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Medical Importance of Arthropods A. Arthropods as direct agents of disease or discomfort -Entomophobia -Annoyance (head and pubic lice) and blood loss. -Envenomization (black widow and brown recluse spiders). -Dermatosis (scabies mite). -Myiasis and related infestations. -Allergy. B. Arthropods as vectors of developmental hosts -Mechanical carriers. -Obligatory vectors (involves some degree of development within the arthropod). Examples of diseases transmitted by arthropods : - Tick borne: as Lyme disease or Lyme arthritis, Babesiosis and Rocky Mountain spotted fever. - Mosquito borne: as Malaria, Dengue fever, Yellow fever, Eastern equine encephalitis, Elephantiasis. - Flea borne : as Bubonic plague and Sylvatic plague - Black flies : as Onchocerciasis or River Blindness - Hemipteran borne: as Chagas disease -Tsetse borne: as African sleeping sickness Some Important Approaches of Control A- Chemical Control 1- No. 2 diesel oil may be applied to the surface of water which leads to deprivation of larvae from air besides its direct toxic effect. 2- Repellants as Deet. 3- Laboratory bioassay of the essential oil extracted from a plant, commonly known as 'Railway creeper', was carried out against the larvae of mosquitoes and it was found to have a mosquito larvicidal effect. 4- Insecticides are placed in the water to eliminate mosquitolarval habitats or sprayed on adult as DDT, Organochlorine, Organophosphorous compounds ect.. 5-Insect growth regulators, which disrupt insect growth and development (Juvenile hormone analogue, Ecdysone inhibitors, Chitin synthesis inhibitors). There are three types of insect growth regulators: Hormonal, enzymatic, and chitin synthesis inhibitors. Juvenile hormone analogs and ecdysone inhibitors both disrupt the ratio of hormones with the young insect. For an insect to molt to the next stage, the correct ratio of juvenile hormone and ecdysone must be present. Juvenile hormone analogs Interferes with molting process which causes premature molting. Also causes deformations of wings and reproductive parts. Ovaries produce infertile eggs. Ecdysone is a primary molting hormone that is necessary for insects to go from the larval to pupal stage. If we change the ratio of one to the other, the insect will not become an adult---thus reducing reproduction and population increase. Ecdysone inhibitors Blocks ecdysone which signals the insect to molt. This causes pupae to die. The chitin synthesis inhibitor Since insects molt many times, the ability to create chitin is vital to form the insect’s exoskeleton. Without the proper cuticle (exoskeleton), the insect will die. B- Physical Control 1- Filling or drainage swampy areas. 2- Removal of aquatic plants (deweeding). 3- Using irrigation water properly. 4- Screening of houses and use of mosquito netting at night 5- Using of barrier animals (zooprophylaxis). 6-Raising houses reduce mosquito bites. 7- Traps and Sound generators with certain frequencies drive away mosquitoes. 8- The use of expanded polystyrene (EPS) beads The use of expanded polystyrene beads (as a blanket over the water surface) is a potential alternative safe applicable method. EPS beads not only prevent oviposition but also kill the immature by forming a thick blanket on the water surface.. The EPS beads are the expanded form of polystyrene granules which are (petrochemical) hard translucent glass like beads with diameter ranging from 0.6 to 2.5 mm. The unexpanded beads contain an expanding agent (pentan). When exposed to super heated steam, they expand about 35 to 40 times of their original volume. The EPS beads are light in weight, inert, nontoxic, and resistant (do not interact with) to water content. Applied at the rate of 500g to 1 kg/m2 in different habitats, float on the water surface in several layers, physical barrier formed by the floating blanket of EPS beads prevent egg laying. Furthermore immature stages of mosquito trapped under the layer of the beads die of suffocation. C- Biological Control Agents of biological control (natural enemies) of insects include predators, parasitic insects, and insect pathogens. -Predators may be insects or other insectivorous animals, that consumes many insect preys during its lifetime. such as Gambusia fish, Toxorhnchites mosquito larvae. -Parasites (also called parasitoids) of insects are other insects which lay their eggs in or on the host insect. When they hatch, the young parasite larvae feed on the host (the pest) and kills it. As Trichogramma. - Insect pathogens include viruses, bacteria, fungi, nematodes, and other microorganisms that cause insect diseases. - Bacterial insecticides such as Bacillus thuringiensis israelensis and B. sphaericus, -Mermithid nematodes such as Romanomermis culicivorax. -Microsporidia such as Nosema algerae, Entomopathogenic fungi such as Lagenidium giganteum. Success in infecting Glossina. D- Genetic Control: The principles of genetic control are based of sterility or other