By walking barefoot on fine volcanic soils, African farmers may suppress their immune response and contract cancer--in their feet.
Kaposi’s sarcoma is a rare form of cancer that first gained wide attention in the United States as a hallmark of AIDS in gay men. But in certain regions of Africa and the Mediterranean it was quite common long before AIDS. Epidemiologists believe it is caused by an opportunistic infection--probably a virus, unrelated to the AIDS virus, that attacks someone whose immune system is suppressed. Compared with AIDS-related Kaposi’s, which spreads throughout the body, the disease that is endemic in Africa is much less aggressive: it usually attacks only the legs and feet, and it is rarely fatal. John Ziegler, a physician specializing in tropical diseases at the University of California at San Francisco, thinks he knows why. He says the victims of endemic Kaposi’s in Africa suffer local immune suppression in their feet from walking barefoot on fine volcanic soils.
Kaposi’s sarcoma is a very common malignancy in sub-Saharan Africa but uncommon in virtually every other part of the world, says Ziegler. The Kaposi’s hotbed, it turns out, is in western Uganda, eastern Zaire, Rwanda, and Burundi--all of which lie along the volcanically active East African rift valley. Ziegler noticed that most Kaposi’s victims in those countries are farmers who cultivate fields in their bare feet.
He noticed further that in those same regions a disease called podoconiosis is also prevalent. Podoconiosis, or mossy foot, is thought to develop when tiny particles of volcanic soil--as little as a tenth of a micron across, which is roughly the size of a virus--penetrate the bottoms of the feet, either through pores or through abrasions in the skin. The particles work their way into the lymphatic system, a network of drainage vessels that collects cellular waste products and proteins and funnels them into the bloodstream. Reacting to the invasion as if it were a wound, the body makes fibrous tissue that clogs the lymphatic vessels and causes the legs and feet to swell.
The lymphatic vessels, however, are also home to immune cells-- and in particular to macrophages. The macrophages, Ziegler thinks, respond to the invasion of soil particles as well. They take this stuff up because that’s what they’re trained to do, he says. And then they die because they just can’t cope. Experiments with rabbits have shown that macrophages are poisoned by soil particles, probably by the metals in them or on their surface.
Macrophages are the roving police of the immune system: they travel around arresting alien invaders and transporting them back to the lymph nodes, which may then launch a full-scale immune response. But like cops walking the beat, macrophages tend to stay in a particular region of the lymphatic system. So when they are killed by soil particles invading through the feet, says Ziegler, the result is a local suppression of the immune response. If the unfortunate person then happens to be infected by whatever microbe causes Kaposi’s, he would fall victim as well to its characteristic purplish lesions. But he would get those lesions only in the feet and legs.
In contrast, immune suppression in AIDS is not a local matter; eventually the patient’s entire immune system collapses under the assault of the AIDS virus. That is why Kaposi’s sarcoma in an AIDS patient spreads throughout the body. It attacks gay men almost exclusively--rarely affecting those who contracted AIDS through blood transfusions or intravenous drug use--which suggests that it is transmitted sexually. For a long time the Kaposi’s agent was probably confined to isolated regions of Africa and Europe. (Some of the European areas, Ziegler notes, such as those in Corsica and Sardinia, are also volcanic zones.) When AIDS came in and caused such severe epidemic immune suppression, Ziegler says, the Kaposi’s microbe began to spread.
Interestingly, though, the incidence of Kaposi’s among gay men with AIDS has plunged dramatically. In 1981, 77 percent of gay men with AIDS in the United States had Kaposi’s; by 1992 that number had fallen to 10 percent. It is not clear why. As for the barefoot farmers of Africa, Ziegler counsels safe walking. If there’s a connection between Kaposi’s and volcanic soil, he says, all they need to do is wear shoes.