A randomized controlled trial of interventions to impede date palm sap contamination by bats to prevent nipah virus transmission in Bangladesh

Abstract

Background: Drinking raw date palm sap is a risk factor for human Nipah virus (NiV) infection. Fruit bats, the natural reservoir of NiV, commonly contaminate raw sap with saliva by licking date palm's sap producing surface. We evaluated four types of physical barriers that may prevent bats from contacting sap.

Methods: During 2009, we used a crossover design and randomly selected 20 date palm sap producing trees and observed each tree for 2 nights: one night with a bamboo skirt intervention applied and one night without the intervention. During 2010, we selected 120 trees and randomly assigned four types of interventions to 15 trees each: bamboo, dhoincha (local plant), jute stick and polythene skirts covering the shaved part, sap stream, tap and collection pot. We enrolled the remaining 60 trees as controls. We used motion sensor activated infrared cameras to examine bat contact with sap.

Results: During 2009 bats contacted date palm sap in 85% of observation nights when no intervention was used compared with 35% of nights when the intervention was used [p<0.001]. Bats were able to contact the sap when the skirt did not entirely cover the sap producing surface. Therefore, in 2010 we requested the sap harvesters to use larger skirts. During 2010 bats contacted date palm sap [2% vs. 83%, p<0.001] less frequently in trees protected with skirts compared to control trees. No bats contacted sap in trees with bamboo (p<0.001 compared to control), dhoincha skirt (p<0.001) or polythene covering (p<0.001), but bats did contact sap during one night (7%) with the jute stick skirt (p<0.001).

Conclusion: Bamboo, dhoincha, jute stick and polythene skirts covering the sap producing areas of a tree effectively prevented bat-sap contact. Community interventions should promote applying these skirts to prevent occasional Nipah spillovers to human.

Figure 1. A picture taken by infrared night observation showing a small fruit bat (in circle) licking sap from the shaved surface of a date palm tree without any intervention during the winter of 2010.

Figure 2. Interventions to prevent bat drinking date palm sap during session two: 1. Bamboo skirt; 2. Dhoincha skirt; 3. Jute stick skirt; and 4. Polythene skirt covering the sap producing areas of the date palm tree.

In the later pictures, we can see sap harvester setting up an infrared camera.

Figure 3. Identifying bats visiting date palm tree on a foggy night: a. 7∶08 PM, no fog, bats are easily identifiable; b. 3∶00 AM, fog blurs visibility but bats are somewhat identifiable; c. 3∶50 AM, fog starts to clear and bats appear clearly in the photos.

Each image shows bats in a circle, observation date, time, and ambient temperature when it was taken.

Figure 4. Average number of bat visits per camera-night around the date palm tree (without intervention) during the date palm sap harvesting season in2009 and 2010.