MEDICAL ENTOMOLOGY Medical entomology is the science that

MEDICAL ENTOMOLOGY Medical entomology is the science that dealing with arthropods that may be directly causing harm or transmit diseases to human as well as acting as intermediate host (victor) for parasites or irritate human tissue by their bites and/or secretions. Arthropods: “Arthro” means joint and “Poda” means legs. Arthropods are invertebrate animals with jointed-legs and identified by their peculiar characteristics.

General characters of arthropods : Multicellular and bilaterally symmetrical animals. Body is segmented, each segment bears a jointed appendage. Body composed of head, thorax and abdomen. In some species, head and thorax are fused together forming cephalothorax as in class Arachnida. Body is covered by exoskeleton, which is made by chitin. The exoskeleton project inside, this projections are useful for muscle attachment. Exoskeleton is cast off periodically during development and a new one is secreted in a process called molting.

Digestive system is divided into foregut, midgut and a hindgut. Foregut is composed of esophagus, croup, proventriculus and a pair of salivary glands opening into the buccal cavity. Midgut is tubular stomach, which may have two or more ceca emerging from its anterior part. Hindgut is divided into intestine and rectum; its function is evacuating food waste as well as regulating water and ions in insect’s body. Body cavity is continuous and is called hemocoele and is filled with colorless blood (hemolymph).

Circulatory system consists of dorsal tubular heart and pericardial isthmus. Circulatory system is open type or lacunar type. Nervous system shows nerve ring and double ventral nerve cord. Excretion is by Malpighian tubules, green glands and coxal glands. Respiration is by gills, or by trachea. Sexes are separate, gonads and their ducts are paired. External genitalia of male (aedeagus and paired claspers) have a taxonomic value.

Medical importance of arthropods: 1 -They may cause disease: A-Adults: 1 -Inoculation of allergic or poisonous substances through their bites, leading to serious, sometimes fatal symptoms e. g. ticks and scorpions. 2 -Invasion of skin causing different cutaneous manifestations as scab mites. 3 -Annoyance of domestic animals. B-Larvae: larvae of some dipterous flies invade different tissues of animals or man causing myiasis.

2) They may transmit diseases: 1 -By inoculation of a pathogen into the bite wound (anterior station) e. g. Glossina in trypanosomes. 2 -By contamination of bite wound by the insect faeces containing pathogens (posterior station) e. g. winged bugs in Trypanosoma cruzi. 3 -By crushing arthropods on skin. Mode of transmission of diseases by arthropods: 1 -Mechanical: Contaminative by their legs or stools e. g. ' house flies and Entamoeba histolytica cysts or passive carrier by their wet proboscis e. g. stable flies and blood parasites.

2 -Biological: 1 -Propagative: Organism multiplies without development; e. g. all arthropod-borne bacterial, rickettsial and viral diseases as fleas in Yresinia pestis. 2 -Cyclodevelopmental: Organisms develop without multiplication e. g. all helminth parasites transmitted by arthropods as mosquitoes in filarial worms. 3 -Cyclopropagative: Organisms undergo multiplication and development e. g. all blood protozoal parasites transmitted by arthropods as sand flies in leishmaniasis.

4 -Transovarian: Organisms are transmitted to the offsprings through the eggs e. g. tick-borne diseases especially in one host tick as in babesiosis. Metamorphosis (Development) of arthropods: 1 -Complete metamorphosis (holometabola): Egg, Larva, Pupa and adult. Larva and pupa are completely different in morphology and habitat as well as habit of feeding e. g. mosquitoes. 2 -Incomplete metamorphosis (hemimetabola): Egg, nymph and adult. Nymph is similar to the adult but smaller in size, paler in color and sexually immature; feeding habits are similar to adult as e. g. bugs and lice.

Classification of Phylum Arthropoda 1 -Class Insecta (Hexapoda): 1 -Body consists of head; thorax and abdomen. 2 -One pair of antenna. 3 -Three pairs of legs. 4 -Two pairs of wings (sometimes two wings or none). 2 -Class Arachnida (Octopoda): y consists of cephalothorax and abdomen (sometimes fused). 2 -No antennae. 3 -Four pairs of legs. 4 -No wings. 3 -Class Crustacea: 1 -Body composed of cephalothorax and abdomen. 2 -Two pairs of antennae. 3 -Four to five pairs of biramous legs. 4 -No wings

I-Class Insecta: General characters: a) Head is globular. It carries the eyes, the antennae and the mouth parts or proboscis. b)Thorax is composed of pro, meso and meta thorax to each part a pair of jointed legs is attached. C)Abdomen consists of eleven segments; some of these are only visible. Respiratory spiracles on abdominal segments are variable. Anus is at the end of the abdomen. Genital opening and accessory genitalia are diagnostic.

