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Malaria is an infectious disease caused by a parasite that's transmitted by mosquitoes. The illness results in recurrent attacks of chills and fever, and it can be deadly.

Although malaria has been virtually eradicated in countries with temperate climates, it's still prevalent in tropical and subtropical countries in Africa, Asia, the Middle East, South America and Central America. Evolving strains of drug-resistant parasites and insecticide-resistant mosquitoes continue to make this disease a global health issue. Malaria remains one of the world's leading infectious killers, particularly of children in sub-Saharan Africa.

Most American cases of malaria develop in travelers who have recently returned from parts of the world where malaria is widespread. Generally, Africa carries the highest risk for the most serious cases.

If you're traveling to malaria-endemic places take precautions before, during and after your trip. Treatment for malaria is with antimalarial drugs.

A malaria infection is generally characterized by recurrent attacks with the following signs and symptoms:

• Moderate to severe shaking chills
• High fever
• Profuse sweating as body temperature falls
• General feeling of unease and discomfort (malaise)

Other signs and symptoms include:

• Headache
• Nausea
• Vomiting
• Diarrhea

A one-celled parasite, plasmodium, causes malaria. About 170 species of plasmodium exist, but only four cause malaria in humans:

• P. falciparum. This species, predominant in Africa, produces the most severe symptoms and is responsible for most malaria deaths.
• P. vivax. This species, found mostly in tropical areas of Asia, produces less severe symptoms but can remain in your liver and cause relapses for up to three years.
• P. malariae. This species, found in Africa, can cause typical malaria symptoms, but on rare occasions it can remain in your bloodstream for years without producing symptoms. In these cases, you may pass on the parasite to a mosquito or to another person through a blood transfusion.
• P. ovale. This species is found mostly in West Africa. Although rare, it can also cause relapses.

The process of transmission
The transmitter (vector) of plasmodium to humans is a female anopheles mosquito. When a mosquito bites a person infected with malaria, it ingests male and female versions of plasmodium (gametocytes). The gametocytes unite in the stomach of the mosquito to form a structure called an oocyst. The oocyst takes about a week to mature and then ruptures, sending out thousands of cells called sporozoites to the mosquito's salivary glands.

When this mosquito bites a human, it injects the sporozoites into that person's bloodstream. The sporozoites migrate rapidly to the liver, where each one develops over the next week or so into a structure housing thousands of cells called merozoites. In some cases of P. vivax or P. ovale infection, these structures can remain inactive in the liver for extended periods of time. Later, reactivation of the parasite's life cycle causes a relapse.

Upon maturation, the infected liver cells burst, sending the merozoites into the bloodstream, where they invade red blood cells. Within the red blood cells, they reproduce further, developing into trophozoites, another form of plasmodium, and gametocytes — available to be ingested by a mosquito, thus renewing the transmission cycle.

When infected red blood cells burst, the tiny parasites invade even more red blood cells. As each wave of blood cells ruptures — about every 48 to 72 hours, depending on the type of plasmodium — the person experiences an attack of chills, fever and sweating. Typically, signs and symptoms begin 10 days to four weeks after the initial mosquito bite, although they can appear as early as eight days or as late as one year later.

In many cases, medication or the immune system eventually helps stop the infection. But in other cases, particularly in children whose immune systems may not yet have adapted to the parasite, complications of the infection may lead to death. In addition, P. falciparum is capable of invading a much greater number of blood cells than are the other types of plasmodium, and the infection can be fatal within a few hours of initial red blood cell rupture.

A pregnant woman can transmit the infection to her unborn baby. Malaria also can be transmitted through blood transfusions. In the United States, steps have been taken to prevent this type of transmission. People who have been in a malaria-endemic area are prohibited from donating blood for a year after returning from the malarious area, or three years if they've been a resident of the malarious area or have been treated for malaria.

People who have little or no immunity to malaria are most at risk for serious illness. Residents of a malaria region may acquire some immunity to the disease during their lifetime, but young children and infants who have yet to acquire any immunity are at risk, as are travelers coming from areas with no malaria. A pregnant woman is more vulnerable than other people are to P. falciparum malaria, as is her unborn child. Poverty, lack of knowledge and little or no access to health care also contribute to malaria deaths worldwide.

