Surveillance and outbreak alert

Nipah virus outbreaks in the WHO South-East Asia Region

Nipah virus (NiV) encephalitis is an emerging infectious disease of public health importance in the WHO South-East Asia Region. Bangladesh and India have reported human cases of Nipah virus encephalitis. Indonesia, Thailand and Timor-Leste have identified antibodies against NiV in the bat population and the source of the virus has been isolated. The status of NiV infection in other SEAR countries is not known although flying bats are found throughout the region.

The first identification of Nipah virus as a cause of an outbreak of encephalitis was reported in 2001 in Meherpur district of Bangladesh. Since then, outbreaks of Nipah virus encephalitis have been reported almost every year in selected districts of Bangladesh. The Nipah outbreaks have been identified in Naogoan (2003), Rajbari and Faridpur (2004), Tangail (2005), Thakurgaon, Kushtia and Naogaon (2007), Manikgonj and Rajbari (2008), Rangpur and Rajbari (2009), Faridpur, Rajbari and Madaripur (2010) and Lalmohirhat, Dinajpur, Rangpur and Comilla (2011) and Joypurhat, Rajshahi, Rajbari and Natore (2012). Repeated outbreaks of Nipah virus encephalitis were established in some districts. Sporadic cases of Nipah virus encephalitis have been reported, mostly from the west and north-western regions of Bangladesh almost every year, with high mortality and constituting a public health threat. Up to March 31, 2012 a total of 209 human cases of NiV infection in Bangladesh were reported; 161 (77%) of them died.

India reported two outbreaks of Nipah virus encephalitis in the eastern state of West Bengal, bordering Bangladesh, in 2001 and 2007. Seventy one cases with 50 deaths (70% of the cases) were reported in two outbreaks. During January and February 2001, an outbreak of febrile illness with neurological symptoms was observed in Siliguri, West Bengal. Clinical material obtained during the Siliguri outbreak was retrospectively analyzed for evidence of NiV infection. Nipah virus-specific immunoglobulin M (IgM) and IgG antibodies were detected in 9 out of 18 patients. Reverse transcription-polymerase chain reaction (RT-PCR) assays detected RNA from NiV in urine samples from 5 patients. A second outbreak was reported in 2007 in Nadia district of West Bengal. Thirty cases of fever with acute respiratory distress and/or neurological symptoms were reported and five cases were fatal. All five fatal cases were found to be positive for NiV by RT-PCR.

The morbidity and mortality data of human NiV infection in India and Bangladesh from 2001 to 2012 is presented in Table 1. So far, NiV has infected 263 people and resulting in 196 deaths since 2001. The case fatality rate of Nipah virus encephalitis ranges from 0-100 and average case fatality rate is 74.5%. The case fatality rate has remained high during 2008 – 2012 despite a public awareness campaign and establishment of a referral system for better treatment and nursing care of patients in potential outbreak areas in Bangladesh.

There was no involvement of pigs in NiV transmission as was observed in Malaysia during an outbreak in 1998-99. Consumption of raw date palm sap contaminated by flying bats was the primary source of human NiV infection in Bangladesh.

Nipah cases tend to occur in a cluster or as an outbreak, although 18% of cases in Bangladesh were isolated. Strong evidence indicative of human-to-human transmission of NiV was found in Siliguri (India) in 2001 and in Bangladesh in 2004.

Outbreaks of Nipah in South-East Asia have a strong seasonal pattern and a limited geographical range. The geographical distribution of NiV outbreaks is shown in Fig. 1. All the outbreaks occurred during winter and spring (December-May). This could be associated with several factors like the breeding season of the bats, increased shedding of virus by the bats and the date palm sap harvesting season.

First outbreak of Nipah virus in Malaysia and Singapore

Nipah virus infection was first recognized in a large outbreak of 265 suspected cases in peninsular Malaysia during September 1998 to April 1999. Most patients had contact with sick pigs or had been in close physical contact with Nipah virus infected patients and then presented primarily with encephalitis. The outbreak was initially thought to be due to Japanese encephalitis, but it was later identified as Nipah virus encephalitis. This outbreak caused widespread panic and fear in Malaysia leading to considerable social disruptions and tremendous economic loss because of the mass culling of over one million pigs. In addition, eleven abattoir workers in Singapore developed a febrile illness caused by Nipah virus during March 1999 following close contact with imported pigs from Malaysia. The presentation of Nipah virus infection has been variable, ranging from the high mortality observed in the original Malaysian outbreak to an outbreak of low mortality disease among abattoir workers in Singapore, which presented as neurological illness and atypical pneumonia. No new outbreaks have been reported from these countries since May 1999.

Chronology of Nipah virus outbreaks in South-East Asia, 2001-2012

Morbidity and mortality due to Nipah or Nipah-like virus encephalitis, South-East Asia Region, 2001-2012

Year/Month Location No. cases No. deaths Case Fatality Rate
Jan-Feb 2001 Siliguri (India) 66 45 68%
Apr-May 2001 Meherpur (Bangladesh) 13 9 69%
Jan-03 Naogaon (Bangladesh) 12 8 67%
Jan  2004      Rajbari( Bangladesh) 31 23 74%
Apr-04 Faridpur (Bangladesh) 36 27 75%
Jan-Mar 2005 Tangail (Bangladesh) 12 11 92%
Jan-Feb 2007 Thakurgaon (Bangladesh) 7 3 43%
Mar-07 Kushtia,Pabna,Natore (Bangladesh) 8 5 63%
Apr-07 Naogaon (Bangladesh) 3 1 33%
Apr-07 Nadia (India) 5 5 100%
Feb-08 Manikgonj (Bangladesh) 4 4 100%
Apr-08 Rajbari and Faridpur (Bangladesh) 7 5 71%
Jan-09 Gaibandha, Rangpur and Nilphamari (Bangladesh) 3 0 0%
Rajbari (Bangladesh) 1 1 100%
Feb-Mar 2010 Faridpur, Rajbari,Gopalganj,Madaripur (Bangladesh) 16 14 87.50%
Jan-Feb 2011 Lalmohirhat, Dinajpur, Comilla, Nilphamari and Rangpur (Bangladesh) 44 40 91%
Feb-12 Joypurhat, Rajshahi, Natore, Rajbari and Gopalganj (Bangladesh) 12 10 83%
Total 280 211 75%