Application of Wolbachia in Invertebrate Vector Control
By: Chinagozi Ugwu
The race to decrease or even totally eliminate the persistence of vector-borne infectious diseases has been ongoing for years if not generations. A parasitic new champion has been found in the endosymbiont bacterium by the name of Wolbachia pipientis. The α-proteobacteria Wolbachia is a gram-negative intracellular parasite. Wolbachia is naturally present in over 20% of all insects. It was first discovered in the ovaries of Culex pipiens mosquito. Wolbachia forms either an obligate or a facultative relationship with its host and these interactions have many effects. This endosymbiont bacterium is in the family of Rickettsiacea, however, Wolbachia has not been found to be directly pathogenic to humans. The natural transmission of Wolbachia can be either vertical or horizontal. Transmission of this bacterial infection to offspring occurs vertically while, members of other species become infected horizantally.
Many invertebrates are infected by Wolbachia and the bacteria’s success may be credited to the diverse phenotypes that result from infection. The phenotypes range from mutualism to reductive parasitism.Inheritance by maternally transmitting Wolbachia through cytoplasm of eggs is also an effective infection pathway used by the bacteria. In the fruit fly, Drosphila, Wolbachia pipientis is vertically transmitted through the fly’s egg cells. Wolbachia has many physiological and reproductive effects, which may reduce male presence of its host species. Wolbachia can feminize infected males so that they can produce eggs with bacterial cells. The bacterium has the ability to change chromosomal sex determination, kill males selectively, influence sperm competition and generate cytoplasmic incompatibility in early embryo, etc.
In arthropods and many other invertebrates, Wolbachia infection can modify host reproduction in a variety of ways such as: reproductive incompatibility in most species; thelytokous parthenogenesis in haplodiploid species, male-killing in several insect and feminization of genetic males in isopod crustaceans. The facultative endosymbiont relationship between Wolbachia pipientis and host organisms allows for increased bacterial propagation and persistence in host populations but Wolbachia are hardly ever found to be beneficial to their. The only instance in which their is obligate relationship between Wolbachia and its host, is in the case of filarial nematode. The removal of the Wolbachia strain present in pathogenic nematodes is disastrous for the nematode. Without the intracellular bacteria, the filarial nematode losses most of its reproductive and pathogenic function amongst other things. Recently, Wolbachia, was found to be medically important vector borne infections. These bacteria could be used for population replacement and suppression of vector organisms such Aedes aegypti mosquito which spreads dengue and yellow fever, Culex pipiens mosquito which spreads West Nile Virus and anopheles mosquito that act as the malaria and filarial vectors.
Physiological and reproductive Effects of Wolbachia
The parasitic endosymbiont Wolbachia pipientis induces a variety of reproductive disorders in their host organisms. These include parthenogenesis in Hymenoptera and Thysanoptera, feminization in isopod crustaceans, male-killing in Lepidoptera, Drosophila and Coleoptera, sperm competition and cytoplasmic incompatibility in a diverse array of arthropods. Wolbachia, has many physiological and reproductive effects on arthropods such as mosquitoes which have several species that act as vectors for deadly infectious diseases such as Dengue Fever. Thelytokous parthenogenesis is a form of asexual reproduction in which females develop from eggs that are unfertilized. Most microbe-associated thelytokous are reversible, however, Wolbachia induced thelytokous parthenogenesis are irreversible. The Parasitic bacteria, Wolbachia, induce irreversible parthenogenesis by causing the reduction and loss of genes for sperm maturation, for mating behaviour of females, for a major muscle in female sperm reservoir, and the spermatheca. If females infected by parthenogenesis-inducing Wolbachia are successfully cured they can then produce male offspring. The male offspring most often are not sexually functional and the infected females become unable or unwilling to mate. Wolbachia induces feminization in isopod crustaceans through conversion of genetic males into functional females. These converted females produce higher female densities in the population. This feminization process is mediated by Wolbachia feminization strains that transferred through eggs. Male-killing strains of Wolbachia are elicited in organism of the order Lepidoptera, Droosphila and Coleoptera. The reduction of the male population in a species by Wolbachia is to increase the chances of this bacteria being transmitted down the generational line. Males in a population are unimportant because the male sperm cells are too small to