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 to 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.

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.

How does Nipah affect the body?

A person can contract the Nipah virus either by coming into contact with contaminated bat saliva or excreta on fruit or trees or by handling other animal hosts like infected pigs. A person can also get Nipah from another infected person. The virus has an incubation period of between five and 14 days after which the person starts to exhibit symptoms of infection.

The virus first attacks the respiratory system. A study by infectious disease researchers in the United States published in the journal PLOS in 2016 described experiments with hamsters in which they found that Nipah virus on to the lining of the nasal cavity, respiratory tracts and the lungs and starts replicating, sometimes as early as eight hours after infection. In the first 48 hours, the virus is able penetrate into underlying tissue and get in to blood vessels that can carry the virus through the body. Similarly in the virus replicates in the nasal cavity, which is possibly the first step towards the virus infiltrating the nervous system, using the olfactory nerve to access the brain and spinal cord.

Other research suggests that the Nipah virus is able to increase the permeability of the blood brain barrier, which is a semipermeable membrane separating the blood from the cerebrospinal fluid. The virus does this by disrupting certain tight junction complexes in this membrane. Still more studies suggest that the Nipah virus, like Ebola, is able to disrupt the immune system by co-opting the function of some of its components to enhance its own replication.

A person infected with Nipah will initially experience flu-like symptoms, with fever, headache, sore throat and muscle pain. Once the virus takes over a person’s vascular and neurological systems, he may experience drowsiness, disorientation, signs of brainstem dysfunction, convulsions, or go into a coma. Some patients may not may not develop neurological signs but have pneumonia or acute respiratory distress syndrome. Severely ill patients may also have septicemia, bleeding from the gastrointestinal tract and renal impairment. Patients who survive Nipah virus encephalitis may have mild to severe residual neurological deficits. The disease has a high fatality rate, with major reported outbreaks claiming the lives of half of those infected.

The Nipah virus in pig lung from Malaysia. Credit: Dr Peter Hooper, CSIRO/Wikimedia Commons

How can Nipah be treated?

International health organisations like that World Health Organisation and the Centre for Disease Control in the United States maintain that Nipah patients must be intensive supportive treatment since there is no vaccine or drug to cure the disease. This entails managing the respiratory and neurological symptoms till the infection subsides.

However, researchers have been looking to develop vaccines and drugs to be used on patients who have already been exposed to the virus. An experimental drug called Favipiravir has shown potential to protect against Nipah infection in animal models. In fact, Favipiravir is one of three experimental drugs that health organisation are considering for use against Ebola in the Democratic Republic of Congo. However, this drug has not yet been proved for safety and efficacy through human clinical trials. Another drug called Ribavirin showed some promise in containing some outbreaks, but experiments on animal models showed that it had little or no effect on the virus and its efficacy is considered to be uncertain. Researchers are also looking into whether administering antibodies of the virus can help contain infection.


For now, people who have not been infected by Nipah can protect themselves by maintaining good personal hygiene and washing their hands after being out in public and before eating. Doctors also recommend avoiding drinking any unpasteurized juices in regions with Nipah outbreaks and fruit that have fallen from trees or show any signs of bites. Fruit should be washed thoroughly, peeled or cooked before eating.

Close contact with fruit bats and their secretions and excretions should be avoided. Since, pigs are the host in some Nipah virus infections, any sick animals should not be used for food, even if the meat is to be cooked. The process of slaughter itself can increase human exposure to virus.