The tiny worm that causes river blindness, Onchocerca volvulus, is a classic parasite. It infects a person via the bite of black flies, survives in a body for up to 15 years, and upholds the cardinal rule of parasites—don’t kill the host. O. volvulus can grow to the size of vermicelli pasta and produce millions of offspring. These larvae float in the lymph or blood systems, crawl under the skin, invade the eyes, scar the cornea, and eventually cause blindness.
In a test on cows, a team of British, German, and Cameroonian scientists now reports that a very close cousin of O. volvulus falls prey to the drug oxytetracycline. Although this worm infects cattle, not people, it is spread by the same blackfly that distributes O. volvulus.
The study suggests that the antibiotic might kill O. volvulus equally well because it works by eradicating a bacterium that lives in both worms, says study coauthor Nigel G. Langworthy, a veterinarian now in private practice in York, England.
In the 1980s, a pharmaceutical company developed a drug called Mectizan that suspends eye damage in infected people by frustrating the worms’ ability to reproduce and unleash their destructive larvae. Mectizan doesn’t kill O. volvulus, however. While millions of people have benefited from Mectizan, provided free by Merck and Co. of West Point, Pa., patients need yearly treatments indefinitely to keep the parasites at bay.
“Using an antibiotic like tetracycline to eliminate an intracellular [bacterium] and thereby killing the parasite is a remarkable development in this field,” says David Abraham, a parasitologist at Thomas Jefferson University in Philadelphia.
Working in northern Cameroon, the researchers studied six cows that had telltale nodules under their hides indicating infestation by the worm Onchocerca ochengi, whose life cycle is similar to that of O. volvulus. Three of the cows received injections of oxytetracycline for 6 months. The researchers analyzed 20 nodules from each of the six cows at regular intervals during the treatment. The average number of worms per nodule dropped from 33 to 4 in the
treated group, whereas the count held steady between 21 and 36 worms per nodule in the untreated cows, the team reports in the June 7 Proceedings of the Royal Society of London B.
By attacking a bacterium that resides in both of the worm species, the researchers have “taken a unique approach to treatment,” Abraham says. The bacterium, Wolbachia, can be found in insects and various other animals (SN: 11/16/96, p. 318: http://www.sciencenews.org/sn_arch/11_16_96/bob1.htm). While it’s clearly essential to O. ochengi, its precise role in worm cells remains unclear, Langworthy says.
If the new strategy were to work in people with river blindness, oxytetracycline or a related antibiotic would have the advantage over Mectizan of curing the disease, not simply stalling it, says Juliet A. Fuhrman, a biologist at Tufts University in Medford, Mass.
A drug that kills the adult worm in people would be a first, Abraham says. People of all ages could be treated to wipe out any worms in their body, he says.
Such an approach would interrupt the long process that leads to blindness, especially near rivers where blackfly-control projects can’t keep up, says Langworthy. The World Health Organization is currently considering testing tetracycline against river blindness in West Africa, he adds.
Tetracycline is an inexpensive drug that requires no refrigeration, making it ideal for use in developing tropical countries, where services, money, and refrigeration are often in short supply, says Magdi M. Ali, an immunologist at Michigan State University in East Lansing.
However, bacterial resistance to tetracycline is already present in many areas that are affected by river blindness and so could lessen the drug’s effectiveness against Wolbachia and the parasite, says Ali. Tetracycline might work best in combination with Mectizan, Abraham says.