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.
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.
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.
As the second-most studied pests of arthropods, black flies affect animals’ health and have medical importance for human beings.
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.