Insects of medical importance belong to 4 orders: 1 -Order Diptera (flies). 2 -Order Hemiptera (Bugs). 3 -Order Siphonaptera (fleas). 4 -Order Anoplura (lice). 1 - Order Diptera General characters: 1 -Two mesothoracic wings, while the metathoracic pair is rudimentary. 2 - Proboscis is adapted for piercing and sucking e. g. mosquito, or sucking and lapping e. g. house flies. 3 -Metamorphosis is complete.

Diptera of medical importance are divided into 3 suborders: 1 -Suborder: Nematocera (Mosquitoes) 2 -Suborder: Brachycera. 3 -Suborder: cyclorrhapha. uborder Nematocera Family Culicidae Life cycle of Mosquitoes: 1 -Eggs are laid on suitable collections of water. 2 -After few days, eggs hatch & larvae come out within 1 -3 days (according to temperature). 3 -Larvae live in water feeding on various organic matters suspended or floating.

Larvae molt three times ending with the 4 th stage, which become pupae. Larva and pupa breath atmospheric air and appear as if hanged from the surface of water. Pupa can move only in a jerky manner but it does not feed. The larval period varies from 5 to 10 days and the pupal period lasts for 2 -3 days depending on atmospheric temperature. 4 - Adult mosquitoes emerge from the pupae through a Tshape slit. Then males and females fly together and mate. 5 -Fertilized female always needs a blood meal before egg laying. In a week, the female begins to lay its eggs.

Egyptian mosquitoes: Anopheline: 1 -Anopheles pharoensis: The chief vector of malaria in Egypt. It is common in rice-growing areas. It is more common in summer and autumn. It enters houses and prefers to bite by night. 2 -Anopheles sergenti: Found most commonly in Oasis, Sinai, and Fayum. Also breeds in rice fields and pools. 3 -Anopheles multicolor: Most common in Delta and Cairo. It breeds in small pools with or without weeds 4 -Anopheles gambia: This is not an Egyptian mosquito but it visited Egypt some years ago causing a severe epidemic of malaria.

Culicine: 1 -Aedes aegypti: It was found throughout Egypt. It is a domestic mosquito and can breed in water tanks as well as in latrines. It prefers to bite through the day. It has been exterminated. 2 -Culex pipiens: Very abundant species in Egypt. It can breed almost in any collection of water in and outside houses. Medical importance: 1 -Anopheles mosquitoes: Transmit malaria and in some countries "Bancrofti's" filariasis. Not all anopheline mosquitoes are capable of transmitting malaria. 2 -Culex mosquitoes: Transmit "Bancroft's filariasis", bird malaria and encephalitis.

3 -Aedes mosquitoes: Transmit yellow fever, Dengue fever as well as Bancroft's filariasis in certain countries. a) Yellow fever: It is a viral disease of high mortality, at times producing epidemics in non- immune population. The disease may involve the viscera causing vomiting of blood, Jaundice and hemorrhage in the skin and extensive necrosis in the liver and kidneys "Viscerotropic strain". The virus may invade the nervous system "Neurotropic strain". In very rare cases, both invasions may take place "Pantropic strain".

b)Dengue fever: Viral disease, "break bone fever" and it causes complete temporary incapacitation of the patient. Control of mosquitoes: 1 -Measures against adult mosquitoes: 1 -Use of wire screening and mosquito nets: The following points should be noticed in screening: 1 - Size of mesh and quality of material. 2 - Screened doors should be opened outwards, never inwards. 3 - Better to use sliding than hinged or swinging windows. 2 -Spraying with chemicals: Kerosene Pyrethrum extracts, "D. D. T. " or "Gammexane" are good insecticides.

"Pyrethrum" has a knocking down effect but has no residual effect; while "D. D. T. " and "Gammexane" are particularly of great value because of their prolonged residual killing effect (may be as long as three months). 3 -Using of mosquito repellents: Applied to exposed parts of the skin repel mosquitoes from biting e. g. "oil of citronella", "indalone" and "dimethyl phthalate". 4 -Animal barriers: This depends on the feeding habits of the local mosquitoes. Anthropophilic mosquitoes (feeding only on human blood) zoophilic mosquitoes (feeding on animal blood) could be attracted to animals rather than to man.

1 -Measures against larval stages: 1 -Draining off collections of water. 2 -Filling up of ponds and water collections. 3 -Changing the growth requirements of larvae. 4 -Spraying oil on breeding places. This suffocates and poisons larvae and pupae. 5 -Dusting with "Paris green" (copper acetoarsenite). It is a stomach poison and is particularly effective against Anopheles larvae because they are surface feeders. It has no effect against pupae as they do not feed.

6 -Spraying of "D. D. T. " This is a contact toxic material, directly affecting the peripheral nervous system and also muscles causing irritability, paralysis and death. It has no effect on pupae. 7 -Introduction of natural enemies as "Gambusia" fishes. These fishes are top feeders, feeding preferably on larvae. They possess marked fecundity and rapid rate of reproduction. They withstand handling and transport. These fishes are worthless as food for man.

