The authors have declared that no competing interests exist.
Conceived and designed the experiments: SUK ESG MJH NN MYS SPL. Performed the experiments: SUK. Analyzed the data: SUK EGH MYS SPL. Contributed reagents/materials/analysis tools: SUK ESG MJH SPL. Wrote the paper: SUK. Reviewed the drafts of the manuscript: ESG MJH NN MYS SPL.
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.
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,
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),
Bamboo,
Nipah virus (NiV) causes seasonal outbreaks in humans in Bangladesh that coincide with the date palm sap harvesting season, November to March
Exploratory studies on date palm sap production and harvesting processes identified several interventions practiced by the sap harvesters to prevent bats feeding on date palm sap
To evaluate the efficacy of these interventions to protect sap from bats, we conducted randomized controlled trials of the locally identified interventions that formed a physical barrier on the sap producing surface of date palm trees.
We conducted two trials in Poromanandopur village (N23° 33′ 49.3″ E89° 41′ 55.2″) in Faridpur district during two sap harvesting seasons: first from January 2009 through February 2009 and we label it as the 2009 data collection period and second was from November 2009 through March 2010, and we label it as the 2010 data collection period. The interventions included bamboo,
In the later pictures, we can see sap harvester setting up an infrared camera.
During 2009 we used a crossover design for applying interventions to the trees. We identified 54 date palm sap producing trees prepared for sap harvesting by a sap harvester in the village. The trees we selected were tall enough so that terrestrial animals, including dogs and foxes, could not reach the sap producing surface and were trees that the harvesters frequently noticed bats visiting. We assigned a unique identification number to each of these trees. Using Microsoft Excel we generated 20 random numbers between 1 and 54 in order to select 20 trees. Each tree was observed for one night with the intervention and for one night without any intervention.
A sap harvester from the study village estimated the average size of the sap collecting area of a date palm tree and made the bamboo skirts that we used for the intervention. Then the harvester climbed the trees and fastened the bamboo skirts over the shaved part, sap stream, tap and opening of the collection pot hung on the trees (
Ten trees were randomly selected to be observed with intervention and 10 were observed without interventions during the first 10 nights of data collection. During the second 10 nights of data collection, the trees were assigned to the opposite group. Every night, we observed two newly shaved date palm trees: one with the bamboo skirt intervention and the other without any intervention so that the observations were matched in terms of time. The date palm sap harvester mounted two infrared cameras (Silent Image™ Model RM30 digital cameras (Inclusion of trade names is for identification only and does not imply endorsement by ICDDR,B, by CDC or the Department of Health and Human Services)) on the trees before dusk, one camera per tree. The cameras were triggered by a motion sensor and were focused on the shaved surface, sap stream, tap and the opening of the collection pot. This way, we observed all 20 trees for two nights each; one night with the bamboo skirt applied and one night without the intervention.
During 2010, we identified 277 tall date palm sap producing trees prepared for sap harvesting by eight sap harvesters in the same village. From those, we selected 60 trees as controls by the random selection process described above. From the remaining 217 trees, we identified potential matches for the controls by comparing their apparent height, shaving pattern of the trunk of the tree
During this season, we hired the same sap harvester who worked during 2009 to prepare skirts and mount cameras on trees prepared by 10 different date palm sap harvesters in the village. The length of the shaved surface, sap stream, location of the tap and collection pot varied with the diameter of the trunk of the tree. Accordingly, we made bamboo,
Details of the observation and data extraction process were described elsewhere
During the data extraction process we counted all species of fruit bats and defined a “bat visit” as an instance where we could identify a bat flying and/or landing on and around the tree. We defined an event of “bat-sap contact” as an instance of a bat landing, licking (i.e. bat’s tongue contacted the date palm sap), or urinating either on the date palm’s shaved surface, sap stream, tap or collection pot that comes in contact with the sap. We categorized the camera-nights of observations into those with and those without bat-sap contact.
Trained technicians reviewed all of the camera images and recorded the camera data in a structured field data sheet, which included the frequency of bat visits, and their duration and methods of bat-sap contact, and ambient temperature (
Each image shows bats in a circle, observation date, time, and ambient temperature when it was taken.
Field workers also used a structured questionnaire to ask the sap harvester(s) to assess the volume of sap produced; its appearance (clear or turbid); and the presence or absence of any physical debris originating from the tree itself, insects, and fecal materials of birds, bats or other animals. Field workers asked the sap harvesters how much money he would receive from selling the raw sap for either human consumption or for molasses production.
