Vector Of Medical Importance Para 08

VECTOR OF MEDICAL IMPORTANCE

Learning objectives

• Define the terms; Entomology, Vector, Biological vector and Mechanical vector.
• Classify vectors of medical importance
• List vectors of medical importance
• Describe characteristics of vectors of medical importance
• Explain feeding and breeding habits of vectors of medical importance
• Describe life cycles of each vector of medical importance

Describe control measures of each vector of medical importance

Definition of terms;

Medical entomology: Is a study of arthropods of medical importance.

Vector: Is an organism that conveys pathogens from one host to another. Vectors can be biological or mechanical.

A biological vector: Is the one in which the pathogens undergo development into infective stage.

Mechanical vector: Is an arthropod vector that transmits the infective organisms from one host to another but is not essential to the life cycle of the parasite.

Classify vectors of medical importance

Vectors of medical importance can be classified into two groups;

Insecta: Diptera (such as flies), Siphonaptera( fleas) and Heteroptera, or Hemiptera( bugs), Anoplura( lice).

Arachnids: Ticks and Mites)

Vectors of medical importance

• Mosquitoes
• Tsetse flies
• Simulium flies species.
• Pulex irritant
• Xenopsylla cheopsis

Ornithodorus moubata

Characteristics of vectors of medical importance

• All vectors transmit pathogens either biologically or mechanically.

For transmission of pathogens to occur vectors need to come into contact with hosts.

All vectors are arthropods though snails (which are intermediate hosts) are conveniently considered as vectors for medical purposes.

Biological vectors seek their host for blood meal while mechanical vectors do not.

Feeding and Breeding habitats of Adult Mosquitoes

Only females suck blood, they can feed during Diurnal(the day time) or nocturnal (the night) Breeding habitats; after blood meal, the female mosquitoes lay eggs directly on or near water, soil and at the base of some plants in places that may fill with water.

Resting: Endophilic (After feeding some blood, mosquitoes prefer to rest indoors) or exophilic (resting outdoors) All these behaviours have implications in disease transmission and control methodologies

Life Cycle

• Female anopheles mosquitoes feed on blood before they lay eggs.

After blood meal, they lay eggs (oviposit). This process takes three days.

Usually a single mosquito lays 30-300 eggs at each oviposition. Eggs are laid singly or in rafts on water or wet surfaces, most do not survive desiccation.

Eggs hatch into larvae in stagnant water. Mosquito eggs hatch in three days if the environment is humid and temperature is high.

Larvae develop into pupae.

• Pupae are comma shaped and motile.
• Pupae release adult.
• Mosquitoes mate shortly after emergence from pupae.

The whole life cycle takes 7 days under favourable conditions

Figure 1: Life Cycle of Mosquito

Source: Winipeg City, 2009

Control Measures of Mosquitoes

• Adult mosquitoes
• Are controlled by using insecticides
• Use of Insecticides Treated Nets (ITNs)
• Organochlorides (OCS) e.g. DDT
• Organophosphates (ops) e.g. malathion
• Fenitrothion
• Chlopyriphos

Pyrethroids; carbamates.

Larval Measures

• Environmental management
• Source reduction
• Destroy breeding places
• Filling in ponds
• Drain and remove stagnant water
• Burry or cover containers
• Intermittent flushing and flooding
• Sanitation systems
• Polystyrene beads

Removal of aquatic vegetation

Biological control:

• Use of predators and pathogens
• Fish: Gambusia, Tilapia and Northobranchius

Mosquitoes: Toxorhynchites

• Fungi
• Nematodes
• Larvicides
• Organophosphates

Pyrethroids: Permethrin, Deltamethrin

• Insect Growth Regulators(IGRS) e.g. Methopren, Diflubenzuron
• Biocides: Bacillus thuringiensis, Bacillus Sphaericus.

Use of herbicides: Diquat or Pentachlorophenol

Feeding site.

• Tsetse flies feeds on blood of human and animal.

Breeding site

The breeding sites/habitat of Tsetse flies are in the forest, woodland, riverine, bushes/shrubs and swampy areas.

Feeding and Breeding habitats of Tsetse flies

Life Cycle of Tsetse flies

The breeding grounds for the riverine species are sandy beaches and loose soil near water.

The breeding grounds for the bush species are loose soil near fallen trees or low branching limbs of trees.

