Insect Borne-Diseases:

Malaria:

Yellow Fever:

Yellow fever is a viral illness which is spread by the bite of a mosquito. The species of mosquito that is responsible for spreading yellow fever is Aedes Aegypti and is a different species from the anopheles mosquito that transmits malaria.

Essentially yellow fever is a disease of monkeys living in tropical rain forests. The virus which causes the disease is one of a group of viruses known as arboviruses. Humans are infected by being bitten by rain forest mosquitoes carrying the yellow fever virus.

As their name implies "arboviruses" are transmitted to humans by insects (arthropod borne virus). The mosquito in particular is well suited to the transport and spread of the infection due to its wide distribution throughout the tropics.

The geographical distribution of yellow fever is confined to Equatorial Africa and Central South America as shown on the maps as shown.

Strangely, yellow fever is unknown in Asia despite the presence of mosquitoes capable of spreading the virus.

Arbovirus illnesses usually have two characteristic phases, the first when the virus is invading the host cells, and the second a few days later when the body's immune system is fighting the infection. The antibodies produced during the second phase of illness can cause damage to the blood vessels which explains why arboviruses often cause bleeding.

Many yellow fever infections are mild and go unrecognized but severe and life threatening illness is not uncommon. After an incubation period of about three to six days fever, headache, abdominal pain and vomiting develop. After a brief recovery period, shock, bleeding and signs of liver and kidney failure develop. Liver failure is associated with jaundice hence the name "yellow fever".

There is no drug available to cure yellow fever hence treatment is aimed at symptomatic relief. Overall about 5% of patients die. Those who recover do so completely and are immune thereafter.

Fortunately yellow fever is one of the few arboviruses for which a vaccination is available. A single injection of a live, weakened (and harmless) virus stimulates the body's immune defenses and confers effective immunity for ten years.

Dengue Fever:
This is an unusual arbovirus infection since no other animals except humans and mosquitoes play a significant part in perpetuating the infection. It is present in Africa, South East Asia, the Pacific area and northern South America.

The disease is spread from person to person by the bite of a mosquito (Aedes aegypti) and after about five days incubation period there is a sudden onset of fever, headache and severe joint and muscle pains. The initial fever resolves after about three to five days only to recur with the appearance of a rash consisting of small white spots which starts on the trunk and spreads to the limbs and face. Within a few days the fever subsides and recovery follows.

Although dengue is a very unpleasant illness, complications are uncommon and recovery is usually complete.

There is also a more severe and life threatening hemorrhagic form of the disease which has appeared with dramatic outbreaks. Fortunately this form occurs only rarely.

This is thought to be the result of a second infection where there is some remaining immunity from a first attack causing a vigorous immunological response in which severe blood vessel damage occurs.

Unfortunately, immunity to infection does not last long and subsequent attacks are possible. There is no vaccine available. Prevention is by avoiding mosquito bites.

Japanese Born Encephalitis:

This is a rare but serious arboviral infection with a 20% fatality rate. It occurs in most of the Far East and South East Asia. The endemic zone extends from India and Nepal across the whole of South East Asia to Japan and Korea in the Far East.

The risk of infection is greatest in long term visitors to rural areas, and the risk to short term visitors and visitors to major cities is small. Precautions against mosquito bites are essential.

Japanese B Encephalitis is transmitted by rice field breeding mosquitoes (of the Culex group) that become infected with Japanese encephalitis virus.

Mosquitoes become infected by feeding on domestic pigs and wild birds infected with the Japanese encephalitis virus. Infected mosquitoes then transmit the Japanese encephalitis virus to humans and animals during the feeding process.

The incubation period is normally between 5 and 15 days. The illness cannot be passed on from person to person. There is no specific treatment. Intensive supportive therapy is indicated.

Mild infections can sometimes occur without apparent symptoms other than mild fever with headache. More severe infection is marked by quick onset, headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, occasional convulsions (especially in infants) and spastic paralysis.

Tick Born Encephalitis:

Ticks are blood feeding external parasites of mammals, birds, and reptiles throughout the world. Ticks are not insects like fleas, but arachnids like mites, spiders and scorpions.

