Of the many diseases spread by insects, very few are actually caused by the insects themselves but raher, by other organisms passed on when they feed or bite.

Insects are capable of spreading diseases caused by many different types of micro-organisms including bacteria, viruses, protozoans, etc. In these instances it is the micro-organism that is the pathogen (disease causer) and the insect involved is known as the vector.

Biting insects are active at all times of the day and night. However, some prefer differnt times of day eg.the mosquitoes that spread malaria are mainly active after dark but the mosquitoes that spread yellow fever and dengue fever are active during daylight hours.

The following table illustrates the global distribution of the more well known insect borne diseases:-

	When entering a region where any of the above diseases are endemic, you should always take the following precautions: Avoid bites after sunset by wearing long-sleeved clothing and long trousers. Most insects can bite through thin clothing, so spray an insecticide or repellent on them. An insect repellent should also be used on any exposed skin.
Spraying insecticides in the room, burning pyrethroid coils and heating insecticide impregnated tablets all help to control mosquitoes. If sleeping in an unscreened room, or out of doors, a mosquito net (impregnated with insecticide) is a sensible precaution. Portable, lightweight nets are available. Things like Garlic, Vitamin B and ultrasound devices do not prevent bites.

Filariasis is a worm infestation confined mainly to the tropics and spread from person to person by the bites of mosquitoes. Many types of mosquito are involved in transmission. The severity of the disease depends on the number of worms present.

Filariasis affects an estimated 120 million persons in tropical areas of the world, including sub-Saharan Africa, Egypt, southern Asia, the western Pacific islands, the northeastern coasts of South and Central America, and the Caribbean Islands.

The disease is caused by long thread like worms up to 50cm in length which live under the skin and in lymphatic tissues. The female worms release larvae that circulate in the peripheral blood vessels and are ingested by mosquitoes; thus, infected mosquitoes transmit the infection from person to person. Filarial worms can live up to twenty years in humans producing larvae that infect insects but the worms do not multiply in the body.

Short-term travelers to endemic areas are at low risk of contracting filariasis. However, travelers who visit endemic areas for extended periods of time and who are intensively exposed to infected mosquitoes can become infected. No vaccine is available.

Protective measures include avoidance of mosquito bites through the use of personal protection measures

Symptoms vary widely between individuals but severe cases include inflammatory swelling of the arms and legs. Some forms of the disease can cause blindness. In all cases, drug treatment is effective if given before the disease becomes advanced.

Lyme Disease

An infectious disease caused by a bacterium which is spread to humans by tick bites. It is characterised initially by an expanding red rash at the site of the bite often accompanied by headache, muscle and joint aches and fever.

Lyme disease occurs in temperate forested regions of Europe (including the UK) and Asia and in the northeastern, north central, and Pacific coastal regions of North America, Australia, China and Japan. It is not transmitted in the tropics.

Travelers to endemic areas who have frequent or prolonged exposure to tick habitats could be at increased risk for Lyme disease and should be advised to avoid tick habitats if possible. If exposure to tick habitats cannot be avoided, the application of repellents to skin and clothing, as well as regular daily checks for any attached ticks, can reduce the risk of infection. Prompt removal of any attached ticks will help prevent infection.

Repellents containing DEET have been found to be useful but if an embedded tick is discovered, slowly pulling it out with tweezers is the best method of removal and the affected person should remain alert to the development of any symptoms. If a rash appears at the site of attachment, immediate medical attention should be sought. Lyme disease is effectively treated with oral penicillin or tetracyclines.

A safe and efficacious vaccine was, until recently (Feb 2002), available for protection from Lyme disease. However, production of the vaccine was discontinued by the manufacturer because of low demand and is no longer commercially available.

Leishmaniasis

Is caused by single celled organisms called leishmania which are passed on to humans by the bite of the sand fly (phlebotomus). There are two forms of the disease: Cutaneous Leishmaniasis (oriental sore) and Visceral Leishmaniasis (kala azar).

Cutaneous: the infection is restricted to the skin at the site of the bite where it manifests as skin nodules on the face, arms and legs which may form ulcers. The disease is normally self limiting but can leave the victim with disfiguring scars. The disease is found throughout North Africa, the Middle East, Asiatic Russia, Iran and tourist resorts around the Mediterranean.

Visceral: in this instance the parasites invade the liver, spleen and bone marrow causing serious illness, which, if untreated can lead to death. The main problem being failure in diagnosis. It is sometimes misdiagnosed as leukaemia or lymphoma due to enlargement of the spleen, anaemia and weight loss. It occurs in the same regions as the cutaneous form but is more widespread in parts of Africa, India and South America. Children are especially at risk.