We used descriptive statistics to summarize the frequency of bat visits with and without interventions placed in the trees. We used conditional logistic regression to assess the association between frequency of the events of bat-sap contact with the presence or absence of interventions; and calculated odds ratios. We used a paired t test to assess the difference of the volume of sap production and price with or without interventions. We also used point bi-serial correlation to correlate sap quality (clear or turbid appearance, and presence or absence of debris in sap) with quantity of sap (volume and price) and ambient temperature
We explained the objectives and the methods of this study to the sap harvesters and the tree owners and obtained informed consent from them before conducting fieldwork. This study was approved by ICDDR,B’s Ethical Review Committee.
The number of camera-nights where bats contacted date palm sap (35% versus 85%), p<0.001] and the number of bat-sap contact events (mean 2 versus 32 bat-sap contact per tree per night, p = 0.01) was lower when trees were protected with bamboo skirts compared to when no intervention was used (
Observations in 2009 | Observations in 2010 | ||||||
Types of intervention and control | Bambooskirtn = 20 | Withoutskirtn = 20 | Bambooskirtn = 15 | Jute stickskirtn = 15 | Polytheneskirtn = 15 | Withoutskirtn = 60 | |
Mean bat visits per camera night of observation[95% Confidence Interval (CI)] | 3 [2.6–4.3] | 39 [36–42] | 12 |
3 |
8 |
8 |
77 [75–79] |
Frequency – landed on the tree | 8% (n = 50) | 92% (n = 579) | 1% (n = 36) | 0.2% (n = 8) | 1.5% (n = 54) | 0.3% (n = 12) | 97% (n = 3601) |
Bat-sap contact during camera-nightsof observations | 35% (n = 7) | 85% (n = 17) | 0% (n = 0) | 0% (n = 0) | 7% (n = 1) | 0% (n = 0) | 83% (n = 50) |
Mean bat-sap contact per cameranight [95% CI] | 2 [(−0.4) –5] | 32 [7–57] | 0 [0–0.2] | 0 [0–0.2] | 1 [0.3–1.3] | 0 [0–0.2] | 59 [35–84] |
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Shaved surface | 29 [26–31] | 2 |
0[0–0.2] | 0[0–0.2] | 1 [0.4–1.3] | 0[0–0.2] | 51 [49–53] |
Sap stream | 0 [0–0.2] | 3 |
0[0–0.2] | 0[0–0.2] | 0[0–0.2] | 0[0–0.2] | 7 |
Tap | 0 [0–0.2] | 0.05 [−0.05–0.15] | 0[0–0.2] | 0[0–0.2] | 0[0–0.2] | 0[0–0.2] | 2 [1.5–2] |
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Clear appearance of sap (%) | 85 | 85 | 67 | 73 | 67 | 87 | 62 |
Mean volume of sap/night in liters [95% CI] | 3.3 [2.5–4.0] | 3.1 [2.3–3.9] | 3.1 [2.3–3.8] | 2.9 [2.0–3.8] | 3.6 [2.5–4.8] | 2.2 [1.6–2.9] | 2.8 [2.3–3.3] |
Presence of debris (%) | 25 | 85 | 80 | 80 | 86 | 60 | 95 |
The infrared photography demonstrated that bats were able to access the shaved surface of trees and contact sap when the placement of bamboo skirts did not entirely cover the sap producing surface of the tree. Among the 20 skirts placed on trees, 13 (65%) were wide enough to cover the shaved part and had been properly applied. When these 13 were placed on trees, bats were unable to contact the sap.
