Scientists at India's National Institute of Virology have for the first time detected the presence of the Nipah virus in two species of bats in the southwestern province of Karnataka. The virus is in the top 10 priority diseases for research by the World Health Organisation (WHO) because of its high health risk, epidemic potential and the absence of a treatment or vaccine against it.
Supaporn Wacharapluesadee, director of the Thai Red Cross Centre for Health Sciences and Emerging Infectious Diseases in Bangkok, and her team detected the first case of COVID-19 outside China. They discovered that the virus originated in bats, which carry other viruses lethal to humans such as Nipah, SARS and Ebola. Wacharapluesadee told the BBC that Nipah is one of the viruses his centre is most concerned about because of its epidemic potential and high mortality rate.
The virus was first recognised in 1999 in Malaysia during an outbreak among pig farmers that killed more than 100 people, since when no new outbreaks have been identified in the country. Researchers have concluded that forest fires and local drought had driven the bats, the natural carriers of the virus, from their natural habitat and forced them to seek out fruit trees, trees grown on the same farms as the pigs.
The combination of being forced to relocate and being in close contact with a species they would not normally interact with allowed the virus to pass from the bats to the pigs and then to the farmers.
It was first recognised in Bangladesh in 2001, and the country has suffered almost annual outbreaks since then. The disease has also since been identified periodically in eastern India.
In infected persons it has diverse clinical manifestations, ranging from asymptomatic or subclinical infection to acute respiratory infection and lethal encephalitis.
It has so far only been detected in the South East Asian region. It can also cause severe disease in animals such as pigs, resulting in significant economic losses for farmers.
Other regions may be at risk of Nipah virus infection, as serological evidence of its presence has been found in bats of the family Pteropodidae (flying foxes) and several other bat species in several countries, including Cambodia, Ghana, the Philippines, Indonesia, Madagascar and Thailand.
Nipah virus is a zoonotic virus; transmitted mainly from animals to humans and sometimes also from person to person. The most common form of animal-to-human transmission is through consumption of contaminated food that has not been properly disinfected.
During the initial outbreaks in Malaysia, which also affected Singapore, most human infections were due to direct contact with sick pigs or their contaminated secretions. Transmission is believed to have occurred through respiratory droplets or contact with nasopharyngeal secretions or tissues of sick pigs.
In the outbreaks in Bangladesh and India, the most likely source of infection was the consumption of fruit or fruit products (e.g. date palm juice) contaminated with urine or saliva from infected bats. This is particularly problematic for the open-air markets so common in this region, where bats hang from the trees above the stalls waiting for a bite of fruit.
Veasna Duong, head of the virology unit at the Institut Pasteur scientific research laboratory in Phnom Penh and a collaborator of Wacharapluesadee, studies these places with high potential for contagion. This is the case of the morning market in Battambang, Cambodia, a market like many others where close contacts between fruit bats and people occur. "People and stray dogs walk under [bat nests in trees] exposed to bat urine every day." "This kind of exposure could allow the virus to mutate, which could cause a pandemic," Duong tells the BBC.
Limited person-to-person transmission has also been reported among family members and caregivers of Nipah virus-infected patients. During subsequent outbreaks in Bangladesh and India in 2001, Nipah virus spread directly from person-to-person through direct contact with human secretions and excretions. In Siliguri, India, transmission of the virus was also reported in healthcare settings, with 75% of cases occurring among hospital staff or visitors. From 2001 to 2008, about half of the cases reported in Bangladesh were due to person-to-person transmission through care of infected patients.
Infected persons initially present with influenza-like symptoms (fever, headache, myalgia, vomiting and sore throat). These non-specific symptoms complicate the possibility of early diagnosis, which encourages the spread of the disease.
These may be followed by dizziness, drowsiness, altered consciousness and neurological signs indicating acute encephalitis. Some people may also experience atypical pneumonia and severe respiratory problems, such as acute dyspnoea. In severe cases, encephalitis and seizures occur, progressing to coma within 24-48 hours.
The incubation period (interval between infection and onset of symptoms) is thought to range from 4 to 14 days. However, incubation periods of up to 45 days have been reported.
The case fatality rate estimated by WHO is 40-75%, but may vary from outbreak to outbreak, depending on local capacity for epidemiological surveillance and clinical care. In 11 different outbreaks of Nipah in Bangladesh between 2001 and 2011, 196 people were found to have Nipah; 150 died. In the last outbreak in the Kerala region of India in 2018, 17 people died out of 18 confirmed cases.
There are no specific treatments or vaccines for humans or animals. In humans, treatment consists only of supportive measures. According to WHO, the risk of international transmission through fruit or fruit products (e.g. date palm juice) contaminated with urine or saliva of infected fruit bats can be avoided by thorough washing and peeling of the fruit. Fruit showing signs of having been bitten by bats should also be discarded.
The Nipah virus vaccine is currently under development and testing at several pharmaceutical companies. Moderna announced earlier this year that it was trying to create a vaccine using the same technology used for COVID-19, with messenger RNA.