Major health organizations around the world are celebrating the elimination of river blindness in Ecuador, the second Latin American nation to announce the milestone. Dr. Thomas Unnasch, chair of the Department of Global Health at the USF College of Public Health, has been involved with numerous organizations in a long fight to eliminate the disease.
“I worked with a whole bunch of different control programs,” Unnasch said, “the Carter Center and a lot of the African programs directly: the African Programme for Onchocerciasis Control, and before that the Onchocerciasis Control Programme in West Africa.”
Unnasch said he started working with the OCP in 1991 and continued with them until the program ended in 2003. It was one of the first large international disease control programs. Its vector control program began in 1975.
“It was concentrated where blinding oncho was the most problematic,” he said. “It was an area about the size of the United States east of the Mississippi River – parts of 11 countries in West Africa in the savanna regions.”
APOC, Unnasch explained, was an expansion of OCP. APOC began operating in 1995, and Unnasch has served as a technical adviser to the organization since 1998. He also has worked as a technical adviser with the Onchocerciasis Elimination Program for the Americas, a program of the Carter Center, since 1997.
“My lab has been really involved in the whole question of elimination,” he said. “The way all of these programs are developed, the first thing you want to do is interrupt transmission. The idea is, if you can interrupt transmission and then maintain that for a period of time that all the female worms in the population die off, then you can break the transmission cycle permanently. The real catch is to know when you’re going to start the clock, when you have actually broken the transmission cycle, which means you have to show that there’s no more transmission.
“That’s difficult to do,” he said, “because it’s difficult to prove a negative. It’s hard to prove that something’s not there. All you can say is that we have a high probability that something’s not there. In order to do that, obviously, you have to look at a lot of samples. You can’t just look at five flies and say, ‘We’re done,’ or a hundred flies and say, ‘We’re done.’ You’ve got to look at thousands and thousands of flies.
“So, we’ve been developing methodologies to screen the flies, and also screen children, who we use to act as sentinels for exposure to the parasite because they’re living out there. We’ve been introducing these methodologies and setting up laboratories around the world in the endemic countries that allow them to screen the samples as they come in, and then we provide quality control, technical support and logistical support to those labs on an ongoing basis. We also have developed the statistical methods: How many samples do you need to test? How do you collect those samples? How do you design your sampling schemes?”
Mass distribution of the anti-parasitic drug Mectizan has played a major role in the elimination, Unnasch said. According to the Carter Center website, Merck Pharmaceuticals manufactures the drug and donates it for mass distribution. The drug kills the parasite larvae in the human body. The virus, Onchocerca volvulus, breeds in rapidly flowing rivers and is spread to humans via the bites of black flies of the genus Simulium. It can live in a human host for up to 14 years.
The Carter Center and its partners have distributed more than 170 million doses of Mectizan in Africa and Latin America, and according to the center and Unnasch, it’s working. The best news is from Latin America, where river blindness – which, according to the Centers for Disease Control and Prevention, is second only to trachoma as a cause of infectious blindness worldwide – has been declared eliminated in Colombia and, more recently, Ecuador.
“It’s a drug that, if it’s used effectively and you get high coverages in the population, can interrupt transmission,” Unnasch said. “In Latin America, they started the program in ’95, giving the drug twice a year throughout the continent.”
The initial belief was that it would take 12 years of high coverage to break the transmission cycle, Unnasch said, but results materialized much sooner.
“It was discovered that, after something like six or seven treatments with the drug, the adult females starting getting sterilized. So really, you only had to do it for five or six years, and at that point, the adult females were either dead or they were sterile, and that was it.”
However, getting from that point to making the announcement is another involved and lengthy process.
“We went in and identified when the transmission had dropped to zero by monitoring the flies and the children, allowed that to continue for five years, continuing the treatments during that period, and then, at the end of that period, we stopped treatment,” Unnasch explained. “We did nothing for three years and held our breath to see if it would come back, and then went back in and did what’s called a post-treatment surveillance, large-scale studies in these countries to see if we could find evidence of transmission that had started again.
“In the countries we’ve done so far,” he said, “we haven’t seen any evidence after that three-year period of any other transmission coming in, so that’s how it’s declared that it’s been eliminated. All that data gets put together and put into a dossier that’s sent to the World Health Organization, and the World Health Organization goes back and does a site visit, and if they’re happy, then they’ll verify that the transmission has been eliminated.”
Two other Latin American countries, Mexico and Guatemala, are putting their dossiers together and preparing to send them to WHO, Unnasch said. If river blindness is declared eliminated in those countries, he said, it will remain a threat only in Venezuela and Brazil, where it is limited to an area in the upper Amazon region. It’s a relatively small area, he said, with only about 30,000 people at risk. About 20,000 others in the region are receiving Mectizan treatments, he said.
“We could do a lot better than that, but that’s a difficult population,” he said. “They’re migratory, they live in the rainforest, it’s tough to get to them. They often don’t know how old they are. They’re small, so when you measure them, you can’t tell how old they are based on how big they are, and getting the same population twice a year, since they’re not sedentary, can be tough.”
While Africa continues to be afflicted with river blindness, recent efforts are beginning to pay off there, as well, and Unnasch also is part of that effort.
“Most of what we’ve been involved with is developing new and improved blackfly traps to help eliminate the flies in the communities and try to bring the vector numbers down that way,” he said.
Unnasch was in Ethiopia in early October for the first meeting of an advisory committee. He chairs advisory boards there and in Uganda, where a combination of mass drug distribution and vector control has proven successful thus far. Of 17 foci, he said, transmission appears to have been interrupted in 15.
“There are only two that are really causing problems still,” he said, “so they’re making tremendous progress.”
Story by David Brothers, College of Public Health. Photos courtesy of Dr. Unnasch, Eric Younghans, USF Health Commuications, and the Houston Chronicle.