MOSQUITOES
Dengue Fever (aedes) - Dengue fever is a virus spread by the Aedes species of mosquito, identifiable by its black-and-white body and its penchant for biting shadowed areas of our bodies during daylight hours - usually in early morning or late afternoon. The mosquito is rarely found above elevations of 4000 feet (approx. 1200 metres). You can't catch dengue from another person. A preventative vaccination is being developed, but for now your only means of avoidance is vigilant bite prevention. Although contracting the infection confers immunity, there are 4 separate subtypes of the virus, and it is unfortunately that immunity to one does not extend to the other strains. A rare but serious complication, which mainly affects children under 15, is dengue haemorrhagic fever (DHF). The predisposing factors to dengue haemorrhagic fever are mainly the particular viral subtype and epidemic strain, recurrent infections, age, general level of health, geographic region (a preponderance of cases are in Southeast Asia) and possibly genetic factors. It is particularly common in people who have suffered a previous attack with one of the viral subtypes and who are then exposed to another subtype. The victim may become shocked (pale, clammy faint and nauseous with a weak, rapid pulse and shallow rapid breathing), and develop abnormal bleeding into the skin, and from the nose, mouth and rectum. This is a life-threatening complication and requires urgent medical attention.

Symptoms - The symptoms of dengue fever are remarkably similar to malaria and usually appear between 5 and 8 days after being bitten. The onset is usually abrupt and includes high fever, malaise, headache (characteristically behind the eyes), joint pains and backache (hence dengue has been nicknamed `break bone fever'). Diarrhoea is fairly common after the first day and, although often being explosive in nature, it doesn't normally last more than a couple of days. A fine rash is likely to appear after a couple of days, initially affecting your torso before spreading to the extremities. It can be maddeningly itchy. The high temperature usually normalizes for a day or two before briefly recurring with the original symptoms. Abnormal bleeding can occur and will need medical assessment. The acute illness does not normally last for more than 10 days, but the convalescence (get better period) can be prolonged, with severe fatigue and depression that may last for several weeks afterwards.

Eastern equine encephalitis - occurs in isolated outbreaks along the eastern seaboard of the US, Canada, the Caribbean and parts of Central and South America. It can occur all year around in the tropics but further north tends to be a summer illness, with the majority of cases occurring between May and August, often in swamp areas. The disease is extremely serious, but by the same token very rare among humans- it usually strikes birds and horses. The virus is spread to humans by mosquitoes. According to the American CDC, fewer than 10 human cases on average occur each year in the US. Although a vaccine exists for horses, there is no preventative vaccine for humans.

Symptoms - An abrupt-onset high fever is usually accompanied by headache, lethargy and vomiting. Progression of the disease leads to drowsiness, neck stiffness, fits and coma. The mortality rate of those severely affected is high and long-term effects in survivors are common, although adults tend to be more resilient than children.

Filariasis (anapheles, culex, aedes)
Widespread in the tropics, where they cause millions of people significant health problems, filarial infections are caused by parasitic worms and spread by biting insects. There are three distinct diseases caused by filarial worms:-

Onchocerciasis (Mal morado, River blindness, Roble's disease, Volvulosis) -
Affecting more than 18 million people globally, onchocerciasis is a major cause of blindness in the developing world. The disease is found mainly in tropical Africa, but some cases also occur in tropical South and Central America. Onchocerciasis is caused by a worm parasite that is transmitted to humans via the bite of a black fly generally found alongside stretches of fast-flowing water. The black fly's bite deposits the larvae, which penetrate into the superficial tissues beneath the skin. A year elapses before the worm matures and starts to reproduce large numbers of tiny offspring - microfilariae - that migrate throughout the body. The adult worm can live for many years. There is no effective vaccination or drug prophylaxis, so taking protective measures against insect bites in high-risk areas is crucial. Fortunately, even in high-risk areas, short-term travellers (staying less than 3 months) rarely acquire onchocerciasis.

Symptoms - The most prominent symptom caused by the large numbers of microfilariae produced by the mature adult worms is a widespread, red, maddeningly itchy rash. Skin nodules develop at the site where the adult worm is lodged. In Africa, the lesions are usually on the lower part of the body, while in the Americas they tend to be on the head, neck, shoulders or upper trunk. Other symptoms are caused by the microfilariae liberated around the body and include fever, headache, lymph, gland swelling and tiredness. During migration, the microfilariae can lodge in the eyes, causing initial redness and irritation which if left untreated leads to blindness. Such serious eye complications, however, are only likely to affect people who are repeatedly infected over the course of many years.

