Malaria, a disease that comes to humans through tropical mosquitoes, is posing a new threat. During the past five to 10 years, the disease has reappeared in regions where health authorities once thought it was under control, and it is emerging for the first time in countries that previously had no problem with the disease.

Malaria is a disease transmitted through the blood to humans by an infected female anopheles mosquito. The primary symptoms are severe chills and fever. If malaria is left untreated, it is often fatal. The infection is most severe in very young children and in pregnant women. The World Health Organization (WHO) estimates that between 1.5 and 2.7 million deaths occur worldwide each year from malaria, and between 300 million and 500 million malaria illnesses occur annually. Out of all the cases worldwide, nine out of 10 infections and deaths occur in Sub-Saharan Africa, where some regions have seen a sevenfold increase in the disease during the past several years.

"If you live in a village in Africa, you pretty much have to deal with malaria on a daily basis, so there's some level of acceptance to the fact that you and your children are going to get a malaria infection several times a year and get sick from it," says Dr. Trenton Ruebush, chief of the Malaria Epidemiology Section at the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia. "Malaria is a huge problem in this region, and it's going to take a lot of work to make major inroads on it."

Along with Sub-Saharan Africa, the Indian subcontinent (India, Pakistan and Sri Lanka) has always had a problem with malaria. "For many years through effective malaria control programs, the region had the disease under fairly good control," Dr. Ruebush explains. "But then, partly because of reduced funding, perhaps partly because of complacency on the part of the Ministry of Health, partly because of the increases in drug resistance and so forth over the last 10 years or so, the disease has increased greatly in these countries."

Malaria extends its reach

Malaria is also occurring at an increasing rate in industrialized nations, including Europe and the United States, areas that haven't had to face the problem before.

In the United States between 1,000 and 2,000 cases of malaria are reported annually, although the CDC has estimated that those figures represent only half of the actual incidence of the disease. The CDC estimates that around 97 percent of these cases are "imported," meaning they are acquired either by an American who's traveling abroad or by a foreign visitor or immigrant to the United States. Often travelers can carry an infection from one country to another before they know they are ill.

According to the CDC, an increase in the number of "locally acquired" cases of malaria in the United States has occurred during the past decade. Locally acquired cases are those in which transmission of the disease occurs in the United States. This could be through blood transfusion or organ transplantation or through drug addicts sharing needles and syringes (in which case some contaminated blood is left in the needle or syringe and transmitted from one person to another).

Dr. Ruebush says a small number of malaria infections have occurred each year because of mosquitoes that live in the southern two thirds of the United States, but cautions that the risk for a large outbreak of malaria in the United States is still low.

"A large number of factors would have to come together for an outbreak to occur in the United States," he explains. "A mosquito would have to bite and infect a person, the weather needs to be just right so the mosquito can live long enough for the parasites to undergo the development in the mosquito, and then they manage to find someone else to bite and transmit the infection. Then other mosquitoes would have to bite those one or two, get infected, and then each of them bite more people and continue that on for several generations."

The battle goes on

WHO and CDC scientists have several malaria vaccines under development and hope to have at least one vaccine ready within the next five to 10 years. Work continues on improving antimalarial drugs and developing new ones. Another malaria-control measure is the use of mosquito netting hanging over people's beds, particularly if it is treated with insecticide.

"We have many of the tools; the problem is, in the regions in the world where malaria is a really big problem, the health infrastructure is very poor, many of these areas are difficult to gain access to, the health budgets in these countries are very limited to buy the drugs," Dr. Ruebush says. "So, technically, malaria is less of a problem than administratively and in terms of logistics."