It's also possible to lose your immunity if you're no longer frequently exposed to the parasite. So even if you've previously lived in a malarious region, take antimalarial precautions if you return to such an area after an extended period away.

Because malaria infection often initially appears to be a flu-like illness or some other viral disease, be wary if you develop an illness with fever while living in a malaria-endemic area or within 12 months after traveling to a high-risk malaria region. See your doctor as soon as possible, and tell your doctor where you've traveled. Left untreated, a malaria infection can cause serious, potentially life-threatening health problems.

After noting your symptoms and travel history, your doctor will likely obtain a sample (smear) of your blood for observation under a microscope. Two blood samples, taken at six- and 12-hour intervals, can usually confirm the presence of the malaria parasite and its type. It's possible to be infected by more than one plasmodium at the same time.

Most complications of malaria are associated with infection by P. falciparum. Among the complications is an extensive destruction of red blood cells, which can result in severe anemia. In addition, if parasite-filled blood cells block small blood vessels to your brain (cerebral malaria), swelling of your brain or brain damage may occur. Other complications may include:

• Breathing problems, at times severe in the form of accumulated fluid in your lungs (pulmonary edema)
• Dehydration
• Liver failure
• Kidney failure
• Rupture of the spleen

If untreated, P. falciparum malaria can be fatal within a matter of hours.

A malaria infection, particularly with P. falciparum, requires prompt evaluation and treatment. In most cases, doctors can treat malaria effectively with one or more of the following medications:

• Chloroquine (Aralen)
• Quinine sulfate
• Hydroxychloroquine (Plaquenil)
• Combination of sulfadoxine and pyrimethamine (Fansidar)
• Mefloquine (Lariam)
• Combination of atovaquone and proguanil (Malarone)
• Doxycycline (Doryx, Vibramycin, others)

Another class of antimalarial drugs, often prescribed in Asia and now in other parts of the world, is derived from artemisinin, a sweet wormwood extract. Artesunate is an example of an artemisinin derivative.

Doctors sometimes use halofantrine for treatment of malaria, although it's not marketed in the United States. If you've been taking mefloquine for prevention of malaria or if you have heart problems, don't take halofantrine because it can be dangerous and possibly fatal.

Primaquine may be given to fight the dormant liver form of the parasite and prevent relapses. However, the Centers for Disease Control and Prevention (CDC) has warned against taking primaquine if you're pregnant or have an enzyme deficiency called G6PD (glucose-6-phosphate dehydrogenase) deficiency. Don't take primaquine until you've passed a screening test for G6PD deficiency.

Which drug you take and the length of treatment depend on the type of malaria, where you were infected, your age and how sick you were when treatment began. Drugs are given either orally or intravenously, depending on the severity of illness. In some countries, they may be given in suppository form. After treatment, you may feel very weak and tired for a few weeks.

The problem of drug resistance
The history of antimalarial medicine has been marked by a constant struggle between evolving drug-resistant parasites and the search for new drug formulations. In certain parts of the world, for instance, resistance to chloroquine has rendered the drug ineffective.

Currently, anti-malaria experts are focusing on therapies that combine artemisinin derivatives with other companion drugs, such as lumefantrine, sulfadoxine-pyrimethamine, amodiaquine (a drug similar to chloroquine) and mefloquine. This is referred to as artemisinin-based combination therapy (ACT). This type of drug combination therapy is often more effective than are drugs administered individually, especially in areas of drug-resistant parasites. In many countries ACT has become the first line of therapy for malaria.

Artemisinins act quickly in your bloodstream, rapidly clear away parasites and makes you feel better faster. They may also help reduce transmission of the disease by reducing the number of gametocytes — the infective version of the parasite — in your bloodstream. There's little documented resistance to artemisinins, and their combination with other drugs may slow resistance to these companion drugs as well. In addition, ACT has few known side effects and shortens the treatment course from a week to a few days.