Out of 60 camera-nights observation at control trees, bats made contact with sap on 50 (83%) nights, with a mean of 50 (SD 94) bat-sap contacts at the shaved part per night. Out of the 45 camera nights observed for three of the interventions, we did not observe any bat contacting sap in trees with bamboo,
The mean frequency of bat visits to date palm trees without interventions were highest during November, the beginning of sap harvesting season (mean: 242; SD 205 visits) and then decreased in subsequent months reaching the lowest number of visits during March, the end of sap harvesting season (mean: 6; SD 7 visits). In the trees with no intervention, the proportion of bats date palm tree visits decreased 83% each month between November and March (
During 40 camera-nights of observation in 2009, most (85%, n = 34) of the trees produced clear sap. Date palm trees with and without bamboo skirt were equally likely to produce clear sap [odds ratio (OR): 1; 95% Confidence Interval (CI) 0.2–5.0] (
Compared to the trees without intervention, the trees with intervention in both 2009 [OR: 0.06; 95% CI 0.01–0.3] and 2010 [OR: 0.2; 95% CI 0.05–0.6] were less likely to have debris in their sap. The differences were similar for each individual intervention (
Volume of sap produced Price of sap Debris in sap | |||||||
Interventions and control | Number ofnights ofobservation | Mean volume ofsap in liter/night(Standard error) | 95% ConfidenceInterval (CI)of mean | Sap price/liter (USD)(standard error) | 95% CIof mean | Odds of havingdebris in sap | 95% CI of Odds |
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Bamboo skirts | 20 | 3.9 (0.4) | 2–4 | 0.12 |
0.12–0.13 | 0.06 | 0.01–0.3 |
No intervention | 20 | 3.1 (0.4) | 2–4 | 0.13 (0.003) | 0.12–0.14 | Ref | - |
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Bamboo skirts | 15 | 3.1 (0.4) | 2–4 | 0.13 (0.006) | 0.11–0.14 | 0.2 | 0.03–1.2 |
15 | 2.9 (0.4) | 2–4 | 0.13 (0.006) | 0.12–0.16 | 0.2 | 0.03–1.2 | |
Jute stick skirt | 15 | 3.9 (0.5) | 3–5 | 0.12 (0.009) | 0.10–0.15 | 0.3 |
0.05–2.3 |
Polythene skirt | 15 | 2.6 (0.4) | 2–3 | 0.14 |
0.12–0.15 | 0.08 | 0.01–0.4 |
No intervention | 60 | 2.9 (0.2) | 2–3 | 0.13 (0.003) | 0.12–0.13 | Ref | - |
Statistically significant difference at 5% level (two tailed).
This study evaluated interventions that may impede NiV transmission form bats to humans through date palm sap. In almost all cases, bats did not contact sap in trees that were covered with skirt interventions. Although we observed a decrease in debris falling into the sap after applying the interventions, the modest improvements in the appearance of sap, and unit price in sap collected from intervention trees was not significantly different from what was observed from the trees without interventions. Since the interventions do not influence the quality and quantity of the sap that much, there is a need to identify specific motivators, which may encourage the sap harvesters to put skirts on trees and harvest disease risk free sap for human consumption.
During 2009, the width of the shaved part of the tree exceeded the width of the bamboo skirt in a few cases and the bats could gain access to the sap from the left or right side of the shaved surface. The length of the shaved surface, sap stream, and the position of the tap and collection pot varies according to the circumference of the date palm tree. Therefore, skirts of one fixed size may not entirely cover the sap producing surface of a tree and protect the sap from bats. Promoting skirts of three different sizes may provide the tree owners and harvesters with the ability to entirely cover the shaved surface, sap stream, tap, and collection pot of trees with various trunk circumferences. Additionally, the materials used for the interventions showed similar efficiency and the harvesters from different regions may use locally available and convenient material to weave skirts to protect sap.
The event of bat-sap contact during one camera-night of observation with the jute skirt intervention could have been due to the structural characteristics of jute stem
The interventions we proposed may prevent physical debris from falling into the sap but the interventions did not influence the clarity or turbidity of sap, or the volume of production. Although several studies reported the volume of sap production and its physical appearance may vary with temperature
We did not see a difference between sap from trees with and without interventions. Before selling the sap to consumers, the harvesters filter their sap with a piece of cloth to get rid of the debris, which may have masked the difference in sap quality produced from trees with and without interventions. However, it remains possible that if the community starts demanding protected sap for drinking, the harvesters may become motivated to exert additional efforts applying interventions and demand more money for their effort.
There was a sharp decline in the frequency of bat visits to the date palm sap producing trees over the sap harvesting season. Bats feed on naturally available food sources throughout the year in Bangladesh
Although observing bat feeding behavior through the infrared camera is a useful method, this is subject to a few limitations, which are described elsewhere
Efficient application of bamboo,
The authors thank our field research officer Md. Ariful Islam, and date palm sap harvester, Mr. Chitta Ranjan, for their efforts in the fieldwork. The authors also thank Dorothy Southern for assistance with writing the manuscript.