Of the many diseases spread by insects, very few are actually caused by the insects themselves but rather, 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:-

Filariasis

Filariasis is a worm infestation confined mainly to the tropics and spread from person to person by the bites of mosquitoes and other flying insects. 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 but travelers who visit endemic areas for extended periods of time and who are intensively exposed to infected mosquitoes etc. can become infected. No vaccine is available.

Protective measures involve 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.

Loa loa is the filarial nematode (roundworm) species that causes Loa loa filariasis. It is commonly known as the "eye worm".and is one of three parasitic filarial nematodes that cause subcutaneous filariasis in humans.

Loa Loa (African Eyeworm)

Loa loa, also known as African eye worm, is caused by the parasitic worm Loa loa. It is transmitted through the repeated bites of mango flies (or mangrove flies also known as deerflies ) of the genus Chrysops. The flies that transmit the parasite breed in the rain forests of West and Central Africa. In addition to eye worm, the infection is most commonly associated with recurrent episodes of itchy swellings known as Calabar Swellings.

The flies that transmit Loa loa bite during the day. The infective larvae are present in the mouth parts of the fly and enter the wound made in the skin by the fly when it takes a blood meal from a human.
Once larvae enter the human body they usually mature to adult worms in around 5 months. Adult worms live between layers of connective tissue under the skin. Fertilized females can produce thousands of larvae a day. The larvae then migrate into the lymphatic system and accumulate in the lung.

These larvae are intermittently released into the blood stream. It usually takes 5 to 6 months or longer after initial infection for the larvae to be found in the bloodstream. The larvae can survive up to one year in the human body. Adult worms may live up to 17 years in an infected person and can continue to release new larvae into the body for much of this time.

If a mango fly takes a blood meal from an infected human and ingests larvae, the larvae will infect cells in its abdomen. After 7 to 12 days the larvae mature into larvae that are infective for humans.

Most people with Loa loa do not experience signs or symptoms of infection, though persons who do not live in areas where the parasite is found, such as travelers to the area, are more likely to experience them. The most common manifestations of the disease are Calabar swellings and eye worm.

Calabar swellings are localized, non-tender swellings usually found on the limbs and near joints. They are associated with itching that occurs in the area of the swelling or is generalized to the large areas of the body.

Eye worm is the visible migration of the adult worm across the surface of the eye. Eye worm can be accompanied by eye congestion, itching, pain, and light sensitivity. Although eye worm can be very distressing, it usually lasts less than one week (often just hours) and commonly causes minimal damage to the eye.

The main methods of diagnosis include the presence of microfilariae in the blood, the presence of a worm in the eye, and the presence of Calabar skin swellings.

Treatment of Loa loa can be difficult and should be made in consultation with an expert in infectious diseases or tropical medicine. The most straightforward treatment is surgical removal of adult worms migrating under the skin or across the eye. This can be done with local anesthesia and offers immediate relief, though this only relieves local symptoms and does not cure the patient of the underlying infection.

There are two antiparasitic agents that can be used to treat the infection and manage the symptoms; diethylcarbamazine and albendazole. Diethylcarbamazine kills both the larvae and the adult worms and albendazole, which kills the adult worms.

Diethylcarbamazine is the preferred treatment but there is a risk of fatal brain inflammation which can be reduced through appropriate testing and management. In some cases, treatment is not recommended.

There are no vaccines available to prevent infection with Loa loa. Diethylcarbamazine 300mg taken once a week is effective at preventing it in long-term travelers to affected areas. Since the mango flies breed in muddy, shaded areas along rivers and are attracted to smoke from wood fires, avoiding those areas may reduce the risk of infection.

Other prevention efforts include personal protection measures against biting insects. This includes wearing insect repellant such as DEET on exposed skin, wearing long sleeves and long pants during the day when deer flies bite, and wearing permethrin treated clothing.

Lyme Disease

Lyme disease is a bacterial infection that is spread to humans by infected ticks. Ticks are tiny arachnids found in woodland areas that feed on the blood of mammals, including humans.

Tick bites often go unnoticed and the tick can remain feeding for several days before dropping off. The longer the tick is in place, the higher the risk of it passing on the infection.

The disease is characterised initially by an expanding red rash at the site of the bite often accompanied by headache, muscle and joint aches and fever.

The earliest and most common symptom of Lyme disease is a pink or red circular rash that develops around the area of the bite, three to 30 days after someone is bitten. The rash is often described as looking like a bull’s-eye on a dart board.

There may also be flu-like symptoms, such as tiredness, headaches and muscle or joint pain.

