Maryland Department of Agriculture

Malaria is a mosquito-borne infection caused by protozoa of the genus Plasmodium. Four species of these blood-borne parasites, Plasmodium falciparum, P. vivax, P. malariae and P. ovale commonly infect humans. Malaria is one of the most common diseases in the world and one of the leading causes of death in children, especially in developing or third world countries in the equatorial regions. Over half the world's population lives where malaria is endemic. However, travel to these areas continues to be an avenue of parasite reintroduction into temperate areas with highly susceptible human populations. This is already clear in the United States where in the last two years endemic malaria cases have occurred in California, New York and New Jersey. Imported cases are on the rise and many occur in Maryland.

Malaria is an ancient scourge of humankind. It can be found in ancient writings from China, India and the Middle East. They often attributed it to gods or evil spirits. However, Hippocrates, in 500 B.C. differentiated the clinical symptoms. Ancient cultures didn't associate the disease with mosquitoes but believed swamp fumes caused it, hence the name "Malaria," which literally means "Bad Air." They found that disease incidence decreased with swamp draining and such efforts persist in endemic areas today.

It was not until 100 years ago that we knew the life cycle of the malarial parasite and the role mosquitoes play as vectors of this disease. Once we understood this, a massive effort to control the disease began but has not been successful due to insecticide resistance, drug resistance, lack of funding and impoverished conditions of endemic areas. Research on the life cycle of the malarial parasites continues but it is still not fully understood.

Mosquito control and chemotherapy have been the thrust of most malarial control programs. Mosquito control began with swamp draining and was augmented by applications of pesticides. Chemotherapy began to treat symptoms. Chemotherapy began with ancient cultures, most notably South American tribes near modern day Peru. They devised concoctions from the Peruvian Bark, cinchona, the fever bark tree. This brought relief to malarial sufferers but it was not until 1820 that European researchers isolated the antimalarial alkaloids quinine and cinchonine. This led to synthetic production of these compounds and their more active chemical relatives chloroquine and primaquine. Insecticide applications were not used until the beginning of the century with Paris Green and DDT being most effective. Today, DDT is still used but malathion and the synthetic pyrethroids are becoming more popular due to resistance to DDT.

Species of Anopheles mosquitoes transmit malaria. When the mosquito bites a human host infected with malaria, it ingests the male and female gametocyte life stage of the parasite. Fertilization between parasite gametes takes place in the mosquito producing sporozoites. When the mosquito bites another human host, it injects sporozoites into the host's blood stream. Once inside, the sporozoites migrate to the liver where they undergo asexual reproduction producing thousands of new individuals and transform into merozoites. This stage enters the blood stream and infects red blood cells. The merozites deform the red blood cell, feed on its hemoglobin, and reproduce until the cell ruptures (hemolyses) and releases thousands of merozoites to feed on other red blood cells.

The disease is cyclic and periodic: P. falciparum, P.vivax, and P. ovale complete development in the red blood cells within two days, better known as the tertian period. Plasmodium malariae, the cause of quartan malaria, requires 72 hours to complete development.

Malarial parasites can persist in the liver if you are infected with the P. vivax or P. ovale parasites, even after we eradicate all parasites from the blood stream. The parasite can become dormant for years in our liver in a form called a hypozoite. It can suddenly produce merozoites that infect the blood causing a relapse.

The most pronounced clinical manifestations of malaria are the periodic chills and sweats. Fever, headache and muscle aches have usually accompanied these. Vomiting is common. A fever may exist for several days before you reach a cycle of chills and fever. Symptoms typically take 10-15 days to appear after the bite of an infected mosquito. Malaria causes death by blockage of brain capillaries (cerebral malaria) or necrosis of the spleen, liver and/or kidneys leading to disfunction in these organs. Untreated malaria can be fatal in the first attack, especially in children. This is most common in cases of falciparum malaria. Untreated forms, if not fatal, are chronic, causing repeated attacks. We can develop immunity within one year in the case of falciparum malaria, five years with vivax and ovale and up to 30 years with P. malariae.

We can also get malaria infection as the result of blood transfusion, contaminated needles and congenitally from mother to child.

E-Mail Jeannine Dorothy, Maryland Department of Agriculture Mosquito Control