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Medical Entomology

Insects and their relatives can affect our health and that of our livestock and pets in many ways-

Nuisance

e.g. bush flies, ants

Phobia

Spiders, cockroaches, moths

Biting, sucking

Ants, bed bugs, March flies, Horse flies, mosquitoes, sand flies, fleas

Venoms and allergies

Wasps, ants, bees, spiders, ticks, dust mites, cockroach faeces

Blistering, inflammation and urtication (irritating hairs)

Blister beetle, 'Spanish fly', caterpillars, spiders

Disease, infestation

Head, body and pubic lice, mites (scabies), Myiasis (fly strike), bot flies

Disease vectors

Whereby an insect or other arthropod does not cause actual disease but transports the organism responsible.

Two vector methods:

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ORGANISMS TRANSPORTED

Protozoa - malaria, sleeping sickness, Chagas' disease, Leishmania

Arboviruses - myxomatosis, equine encephalitis, yellow fever, dengue, blue tongue virus

Rickettsias - endemic typhus, murine typhus, scub typhus

Bacteria - plague

Nematodes - elephantiasis (filariases), river blindness

Factors affecting disease transmission

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SPECIFIC DISEASES

MALARIA

Disease organism - protozoan

Vector - Mosquito (Dusk and night biting)

Hosts - Man

Distribution - Tropical Asia, Africa, South America, Pacific

Method -

DDT etc

Numbers affected

Control -

SLEEPING SICKNESS

Disease organism - protozoan

Vector - Tse tse fly

Hosts - Man (preferred), cattle

Distribution - arid Africa

Method - The protozoan multiplies in the tse tse fly's blood and then migrates to the salivary glands from where they are transmitted to humans or cattle by biting. In humans, the protozoan multiplies in the blood (stage 1), then in the lymph nodes (stage 2). The disease can also enter the central nervous system. Death by toxins follows in a few months, preceded by emaciation and weakness/sleeping.

There is no cure.

Control -

CHAGAS DISEASE

Disease organism - protozoan

Vector - reduviid (assassin) bug

Hosts - Man, monkeys, armadillos, various mammals

Distribution - Dry marginal agricultural areas of South America

Method - The bug ingests infected blood of a mammal. The disease organism is deposited in the faeces of the bug onto the skin of the next host mammal. It enters the mammal's blood via the skin or eye, is engulfed by a host macrocyte and divides within it. It then circulates within the blood causing toxaemia. It may also lodge and multiple in soft tissues and organs (heart, brain, kidney) and cause either death or chronic illness.

Control -

LEISHMANIA

Disease organism - protozoan

Vector - sandflies

Hosts - Man, dog, rat

Distribution - Generally arid areas in Africa, Middle East, South America,

Method - the disease organism multiples in the fly's midgut, migrates to the pharynx and after 3-5 days is ready for transmission to another mammal. In a mammal, the organism is engulfed by a phagocytic cell where it multiples again in several cycles

Injury is mostly minor but disfiguring, particularly for infants and infections on mucous membranes. It may also cause liver and spleen enlargement.

Control -

PLAGUE

There is currently no content for plague.

ELEPHANTISIS (FILARIASIS)

Disease organism - nematode

Vector - a day-biting mosquito (Culex quinquefasciatus)

Host - Man

Distribution - Asia, PNG

Method - The worm develops to infective stage in the flight muscles of the mosquito, migrates to the head and completes development in the human host. The worm takes several months to maturity and can live in human blood for many years.

Injury - Currently 120 million people are affected, 40 million seriously.

The nematode damages lymph vessels, preventing proper drainage from limbs, breasts and genitalia, leading to massive swelling. There is no cure for the injury and preventative medicines available are useful only in presymptomatic stages.

Control - Standard mosquito control (see malaria) except for netting on beds

RIVER BLINDNESS

There is currently no content for river blindness.

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