desirable genetic factors in successive generations. The general goal is birth control through reduction or replacement of a population following release of a desirable genotype. Basis of genetic control: 1- population suppression 2- population replacement 1-Population suppression The most common strategy, It include the following following-A-The sterile insect technique (SIT) B-Release of insects with a dominant lethal gene (RIDL). C-Inherited or delayed sterility. D- Autocidal control methods: a-conditional lethal mutations b- simple inherited mutations. E-Cytoplasmic incompitability. A- Sterile insect technique (SIT) Or sterile insect release method (SIRM) This technique relies on the mass rearing of the target insect in large biobio-factories, their sterilization by ionizing radiation or chemical sterilants and then aerially releasing them in the field in sufficient numbers. The wild female will have no offsprings following mating with the released sterile male, leading to reduction in the natural pest population. The major technical aspects of SIT are: i-Automated mass rearing (Breeding biofactory) ii--Genetic sexing (only male) ii iii--Sterilization (irradiation or chemically) iii iv--Release (in the air as pupae) iv Colonization of target insect Mass rearing in Insect Factory----Factory-----Breeding Breeding population Genetic sexing (Males reared) Sterilization Release Mate with wild insects Reduced reproductive potential Population Suppression/ Eradication Flowchart of SIT Applications of the SIT: 1-Successful areaarea-wide SIT program has been conducted against the Screwworm fly Cochliomyia hominivorax in Libya (1988 (1988--1991 1991). ). Adult Cochliomyia hominivorax Larvae of Cochliomyia hominivorax in wound Larvae of Cochliomyia hominivorax in wound 3-The SIT has been conducted against the Mediterranean fruit fly (Medfly) Ceratitis capitata,, the Stable fly Stomoxys calcitrans capitata and the pink Bollworm, Pectinophora gossypiella. 4-Application of SIT against mosquitoes. The production of rearing mosquitoes does not exceed 7 million per week. This compares with 500 million per week for the screwworm fly, and natural mosquito densities are usually much higher than those of screwworm flies. Major improvements will need to be made in larval rearing and pupal collection. Also other techniques involving the release of fragile sterile pupae need to be considered. 2-The SIT was tried in the control of Glossina transmitting animal trypanosomes in South Africa, Zinzibar and now tried in Ethiopia. 5-Application of SIT against Glossina transmitting sleeping sickness: The use of SIT is unfortunately impractical with the present technique. Release of Tsetse flies (males or females) close to human habitations leads to an unacceptable increase in biting. B- Release of insects with a dominant lethal gene (RIDL) The transgenic female insects will be reared on a medium supplied with special additive to develop and express certain reporter gene. then when they are released to the natural environment free from the additive used to suppress the system in the rearing facility will result in females death. The mating of released RIDL males with wild females will result in viable male offspring, but no viable daughters as the dominant lethality would be expressed in the natural environment. Moreover, any males produced by RIDL males would contribute to the elimination of females in later generations. generations. The RIDL system has been exemplified in Drosophila and is under development for Aedes aegypti, aegypti, the vector of dengue fever and yellow fever. C- Inherited or (Delayed) Sterility "Inherited sterility" is an alternative genetic strategy that generally requires fewer insects to be reared and released. It also requires lower doses of irradiation than those necessary for sterilization. It involves the transmission of aberrant chromosomes (usually in the form of translocations) from the released population to the native population. When the aberrant chromosomes are passed on to the native populations, the individuals carrying the aberrations in the heterozygous state will show sterility. sterility. D- Autocidal control methods : 1-Conditional lethal mutations In this technique, strains of insect are produced (by genetic manipulation) carry traits that are detrimental (lethal) to the species in the native environment, but which are not detrimental under laboratory conditions. For example, the inability to diapause would be a conditional lethal in an insect that had to go into diapause to survive a host--free period, but in the laboratory no diapause would be host necessary.. necessary 2- Simply inherited mutations Genetic changes in a single gene could lead to decreased fitness in the native if the gene could be forced into the native population in large numbers. Some examples of such changes would be recessive lethal mutations, eye colour changes (to increase or decrease light sensitivity), pupal or adult body colour changes, and