harbor the bacteria therefore but the female egg cells are not therefore females are favored. Wolbachia infected males in some host species experience decreased sperm competition compared to uninfected males of the same species. Significantly fewer sperm is produced by males that are infected with Wolbachia due to reduced sperm competitive ability. Cytoplasmic incompatibility is the most prominent feature used for reproductive manipulation by Wolbachia. The endosymbiont, Wolbachia imposes cytoplasmic incompatibility in many arthropods resulting in embryonic mortality. When an infected male mates with an uninfected female unidirectional cytoplasmic incompatibility arises, however uninfected males can successfully mate with infected females and so can two infected parents. At the other end of the spectrum, bidirectional CI occurs between individuals infected with different Wolbachia strains mate. Wolbachia affects mitotic division as a result of spermatogenesis paternal chromosome modification. This modification thus causes a loss in mitotic synchrony. The importance of cytoplamsic incompatibility induced by Wolbachia to stopping mosquito vector disease transmission. Wolbachia can manipulate reproduction by causing a form of sterility known as cytoplasmic incompatibility (CI), which prematurely stops the development of early embryos. If a Wolbachia-infected male mates with an uninfected female or a female with a different Wolbachia type then cytoplasmic incompatibility occurs. Some Wolbachia strains stimulate an extremely high cytoplasmic incompatibility (CI) which leads to increased population invasion and also increased mortality in the mosquito population. Strains such as wRi Wolbachia strain spread at a rate of 100km/yr as a consequence of CI. The increased mortality of vector mosquitoes has no impact on the Wolbachia strains because the host organism reproduce prior to death. The life-shortening strain spreads due to the benefit of higher reproductive success that arises from cytoplasmic incompatibility of individuals with Wolbachia bacterial infection. The most virulent Wolbachia strain, popcorn (wMelpop) associated with brain degeneration in its host fly, was isolated from the organism D. melanogaster. This strain, wMelpop, reduces the life-span of the infected flies to about 50% compared to uninfected flies. Density of wMelpop strain in different tissues at the adult- stage is drastically higher than during the earlier stages of life such as the larval and pupal stages. Bacterial cells accumulate at such high densities in different tissues resulting in early death because of the overwhelming host cell pathology. There are several physiological factors that influence how effective wMelpop at shortening the life-span of infected organism. Temperature is a very important physiological factor. At higher temperatures, the virulence of wMelpop increased while the life-span of the bacterial host organism drastically decreased. For instance in the fly D. melanogaster, wMelpop had little to no effect on the life-span at 19 °C, however, at 25 °C there was a considerable decrease in life-span. It is of great medical interest to employ Wolbachia strains such as wMelpop for reduction of Dengue Fever transmission by altering the age structure and life-span of Ae. aegypti. Since only adult female, Ae. aegypti mosquitoes are capable of pathogenesis therefore the introduction of wMelpop strains would eliminate the part of the mosquito population responsible for transmitting Dengue Fever. Theoretical models revealed that transinfection with wMelpop strains of Wolbachia could significantly reduce transmission of disease about 90% or more.
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Pathogen Exclusion in Vector Mosquitoes with Wolbachia
Mosquitoes are flying insects in the Culicidae family, with more than 2000 different species. The males and females mosquitoes both feed on nectar but the females of some species of mosquito are capable of sucking blood from humans and animals. The hematophagic activity of female mosquitoes is essential for production of eggs and has made them one of the deadliest known disease vectors that claim millions of lives each year. 
Mosquitoes act as vectors for a variety of parasites and pathogens. Many mosquito-borne diseases such as West Nile Virus, Dengue Fever, Malaria, Yellow fever are transmitted by different species of mosquitoes. Yellow Fever, West Nile Virus and Dengue Fever are all caused by a family of viruses called Flaviviridae. Culex pipiens mosquitoes act as the primary vectors for the West Nile Virus in America while female Aedes aegypti mosquitoes are the disease vectors that transmit Dengue Fever and Yellow Fever. The vector-borne infectious disease, malaria can be caused by one of four types of Plasmodium parasites that can infect humans, with Plasmodium falciparum causing the most dangerous infection. Malaria is transmitted by infected female Anopheles mosquito. 
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