Tick-borne encephalitis (TBE), is a viral disease affecting the central nervous system. The disease is most often manifest as meningitis or encephalitis or sometimes, even meningoencephalitis. It is caused by the tick-borne encephalitis virus (TBEV), a member of the family Flaviviridae.

TBE is endemic in many European countries, the former Soviet Union, and Asia. It is found mainly in temperate regions. The natural hosts of the virus are small rodents and deer, with humans being accidental hosts. After attaching itself to the host, an infected tick transfers the virus to the host during feeding.

In disease endemic areas, people with recreational or occupational exposure to rural or outdoor settings such as; hunters, hikers, campers, forest workers, farmers, etc. are potentially at risk of infection by contact with the infected ticks.

The incubation period of TBE is usually between 7 and 14 days.

A characteristic biphasic febrile illness follows, with an initial phase that lasts two to four days. It is non-specific with symptoms that may include fever, malaise, anorexia, muscle aches, headache, nausea, and/or vomiting.

After about eight days of remission, the second phase of the disease occurs in twenty to thirty percent of patients and involves the central nervous system with symptoms of meningitis (fever, headache, and a stiff neck) or encephalitis (drowsiness, confusion, sensory disturbances, and/or motor abnormalities such as paralysis) or meningoencephalitis. TBE is more severe in adults than in children.

Mortality is about two percent with deaths occurring five to seven days after the onset of neurological signs.

West Nile Virus:

West Nile Virus is a flavivirus commonly found in Africa, West Asia, the Middle East and the United States. The virus can infect humans, birds, mosquitoes, horses and some other mammals.

The principal route of human infection with West Nile virus is through the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds, which circulate the virus in their blood for a few days. The virus replicates in the mosquito and spreads to the mosquito's salivary glands. During subsequent blood meals, the virus may be injected into humans and animals, where it can multiply and possibly cause illness.

At least 36 species of mosquito are known to be carriers of West Nile Virus. The most common carrier of West Nile in the United States is the Culex pipiens (Northern house) mosquito.

West Nile fever is usually a case of mild disease characterized by flu-like symptoms. It typically lasts only a few days and does not appear to cause any long-term health effects.

More severe disease due to a person being infected with this virus can be West Nile encephalitis, West Nile meningitis or West Nile meningoencephalitis. Encephalitis refers to an inflammation of the brain, meningitis is an inflammation of the membrane around the brain and the spinal cord, and meningoencephalitis refers to inflammation of the brain and the membrane surrounding it.

The incubation period is usually 3 to 14 days. Symptoms of mild disease will generally last a few days. Symptoms of severe disease may last several weeks, although neurological effects may be permanent.

Many people who are infected with the West Nile virus will not have any type of illness. It is estimated that 20% of the people who become infected will develop West Nile fever: mild symptoms, including fever, headache, and body aches, occasionally with a skin rash on the trunk of the body and swollen lymph glands.

The symptoms of severe infection (West Nile encephalitis or meningitis) include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. It is estimated that 1 in 150 persons infected with the West Nile virus will develop a more severe form of disease.

There is no specific treatment for West Nile virus infection. In more severe cases, intensive supportive therapy is indicated, often involving hospitalization, intravenous fluids, airway management, respiratory support (ventilator), prevention of secondary infections (pneumonia, urinary tract, etc.), and good nursing care.

When travelling to areas where the disease is endemic you can reduce the risk of becoming infected with the virus by employing preventive measures such as protecting yourself from mosquito bites:-

Many of the mosquitoes that carry the West Nile virus are especially likely to bite around dusk and dawn. If you are outdoors around these times of the day, it is important to apply repellent. But there are also mosquitoes that bite during the day, and these mosquitoes have also been found to carry the West Nile Virus. 

Filariasis:

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.

Lyme Disease:

An infectious disease caused by a bacterium which is spread to humans by tick bites. It is characterized 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.

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.

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.

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 feces 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.

Typhus Fever:
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.

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.

Rocky Mountain Spotted Fever:

Rocky Mountain spotted fever is transmitted to people by the bite of a tick infected with the bacterial organism Rickettsia rickettsii, or by contamination from the contents of an attached tick when it is removed (from a person or animal). Tweezers should be used to remove ticks, along with a paper towel or rubber gloves to shield contact.

