Medical Entomology and Vector Bionomics

By Dr Utpal Sharma

Assistant ProfessorDept Of Community Medicine

Sikkim Manipal Institute of Medical Sciences

Introduction Medical Entomology is the science that deals with insects,

arachnids and other arthropods which directly or indirectly effects the human beings

Also it is the study of the biology and control of offending arthropods and contributes to both public and individual health and well-being.

The most abundant form of animal life on earth

Out of some one and a quarter million known animal species, about 4/5th belongs to arthropods.

The insects comprises about 70% of known species of all kinds of animals

Cont….

Over 8 lacs species of insects described so far.

Three quarters of a million species of insect have already been described and estimated number of about 1 million to 30 million are still pipelined.

Entomology (from Greek , entomos, "that which is cut in pieces or engraved/ segmented", hence "insect“

The term “Medical Entomology” is generally a misnomer cause not only insects but other members of phylum artropoda are also included in the study.

Characteristics of Artropods The name arthropod actually comes from the Greek

“arthro,” meaning joint, and “pod” meaning foot.

Invertebrates, Multicellular and body being bilaterally symmetrical

Body covered with hard chitinous exoskeleton

True segmentation of the body, each segment bearing a pair of joint appendages.

Have a wide range of chemical and mechanical sensors, mostly based on modifications of the many seatae (Bristles) that project through their cuticles

Characteristics of arthropods

Characteristics….cont..

Muscles are straited

No ciliated epithelium.

Growth by periodical shedding of skin.

Jaws present.

Have compound eye, may be absent in some members

Sexes are separate.

The nervous system is dorsal (belly), the circulatory system is open and ventral (back) and alimentary canal in between.

Body cavity called “Haemocele” have free communication with circulatory system.

Nervous and circulatory system of arthropods

Characteristics…..cont….

Compound eye of arthopods Molding in arthropods

Arthropods of medical importance

Class: Insecta Class: Arachnida Class: Crustacea

MosquitoesAnophelinesCulicines

TicksHard tickSoft tick

Cyclops

FliesHouseflySandflyTsese flyBlackfly

Mites (Chiggers)Leptotrombidium miteTromboculid miteItch mite

Human liceHead & body liceCrab lice

FleasRat fleaSand flea

Reduiid bug

Insects of Medical Importance

Directly cause damage to human tissue

Act as Vectors for disease-causing organisms

-----------------------------------------------------------Provide useful medical services/drugs

Are useful model systems in genetic studies

Features of arthropods of medical importance

Insecta Arachnida Crustacea

Body division Head Thoraxabdomen

Cephalothorax Abdomen(No division in some cases)

CephalothoraxAbdomen

Legs 3 pairs 4 pairs 5 pairs

Antennae 3 pairs None None

Wings One or two pairsSome are wingless

None

Where found On land On land In water

Class: Insecta

Class: Arachnida and Crustacea

Arthopods in relation to human health

AnnoyanceBlood lossInjury to sense organsDermatosis Myiasis Allergy Anaphylactic shockEnvenomization Entomophobia

Why are these arthopods so successful ???

Small size -there are many more niches for small organisms than for large organisms

Short life cycle - this allows many generations within a given time for selection and evolution to take place.

Large reproductive ability - large numbers of offspring support a large variation for selection and evolution to act upon.

Variation in the life style of different stages in an insect's life (e.g. caterpillar versus butterfly) reduces competition for resources within the species.

Cont…

Wings-the ability to fly has allowed them to colonise freely.

Sensory sophistication - the sensory capabilities surpasses most other organisms.

Evolutionary interactions with other organisms – co-evolution leads to greater specialisation and speciation.

Adaptation of appendages - mouthparts, wings and legs have often become highly specialised.

Modes of Disease Transmission by arthropods

Direct Contact-With the direct contact of hosts, the arthropod vectors

gets transferred from one host to the other.

e.g. pediculosis and scabies.

Mechanical transmission : The agent is transmitted mechanically transmitted by the

arthropods on the outside or inside their bodies

e.g. diseases transmitted by houseflies - diarrhoea, dysentery, cholera, hepatitis A & E etc.