If Lyme disease is left untreated, further symptoms may develop months or even years later and can include:

Lyme disease in its late stages can trigger symptoms similar to those of fibromyalgia or chronic fatigue syndrome. This is known as chronic Lyme disease. More research into this form of Lyme disease is needed.

A person with Lyme disease is not contagious because the infection can only be spread by ticks.

Ixodes ricinus, the castor bean tick, is a European species of hard-bodied tick that can reach a length of 11 mm (0.43 in) when engorged with a blood meal, and can transmit both bacterial and viral pathogens such as the causative agents of Lyme disease and tick-borne encephalitis.

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.

The Health Protection Agency (HPA) estimates that there are 2,000 to 3,000 cases of Lyme disease in England and Wales each year, and that about 15%-20% of cases occur while people are abroad.

Parts of the UK that are known to have a high population of ticks include:

Most tick bites occur in late spring, early summer and during the autumn because these are the times of year when most people take part in outdoor activities, such as hiking and camping.

Prolonged exposure to tick habitats increase the risk of Lyme disease and should be avoided 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.

The best way to prevent getting Lyme disease is to be aware of the risks when you visit areas where ticks are likely to be found and to take sensible precautions.

When travelling to other European countries or to North America, where the infection occurs more frequently than in the UK, you should also be aware of the risks.

You can reduce the risk of infection by:

• being aware of ticks and the areas where they usually live
• wearing appropriate clothing in tick-infested areas (a long-sleeve shirt and trousers tucked into your socks)
• wearing light-coloured fabrics that may help you spot a tick on your clothes
• using insect repellents
• inspecting your skin for ticks, particularly at the end of the day, including your head, neck and skin folds (armpits, groin, and waistband)
• checking your children’s head and neck areas, including their scalp
• making sure that ticks are not brought home on your clothes
• checking that pets do not bring ticks into your home in their fur

Oral antibiotics are recommended for treating early, mid- and late-stage Lyme disease. Most people will require a two- to four-week course depending on the stage of the condition. It is very important that the course of antibiotics is completed to ensure that all the bacteria are destroyed.

If symptoms are particularly severe and include arthritis or neurological conditions, intravenous antibiotics may be used. Most people with mid or late stage Lyme disease will require a course of intravenous antibiotics.

Leishmaniasis

Leishmaniasis is a widespread parasitic disease caused by single celled organisms called leishmania which are transmitted by the bite of an infected sandfly (phlebotomus).

The disease occurs in three forms, cutaneous leishmaniasis, mucocutaneous leishmaniasis, and visceral leishmaniasis, each of which varies in incidence and severity. The three predominant forms of leishmaniasis can affect the skin, mucosa, and/or internal organs resulting in severe disfigurement, disability, or death.

Leishmaniasis is endemic in 88 countries across four continents with around 350 million people currently at risk of infection. Each year it is estimated that 1.6 million new infections occur as well as 47,000 deaths.

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. The disease is often called "Oriental Sore".

Mucocutaneous: For unknown reasons, certain species of Leishmania parasites migrate beyond the site of initial sandfly bite to cause the more extensive disease; mucocutaneous leishmaniasis. Mucocutaneous disease occurs when parasites migrate to mucosal surfaces, generally of the nose or mouth.

As with cutaneous lesions, parasites replicate in the tissues causing damage. However, unlike the cutaneous form of the disease, mucocutaneous lesions are not self-limiting and can result in permanent damage or loss of the nose, soft palate, or lips. Mucocutaneous disease is the least common form of leishmaniasis and is primarily limited to South America.

Visceral: (Kala Azar) 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

African Trypanosomiasis, also known as "sleeping sickness," is caused by microscopic parasites called trypanosomes. It is transmitted by the bites of infected tsetse flies and is confined to areas of tropical Africa.

Sleeping Sickness has been a serious public health problem in some regions of sub-Saharan Africa with about 10,000 new cases each year being reported and it is believed that many cases go undiagnosed and unreported.

Sleeping sickness is curable with medication, but is fatal if left untreated.

There are two subspecies of the parasite Trypanosoma brucei that cause the disease in humans.They are found in different regions of Africa (East & West African Sleeping Sickness). At present, there is no overlap in their geographic distribution.

East African sleeping sickness is found in focal areas of eastern and southeastern Africa. Over 95% of the cases of human infection occur in Tanzania, Uganda, Malawi, and Zambia. Animals are the primary reservoir of infection. Cattle have been implicated in the spread of the disease to new areas and in local outbreaks. A wild animal reservoir is thought to be responsible for sporadic transmission to hunters and visitors to game parks. Infection of international travelers is rare, but it occasionally occurs. Most cases of sleeping sickness in travelers are amongst those who went on safari in East Africa.