Lymphatic filariasis (Bancroftian or Malayan filariasis) - Recognized as one of the world's leading causes of permanent disability, filarial lymphangitis is thought to affect more than 100 million people in the Western Pacific Islands, the north-eastern coast of Central and tropical South America, and the Caribbean. The disease is caused by infection with thread-like parasitic worms, which are spread from person to person (and in some strains, animal to human) by mosquitoes. The adult worms reside in the lymph system, liberating into the bloodstream microfilariae , which the mosquitoes ingest during feeding. After a period of development in the mosquito, the larvae are deposited again during feeding and migrate via the bloodstream to the new host's lymph system. There is no protective vaccination against filarial lymphangitis and your only means of protection is bite avoidance, although short-term, casual tourists are rarely affected.

Symptoms - show themselves between 5 and 18 months after being bitten, as the parasitic worms slowly grow in the body's lymph system. The worms' presence in the lymph system causes a local inflammatory reaction, followed by scarring and consequent occlusion of the lymph channels. This in turn causes the accumulation of fluid (lymph) in the tissues and swelling. The swelling of the legs, in its extreme, can lead to a condition known as elephantiasis, so-called because of the skin's resemblance to an elephant's. The resulting disfigurement and reduced mobility are permanent. Scrotal swelling can also occur. Other symptoms include painful, swollen lymph glands, recurrent fever, skin rashes, blindness and a lung condition, tropical pulmonary eosinophilia, characterized by nighttime coughing and wheezing.

Malaria (anapheles) - As treatment changes frequently we would suggest to surf to www.cdc.gov/travel (National Center for Infectious Diseases - American) they have up to date information on malaria and all other diseases or see other travel health links.

Oropouche virus - is a zoonosis with a sloth acting as the natural reservoir. It is transmitted to humans by midges, which breed in piles of the rotting husks of cacao beans. The disease has caused large epidemics in recent years in Brazil, Peru and Panama. Its incidence appears to be on the increase, and this seems to be related to the intense deforestation in these areas, which is altering the local ecosystems and bringing humans into closer contact with the disease vectors. There is no vaccine against the virus so the best means of avoiding the infection is to observe precautions against insect bites. The risk to the traveller is low unless you plan on venturing into an epidemic area.

Symptoms - include sudden-onset high fever with muscle and joint pains, intense headache, nausea and diarrhoea. In severe cases, symptoms of meningitis occur. Diagnosis is confirmed by a blood test. There is no treatment other than rest and symptomatic relief for the fever including drinking plenty of fluids. The presence of meningitis symptoms, however, requires urgent medical assessment.

Ross River virus - Taking its name from the river in northern Queensland where it was first identified, Ross River virus is endemic throughout Australia, with isolated outbreaks occurring in the islands of the South Pacific. Ross River virus is spread by mosquitoes, and outbreaks tend to occur after flooding during the rainy season, when mosquitoes are most active. Direct person-to-person spread does not occur. The disease can also infect domestic and wild animals, and the Australian kangaroo and wallaby populations are believed to be natural reservoir for the infection (it is though that this is the reason that Ross River fever has never been exported to any great extent). There is no vaccination against the disease and your best means of protection against infection is by taking steps to avoid mosquito bites whenever possible. The risk to travellers is generally low unless you visit an area where there is an active epidemic.

Symptoms - Ross River virus causes a remarkably similar illness to Dengue fever. After an incubation period of between 2 and 21 days, symptoms emerge as a flu-like illness with fever, chills, muscle pains, headache, lethargy and occasionally a rash. Painful, stiff and sometimes swollen joints, which are usually worse in the morning, may occur. The symptoms tend to be less severe in children. Although the acute illness resolves fairly quickly, the joint pains, tiredness and sometimes depression can continue for several months.

Venezuelan equine encephalitis - is a mosquito-borne viral disease carried by horses, rodents and humans and occurring in epidemics during the rainy season in Trinidad, Central America (Panama in particular) and northern South America. Outbreaks have also been reported in Florida and other parts of the USA. A vaccination has been developed but is not generally available, being reserved instead for lab staff working with the virus. Generally speaking, the infection is very rare and only likely to be a worry to travellers in the above areas during an epidemic and in close proximity to horses. This being the case, exercises the normal measures of bite avoidance.

