Epidemiology and Etiology

Favism is generally a pediatric illness in which the majority of victims are between 2 and 5 years of age, although cases as young as 6 months and as old as 65 years have been reliably reported. The disease has a marked seasonal cycle corresponding to the harvest of fresh fava beans, a 4- to 5-week period between April and July, although in areas where the bean is dried for later consumption, cases can occur all year. There is evidence that the toxic factor that induces the favism crisis has four characteristics:

1. It is located in the skin of the bean.

2. It is heat stable.

3. It is able to enter the breast milk of lactating women.

4. Its toxicity decreases when the beans are dried and the skin changes color.

Active biochemical agents in the skin of the bean -the pyrimidine oxidant compounds vicine, isoura-mil, divicine, and L-dopa - are probably responsible. These same biochemical agents are believed to provide some protection against malaria for people with normal genotypes when they eat fresh fava beans. Although it is widely believed that inhalation of pollen of the V. faba can trigger cases of favism, recent studies indicate that this is not true.

Boys are much more likely to suffer favism attacks than girls, with male/female case ratios varying from 2.1:1 to 2.7:1. The reason for the increased risk for males is that G6PD deficiency is a sex-linked trait; the gene is located on the X chromosome. Only carrier males (hemizygotes) and homozygous females can suffer from favism. Heterozygote females appear to be at an evolutionary advantage because they have no risk of favism and also enjoy a degree of protection against the malaria protozoa Plasmodium.

The G6PD enzyme, found in all tissues, plays important housekeeping functions in red blood cell metabolism. The cells of enzyme-deficient individuals tend to become oxidant-sensitive, and any exogenous sources of increased oxidants (malaria parasites, antimalaria drugs, or fava beans) can result in the lysis (explosion) of the cell (Katz and Schall 1979). In enzyme-deficient individuals, this process can result in either favism or protection from a severe malaria infection, depending on the context.

The geographic distribution of the many varieties of G6PD-deficient genes has been exceptionally well studied by population biologists. Over 200 varieties of the gene have been identified. Both population genetic data and in vitro studies indicate that the distribution of these genes is correlated with the historical distribution of malaria. Favism occurs in only a small proportion of individuals with the Mediterranean variant of G6PD deficiency, and within that population it follows familial lines. It has been recently recognized that there is substantial genetic variation within the Mediterranean variant (Luz-zatto and Battistuzzi 1985).

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