West African sleeping sickness is found predominantly in central Africa and in limited areas of West Africa. Most of the sleeping sickness in Africa is caused by this form of the parasite. Over 95% of the cases of human infection are found in Democratic Republic of Congo, Angola, Sudan, Central African Republic, Chad, and northern Uganda.

Humans are the important reservoir of infection, although the parasite can sometimes be found in domestic animals (e.g., pigs, dogs, goats). Imported infection by returning travellers is extremely rare, and most cases have occurred in African nationals who have immigrated rather than in returning travelers.

Both forms of sleeping sickness are transmitted by the bite of the tsetse fly (Glossina species). Both male and female flies can transmit the infection.

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.

East African sleeping sickness progresses rapidly. 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 along with headache, muscle and joint aches, and enlarged lymph nodes. Sometimes there is a rash. After a few weeks of infection, if left untreated the parasite invades the central nervous system and eventually causes mental deterioration and other neurologic problems including the characteristic daytime drowsiness. Death ensues usually within months.

West African sleeping sickness progresses more slowly. At first, there may be only mild symptoms. Infected persons may have intermittent fevers, headaches, muscle and joint aches, and malaise. Itching of the skin and swollen lymph nodes. Weight loss is common.

Usually, after one to two years, there is evidence of central nervous system involvement, with personality changes, daytime sleepiness with nighttime sleep disturbance, and progressive confusion. Other neurologic signs, such as partial paralysis or problems with balance or walking may occur, as well as hormonal imbalances. The course of untreated infection rarely lasts longer than six or seven years and more often kills in about three years.

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. Most bites that occur on the African savannah are quite painful, and travellers often recall the bite. Travellers to urban areas are not at risk.

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 are endemic to South and Central America and is spread through contact with the feces of an infected triatomine bug alsoi known as assassin bugs or kissing bugs (a blood-sucking insect that feeds on humans and animals).

Chagas disease is endemic throughout much of Mexico, Central America, and South America where an estimated 8 to 11 million people are infected. The triatomine bug thrives under poor housing conditions (for example, mud walls, thatched roofs), so in endemic countries, people living in rural areas are at greatest risk for acquiring infection. Public health efforts aimed at preventing transmission have decreased the number of newly infected people and completely halted vectorborne transmission in some areas.

American trypanosomiasis is caused by the protozoan parasite, Trypanosoma cruzi.

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 may also harbour the disease.

Chagas disease has an acute and a chronic phase. If untreated, infection is lifelong.

Acute Chagas disease occurs immediately after infection and may last up to a few weeks or months. During this time parasites may be found in the circulating blood. Infection may be mild or asymptomatic. There may be fever or swelling around the site of inoculation. Rarely, acute infection may result in severe inflammation of the heart muscle or the brain and lining around the brain.

Following the acute phase, most infected people enter into a prolonged (chronic) asymptomatic form of disease during which few or no parasites are found in the blood. During this time, most people are unaware of their infection. Many people may remain asymptomatic for life and never develop Chagas-related symptoms. However, an estimated 20% to 30% of infected people will develop debilitating and sometimes life-threatening medical problems over the course of their lives.

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.

The risk of acquiring Chagas disease while traveling is considered to be extremely low. Travellers could be at risk of Chagas disease if staying in poor-quality housing in endemic areas.

Chagas disease could be acquired through blood transfusion in areas with poor screening or by consuming contaminated food or beverages.

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. Clinicians should consult with an infectious disease or tropical medicine specialist to diagnose and treat Chagas disease.

Antitrypanosomal drug treatment is usually recommended for acute and chronic Trypanosoma cruzi infection based on recent data suggesting that a course of antitrypanosomal treatment delays progression of cardiomyopathy and decreases mortality.

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.

Travelers who cannot avoid camping, sleeping outdoors, or sleeping in poorly constructed houses in endemic areas should use insecticide-impregnated bed nets and tuck in the edges to provide a physical barrier to the vectors.


Typhus

There are several varieties of typhus all of which cause fever, severe headache and a skin rash (spotted fever) 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. Ricketsial illness exists world-wide

All age groups are at risk of rickettsial infections during travel to endemic areas. Transmission is increased during outdoor activities in the spring and summer months when ticks and fleas are most active. However, infection can occur throughout the year.

Because of the five to fourteen day incubation period for most rickettsial diseases, tourists may not necessarily experience symptoms during their trip, and onset may coincide with their return home or develop within a week after returning. Epidemic typhus is only likely to affect volunteer workers who come into close contact with locals.