Symptoms - After an incubation period of less than a week, the disease usually manifests as a flu-like illness with fever, rigors, severe headache, nausea, vomiting, diarrhoea and muscle pains. The fever normally lasts no longer than 4 days but can be followed by a period of weakness and depression lasting several weeks. A small minority of cases develop encephalitis, which can be fatal.

Yellow fever (aedes) - In South America the majority of cases occur in the Amazon region. It is principally a disease of tropical, forested areas but sporadic urban outbreaks do occur. The virus causing yellow fever is related to dengue fever and Japanese encephalitis. Its natural host is the monkey but is spread to humans by mosquito bite. The disease can reach epidemic proportions in a relatively short space of time and the past 20 years has seen a rise in the number of reported epidemics and the countries affected.
A live vaccination is compulsory when entering some endemic areas and some countries following a visit to endemic areas.

Symptoms - appear after an incubation period of 3-6 days, but are often mild, self-limiting and unrecognisable as yellow fever. The classical picture of yellow fever is of a severe illness with an initial sudden high fever and disabling weakness and lethargy, accompanied by headache, generalized muscle pains, abdominal pain and vomiting. A day relatively slow pulse rate may be present from the second day of the illness. The majority of those affected will improve over a period of about a week and the symptoms will disappear without treatment. However, around 15% of infected people will develop a second and more dangerous phase of the illness after a lull of a day or two. The vomiting returns together with jaundice (thus the name), a reduced flow of urine (a sign of failing kidneys), convulsions, signs of bleeding (eg. nose bleeds, bruising under the skin, bleeding gums, etc) and shock. The fatality rate is high in those with complications (up to 50%) and usually occurs as a result of liver or kidney failure.

TICKS
Lyme disease - occurs mainly in rural areas of North America and Europe. Deriving its name from Lyme, Connecticut, where the first case was described in 1975, there has been a 25-fold increase in reported cases in the US over the past 15 years, although this may be largely attributable to increased public awareness. The microbes causing Lyme disease are transmitted to humans via the bite of blood-sucking deer-ticks. The ticks are active in the spring, summer and early autumn, when they can be found in long grass and trees waiting for their host animal, deer, to pass by. If you go walking in areas where there are deer, observe measures to avoid tick bites. Infected ticks are unlikely to pass on the disease unless attached for more than 24 hours, so it's important to inspect your hair, skin and clothes for them every few hours. A vaccine currently, available in the US has met with controversy, but if you're going to be living in a high-risk area for any length of time you might want to enquire about it locally on arrival.

Symptoms - Many people experience no symptoms at all, while in other slowly spreading, red ringed ("bull's-eye") skin lesion develops at the bite site, any time from 3 days to 4 weeks after infection. There may also be local lymph gland swelling and fever, generalized aches, pains and tiredness. More serious symptoms are uncommon, frequently delayed by weeks or even months after the initial infection, and mainly affect the nervous system, joints and heart. These can manifest as weakness, paralysis, abnormal sensations, mild meningitis symptoms, joint pains and swelling and palpitations.

Q fever - was first identified in 1935 in Australia, where it was given the name "Query fever" because its cause was initially unknown: the microbes that cause it eventually were identified 2 years later. The illness occurs worldwide, although with a higher incidence in rural areas. Many people have a natural immunity to the microbes, which suggests that the infection is widespread but usually goes undetected. The microorganisms responsible are usually carried by sheep, cattle and goats and are found in particularly high concentrations in the placenta but are also present in the milk, urine and faeces. It's possible to become infected simply by breathing in the microbes from contaminated air, although ticks can carry the disease and may be responsible for a small number of human infections. Farmers, abattoir workers, vets and anyone else in close contact with animals are at greatest risk, with little threat posed to the average traveller. A vaccine against Q fever exists, but is not widely available - discuss it with your doctor if you plan to work with animals while you're away.

Symptoms - After an incubation period of between 10 and 20 days headache, fever, shivering, muscle aches, loss of appetite, nausea and fatigue are the prominent symptoms. A cough and sharp chest pains that are worse on taking a deep breath or coughing can also occur. Symptoms are usually self-limiting, lasting up to 2 weeks. Rarely, the illness can lead to heart, liver or brain complications.