leg, antenna or leg deformities... deformities. Such technique requires significant research data on insect genetics, physiology, behaviour and reproduction . E- Cytoplasmic incompatibility sterility between populations of the same species can be made by microbial infection of the genus Wolbachia Wolbachia.. The bacterium is transmitted through the egg cytoplasm. Sperm, thus can’t fertilize any ova except the one carrying the same kind of Wolbachia In its simplest form cytoplasmic incompatibility results when a Wolbachia Wolbachia--negative female mates with a Wolbachia Wolbachia--positive male, male, no offsprings are produced from such an incompatible cross. In using cytoplasmic incompatibility as a substitute for SIT, the idea would be to release males of a strain that is uniformly incompatible with the local wild population. Such males are considered as functionally sterile without the expense of artificial sterilization. ♀× ♂ ↓ C ♀×♂ ↓ C ♀ ×♂ ↓ C ♀× ♂ ↓ I II Population replacement A-Use of transmission blocking vaccine B- Blocking malaria parasite invasion of mosquito salivary glands C-Transgenesis D-Paratransgenesis A-Use of transmission blocking vaccine Transmission blocking vaccines consist of antibodies that are ingested by the mosquito with blood meal and interfere with parasite development. They block ooknite invasion of the midgut epithelium. B-Blocking malaria parasite invasion of mosquito salivary glands -In general, the three lobes of male salivary glands appear similar to each other and likely all have the same secretory capabilities. Female glands are differentiated into two lateral and one medial lobe. The proximal regions of the lateral lobes are involved in sugar feeding. In contrast, the medial lobe and distal-lateral lobes express genes whose products such as enzymes, anticoagulants and vasodilatory agents are involved in hematophagy. a number of studies have been interpreted to indicate that sporozoites preferentially invade the distal-lateral and medial lobes of the female glands. A certain peptide was found to bind to the distal-lateral and medial lobes of female glands of An. gambiae and An. stephensi, and blocks slightly more than 90% of P. berghei sporozoite invasion in the latter species. The authors concluded that the peptide competes with the sporozoites for a salivary ligand. C-Transgenesis It is chromosomal insertion and expression of a gene derived from one organism in a second heterologous organism. Major components of the transformation system include: 1-Potential DNA vectors (transposable elements or viralviral-transducting agent). 2-A specific refractory gene that express the desired phenotype (i.e a factor that would inhibit transmission of the pathogen). Examples: 1-Transgenic Anopheles stephansi were produced by expressing either of two genes, a tetramer of SMI peptide or the phospholipase A2 gene (PL A2 A2) from honey bee venom. Mosquitoes carrying either of these two transgenes were impaired to Plasmodium berghei transmission. 2-There is recorded genetic variability for mosquito refractoriness against malaria parasites based on Melanotic Encapsulation of its early developmental stages. In encapsulation an electronelectron-dense melanin like substance begins to coalesce around the parasite in the mosquito midgut. Identification of the genes responsible for encapsulation and melanization and the possible mechanisms of introducing these genes into the mosquito genome could be one strategy of genetic control. D-Paratransgenesis It is the extrachromosomal expression of exogenous genes in arthropods by genetically modifying the symbiotic bacteria. 1-Wolbachia pipientis are intracellular bacteria found in many species of arthropods and are maternally transmitted from parents to offsprings. Since Wolbachia are frequently observed in the reproductive tissues of arthropods, one potential approach that they might be used to disrupt transmission of a pathogen. Genetically modified Wolbachia can be used to block transovarian transmission of arboviruses as Rift valley fever virus which is dependant on vertical transmission from adult mosquito to its progeny for maintenance of the virus in nature. The disruption of transovarian transmission is an application that could potentially be adopted for use in a number of species of mosquitoes mosquitoes,, ticks and mites in which transovarian transmission of pathogen is important. 2- In the midgut of triatomine bugs there is a symbiotic bacterium Rhodococcus rhodnii (vital for sexual maturity of bugs). It produce antimicrobial peptide cecropin A renders a bugs refractory to infection by Trypanosoma cruzi. cruzi. This peptide has a strong lytic activity against T. cruzi but no effect on bacteria or gut tissues of bugs (which is vector of Chagas’ disease). Genetically altered symbiotic bacteria can be introduced into aposymbitic first instar nymphs of R. prolixus where they allow normal growth and reproduction while expressing specific gene products of interest.