Diseases and Human Evolution

12: Syphilis: The Great Change Artist

CHAPTER 12 Syphilis The Great Change Artist Syphilis was the most feared venereal disease throughout the civilized world until the mid-1940s when the introduction of penicillin drastically altered the damaging affects and spread of the disease. Prostitution and loose morals came to be identified with the disease by the mid-nineteenth century when the term syphilis came into common use (Hudson 1958). The causative agent of the disease remained unknown until 1905 (Sell and Norris 1983), and the mysterious changing nature of the disease finally began to unfold its secrets in the late 1920s. The Agent The cause of syphilis was identified as a microscopic bacterial parasite that looks like a tiny worm upon magnification. It belongs to a class of bacteria known as spirochetes. They are slender, cylindrical bacteria that have the ability to move about on their own. Spirochetes occur everywhere in nature, living off dead or decaying organic matter. Some have adapted to parasitic life on living organisms, often as noninvasive commensals, thriving on dead cells shed by the host. Harmless spirochetes thrive in our mouths, clinging to deep mucosal crevices between our teeth and gums. Some cling to mucosal crevices in our lower gut, and open surface wounds invite these little “critters” to dine on dead cells without causing any harm to living cells (Sell and Norris 1983). Sometimes members of harmless colonies of spirochetes in the mouth can contribute to periodontal disease, while some can combine with other 201 bacteria to cause painful lesions in the mouth and throat, and occasionally settle in the lungs, forming abscesses (Manz and Buck 1997). Three different kinds of spirochetes have become invasive parasites in humans (Eagle 1952). One group, known as Borrelia, can be transmitted by ticks and human body lice, causing intermittent bouts of fever. These illnesses , known as relapsing fevers, include Lyme disease. Another group, known as Leptospira, and carried by rats, can be transmitted through ratinfested waters to cause Weil’s disease, with fever and influenza-like symptoms that progress to jaundice. Deadly symptoms can develop in more susceptible individuals (Forbes and Jackson 1993). Borrelia and Leptospira spirochetes operate as accidental parasites of humans, and people do not directly transmit them to other people. Another group of parasitic spirochetes have become true human parasites, with direct human-to-human transmission. This group is known as Treponema. Morphologically treponemes resemble commensal spirochetes that live in our mouths, gut, and surface wounds (Kolker et al. 1997). Perhaps at some time in human history members of a certain type of these commensal spirochetes managed to penetrate damaged mucous membranes and/or wounded flesh, to survive within the internal microenvironments of human hosts. Those microbes unable to survive within the host died off. Treponemes infecting humans have also been found in monkeys, baboons, and chimpanzees in west and central Africa, suggesting the human treponemes originated there (Fribourg-Blanc 1972). A similar treponeme, known as Treponema cuniculi, adapted to rabbits as an internal parasite, but cannot survive in humans (Hardy 1976). Ironically, those treponemes that did manage to adapt as internal parasites behave as if they would prefer to be outside the host. They prefer living in tiny blood vessels nourishing mucosal or skin surfaces, and they feed off dead cells like their commensal cousins. Most of them cannot tolerate the higher temperatures of the internal organs very well, so they flee to the cooler parts of the body, particularly the skin, whenever possible. High fevers within the host can be deadly to the treponemes (Eagle 1952; Turner 1970). Treponema pallidum identifies the treponemes responsible for the disease of syphilis. This treponeme can change its morphology as it moves around to different microenvironments within its host. Similar changes occur among free-living spirochetes that encounter environmental changes. Following their introduction into a host, parasitic treponemes change shape, 202 CHAPTER 12 depending on where they wind up within the body. Infections near the surface more likely consist of the classic, spirochete form. Treponemes carried to deeper, warmer parts of the body usually are small, rounded or granular forms, sometimes serrated, that reproduce by budding. Numerous small forms frequently join to form cysts that clump together and produce large numbers of classic spirochete shapes. All the stealth forms of treponemes can revert to the spirochete form when conditions favor its survival (Delamater et al. 1951; Mattman 1992; Turner 1970). The treponemes can only enter...