During our exchange, when I made an offhand comment about how disgusting the flies were, he chided me: “Of course we don’t think they are disgusting at all. We love our flies.”
The leish lab, Sacks said, has been working for years on charting every stage in the life cycle of leishmania, looking for chinks in its armor that could be exploited by a vaccine. It’s harder to design a vaccine against a protozoan than against a simpler virus or bacterium; in fact, not one major parasitic disease has a reliable vaccine yet. Leish is very sophisticated in how it infects the body. It is, as one parasitologist said, “the royalty of the disease world.” Instead of wreaking carnage like many viral and bacterial diseases, and thus triggering a massive immune response, the parasites “try to have tea with your immune system.” Sacks and his team have identified the essential proteins the parasite uses during its life cycle inside the sand fly—and they’ve created mutant forms of those proteins that might block development. But figuring out how to exploit those vulnerabilities is hard, and getting from there to a vaccine is even harder.
As is too often true, the biggest hurdle is money. Vaccines cost hundreds of millions of dollars to develop, test, and bring to market. Human trials involve thousands of subjects. “It’s difficult to get companies to partner in trials,” Sacks told me. “They don’t see any market in it, because the people who have leishmaniasis have no money.”
Over the past decade the World Health Organization sponsored a series of clinical trials to test a simple leish vaccine, in which parasites were heat-killed and injected into people. Doctors hoped the dead parasites would prime the immune system to attack live parasites when they arrived. The trials failed, but it is unclear why. Other possible vaccines are in the early stages of testing.
One of the biggest discoveries Sacks and his team made was that the leish parasites have sex inside the sand fly. Previously it was thought the parasite could only reproduce by division—clonal reproduction. By having sex they can recombine their genes. This gives them a way to hybridize and adapt. It explains why there are dozens of leish species and why, even within a species, there are so many different strains. The ability to have sex gives leishmania a tremendous evolutionary advantage. It is the main reason it has thrived and spread for a hundred million years, infecting dinosaurs and people, becoming one of the most successful diseases (from its own point of view) in the world.
With leishmaniasis so prevalent in the valley of T1, I wondered how ancient people might have coped with the disease. Could they have controlled it by clearing vegetation or killing the animals that acted as hosts? I posed the questions to Sacks. He pointed out it would have been difficult for the people of T1 to identify the sand fly as the vector and certainly impossible for them to know a host animal was necessary; assailed daily by biting insects, they probably would not have made the link between a sand fly bite and a lesion that developed weeks later. (The link between mosquitoes and malaria, for example, wasn’t made until 1897. Previously, malaria was thought to be caused by the “bad air” of nighttime—which is what mal aria means in Italian.)
Nor could leishmaniasis have been a reason for the abandonment of T1, since the disease in pre-Columbian times was too widespread; there was nowhere the people of T1 could escape to. They would have lived with the disease, just as hundreds of millions do today.
When our team members were diagnosed, biopsies were taken from our lesions and sent to another lab at NIH, called the Molecular Parasitology Section, where the lab’s director, Michael Grigg, had originally identified the parasite as L. braziliensis by sequencing part of its genome. I called up Grigg to find out if he had found out anything unusual.
“The type of leish you have was very hard to grow,” he recalled. In fact, like some difficult strains, it wouldn’t grow at all. He smeared tissue samples from our biopsies on blood agar plates, but the parasites refused to multiply. Because of that, his lab wasn’t able to tease enough parasites clear of human tissue to sequence the entire genome at the time—there was too much human DNA mucking up the works.
Instead, he explained, they initially sequenced one gene or marker: a characteristic one that reveals the species. That marker matched braziliensis. But later, Grigg sequenced five markers—which he described as “five little windows into the parasite.” He got a big surprise. In two “windows” he found genetic sequences different from any known species of leish. In another window, he found that the DNA resembled another species called L. panamensis, an equally bad mucosal strain. But that gene also had a couple of mutations.
Our parasite, he said, might have been a hybrid between panamensis and braziliensis, in which the two species mingled in a sand fly gut, mated, and produced hybrid offspring. That hybrid was then isolated and began to evolve into a new strain or possibly even a new species. There were enough mutations, called “snips,” at the five sites to indicate that this particular species had been isolated for a period of time.
How long? I asked.
“That’s a tough one. There are not a lot of snips, so I’d say it’s been hundreds of years, not thousands or tens of thousands.”
I had a sudden idea. I explained to Grigg that the valley had once been the site of a bustling city with active trade networks. But about five hundred years ago, the city had been abandoned and the valley suddenly cut off from the rest of the world, with people no longer coming and going to spread the disease. Could that abandonment be the moment when the parasite was isolated? And if so, could the parasite’s molecular clock be used to date the time of abandonment?
He thought about it and declared it a reasonable hypothesis. “When you get rates of change of one or two snips, we’re looking at an isolate of hundreds of years. It’s relatively recent. It’s consistent with your theory.”
All species have what is known as a molecular clock. This clock measures how fast random mutations accumulate over the generations. Some species, like cold viruses, have fast clocks, while some, like humans, have slow. By counting the number of mutations, the molecular clock will tell how long that species has been isolated. It’s like the game of telephone; you can tell how far you are from the original message by hearing how garbled it has become.
Later I sketched out to Sacks the same idea about dating the death of T1 by using the parasite’s molecular clock.
“That would make sense to me,” he said. “These phylogenetic trees are published, so when you find a new species you stick it on that tree to find the genetic distance.” And that would give you the time period of isolation.
If true, this might be the first instance in which an archaeological site could be dated by molecular clock; our disease might actually hold clues to the fate of T1. The research, however, has yet to be done.
CHAPTER 26
La Ciudad del Jaguar