Relapsing fever - is a bacterial infection, which derives its name from the relapsing nature of the untreated illness. There are two distinct varieties: louse-borne or tick-borne. The tick-borne variety is found in Africa, southern Europe, the Middle East, Asia and the Americas (including the western US and Canada). The risk of infection to the traveller is generally low, especially if the standard insect bite-avoidance tactics are observed. Person-to-person spread of relapsing fever does not occur (unless via body lice).

Symptoms - After an incubation period of 2-10 days, initial symptoms consist of an abrupt-onset fever, rigors, muscle pains, headache and extreme weakness lasting for up to a week. A remission period of similar length follows before the symptoms relapse. Without treatment, further periods of regression and relapse occur. A cough and swelling of the lymph glands, liver and spleen may develop, and in severe cases (particularly the louse-borne variety) jaundice and a purpuric rash. Each time the fever abates a dangerous drop in blood pressure can occur. Progression of the disease can lead to dangerous swelling of the brain and inflammation of the heart muscle. Overall, relapsing fever is a dangerous, potentially fatal illness although there is usually a good response to treatment.

Rocky Mountain spotted fever (RMSF) - is a tick-borne illness affecting 600-800 people annually across the USA, with cases most commonly occurring in the southeastern states. It also occurs in parts of Central and South America. The risk is higher in the spring and summer when the ticks are most active. Children between the ages of 5 and 9 are most commonly affected, for the simple reason that they're more likely to indulge in pastimes that put them in contact with the ticks. A form of typhus, RMSF is caused by the rikettsial bacteria transmitted by the bite of dog ticks (in the eastern USA) and wood ticks (in the western USA). The longer the tick remains attached to the body, the greater the chance of infection. Careful removal of ticks is important to avoid leaving body parts in the skin and because the infection can be contracted from the body juices of a crushed tick coming into contact with broken skin. There is no vaccination against RMSF so the best way to avoid the illness is to avoid being bitten by tick. Person-to-person spread does not occur.

Symptoms - Between 1 and 2 weeks after the tick bite, the symptoms are usually sudden in onset and include high fever, chills, muscle pains, severe headache and vomiting. The eyes may become painful and there may be generalized swelling of the body. A crusted, raised lump can develop at the site of the bite, while nearby lymph glands are likely to become swollen and tender. The rash synonymous with the name begins between 1 and 10 days after the onset of the fever. Small red spots or blotches begin peripherally (hands, feet, wrists, and ankles) and spread up the limbs to the trunk (the face is usually spared). As the illness progresses the rash changes to look like bruising and blood blisters (which do not blanch under pressure, ie. purpuric). Complications of RMSF can affect the brain, liver, kidneys and lungs, and the disease can be fatal without appropriate antibiotic treatment.

Other spotted fevers
Elsewhere in the world there are several varieties of tick-borne spotted fevers caused by rikettsial bacteria. The kind of tick may differ from country to country but, broadly speaking, the symptoms, diagnosis and treatment are the same. RMSF remains the most dangerous, followed by Mediterranean spotted fever and Siberian tick typhus. Like RMSF, most of the fevers are seasonal, occurring in the summer when the ticks are most active. The fevers' names generally hold clues to the parts of the world where they are prevalent. Thus:
- Mediterranean spotted fever
- Kenyan tick typhus
- African tick bite fever
- Israeli spotted fever
- Astrakhan fever (Caspian Sea)
- Siberian tick typhus
- Indian tick typhus
- Japanese spotted fever
- Queensland tick typhus
- Flinders Island spotted fever (Australia)

Tick-borne encephalitis - is a viral illness that occurs mainly in forested areas of northern, eastern and central Europe. A related illness known as Russian spring-summer encephalitis (RSSE) is found in Siberia, northeast China and Korea. The disease tends to be seasonal, occurring mainly between May and September (when the ticks are most active) and is caused by a virus, which is transmitted by a tick bite, closely related to dengue, yellow fever and St Louis encephalitis. Walking or working in wooded areas puts you most at risk, but the chance of contracting the illness from a single tick bite is very low. The illness may also be transmitted to humans in the unpasteurized milk of cows, goats or sheep. An effective preventative vaccination is available, although it is often has to be specially ordered as required and can be expensive. Always observe the standard measures to prevent tick bites when walking through endemic areas and avoid unpasteurized cow, goat or sheep dairy products.