The female produces a single, large, mature, 3rd stage larva at intervals of about 10 days.

• Glossina palpalis yields a total of nine larvae.
• The larva burrows to a depth of 2 inches in the ground and immediately pupates.

The adult fly emerges in about 5 weeks.

Control of Tsetse Flies

• Riverine tsetse flies
• Clearing trees and bush from stretches of river banks
• Selective removal of shrubs and trees along the stream
• Adult flies can be reduced by trapping and by selective insecticiding.

Male sterilization

Woodland tsetse

• Clearance of tracts for agricultural purposes
• Selective bush clearance
• Trapping of flies
• Use of insecticides e.g. Dieldrin, BHC, and DDT to eliminate residual flies
• Sterile male technique

Spraying of vehicles entering and leaving the tsetse fly infested areas

Feeding and Breeding habitats of Simulium

• Feeding site.
• Simulism, they feed on blood of human and animal.

Breeding site

The breeding sites/habitat of Simulium are in moderately swift woodland streams in upland regions and They remain near, or move along, these shaded watercourses.

Simulium

• Simulium breeds in moderately swift woodland streams in upland regions.

They remain near, or move along, these shaded watercourses.

The females bite during the daytime, especially in the morning and toward evening in open places at the edge of thick vegetation.

• They may enter darkened houses, and they bite humans in the vicinity of buildings.

Fishermen are particularly at high risk.

Vector Of Medical Importance Para 08

Eggs are laid in batches of 300 – 500 and are attached by a gelatinous secretion to stones, leaves, submerged plants, stakes, and branches.

In 3 to 5 days, a yellowish-green, cylindrical larva emerges and attaches itself in an upright position to rocks, aquatic vegetation, fresh-water crabs, and other debris.

• It moults seven times in 13 days to become a pupa.
• The adult emerges in about 3 days.
• Emerging adults live from two to three weeks, to as long as 85 days.

Life Cycle

Control measures of simulium

• Control of black flies is difficult because of the number of potential breeding sites.

Mechanical destruction of breeding places is effective but expensive.

Fine head nets, tight sleeves, and trousers, and repellents are used for personal protection.

1. Source Reduction:

Habitat Modification – Identify and modify breeding sites. Simulium larvae typically develop in fast-flowing, oxygenated water. Environmental engineering could involve altering water flows or streams to disrupt breeding sites.

Vegetation Management – Clear or manage vegetation near water bodies to reduce larval habitats.

Control measures of simulium

2. Chemical Control:

Larvicides – Use of larvicides like Bacillus thuringiensis israelensis (Bti), which is biologically specific and safe for aquatic environments. Application of larvicides is usually aerial, with targeting of specific breeding habitats in fast-flowing water.

3. Biological Control:

Natural Predators – Encourage the habitat of natural predators such as certain fish species that consume larval stages.

Control measures of simulium

4. Community Involvement:

Health Education – Educate communities on the importance of controlling blackflies for preventing river blindness. Involvement of community in larviciding programs and monitoring efforts is crucial for sustainability.

5. Personal Protection:

Protective Clothing – Wearing long-sleeved shirts and trousers to minimize skin exposure, especially near breeding sites.

Insect Repellents – Application of insect repellents containing DEET on exposed skin areas.

Control measures of simulium

The medically important fleas belong to two general category;

• Pules
• Xenopsylla

Fleas of Medical Importance

Feeding and Breeding habitats of fleas

• Feeding site.
• fleas they feed on blood of human and animal.
• Breeding site

The breeding sites/habitat of fleas are nest or farming areas near pig’s famers.

Life Cycle of Pulex and Xenopsylla Species below.

• Eggs are shed by the female in the environment
• Eggs hatch into larvae in about 3-4 days and feed on organic debris in the environment.
• The number of larval instars varies among the species.
• Larvae eventually form pupae

Life Cycle of Pulex and Xenopsylla Species

The larval and pupal stages take about 3-4 weeks to complete.

Afterwards, adults hatch from pupae and seek out a warm-blooded host for blood meals.

The primary hosts for Ctenocephalides felis and Ctenocephalides canis are cats and dogs, respectively, although other mammals, including humans, may be fed upon.

• The primary hosts for Xenopsylla cheopis are rodents, especially rats.