The primary carriers of Rocky Mountain spotted fever in the United States are the American dog tick, Dermacentor variabilis, and the Rocky Mountain wood tick, Dermacentor andersoni.

First recognized in 1896 in Idaho, Rocky Mountain spotted fever has a misleading name because the United States' highest incidence of the disease occurs in North Carolina and Oklahoma. In fact, relatively few cases are reported in the Rocky Mountain states. Each year from April through September, approximately 800 to 1,200 cases are reported in the United States, more than half of which occur in the Southeast (Maryland to Florida).

Rocky Mountain spotted fever is a serious disease, and it is likely that many cases are not reported. Initial symptoms, which can resemble other illnesses, usually appear about 5-10 days after a tick bite and can include fever, nausea, vomiting, severe headache, muscle pain and lack of appetite. Further symptoms include a rash, abdominal pain, joint pain and diarrhea. The rash generally appears 2-5 days after the onset of fever. Children and young people typically develop a rash earlier than older persons, but about 10-15% of patients never develop a rash.

Rocky Mountain spotted fever usually requires hospitalization. Cells lining blood vessels are infected, and the respiratory, gastrointestinal, renal and central nervous systems can be affected. The occurrence of severe or fatal Rocky Mountain spotted fever has been linked to advanced age, male sex, African-American race, chronic alcohol abuse and deficiency of a specific enzyme (G6PD). People who survive acute infection can develop chronic health problems including partial paralysis of the lower extremities, gangrene, hearing loss, loss of bowel or bladder control and language disorders.

There is no vaccine for Rocky Mountain spotted fever, but there are laboratory tests to confirm the disease, and early treatment with antibiotics proves effective. In fact, prompt and appropriate treatment is imperative. According to the Centers for Disease Control and Prevention (CDC), approximately 3-5 % of cases are fatal.

What is Rocky Mountain spotted fever and how is it transmitted?
Rocky Mountain spotted fever is caused by a small bacterium, Rickettsia rickettsii, which is transmitted by members of the hard tick family, Ixodiae. The two major tick vectors in the United States include the dog tick, Dermacentor variabilis, and the Rocky Mountain wood tick, Dermacentor andersoni.

The dog tick prefers dogs as hosts, but it feeds readily on other large mammals including people. Larvae and nymphs of the Rocky Mountain wood tick feed on rodents and other small animals, and adults feed on large mammals.

Rickettsia is transmitted through the saliva of an infected tick. It usually takes several hours of feeding for transmission to occur, and the proportion of infected ticks is low-approximately 1%-3% of the population in areas where human cases have been reported.

In people, rickettsia infects the cells that line small to medium blood vessels, causing the cells to become damaged and die. This results in the leakage of plasma and blood into tissues adjacent to the cells-which creates the typical Rocky Mountain spotted fever rash, and can damage tissues and organs. Severe cases may involve the respiratory, gastrointestinal, nervous and renal systems. Long-term complications of Rocky Mountain spotted fever, particularly in those with severe disease, include paralysis of the lower extremities, gangrene requiring amputation of limbs, toes or fingers, loss of bowel and bladder control, as well as movement and language disorders.

Early clinical signs of Rocky Mountain spotted fever resemble those of other viral diseases, and include nausea, headache, muscle pain and lack of appetite. About 2-5 days after onset of fever, a rash appears consisting of small, flat, pink, non-itchy spots on the wrists and forearms. Later symptoms include rash, joint pain, abdominal pain and diarrhea. The characteristic red, Rocky Mountain spotted fever rash is usually not seen until after the sixth day of fever onset, and occurs on the palms and soles of the feet.

Rocky Mountain spotted fever rash

How many cases of Rocky Mountain spotted fever occur annually in the United States?
Between 250 and 1,200 cases of Rocky Mountain spotted fever occur each year.

How is Rocky Mountain spotted fever treated?
Because it is a bacterial infection, Rocky Mountain spotted fever is treatable with antibiotics, usually doxycycline. Patients generally respond well to treatment if started early, and the fever usually subsides within 72 hours. If Rocky Mountain spotted fever is not treated promptly, possible organ damage and failure means patients will need more intensive care. Infection generally confers lifelong immunity.