Symptoms - A very small number of people who become infected with the virus actually develop symptoms (estimated at around 1 in 250). After an incubation period of between 1 and 2 weeks, symptoms classically occur in 2 phases; the first phase presents as a non-specific flu-like illness (headache, fever, generalized aches and pains) while the second phase, following between 1 and 3 days later, affects the central nervous system causing encephalitis (dizziness, tremor, fits, paralysis, psychiatric disturbance) and meningitis. Although the disease is very rarely fatal, it can cause permanent damage to the nervous system.

Typhus - is the collective name for several different diseases. Common features are the causative organisms (rickettsiae bacteria), the mode of transmission to humans (via a tick, louse or flea bite) and the symptoms of high fever followed by a rash. It occurs throughout the world in some or other form. There are 4 main types of typhus infection: epidemic louse-borne typhus, endemic flea-borne typhus, Rocky Mountain spotted fever, and scrub typhus. No vaccination is currently available against any form of typhus, and although risk to the traveller is small, it's essential that you take steps to avoid being bitten by the insect vectors in endemic areas.

Back to top

Endemic flea-borne typhus(murine typhus) - occurs worldwide but prevails around ports and coastal areas. Although rare in developed countries, intermittent outbreaks do occur. The disease-causing microbe is carried by rats and transmitted by humans by the bite of a flea. The faeces of the infectious flea usually contaminate the bite site, although it may also be contracted by inhaling the flea's dried faeces. In the USA (particularly central and south-central parts of Texas), the disease can be carried by fleas on dogs, cats and opossums.

Symptoms - Typically, the onset is gradual over a period of 2 weeks following contact. The symptoms are similar to epidemic typhus only much milder, consisting of high fever (sometimes lasting up to 2 weeks), headache, muscle pains and a rash. Full recovery usually occurs, even without treatment, and fatalities are very rare indeed.

Scrub typhus - occurs sporadically in parts of Southeast Asia, Australia, India and western Pacific Rim. It was responsible for the deaths of thousands of troops fighting in the Pacific in World War II. The illness is transmitted to humans by the bite of infectious mites - today you're most at risk hiking in cleared areas of jungle forest.

Symptoms - Close examination of the skin sometimes reveals tiny darkened scabs where the mites have bitten. Initial symptoms usually occur within 5 to 10 days of being bitten and manifest as a sudden fever and severe headache accompanied by muscle pains, a dry cough and swollen lymph nodes. A rash develops over the torso and limbs and the central nervous system can be affected, sometimes causing permanent damage. The illness can recur despite treatment with antibiotics.

Epidemic louse-borne typhus - occurs in the cool, mountainous areas of Africa, Central and South America, Asia, Eastern Europe and Mexico. It can occur in explosive epidemics, particularly at times of famine or war when large numbers of people live in close proximity and in unsanitary conditions. The microbe is passed to humans by body lice, which live in clothing and feed on the blood. The faeces of the infectious body lice enter the body via fleabite sites, abrasions or through the eyes. It's also possible to inhale the contaminated faeces. The disease does not spread directly from person to person.

Symptoms - High fever, headache, dry cough, rigors, muscle pains, nausea and vomiting usually occur within 2 weeks of being bitten, after which a rash appears on the torso, spreading to the rest of the body. The fever remains constant at around 40C for a few days, and the headache can be severe. If the disease progresses, it can affect the central nervous system, causing permanent damage. There are many complications that can arise from epidemic typhus and without treatment, it can be fatal, although it may also resolve spontaneously. Recurrences many years after the original illness are possible.

SANDFLIES
Bartonellosis / Oroya fever - originates in the Andes of southwest Columbia, Ecuador and Peru. You are most at risk in certain narrow valleys on the range's western slopes between 1000 and 3000 m - outside these geographically isolated areas, the risk to travellers is negligible. The bacteria responsible are transmitted by sand flies, which usually bite between dusk and dawn. There's no vaccination, so if you find yourself in an area of high risk, your only protection is insect-bite avoidance.

