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(Corrects size of sandfly in para 3 from 8mm to 2.8mm)
From a small yet sturdy single-prop plane, Dr. Jill Seaman returns to Old Fangak in South Sudan’s Jonglei State after three days crafting a logistics plan to keep her small clinic up and running.
Months of continuous fighting have forced over 700,000 displaced people to roam from village to village in search of food, health care and safety. But with images of violence so prevalent in this region, it may be difficult to imagine there is a latent killer lurking in the forests - one that is potentially more dangerous than warlords or guns.
Visceral leishmaniasis, known locally as kala-azar, is the second largest parasitic killer in the world. It is spread by the bite of a 2.8mm female sandfly, an insect that flourishes in South Sudan’s acacia and balanites forests.
Symptoms can be silent, but generally include fever and enlargement of the spleen and liver. The mortality rate of leishmaniasis in South Sudan is near 100 percent, but there is hope because proper treatment can lead to recovery for the majority of infected people.
Dr. Seaman has been working in South Sudan for over 25 years and has seen her fair share of deadly outbreaks, including those during the civil war in the 1980s and 1990s that resulted in over 100,000 deaths.
In recent email correspondence via satellite connection from her mud hut along the Nile, she notes that the incidence of kala-azar during recent years has remained sub-epidemic, most likely due to the relative political stability. But she is quick to flag up her concern that the situation will be different this year.
There is a clear connection between political instability and incidence of leishmaniasis. During periods of intense fighting, huts are torched and their residents are forced to flee. Left with nothing except what they can carry, the displaced trudge across the Sudd, the third largest swamp in the world, hoping to find food, medical aid and shelter.
As the African sun beats down, the only shelter is the shade of the acacia and balanites trees - also the prime location for Phlebotomus orientalis, the main sandfly species responsible for leishmaniasis transmission. Among the exhausted people, the need for rest overtakes the ability to swat away the swarms of sandflies, and they become infected with the parasite.
Although movement of people usually precedes an epidemic, there are certain climatic conditions that correlate with increased sandfly activity and the subsequent risk of disease transmission.
A study I co-authored, due to be published in Earthzine, an online publication from the Institute of Electrical and Electronics Engineers’ Committee on Earth Observation (ICEO), explores the relationship between climate and outbreaks of diseases. The study employed a groundbreaking method of comparing remotely sensed climate data from NASA satellites to epidemiological data of leishmaniasis outbreaks obtained from Medicines Sans Frontières (Doctors without Borders).
The study analysed how climatic and environmental variables such as night and daytime temperatures, humidity, soil type, NDVI (a way to measure vegetation from space), rainfall and inundated surface area influenced the incidence of leishmaniasis from 2004-2012.
Admittedly, the study period is too short to establish any concrete evidence of a cause-and-effect relationship between climatic and environmental factors. But the results do suggest that a decrease in rainfall and inundated area during certain months of the rainy season lead to a higher risk of leishmaniasis outbreaks.
The next step for climate researchers is to quantify the connection between climate and leishmaniasis, allowing for future analysis on both short and long timescales. While short-term (same-year) relationships can be used to develop an early warning system, evaluation of long-term links would offer insight into whether shifts in suitable climatic conditions for sandfly development - including temperature and rainfall - are becoming more likely due to climate change.
It can be argued that there is no good time for a civil war - but when evaluated in terms of the risk of a deadly outbreak of leishmaniasis, March to June is the worst for South Sudan.
Historical evaluation of epidemiological data shows that cases of leishmaniasis tend to surface in August and last until December, but due to a three-to-six month incubation period based on the varying strength of individual immune systems and nutrition, studies have found that transmission preceding epidemics usually occurs during the months of March through June.
The results of our study indicate that above-average rain and inundation may quell, at least somewhat, the risk of an epidemic. But recent forecasts on weekly and monthly scales predict below-average and average rainfall respectively, especially over Lake Victoria, which contributes much of the water that causes flooding in South Sudan.
The three-to-six month variable incubation period may make it harder to establish the precise conditions for sandfly development and proliferation, but it does allow time for resources to be allocated to the areas of a predicted outbreak.
If an outbreak does occur in South Sudan - an above-average probability at this point - the ongoing political instability will surely increase response times significantly, leaving many to die while waiting for medical attention.
The final lines of Dr. Seaman’s most recent email echo the concern felt throughout the medical aid community in South Sudan: “We are all so worried because of the tens of thousands of people who have fled the big cities and (who) now reside in the Jonglei State acacia forests - the site of the sandfly vector.”
Andrew Kruczkiewicz works at the International Research Institute for Climate and Society, part of Columbia University's Earth Institute in New York City, where he specialises in remote sensing and applications of meteorological and climatic data for public health and agriculture.