The downside of these combination drugs is that they are more expensive than conventional antimalarials. Also, doctors must be careful in selecting companion drugs for different geographical regions, in order to avoid administering drugs for which resistance is already present and weakening the effect of the combination therapy.

Malaria research
One of the goals of malaria research is to find companion drugs that haven't already been used as antimalarials, thus lessening the risk of drug resistance. More research is also needed to prove the safety and effectiveness of combination therapies, particularly with regard to children and pregnant women.

Scientists have succeeded in mapping the genetic makeup of both P. falciparum and the anopheles mosquito. With these findings, researchers hope to identify drugs that effectively inhibit the life cycle of the parasite and stop infection, as well as better ways to control the mosquito population and prevent spread of the disease.

There's no effective vaccine against malaria. In countries where the disease is endemic, prevention involves keeping mosquitoes away from humans. This has included the use of insecticide-treated mosquito netting and spraying indoor walls with insecticide.

Most drugs used to treat malaria are also used to prevent it. Doctors sometimes use the antibiotic doxycycline to prevent malaria. Two or three months before traveling to an area where malaria is prevalent, talk to your doctor or a tropical disease specialist or visit a travel health clinic to obtain the necessary medications to prevent malaria and to receive travel-related vaccines and information. Explain to your doctor exactly where you're going. The drugs you're prescribed depend on the level of drug resistance within your area of travel.

Consider possible adverse effects
In addition, discuss possible and prior adverse reactions to medications. Some have fewer side effects than others. Also, be sure to review your medical history to help identify any possible side effects of taking a medication. For example:

• Mefloquine may infrequently cause nausea, dizziness, insomnia and vivid dreams. In people with past or present psychiatric disorders, mefloquine can worsen symptoms of mental problems.
• Doxycycline can permanently stain the teeth of children younger than 8. It can also cause sun-induced rash.
• Malarone is dangerous for people with severe kidney impairment.

If you're pregnant, avoid traveling to malaria-endemic regions. If this isn't possible, your doctor can prescribe an antimalarial drug that's appropriate for you, such as chloroquine or mefloquine (during the second or third trimester). Don't take Malarone or doxycycline if you're pregnant, because these drugs can harm the fetus.

For preventive treatment, you generally take the prescribed drug one to two weeks before leaving, throughout your trip, and for four weeks after your return. Overdosage of antimalarial drugs can be fatal, so follow your prescription carefully. Don't miss doses.

Drug quality varies worldwide
Be careful about purchasing antimalarial drugs in other countries. Quality of drugs varies widely from country to country, depending on the level of regulation and quality control practices. For example, some drugs may be substandard in the dose or release of the active ingredient; others may be contaminated with other substances. In some countries, fake (counterfeit) malarial drugs have caused a number of deaths. To avoid questionable products while traveling, the CDC recommends that you:

• Bring your medications with you. Purchase medicines in advance in your home country and take them with you.
• Note drug names. Record the generic and brand names of your drugs and the manufacturers, so that if you run out, you can find the correct replacement.
• Inspect packaging. Make sure any drug you purchase is in its original packaging and that the packaging appears authentic.
• Avoid suspicious drugs. Avoid taking tablets that have a strange smell, taste or color, or that are extremely brittle. This may reflect poor storage conditions, which can affect chemical components of the drug.

Tips to prevent malaria
The CDC also recommends the following measures to help prevent malaria:

• Use repellent. Spray DEET insect repellent on exposed skin, and use flying-insect spray to kill mosquitoes in your sleeping area.
• Wear protective clothing. During active mosquito times, usually from dusk to dawn, wear pants and long-sleeved shirts.
• Use netting. If you're staying in a place that doesn't have screens or air conditioning, sleep under mosquito netting that's been treated with an insecticide called permethrin. This netting is available in many travel and camping supply stores and online.

You may also find other anti-mosquito supplies in stores, such as hats with attached mosquito netting that protects your face and neck.

You can obtain current malaria information by calling the CDC at 877-FYI-TRIP, or 877-394-8747. You can also get current health information for a particular country by visiting the World Health Organization's Web page on international travel and health.