2. Family Psychodidae (Sand flies) 1 -Genus Phlebotomus: In Mediterranean region and West Africa. 2 -Genus Lutzomyia: Occur mostly in Central and South America. Medical importance: 1 -Leishmaniasis caused by Leishmania donovani, L. tropica (Phlepotomus) and L. braziliensis ( Lutzomyia). 2 -Sand fly fever (papatasi fever or 3 days fever) 3 -Bartonellosis "Oroya fever" or "Carrion's disease". 4 -Sand fly bites produces local indurated lesion. 5 -Harrara: Allergic reaction to the bite of sand fly in the form of a papular skin reaction. Control: 1 -Removal of vegetations. 3 -Filling cracks in the wall 2 -Spraying insecticides.

3 -Family Ceratopogonidae Culicoides sp. (Midges) Medical important: Transmit Acanthocheilonema perstans and Mansonella ozzardi. 4. Family Simulidae Simulium spp. (black flies) Medical importance: 1 -Transmits: Onchocerca volvulus to man (river blindness). 2 - Simulium bite is painful followed by an intense itching, raised ulcerative lesions and hemorrhagic spots at the site of bite due to the irritation produced by salivary secretion, fever, nausea and headache(black fly fever). Control: Spray insecticide& Repellants.

5 -Family Tabanidae 1 -Genus Chrysops "Deer fly or Horse fly“. 2 -Genus Tabanus. Medical importance: Chrysops transmits Loa loa; while Tabanus transmits "Tularemia" and "Anthrax" Suborder Cyclorrapha 6 -Family Muscidae 1 -Musca domestica "House fly" Life cycle: 1 -Adult lay eggs on human or animal excreta and decaying organic matter. 2 -Larvae hatch in 12 -24 hours and grow into maggots 10 l 2 mm in 3 -7 days.

3 -Three ecdysis (development) occur during the larval life and the pupa remains in the last larval skin. 4 -Adult emerge within 3 -26 days from pupae through circular opening. Medical importance: (A)- Disease transmission: House fly transmits microorganism to our food or body directly mechanically: 1 -Contaminated legs and hairs of the fly. 2 -Ingestion of human faces by the fly and passed with its faces unchanged.

3 - Fly sucks fluids until its diverticulum becomes filled; when such fly drops on sweet food, it vomits these fluids so as to dissolve it and sucks it again. Organisms transmitted in this way are: Bacteria: Vibrio cholera, Streptococci. Shigella, Salmonella, Escherchia coli. Protozoa: E. histolytica cysts, Giardia cysts, Balantidium cysts. Helminth eggs: As Taenia, Ascaris, Trichuris, Oxyuris, and Hymenolepis nana.

(B)-Disease causation: Larvae of flies may cause accidental intestinal myiasis, urogenital, aural or traumatic myiasis. Control: 1 -Elimination of breeding places. 2 -Residual spraying and dusting inside houses. 3 -Mixing powdered borax with the manure in a ratio of 1 kg per cubic meter will kill 90% of the larvae. 4 -Screening is of utmost importance in protecting food- stuffs from contamination.

2 - Stomoxys calcitrans "Stable fly“ Medical importance: 1. Accidental myiasis. 2. Painful bites. 3. Skin allergy. 4. They cause nuisance to man. 5. They can cause economic losses in cattle raising areas due to vigorous blood sucking. 6. Mechanical transmission of trypanosomes to man and animals. 7. It transmits anthrax, brucellosis, epidemic relapsing fever.

3 -Genus Glossina (G. palpalis & G. morsitans) Distribution: These flies occur in Africa. There are two main species groups which are: 1. G. palpalis group prevalent in West Africa 2. G. morsitans group, prevalent in East Africa. Life cycle and habits: • A female fertilizes only one egg at a time and retains each egg within her uterus to have the offspring develop internally during the first three larval stages. • During this time, the female feeds the developing offspring with a milky substance secreted by a modified gland in the uterus.

• Female gives birth to fully- grown larva (3 rd stage larva). It gives one larva every 10 -14 days. Larva is cylindrical creamy white to pale yellow. It hides itself in loose soil to depth of about 2 cm and turns into a pupa after 1 -2 hours and develops a hard outer shell called the puparial case, in which it completes its morphological transformation into an adult fly within 20 to 30 days. Medical Importance: 1 -G. palpalis is the vector of Gambian ( chronic) sleeping sickness. It occurs in Congo and West Africa. It prefers to live along water courses and lakes and is known as "Wet fly".

2 -G. morsitans is the vector of Rhodesian (acute) sleeping sickness. It is found in Sudan and Rhodesia. It lays larvae in dry open spaces. Control: 1. Changing the nature of breeding places to become unsuitable for the fly and periodic cleaning of riverine vegetation. 2. Collection of larvae and pupae. 3 -Repellents provide temporary protection. 4 - Insecticidal sprays and aerosols in homes and buildings provides partial protection.