Rickettsial infections among travelers include Mediterranean spotted fever from southern Europe and Africa, Indian tick typhus from India, Astrakhan fever from southeastern Europe and central Africa, Israeli tick typhus from Mediterranean countries, Thai tick typhus from Asia and Australia, Queensland tick typhus and Australian spotted fever from eastern Australia, tickborne lymphadenopathy from European countries, north Asian tick typhus from China and Russia, Rocky Mountain spotted fever and Rickettsia parkeri from the Americas, and African tick-bite fever from Africa and the Caribbean islands.

Game hunting and traveling to southern Africa from November through April are risk factors for African tick-bite fever in travelers. Contact with tick-infested dogs in areas endemic for certain rickettsiae may increase the risk of disease. One study estimated that the risk of a traveler contracting a rickettsiosis in southern Africa isfour to five times higher than that of acquiring malaria.

Epidemic typhus occurs in communities and refugee populations where body lice are prevalent. Outbreaks often occur during the colder months when infested clothing is not laundered. Travelers at most risk for epidemic typhus include those who may work with or visit areas with large homeless populations, impoverished areas, refugee camps, and regions that have recently experienced war or natural disasters.

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.

typhus,usually manifests with a sudden onset of headache, chills, fever, and general pains. On the third to fifth day a rash and toxemia develpos. About ten days after being bitten, an infected person experiences headache, loss of appetite, malaise, and a rapid rise in temperature with fever, chills, marked prostration, and nausea. Four to six days after onset, a characteristic rash appears over most of the body; the sick person is flushed, and the eyes are bleary.

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.

Rocky Mountain Spotted Fever

is a tickborne disease caused by Rickettsia rickettsii. This organism is a cause of the potentially fatal human illness in North and South America, and is transmitted to humans by the bite of infected ticks. In the United States, these include the American dog tick, Rocky Mountain wood tick, and brown dog tick.

Typical symptoms include: fever, headache, abdominal pain, vomiting, and muscle pain. A rash may also develop, but is often absent in the first few days, and in some patients, never develops. Rocky Mountain spotted fever can be a severe or even fatal illness if not treated in the first few days of symptoms.

Doxycycline is the first line treatment for adults and children of all ages, and is most effective if started before the fifth day of symptoms. The initial diagnosis is made based on clinical signs and symptoms, and medical history, and can later be confirmed by using specialized laboratory tests. Rocky Mountain Spotted Fever and other tickborne diseases can be prevented.

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. These bacteria are found in many areas of the world, including the United States in rural and semi-rural areas of the western states and is most common in the southwestern states, particularly New Mexico, Arizona, and Colorado. Human plague occurs in areas where the bacteria are present in wild rodent populations. The risks are generally highest in rural and semi-rural areas, including homes that provide food and shelter for various ground squirrels, chipmunks and wood rats, or other areas where you may encounter rodents.

People most commonly acquire plague when they are bitten by a flea that is infected with the plague bacteria. People can also become infected from direct contact with infected tissues or fluids while handling an animal that is sick with or that has died from plague. Finally, people can become infected from inhaling respiratory droplets after close contact with cats and humans with pneumonic plague.

Rat fleas (Xenopsylla cheopis) become infected by feeding on the blood of an infected rodent. The infection is then spread to humans by the bite of the rat flea. 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 three forms; bubonic plague, septicaemic plague and pneumonic plague:-

• Bubonic plague: About seven days after exposure patients develop sudden onset of fever, headache, chills, and weakness and one or more swollen, tender and painful lymph nodes (called buboes). This form is usually the result of an infected flea bite. The bacteria multiply in the lymph node closest to where the bacteria entered the human body. If the patient is not treated with appropriate antibiotics, the bacteria can spread to other parts of the body.

• Septicaemic plague: Patients develop fever, chills, extreme weakness, abdominal pain, shock, and possibly bleeding into the skin and other organs. Skin and other tissues may turn black and die, especially on fingers, toes, and the nose. Septicemic plague can occur as the first symptoms of plague, or may develop from untreated bubonic plague. This form results from bites of infected fleas or from handling an infected animal.

• Pneumonic plague: Patients develop fever, headache, weakness, and a rapidly developing pneumonia with shortness of breath, chest pain, cough, and sometimes bloody or watery mucous. Pneumonic plague may develop from inhaling infectious droplets or from untreated bubonic or septicemic plague that spreads to the lungs. The pneumonia may cause respiratory failure and shock. Pneumonic plague is the most serious form of the disease and is the only form of plague that can be spread from person to person (by infectious droplets).