Climate, Movement, and the Spread of Disease

Grandidier's baobab (Adansonia grandidiera) at dawn near Andavadoaka, Madagascar. February 2016. Photo by Amy Winter.
Grandidier’s baobab (Adansonia grandidiera) at dawn near Andavadoaka, Madagascar. February 2016. Photo by Amy Winter

Setting the Scene

On the southwest coast of Madagascar, the sun burns bright and fierce over white-hot sands and turquoise seas. The plants are small and spiny and hardened against the relentless drought—they store water over long periods in bulging baobab trunks, and open their stomata to drink in CO2 only sparingly in the relative cool of the night.

The people of the southwest too, are dark and weathered and wiry—in the jumbled diversity of ethnicities that characterize the Eighth Continent, they identify as members of the Bara, the Mahafaly, or the Antandroy tribes, and they speak a slurring dialect of Malagasy quite distinct from the nasal twang of the Highlands to which my ear and tongue are trained.

Year by year, as the summer rains come later or not at all, these people of the southwest, the Atsimo-Andrefana, increasingly abandon futile attempts at farming to take to the only livelihood available—that of the sea. There, their ethnic ancestries fade into memory, and they assume the nomadic identity of the Vezo. The changing climate changes where these people go and who they interact with. And when people exchange goods or conversation, they often also exchange disease.

New Research

It’s been a long while since I wrote last, fresh from a year of field research studying bat-borne viruses in Madagascar. The past few months have seen me lost in data analysis and scientific paper writing as part of my PhD studies at Princeton University. Now I’m back once more in Madagascar—with labmates in tow this time—investigating how climate change affects human movement and the spread of disease here.

“Diseases track human migrations all throughout history,” says Amy Winter, a soon-to-be-postdoc under Princeton professor Jessica Metcalf, the Principal Investigator on this project. A bit of reflection convinces me that she is right—I think of rats carrying the Black Death on the heels of traders along the Silk Road and Venetian merchants across the Aegean; I think of HIV spreading silently through prostitution networks along the major highways of East Africa; and most recently, I think of SARS coronavirus leaping across the globe in the giant sealed incubating chambers that we like to call “airplanes.”

A young Vezo diver holds up a freshly captured octopus. Andavadoaka, Madagascar, February 2016. Photo by Cara Brook.
A young Vezo diver holds up a freshly captured octopus. Andavadoaka, Madagascar, February 2016. Photo by Cara Brook

In Madagascar, one of the ten poorest nations in the world, most rural livelihoods depend on agricultural, pastoral, or wild-harvested resources. As climate change alters the once-predictable seasonal availability of these resources, human movement patterns shift to seek provisions elsewhere.

“It hasn’t rained in this region yet beyond a few small drizzles. That’s about two months late,” says Caroline Savitsky, Madagascar Population-Health-Environment Coordinator at Blue Ventures, an environmental NGO, which works with Vezo communities in western Madagascar to sustainably manage marine resources while meeting community needs.

The delayed summer rains mean that more and more people have given up on farming to adopt the sea-faring life of the Vezo, who traditionally migrate up and down the west coast of Madagascar, following abundances of sea cucumber, octopus, and shark. This increased human sea traffic naturally adds pressure to already-sparse marine resources, which, in turn, become progressively scarcer. “Every year, there is less to catch,” says one Vezo man, when I question him about his annual trip to the Barren Isles, off the coast of Maintirano, in Madagascar’s central West. “Sometimes now, we don’t catch enough to travel all the way north,” he adds, gesturing to the wife and two small children who make the yearly pilgrimage with him.

Migrating fisherman also increase contact between previously isolated regions of Madagascar, thus allowing new avenues for the spread of disease. In 2015, Madagascar made world headlines with a re-emergence of childhood polio cases, “many of which,” notes Caroline, “popped up in remote regions all along the South and West coasts.”

I squint out to sea, my eyes on the white sails of the East African-like dhow, which the Vezo favor, and I wonder if these migrating families of fishers, wives, and unvaccinated children might be linked to some of those cases.

Throughout history, diseases track human migrations, Amy rightly said. And as environmental change alters the traditional patterns of human movement, we are likely to see alterations in patterns of disease to follow.

A Vezo fisherman looks northward toward the annual migration. Ampasilava, Madagascar, February 2016. Photo by Cara Brook.
A Vezo fisherman looks northward from his dhow toward the annual migration route. In March or April, after the summer rains have passed, he will typically bend his sails toward Maintirano, not planning to return south before December. Ampasilava, Madagascar, February 2016. Photo by Cara Brook

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Meet the Author
My name is Cara Brook, and I am a postdoctoral fellow with the Miller Institute for Basic Research at UC Berkeley. I study the role of bats as reservoirs for some of the world's most deadly emerging viruses, including Ebola and Marburg filoviruses, Hendra and Nipah henipaviruses, and SARS coronavirus. I bridge field ecology, cellular immunology, and quantitative epidemiology to investigate this question, at both within-host and population levels. I blog from my field site while tracking down fruit bat viruses in central Madagascar. Tonga soa --Cara E. Brook is the recipient of two research grants from the Nation al Geographic Society.