Entomology

Learning objectives

At the end of this lecture student will be able to : 1-Define relevant terms used in medical entomology . 2-Identify arthropods of medical importance & its distinctive characters . 3-Enumerate arthropod-borne diseases . 4-Determine methods of transmission of arthropod-borne diseases . 5-Specify principles of arthropod control . To a level accepted to the accreditation standard of the College .

Medical Entomology

Medical entomology :- Is the study of arthropods of medical importance .It is an important branch of preventive medicine.*Arthropods comprise the most numerous and varied form of the living things in the environment of man.*Some are man’s allies helping in fertilization of flowers, but majority are either of no use to man or his most dangerous enemies.

*Arthropods destroy man’s crops and food reserves ;and some which live close to man act as vectors or carriers of diseases.Arthropods of medical importance: A/Class Insecta:1-MosquitoesAnophelines Culicines

2-Flies

House Flies Sand Flies Tsetse Flies Black Flies 3-Human Lice Head &Body Lice Crab Lice

4-Fleas

Rat Fleas Sand Fleas 5-Reduviid Bugs B/Class Arachnida: 1.Ticks Hard Ticks Soft Ticks

2.Mites (chiggers) Leptotrombidium & Itch trombiculid mite mite C/Class Crustacea: Cyclops



Distinctive Characters of Arthropods of Medical Importance
Crustacea
Arachnid
Insecta
Cephalo thorax and Abdomen
Cephalo thorax and Abdomen
Head
1.Body divisions
Thorax
Abdomen
5 Pairs
4 Pairs
3 pairs
2.Legs
None
None
1 pair
3.Antennae
None
None
One or two pairs
4.Wings
In water
On land
On land
5.Where found



Arthropod –Borne Diseases 1.Mosquito Malaria ,filaria, viral encephalitis, viral fevers (dengue), viral hemorrhagic fever. 2.House fly Typhoid and paratyphoid fever, diarrhea, dysentery, cholera, gastroenteritis, polio, conjunctivitis, trachoma, anthrax, helminthic infestations , amoebiasis. 3.Sand fly Kalaazar, sand fly fever , oriental sore . 4.Tsetse fly sleeping sickness.


5.Louse Epidemic typhus , relapsing fever, pediculosis. 6.Rat flea Endemic typhus ,Bubonic plague, hymenolepis eleminuta. 7.Balck fly Onchocerciasis. 8.Reduviid bug Chagas disease . 9.Hard Tick Tick typhus ,viral encephalitis, viral fevers ,viral haemorrhagic fevers, tularemia, tick paralysis. 10.Soft Tick Q fever ,relapsing fever. 11.Trombiculid mite Scrub typhus, Rickettsial-pox.


12.Itch mite Scabies.13.Cyclops Guinea – worm disease , fish tape worm.14.Cockroaches Enteric pathogens.Transmission of arthropods – born diseases:-1-Direct Contacti.e. arthropods are directly transferred from man to man through close contact e.g. Scabies and pediculosis.2.Mechanical Transmission i.e. The disease agent is transmitted mechanically by arthropod.

e.g. diarrhea, dysentery, typhoid , food poisoning ,trachoma. 3.Biological Transmission i.e. the disease agent multiplies or undergoes some developmental change with or without multiplication in the arthropod host. Three Types of biological transmission: 1.Propagative . i.e. The disease agent undergoes no cyclical change but multiplies in the body of the vector e.g. plague bacilli in rat fleas.


2.Cyclo – propagative .i.e. The disease agent undergoes cyclical change and multiplies in the body of arthropod e.g. malaria parasite in anopheline mosquito. 3.Cyclo – Developmentali.e. The disease agent undergoes cyclical change but doesn’t multiply in the body of arthropod. e.g. filarial parasite in culex mosquito and guinea worm embryo in Cyclops.

In Communicable disease terminology : Vector :- Means arthropods or other invertebrates which transmit infection by inoculation into or through the skin or mucous membrane by biting or by deposit of infective materials on skin or on food or other objects.

Extrinsic incubation period

Means the period of time necessary for the development of disease agent in the arthropod host. e.g. in malaria and filaria the extrinsic incubation period is (10-14)days or longer depending on environmental temperature.


Definitive host :Means the host in which the sexual cycle of the agent occurs e.g. mosquito is the definitive host in malaria.Intermediate host:Means the host in which the asexual cycle of the agent occurs e.g. mosquito in filaria and Cyclops in guinea – worm disease.Infestation :Means the lodgment ,development and reproduction of arthropods on the surface of the body or in the clothing e.g. louse infestation.


Principles of arthropod control 1.Environmental control . 2.Chemical control . 3.Biological control . 4.Genetic control . 1.Environmental control : *It offers the best approach to arthropods control, because the results are likely to be permanent.


*Examples of environmental manipulation : 1.Elimination of breeding places (source reduction). 2.Filling and drainage operation. 3.Carefully planned water management. 4.Provision of piped water supply. 5.Proper disposal of refuse. 6.Cleanliness in and around houses.

2.Chemical control: * A wide range of insecticides are used for vector control. *Vector control by insecticides alone is no longer fully effective because of the appeared resistance in <100 species of arthropods with public health importance . *The dangerous environmental contamination has led to restricted use of most insecticides in some countries.


*To avoid undue environmental pollution ,it is now considered essential to replace gradually the highly persistent compounds such as DDT with compounds which are readily “biodegradable” and less toxic to man and animals such as methoxy chloral compounds .*As there is no alternative control method which is as effective and economical as the insecticides, it is postulated that most of the developing countries will have to depend on organic-chlorine compounds for control of vectors.

Insecticides

Contact Poisons
Stomach Poisons
Fumigants
Hydrogen Cyanide methyl bromide sulfur dioxide Carbon Disulphate
Pyrethrum Rotenone Derris Nicotine Mineral Oils
Synthetic
Paris green Sodium Fluoride
Natural



Synthetic
Organo – ChlorineCompoundsDDTMethoxychlorHCH(BHC)LindaneChlordaneHeptachlorDieldrinAldrinToxapheneKeponeMirex Organo – PhosphorusInsecticidesChlorthionDiazinonDioxathionDemethoateEPNMalathion(OMS-1)Fenthion(OMS-2)Methyl ParathionParathionRonnelTrichlorfonDichlorvosAbate (OMS-786)NaledGardona Carbamates Carbaryl Dimetilan Pyrolan Propoxur (OMS-33)
Synthetic Pyrethroids Resmethrin Bioresmethrin Pothrin
Repellents Benzyl benzoate Indalone Ethyl hexanediol Dimethl phthalate


3.Biological control: *To minimize environmental pollution with toxic chemicals ,great emphasis is now been placed on biological control. *The use of larvivorous fishes especially Gambusia is well known in mosquito control. *A variety of other biological agents (e.g. bacteria, fungi, nematodes, protozoa and viruses) are under study for control of insects. *But the fear exists that the introduction of biological agents may pose a direct hazard to the health of man himself.


4.Genetic control : *Much progress has taken place in recent years in the theoretical and applied aspects of genetic control of arthropods. *Techniques such as sterile male technique, cytoplasm incompatibility and chromosomal translocation have been found to be effective is small field trials. *Newer methods: New &innovative methods are being sought for insects control : a. Insect growth regulators. b. Chemosterilants. c. Sex attractants pheromones.

Integrated approach:Since no single method of control is likely to provide solution in all situations ,the present trend is to adopt an “Integrated approach “ for vector control combining two or more methods with a view to obtain maximum results with the minimum efforts and to void the excessive use of any one method.

The End