ARTHROPODS OF MEDICAL IMPORTANCE Session 22. Classification of arthropods and practical techniques in Medical Entomology

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1 ARTHROPODS OF MEDICAL IMPORTANCE Session 22 Classification of arthropods and practical techniques in Medical Entomology

2 Arthropods are invertebrate animals and they consists of 85% of the known species of animals. Their body is segmented and some segments fuse to form tagma. Entire body covered with a tough covering, the cuticle. They have jointed appendages.

3 Classification Phylum Arthropoda Class Insecta mosquitoes, flies, fleas, lice bugs Class Arachnida spiders, scorpions, ticks, mites Class Crustacea - crabs, lobsters, shrimps, cyclops etc. Class Diplopoda- millipedes Class Chilopoda centipedes Class Pentastomida tongue worms Medically important insects are found in Order Hemiptera Order Pthiraptera Order Siphonaptera Order Diptera Medically important Arachnids are found in Order Scorpiones Order Araneae Order Acari

4 Modes of transmission Arthropods are act as vectors of diseases. A vector is a living transporter or a transmitter of disease causing agents. Modes of transmission Mechanical transmission Biological transmission Trans-ovarial transmission

5 1. Mechanical transmission The vector is only a vehicle carrying the infective agent. eg: Houseflies carrying nematode eggs in their legs and mouth parts. 2. Biological transmission The infective agent changes (eg: filarial worms within the mosquito) or multiplies (eg: dengue virus in the mosquito) as it passes part of its life cycle inside the vector.

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7 3. Transovarial transmission In this special form of transmission, the parasite is transmitted to the progeny of the vector via transovarian route. eg: mites causing scrub typhus- The larval mites are the parasitic and thus become infected when they feed on an infected host, larvae pass through the life cycle as nymphs which are non- parasitic. But these adults give rice to an infected generation of larvae passing the infection to their progeny.

8 Practical techniques in medical Entomology Capturing of vectors is an essential part of entomological surveys. The arthropods with no wings are relatively easy to catch due to restriction of quick movements. Therefore capturing of fleas, bugs, lice, mites and ticks can be relatively easy. But special techniques have been developed for entomological surveys of flies and mosquitoes,

9 In this section we discuss mainly on mosquito collecting methods Objectives of mosquito surveys 1. The prevalent(common) mosquito species in the study area. 2. The vector species prevalent in the area. 3. The proportion of vectors feeding on humans. 4. The proportion of vectors resting indoors of human dwellings. 5. The different types of resting places 6. The peak biting times of each species of mosquitoes. 7. The preferred breeding sites of each species mosquitoes.

10 Sample collection Sample collection is a major part of entomology. This is done through field surveys. 1. Collection of adult mosquitoes 2. Collection of mosquito larvae 3. Collection eggs

11 Surveys involve systematic sampling of mosquitoes by making collections. This involves methods to collect Adult mosquitoes, Larval stages of mosquitoes and Mosquito eggs. Sampling equipment.

12 Collection of adult mosquitoes Adult mosquito surveillance is carried out to evaluate the incidence of mosquitoes within an area, to show the relative abundance of various species. The need for vector control programmes can be determined.

13 Sample collection techniques of adult mosquitoes Landing and biting collections (Direct catches)- using animal bait or human bait Insect sweep net collection. Bait trap collection Animal baited traps Human baited traps

14 Truck trap collection. Daytime resting collection Natural resting places are found indoors and outdoors. Information on species that rests indoors and outdoors are important for planning control measures (Examples of resting places of mosquitoes should be known).

15 Light trap collection Dry ice wrapped in newspaper make the trap effective, trap operates during night time. Gravid trap collection Effective in capturing large number of undamaged gravid Culex mosquitoes. Infusion made from grass, rabbit pellets, manure etc. placed in a black tray draws females to the trap to oviposit.

16 Pyrethrin spray sheet collection Collects mosquitoes resting indoors. White sheet is spread on the floor and a fine mist of pyrethrin solution is sprayed on the walls of well closed room to kill mosquitoes resting inside the house. Window (exit) trap collection Collects species that bite indoors but rests out doors. Useful in studying the effectiveness of insecticide spraying.

17 Mosquito egg surveys Egg survey are carried out to determine the exact breeding location of mosquito species. Common methods Sod sampling Egg separation machines Oviposition traps,(ovi traps) especially for container breeding mosquitoes.

18 Sod sampling Sod samples usually contains 8 cubic inches of soil and vegetation with thickness of about an inch. These are stored for a week or more to allow the embryos to develop within the eggs. The sod samples are then placed in glass jars and flooded with water. The larvae are identified after they hatch.

19 Oviposition raps

20 Collection of mosquito larvae Larval surveys are carried out to determine the exact area in which the mosquitoes breed and their relative abundance. Potential breeding sites of larvae include 1. Small rain pools, hoof-prints, drains and ditches-entire water surface should be examined for larvae 2. Brackish water (where fresh water and salt water mix) 3. Streams-larvae may be found at the edges where there is vegetation and the water moves slowly. 4. Ponds, lakes, swamps and marshes 5. Special sites, such as wells and water containers made of cement

21 Correct equipment should be used Enamel or plastic dipper, (350 mls capacity) with a long or short handle depending on the circumstances, plastic pipettes, larval collection bottles.

22 Larval collection techniques Dipping Larval collection by pipetting. Larval collection by pulling up aquatic plants. Once larvae and pupae are collected they should be Transported as live larvae and pupae to laboratories Killed using hot water between C Preserved using 70% alcohol

23 Collection by dipping

24 Larval indicies House (premises) Index (HI) The percentage of houses or premises with one or more habitats positive for Aedes larvae is referred as house index. HI can be calculated as follows: HI= No of infested houses X 100 No of inspected houses This is the most frequently used method. It is less laborious as when the first positive container is found there is no need to proceed further. High index value tells that transmission can occur easily to adjacent houses. The demerit of this index is that it fails to consider the number of positive containers per house.

25 Container Index (CI) The percentage of water holding containers positive for Aedes larvae is called container index. CI can be computed as follows: CI= No of infested containers X 100 No of inspected containers The undesirable feature of this index is that it only provides data on the proportion of containers that are positive without any reference to infested houses.

26 Breteau Index (BI) This index refers to number of positive containers per 100 houses in a specific locality. BI is calculated as follows: BI= No of infested containers X 100 No of inspected houses Among these indices BI is the most widely used index as it establishes a relationship between positive containers and number of houses. Therefore, it is more qualitative and of more epidemiological significance.

27 Specimen collection techniques for sand fly Mainly adult sand flies are collected during surveys. Methods Resting adults can be aspirated directly or trapped after disturbing them. Trapped using baited animals traps, CO 2 and light traps. Some sandfly genera of the Phlebotominae subfamily are the primary vectors of leishmaniasis and pappataci fever.