May 18, 2013 | 11:29 AM (BD Time)
18 May, 2013 Saturday
Nipah transmission from bats to humans
The most common transmission pathway of Nipah virus from fruit bats to humans in Bangladesh is through drinking raw date palm sap. In July 2011, the Institute of Epidemiology, Disease Control and Research (IEDCR) and icddr,b investigated a cluster of 7 encephalitis patients in Rangpur district; 4 (57%) died. Among 4 collected blood specimens, 1 case had IgM antibodies against Nipah and 3 had anti Nipah IgG. The primary cases regularly purchased and drank traditional liquor (tari) made from date palm sap from a common source prior to their illness. The tari was likely contaminated with bat urine or saliva during sap collection and fermentation, and was the most likely source of Nipah virus infection for these cases. Rural people should be informed about transmission of Nipah virus through drinking tari. Interventions are also needed to limit bat access to tariproducing date palm trees.
Nipah virus is a bat-borne emerging infection that causes fatal outbreaks of encephalitis almost yearly in Bangladesh (1). The most common transmission pathway of Nipah virus from fruit bats to humans in Bangladesh is through consumption of raw date palm sap, a national delicacy consumed during the cool, winter months (2,3). Human to human transmission of Nipah virus has also been repeatedly noted in Bangladesh (1,4). There is some evidence that date palm liquor may also transmit Nipah virus. In 2007, a human Nipah case was identified in India with a history of drinking a traditional liquor derived from the sap of the palm tree, which produces sap similar to the sap of the date palm (5). In Bangladesh, the traditional liquor derived from date palm sap is called tari and low income people from rural areas often drink this liquor. Bats visit date palm trees and occasionally shed Nipah virus through their saliva and urine which can contaminate the sap (6). In bat urine, the virus can survive up to four days, and in sap contaminated with bat urine, the virus is likely to survive for over a day (7). Generally, enveloped (lipophilic) viruses like Nipah are susceptible to alcohol and approximately 60-70% alcohol is recommended to sterilize contaminated objects (8,9).
Since 2006, IEDCR and icddr,b have been conducting encephalitis surveillance in Nipah-prone areas in Bangladesh. As part of the surveillance activities, a physician working in Rangpur Medical College Hospital collected blood from patients admitted with encephalitis during March 2011. The virology laboratory of IEDCR tested the blood samples for Nipah IgM antibodies. We found a case-patient that had IgM antibodies to Nipah virus and investigated his exposures that may have led to the infection. This case-patient (case G) was a 25-year-old male. He was a regular tari drinker and prior to his illness he purchased and drank tari from a commercial source in a periurban area of Rangpur district. While investigating case G, we found three more encephalitis patients in the same area who died within 6 weeks of this patient's death, and who all had a history of drinking tari from the same commercial source. We collected blood samples from sick individuals who came into close physical contact with these cases during their illness. We identified seven Nipah cases through this investigation (Figure 1).
In August 2011, we investigated this cluster to explore in detail the exposure and illness history of the cases. We interviewed the surviving cases, family members and friends of the deceased, and explored the tari consumption pattern and travel history of the cases. In addition, we explored tari production, processing and selling patterns in the community. Our team interviewed date palm sap collectors and tari producers and explored the production, processing and selling patterns of tari. We also explored the presence of bats in the community and their contact with date palm sap.
We identified 7 cases in this cluster; 4 were males. Three cases (A, B and C) developed onset of fever that subsequently led to headache, altered mental status, respiratory distress and oral frothy discharge. Cases A and G were family friends. Cases B and C were also friends and relatives. Cases A, B, C and G regularly purchased and drank tari in the evenings from a common source prior to their illness. None of these cases had either a history of drinking fresh date palm sap nor any exposure to sick humans or animals in the month before they became ill. After caring for case C, three of his family members, including his wife (case D), daughter (case E) and sister-in-law (case F) became infected. All these three family caregivers (cases D, E and F) survived.
This Nipah-affected area is famous for tari and is the largest tari-producing area in Rangpur district. Tari is produced commercially in the area and the producers harvest date palm trees for tari throughout the year. The men who harvested date palm trees and collected sap for tari are known as pashi by the local community. Tari businessmen from different areas come to this community and buy tari at wholesale prices from the pashi. Tari is called rosh (juice) by local residents. They called fresh date palm sap misti rosh (sweet juice). The process of collecting date palm sap for tari was nearly identical to the process for collecting date palm sap for fresh consumption. The villagers reported that they did not have regular access to fresh date palm sap in the community because it was all made into tari. If they wanted to drink fresh date palm sap, they would have needed to inform the pashi at least a week prior to the date they intended to drink it. The pashi mentioned that for fresh date palm sap, they needed to clean and dry the earthen pot after each time they collected it. On the other hand, for tari, they used the same earthen pot for several months without cleaning it to encourage the fermentation of the date palm sap while it hangs on the tree. Over time, a thick white sediment called gad forms at the bottom of the pot, which acts like yeast to ferment the fresh date palm sap into tari as it flows into the pot overnight. There were two bat roosts in the community, and pashi frequently observed bats flying near the date palm trees. The pashi reported filtering tari with a net or cloth before selling it.
Every day between 10 a.m. and noon, the pashi collected the tari from all the hanging earthen pots and combined it in one container, leaving the used pots in the tree. The tari was then immediately sold to consumers from morning to until late at night. We observed the wives of the pashi selling tari at their household premises. Occasionally, the consumers took tari away with them in plastic bottles.
Reported by: Rangpur Medical College Hospital, Rangpur; Institute of Epidemiology Disease Control and Research, Ministry of Health and Family Welfare, the Government of the People's Republic of Bangladesh (GoB); Surveillance and outbreak response research group, Centre for Communicable Diseases, icddr,b.
Supported by: Institute of Epidemiology Disease Control and Research, Ministry of Health and Family Welfare, GoB; The Centers for Disease Control and Prevention, Atlanta, USA, and EcoHealth Alliance, USA.
The findings of this cluster investigation suggest that the illness was caused by Nipah virus. The four primary Nipah cases had neither a history of drinking fresh date palm sap nor any exposure to sick humans or animals that are commonly described as the source of Nipah infection (1,2,4). However, they regularly purchased and drank tari in the evenings from a common tari production area prior to their illness. Our epi-curve suggests that there might be two separate spillover events for these cases. The presence of bat roosts in the community and the frequent presence of bats in the tari-producing date palm trees indicate the implicated tari were likely contaminated with bat urine or saliva during collection and ferm
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