What exactly is the Nipah virus and what does it do when it enters the human body?
The virus, which was relatively unknown till reported infections in Kerala this week, is one of several pathogens that jump from animal to human hosts.
In 1998, an viral outbreak affected parts of Malaysia, first killing a large number of pigs and then infecting their human handlers. They passed on the disease fo their families. Half the people who caught the infection died. The infection was first thought to be Japanese encephalitis but a new virus was identified as the disease causing agent in the town of Kampung Sungai Nipah. The virus was named the Nipah virus.
Do how does the virus spread so quickly and take such a high toll?
Since that first outbreak in Malaysia 20 years ago, researchers have been looking into the viral mechanism in its animal hosts and in humans to find the best ways to counter it.
The Nipah virus causes fever and upper respiratory distress in humans that quickly escalates to encephalitis or inflammation in the brain. In a few cases, infected people have also shown symptoms of myocarditis, which is inflammation of the heart.
Nipah virus encephalitis is a zoonotic disease. This is a class of diseases that are naturally transmitted from vertebrate animals to humans and also the other way around. Zoonotic diseases have been identified for many centuries but they have had a greater impact on public health in recent years with greater mingling of human and animal ecosystems.
The Nipah virus belongs to a class of viruses called paramyxoviruses that cause respiratory ailments and include viruses that cause childhood diseases like parainfluenza, measles and mumps. These viruses comprise of a single strand of RNA within an envelope.
The natural hosts of the Nipah virus are Old World fruit bats found in East Africa, Asia, Australia and the Pacific Islands. Outbreaks of Nipah have so far only occurred in Malaysia, Bangladesh and India. Antibodies for the virus have been found in bats in Indonesia, Thailand and Timor-Leste indicating the presence of the virus in these countries but without any spillover into human populations.
In fact, there are two distinct strains of the virus – NiVM, which refers to the strain found in Malaysia and NiVB, which is the strain found in Bangladesh. NiVb is thought to be more pathogenic than NiVM because of its quicker transmission pattern and higher mortality rate.
The virus is transmitted through contact and transfer of body fluids. So the virus can move from bats to other hosts through fruit or tree sap contaminated with bat saliva or excreta.
As was seen in the first Nipah outbreak in Malaysia, pigs also act as a reservoir for the virus. Researchers who studied the outbreak observed that Malaysia had large intensively managed commercial pig farms with fruit trees where bats could drop partially eaten fruit into pig stalls. Pigs eating fruit contaminated with saliva from a bat that was a carrier of the Nipah virus would in turn become infected. Infected pigs could efficiently transmit virus to other pigs on the densely populated pig farms.
Bangladesh has had repeated outbreaks of Nipah since 2001. Unlike the Malaysia outbreak, the source of infection has been traced to bats and not pigs. In Bangladesh, people often drink raw sap from date palm trees as toddy. Bats also drink the sap of date palm trees and the virus is likely to have reached humans from contaminated bat saliva through this route. Before the outbreak in Kerala this month, India has had two outbreaks in West Bengal – one in Siiguri in 2001 and the other in Nadia in 2007. Both these outbreaks are thought to have been caused by infected bats.