Widespread, heavy, and above-normal amounts of rainfall have created perfect conditions for another, mosquito-borne, Rift Valley fever along the coastal areas of Saudi Arabia and Yemen. This region witnessed its first Rift Valley fever outbreak in 2000. That year, the disease killed more than 160 people and led to large losses of livestock.
Rift Valley fever predominantly affects domesticated animals (particularly livestock). When the virus infects humans, the symptoms are usually mild (flu-like fever, muscle pain, joint pain, and headache). But in 8 to 10 percent of people, the infection can lead to blindness, hemorrhagic fever, or death.
The water accumulates in low-lying areas called dambos, or in agricultural wetlands along the Red Sea coast, providing habitat for disease-carrying mosquitoes to reproduce, EO reports. The same conditions that promote the disease also make it easier to identify when and where conditions are suitable for an outbreak.
Vegetation anomaly (percent), image acquired May 16 - 23, 2016. Credit: Jesse Allen and Joshua Stevens / Jennifer Small and Assaf Anyamba - NASA GIMMS Group at GSFC and the USDA FAS / GLAM Project
The floodwaters that provide ideal mosquito habitat also cause the vegetation to green up. This greening is visible from satellites via the Normalized Difference Vegetation Index (NDVI), a measure of how plants absorb visible light and reflect infrared light. Healthy vegetation with ample water reflects more infrared light and less visible light than stressed vegetation.
The map above shows NDVI anomalies based on data from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA’s Aqua satellite. The map contrasts vegetation greenness from May 8 - 15, 2016, against the long-term average for that period from 2003 – 2011. Greens indicate vegetation that is more widespread or abundant than normal. Brown areas show plant growth that was below normal, and dark grays depict areas where data were not available, usually due to cloud cover.
“There has been a lot of rain showing up in the vegetation index over the same regions that were affected by the 2000 outbreak,” said Assaf Anyamba, a remote sensing scientist with the NASA Goddard Earth Sciences Technology Center. “The current NDVI anomalies are the highest in the past 14 years. It may be worth finding out what may be going on the ground with respect to flooding and the emergence of different vector species. The landscape seems primed.”
8-day average NDVI in Asir, Saudi Arabia, acquired January 1, 2003 - May 31, 2016. Credit: Jesse Allen and Joshua Stevens / Jennifer Small and Assaf Anyamba - NASA GIMMS Group at GSFC and the USDA FAS / GLAM Project
The chart above shows the eight-day average of NDVI in the Asir region of Saudi Arabia. From mid-April to mid-May 2016 (dark green), NDVI soared far above normal (black). For comparison, the light green line shows that NDVI was also above normal from 2006-2007 during the last El Nino period. “We would expect an outbreak to occur in the next 1 - 2 months if there is no vaccination and if no mosquito control is undertaken,” Anyamba said.
Image credits: NASA Earth Observatory image and chart by Jesse Allen and Joshua Stevens, using vegetation anomaly data provided by Jennifer Small and Assaf Anyamba, NASA GIMMS Group at Goddard Space Flight Center and the USDA Foreign Agriculture Service (FAS) Global Agricultural Monitoring Project (GLAM) Project. Caption by Kathryn Hansen.