Cont…

Biological Transmission :

Cyclo-developmental : The agent undergoes cyclical changes but doesn’t multiplies in

the body of arthopods, e.g. W. bancrofti (Filariasis) transmitted in Culex female.

Cyclo-propagative : The agent undergoes cyclical changes and also multiplies in the

body of arthopods r. Example is Plasmodium sp in Anopheles female.

Propagative : The agent undergoes no cyclical changes but simply grows and

multiplies in the body of the vector, e.g. Plague bacilli in rat flea

Vector Bionomics

Introduction Bionomics (Greek: bio = life; nomos = law) is the

comprehensive study of an organism and its relation to its environment.

Establishing the relationship between the epidemiology of a disease and the ecological status of its vector.

To devise effective measures to control the vector with understanding this relationship.

Why is it important ???..

Cont…

Bionomics of vectors include the development of immature stages to adults under the influences of….

Environmental conditions.Genetic factors, Ecological factors, which may produce different type of

reaction in a population having the same genetic characteristics.

Each vector occupies a separate ecological habitat, and has specific requirements to be able to…..

… Breed, Develop, Feed, Mate and Lay Eggs

Factors influencing vector bionomics

Temperature: Between species, variations exist in temperature tolerance eg Plasmodium survives in the vector at 20º to 30º C

Humidity: Direct effect on the longitivity of the vector eg. A relative humidity of 60 is necessary for the mosquito and flea to survive through its normal life span.

Rainfall: This increases the relative humidity and hence chances of survival.

Topology: Found at the height 14000 ft above to 3760 ft below sea level, however Anophilines doesn’t survies above 2000 to 2500 mt

Factors effecting….

Water movement: Species differ in their tolerance of current and wave

action eg. An fluviatilis.

Salinity: Some species are fresh-water species; others prefer

brackish water eg. An sundaicus.

Pollution: Some species react differently to oxygen tension and

the presence of organic matter eg Culex

Cont…

Turbidity: The depth to which sunlight penetrates can be a factor

Microflora: Different species prefer different compositions of microscopic plant life as the source of larval food, which must be present in abundance.

Macroflora: Emerging adults have specific requirements of shade, protection, and resting places.

Fauna: The presence of predators and parasitizing organisms reduce the number of immature forms.

Cont…

Gonotrophic cycle: This is the cycle in the life from taking a blood-meal to oviposition.

After feeding, gorged with blood, rest for 24-48 hrs need for meal to get digested and mature eggs have formed in the ovary.

Dispersal: Under normal atmospheric circumstances, flights range from 1 to 11km.

Dispersal is mostly downwind, and strong winds may carry mosquitoes much further.

Passive dispersion through boats, buses, trains, and aircraft is common nowadays.

Cont…

Biting behaviour: Flight, host-seeking, and feeding generally take place in a

warm and humid environment.

Many principal malaria vectors, bite in the latter half of the night when relative humidity is high.

Species may bite during daytime or at dusk eg Aedes

Some mosquitoes prefer biting inside houses (Endophagic) others bite outdoors (Exophagic).

Host preference: Anthropophilic: The preferred vertebrate host may be man

eg Culex

Zoophilic: The preferred vertebrate host maybe animals.

Cont…

Resting habits: Endophilic: The vector prefer resting inside the houses

Exophilic: The vector prefer resting outside the houses.

The design and construction of houses can greatly influence the extent to which they are entered by mosquitoes.

Seasonal prevalence: Some arthropods Hibernate to survive winter temperatures,

either in the egg or the larval stage e.g. moth, grasshoppers.

Hibernation doesn’t occurs in pupal stage in mosquitos.

Aestivation: Some vectors, including An. Gumbiue and fleas, are able to survive hot, dry, and apparently waterless periods.

Cont…

Longevity: Climatic factors greatly influence the vector longevity and

mortality.

Also heavy parasitic infections shorten their life expectancy eg blocked flea, mosquitoes.

The vectorial status of arthropods depends on the margin by which longevity exceeds the Extrinsic incubation period.

Susceptibility to insecticides: Insecticide resistance is induced through the selection of

vectors surviving dosages of insecticides which kill susceptible individuals.

Once induced, resistance is inherited by the next generation.

Thank you