Humans are the primary host for Pulex irritans.

Vector Of Medical Importance Para 08

Killing of rodents should be done after killing fleas, otherwise the fleas will leave the dead rodents and jump onto man and result in increased disease transmission.

• Fleas may be controlled in the following methods:

Chemical Control

For the control of rodent fleas xenopsylla cheopis and Pulex irritans, insecticides can be applied to the floors of houses, and runways of rodents.

• The following organochlorides are used: DDT, HCH, or Dieldrin.

Control Measures of Fleas

Also insecticides dust can be blown into rodent burrows.

In situations where fleas are resistant to organochlorides, use organophosphate and carbamate insecticides such as:

• Diaznon

Fenthion (Baytex)

• Malathion
• Fentrothion (Sumithion) or

Carbaryl (Sevin)

• House fumigation with insecticides

Use of pesticides

Personal Protection

To protect humans from flea bites, repellents, e.g. Diethyl-meta-toluamide (DEET), Diisopropyl methylphosphonate (DIMP) or benzyl benzoate emulsion (BBE) may afford personal protection.

Feeding Site

Primarily feed on humans and domestic animals, including pigs, leading to the transmission of pathogens. Preferential biting sites include soft and warm areas of the body such as the armpits, groin, and inside of the ears. As nocturnal feeders, they tend to bite hosts during the night.

Breeding Site

Often found in rustic dwellings, animal burrows, or similar environments where they can live close to hosts. They thrive in crevices, cracks in flooring, or inside wall spaces of mud-and-thatch homes, commonly found in rural areas of East Africa. Females lay eggs in these sheltered environments after a blood meal Feeding and Breeding habitats of Ornithodorus moubata

Both sexes are blood suckers

After taking a blood meal, the female tick, drops off the host to deposit its eggs in soil It oviposits 2000-8000 eggs and then dies Soft ticks lay 100-200 eggs in several batches following successive blood meals After 2-7 weeks, eggs hatch into hexapod larvae (seed ticks) Seed ticks attach themselves to small animals for a blood meal, and then drop off to Life cycle of Ornithodorus moubata

moult into nymphs with four pairs of legs

• The life cycle of ticks is completed in 1-2 years

Hard ticks have a single nymphal stage but soft ticks have several Various modifications of the cycle such as change of host, length of time on the host, number of moulds, and frequency of oviposition occur in different species Life cycle

Vector Of Medical Importance Para 08

During the larval, nymphal, and adult stages, ticks are intermittent parasites of animals and spend most of their existence on the ground Some species e.g. boophilus, however, spend most of their lives on the animals Life cycle

Soft ticks are best controlled by destroying their nests or lairs

Floors and walls should be plastered to eliminate the crevices Spraying with DDT or benzene hexachloride (BHC). More than one application is required, since these insecticides are ineffective against the eggs Control measures of soft Ticks

Key points

• Describe feeding and breeding habits of vectors of medical importance
• Explain life cycles of each vector of medical importance

Describe control measures of each vector of medical importance

References

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Hagner, R. Root, M. Augustine, L. & Huff, G. (1939). Parasitology, with Special Reference to Man and Domestic Animals. New York Inc: D. Appleton-Century Company.

Harwood, R.F. & James, M.T. (1979). Entomology in Human and Animal Healthc (7th Ed.). Pulman: Washington State University.

Mike, S. (2004). Medical Entomology for students. London: Oxford University Press.

Monica, C. (1987). Medical Laboratory Manual for Tropical Countries. Volume 1 (2nd Ed.). Heinemann Ltd, Oxford: ELBS Butterworth.

Monica, C. (1998). District Laboratory Practice in Tropical Countries. Part 1. Tropical Health Technology. NOIDA, India: Gapson Papers Ltd 38

Vector Of Medical Importance Para 08

Monica, C. (2000). District Laboratory Practice in Tropical Countries. Part 2. Tropical Health Technology. Cambridge University Press UK.

Service, M.W. (1986). Lecture Notes on Medical Entomology for students. London: Blackwell Scientific Publications.

Service, M.W. (2004). Medical Entomology for Students. London: Oxford University Press.

United Republic of Tanzania. (2007). Training Course on Laboratory Diagnosis of Malaria: Malaria Control Series 17. National Malaria Control Programme of the Ministry of Health and Social Welfare.

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References