Malaria is primarily transmitted by an insect vector specifically the female mosquito of the Anopheles genus. Thus, males do not transmit the disease. The female insect prefers to feed at night. They usually start searching for a meal at dusk and will continue throughout the night until they succeed. When a mosquito bites an infected person, a small amount of blood is taken. This contains microscopic malaria parasites. About one week later, when it takes its next blood meal, these parasites mix with the mosquito saliva and are injected into the person bitten.
However malaria parasites can also be transmitted by blood transfusion (Marcucci et al., 2004).This is a rare occurrence but potentially is a source of concern for blood recipients. During 1958-1998 a total of 103 cases of transfusion transmitted malaria were reported to the centre for disease control (CDC) in the United States. The frequency of post transfusion malaria has been estimated to vary from less than 0.2 cases per million in non-endemic countries to 50 or more cases per million in endemic countries (Bruce-Chwatt, 1985)
Thus the frequency of reported transfusion malaria is relatively low in non-endemic countries. For instance, there have only been about 8 cases in the United Kingdom in 50 years; 26 cases in the United States of America in 10 years from 1972 to1981 and in France, 110 cases in 20 years (Bruce-Chwatt, 1985). But in Saudi Arabia there have been 12 cases in just four years in one centre in Ridyadh as well as at least 20 unreported cases from other centres in the kingdom.
In another study carried out in 1987, it was reported that P. falciparum was present in the blood of donors who were exposed to malaria 20 months to 3 years previously. P.vivax has been transmitted by transfusion from donors infected four years previously and P. malariae from donors infected 17 years earlier (Chikwen, et al., 1997). Also McClure and Lam (1945) reported that quartian malaria (P. malariae) has been accidentally transmitted in blood previously stored for five days (McClure et al., 1993) and in yet another case reported by Black (1960), a donor had carried P. malariae parasite for 10 years (Button et al., 1993).
These cases clearly implicate blood transfusion and qualify it as a second source of malaria transmission. The transmission of malaria by blood transfusion is a serious risk as the diagnoses of malaria in the recipient is unexpected and this is often missed (Kinde-Gazard et al., 2000). Asymptomatic carriers are generally the source of transfusion transmitted malaria. Since healthy blood donors are often selected for blood donation, density of parasites is usually very low, if present and hence may be missed (Kaur et al., 2005).
In another report it was stated that donors who have been implicated as infection source in transfusion-transmitted malaria cases typically present with very low level of parasitemia that may be undetectable, even with microscopic examination of several thick films. Out of the 60 cases reported in United States during 1963-1998 where a blood smear was obtained, only 18 (30%) of implicated donors had Plasmodium parasites detected on the blood smear. Detection of malaria antibodies provides evidence of an immune response to current and past infection, but these tests may remain positive for more than 10 years long after parasitemia has resolved; therefore the use of malaria antibody detection to screen blood donors would result in the exclusion of otherwise healthy volunteers. However, polymerase chain reaction (PCR) has increased the sensitivity over blood film examination, positively indicating current malaria infection (Vu et al., 1995).
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