Symptoms - The disease has two phases; an initial, acute illness characterized by loss of appetite, thirst bone pains, fatigue resulting from anaemia and high fever. The fever is particularly high at night and may last for up to 6 weeks. This phase is followed by wart-like eruptions on the skin, which is particularly dense on the face and limbs, and bleeds easily. They do heal without scarring, although sometimes this takes up to 12 months. Victims of bartonellosis are unusually susceptible to salmonella septicaemia, especially in the second week of the illness, so prompt treatment or medical attention is important.

Leishmaniasis - The WHO estimates that around 12 million people (mainly indigenous) in 88 countries across the Mediterranean basin (mainly southern and eastern), central Asia, eastern Africa, China and Central and South America are currently infected with leishmaniasis. Female sand flies transmit this portozoal infection to humans, usually from dogs' rodents or, in India, other humans. Transmission of the disease may also occur through blood transfusions, sexual contact with infected individuals and from mother to child. The flies, which inhabit dry regions, normally bite at dusk, dawn and during the night. Because they are low-altitude fliers the best way to avoid sand flies is by sleeping above ground level (in a hammock if you can't get higher than the ground floor). Bear in mind they are small enough to pass through most mosquito nets. As yet there is no vaccination against leishmaniasis, although one is being developed. Many varieties of leishmanisias exist, but for practical purposes they are grouped into cutaneous, visceral and mucocutaneous forms, affecting the skin, the internal organs and mucous membranes respectively.

Cutaneous leishmaniasis - It's estimated that between 1 and 1.5 million people contract cutaneous leishmaniasis each year. The sand flies' bites first appear as red patches, which gradually enlarge and ulcerate. The resulting nodules are painless but itchy, firm and may resemble burn marks. Multiple nodules may coexist in different areas of the body reflecting the insect's bite pattern. Without treatment, the lesions heal very slowly (sometimes over many years) and often leave permanent scarring. Cuteneous leishmaniasis is usually identified from a biopsy of a skin nodule. Note that many doctors in developed countries will not think of investigating for leishmaniasis on the basis of the skin lesions alone, so if you suspect the illness, makes sure your doctor knows where you have been and under what conditions you have been living. A 10-day intravenous course of pentavalent antimony (sodium stibogluconate) is partially effective but side effects require specialist monitoring. Relapses are common and although the disease is not life-threatening, long-term scarring may occur.

Visceral leishmaniasis - "Black sickness", the disease sometimes causes darkening of the extremities, face and abdomen. Visceral leishmaniasis tends to affect young people, with an incubation period that ranges from months to years. Onset of symptoms is usually insidious, with a mild, intermittent fever and bouts of profuse sweating. As the disease progresses, weight loss, fatigue, poor appetite, nausea, abdominal pain and diarrhoea can develop. Over time the liver, spleen and lymph glands swell. Left untreated, the disease is usually fatal.
A blood test will identify the presence of visceral leishmaniasis. Again, an intravenous course of pentavalent antimony drugs can be used as treatment but side effects may be unpleasant. In resistant areas, it may be necessary to surgically remove the spleen.

Mucocutaneous leishmaniasis - Occurring in Central and South America. It has both short-term and long-term effects. The initially painful, itchy nodules, usually on the legs, are likely to resolve spontaneously within a few months. Years later, however, up to 40% of those infected can develop ulcers around the nose and mouth, which cause permanent scarring and disfigurement. Secondary infection of these lesions has serious consequences. Diagnosis of mucocutaneous leishmaniasis is usually made from a biopsy of the ulcers, but blood tests may also help. Pentavalent antimony compounds can again be used as treatment but cure rates are unimpressive. Reconstructive surgery is often needed for facial deformities.

Sandfly fever (Dog disease, Phlebotomus fever, Pym's fever) - occurs across dry areas of the Balkans, throughout the Mediterranean littoral, the Middle East, Central and Southeast Asia, and Central America. It is a viral illness spread by sand flies, which tend to bite at night and are also responsible for the transmission of leishmaniasis. There is no preventative vaccine so observe the usual precautions against insect bites. Bearing in mind that sand flies don't fly above 3 metres, sleeping above the ground floor is safest.

Symptoms - Sandfly fever is very similar to Dengue fever; after an incubation period of between 3 and 8 days, there is an abrupt onset of fever, rigors, headache (usually behind the eyes) and muscle and joint pains. Although symptoms can be quite intense and are usually followed by a period of weakness and lethargy, complete recovery is the norm.