Most cases of leishmaniasis respond well to treatment with an antimony drug (sodium stibogluconate) if diagnosed early enough.

Sleeping Sickness

Also known as African Trypanosomiasis is caused by tiny organisms called trypanosomes which are spread by the bites of infected tsetse flies and is confined to areas of tropical Africa.

Tsetse flies breed alongside rivers and transmit the disease between wild animals, cattle and humans. It is also responsible for deaths amongst livestock and is therefore of economic significance. Epidemics usually occur after outbreaks of social or political turmoil.

The first sign of the illness is a boil-like swelling which arises five or more days at the site of the bite (any sooner is probably an allergic reaction). Fever follows within two or three weeks and a serious illness follows which unless treated, affects the nervous system resulting in the characteristic daytime drowsiness.

Due to the unfamiliarity of the disease it can remain undiagnosed for some time. Travellers who become unwell after returning home from an endemic area must inform their doctor of their itinerary.

People on wildlife safaris are most at risk. The flies are less attracted to persons on foot than on horseback and then they are more likely to attack the horse than the rider. They will even follow vehicles travelling slowly through infested areas so windows should be kept closed and any insects that enter should be killed with a knock down insecticide spray.

There is no vaccine available and prophylactic drugs are not recommended since they mask symptoms. Prevention depends largely on avoidance of bites. Effective drug treatment is hazardous and highly specialised.


Chagas Disease

Also known as American Trypanosomiasis, Chagas is a potentially serious disease also caused by trypanosomes which is endemic to South and Central America which is spread by biting bugs called assassin bugs or kissing bugs.

The disease is present in most rural areas particularly where there is a human presence. This is because the bugs that spread the disease live in the walls of mud huts where they venture out, only at night to feed.

Travellers to these areas are most at risk but avoidance is straightforward. True jungle areas of the Amazon are mainly free of the disease and visitors to cities or remote jungle ruins are not at risk.

The bugs become infected by feeding on the blood of someone who has the disease. They then excrete infective trypanosomes in their faeces which enter the blood of a subsequent victim through the bite wound.

Chickens, dogs and opossums also harbour the disease.

Symptoms include swelling around the site of the bite followed by enlargement of the lymph glands and fever. Long term symptoms include damage to the heart causing sudden death and paralysis of the gut causing difficulty in swallowing and severe constipation.

There is no vaccine or preventative drug treatment available and treatment of the disease is difficult since no drug is currently able to kill the parasites without harming the host.

Avoidance of the bites is the best strategy. When travelling through an endemic region try not to sleep in adobe huts where the locals sleep, keep away from walls when sleeping and use mosquito nets. Spraying the insides of rooms with an insecticide spray is also a good idea.


Typhus

There are several varieties of typhus all of which cause fever, severe headache and a skin rash and the severity of the illness varies greatly among the different types. In the past, outbreaks of typhus fever have been responsible for thousands of deaths.

All forms of typhus fever are caused by tiny organisms called rickettsiae which are passed on to humans by various types of insects including lice (epidemic), fleas (endemic), mites (scrub) and ticks.

The illness exists world-wide but nowadays, most cases of typhus are confined to areas of Central and Southern Africa, South America and Central and South East Asia. Epidemic typhus is only likely to affect volunteer workers who come into close contact with locals.

People who walk through tropical bush should inspect their skin for any attached ticks and carefully remove them. There are no vaccines available for any form of typhus.

Symptoms of the disease include; severe headache, fever and skin rashes. The severity of the illness depends on the variety of typhus. The disease can be fatal but responds well to antibiotic therapy using tetracyclines if given early enough.

Plague

Otherwise known as The Black Death, this disease is one of the oldest known and is found world-wide including the USA. In the fourteenth century it spread across Europe and was responsible for wiping out one third of the population.

The causative organism is a bacterium (Yersinia Pestis) and it is primarily a disease of rodents especially rats. The infection is spread to humans by the bite of the rat flea (Xenopsylla cheopis).

Fleas become infected by feeding on the blood of an infected rodent. When the rodents die the fleas go in search of a new host. They can then transmit the disease to humans who become their hosts.

The disease manifests itself in two forms; bubonic plague and pneumonic plague:-

The symptoms of bubonic plague appear suddenly about seven days after exposure and include high fever, headache,muscular aches,shaking chills and pain in the groin and armpits due to the formation of buboes or swollen lymph nodes.

Untreated, the disease can spread rapidly throughout the body via the lymphatic system. When the lungs become infected an infectious pneumonia may develop.