Nipah virus was identified in 1999 when it caused an outbreak of neurological and respiratory disease in pigs on farms in peninsular Malaysia, resulting in 105 human deaths and the culling of one million pigs.
It is believed that bats are the natural hosts of Nipah virus, meaning that they carry the virus by are not affected by it. Because around 90% of those infected in the 1998–1999 infection were pig farmers or had contact with pigs, pigs were tested and found to be infected with the virus.
Other outbreaks of Nipah occurred in 2004, 47 people in Bangladesh became infected with Nipah in two outbreaks, with 35 of them dying.
In humans, the infection presents as fever, headache and drowsiness. Cough, abdominal pain, nausea, vomiting, weakness, problems with swallowing and blurred vision are relatively common. About a quarter of the patients have seizures and about 60% become comatose and might need mechanical ventilation. In patients with severe disease, their conscious state may deteriorate and they may develop severe hypertension, fast heart rate, and very high temperature.
Nipah virus is also known to cause relapse encephalitis. In the initial Malaysian outbreak, a patient presented with relapse encephalitis some 53 months after his initial infection. There is no definitive treatment for Nipah encephalitis, apart from supportive measures, such as mechanical ventilation and prevention of secondary infection. Ribavirin, an antiviral drug, was tested in the Malaysian outbreak and the results were encouraging, though further studies are still needed.
In animals, especially in pigs, the virus causes porcine respiratory and neurologic syndrome also known as barking pig syndrome or one mile cough.
Hendra and Nipah viruses are the only members of the genus Henipavirus of the family Paramyxoviridae.