species. Therefore, unlike using cytotaxonomy to explore taxonomic placement and phylogenetic relationships in the superfamily in the past, the latest research trends worldwide change focus from cytogenetic to molecular investigation. It provides a unique opportunity to study genomes of disease-transmitting vectors and vector competency (Adler et.al, 2005; Adler et.al, 2010). The genome and transcriptomic sequences can provide markers including single-nucleotide polymorphisms, microsatellites, and exon-primed markers (Adler, 2005). Through these ways, researchers generate more complete genome-sequence data to generate a genetic map. These genetic data provide an epidemiological and genetic framework. They delimit populations and effective population sizes, which is beneficial to controlling diseases such as onchocerciasis.
For example, in African Programme for Onchocerciasis Control (APOC), genome-sequence data of the African vectors estimate effective sizes of transmission zones and distinguish the effective population of black flies. Besides being markers for monitoring epidemiological traits, genome-sequence data help to identify promising species and other species in similar families. By using sequence similarity and Hidden Markov Model-driven orthology, well-characterized species aid the gene regulation analysis of promising species (VectorBase, 2011). Other species in parallel families such as mosquitoes also benefit from black flies’ genome sequences. ConclusionDue to black flies’ crucial place in the family of Simuliidae, researchers have been studying their special characteristics. In ecology, black has broad geographic distributions due to successful evolution and life cycle. In biology, they have smart survival strategies, multiple transmissions, and blood-feeding ability. In pathology, they have medical importance because they are disease vectors to transmit causative agents. As for future research, researchers shift the emphasis of future research from genetics, phylogeny, and vector biology to genetic analysis. Building more complete genome-sequence data is necessary for the prevention of diseases. The more extensive and complete the genome-sequence data are, the greater value they have.