Transmission Mode of Malaria

The female anopheles mosquito is the vector for human malaria. Some 60 species of this mosquito have been identified as vectors for malaria, and their distribution varies from country to country.

The infection is transmitted by the bite of an infected female mosquito – Anopheles. An. culicifecies in Rural area & An. stephensi in urban area. The mosquito usually bites during dawn & dust time. The mosquito becomes infected by biting a patient with malaria infection. When a mosquito bites an infected individual, it sucks the gametocytes, the sexual forms of the parasite, along with blood. These gametocytes continue the sexual phase of the cycle and the sporozoites fill the salivary glands of the infested mosquito. Once the mosquito becomes infected, it remains so far life. The female mosquitoes can survive upto 4 weeks under normal temperature i.e. 28°C to 30°C and humidity i.e. 60 to 80%. When this female mosquito bites the man for a blood meal, which it needs to nourish its eggs, it inoculates the sporozoites into human blood stream, thus spreading the infection.

Other modes of transmission Rarely malaria can spread by the inoculation of blood from an infected person to a healthy person. In this type of malaria, asexual forms are directly inoculated into the blood and pre–erythrocytic development of the parasite in the liver does not occur. Therefore, this type of malaria has a shorter incubation period and relapses do not occur.

1. Blood transfusion (Transfusion malaria) This is fairly common in endemic areas.

Following an attack of malaria, the donor may remain infective for years (1–3 years in P. falciparum, 3–4 years in P. vivax, and 15–50 years in P. malariae.)

Most infections occur in cases of transfusion of blood stored for less than 5 days and it is rare in transfusions of blood stored for more than 2 weeks. Frozen plasma is not known to transmit malaria.

The clinical features of transfusion malaria occur earlier and any patient who has received a transfusion three months prior to the febrile illness should be suspected to have malaria.

Donor blood can be tested with indirect fluorescent antibody test or ELISA, and direct examination of the blood for the parasite may not be helpful.

In endemic areas, it is safe to administer full course of chloroquine to all recipients of blood transfusion.

In transfusion malaria, pre–erythrocytic schizogony does not occur and hence relapses due to dormant hepatic forms also does not occur. Therefore, treatment with primaquine for 5 (or14) days is not indicated.

2. Mother to the growing fetus (Congenital malaria) Intrauterine transmission of infection from mother to child is well documented. Placenta becomes heavily infested with the parasites. Congenital malaria is more common in first pregnancy, among non – immune populations.

3. Needle stick injury Accidental transmission can occur among drug addicts who share syringes and needles.