Nipah virus
المؤلف:
Baijayantimala Mishra
المصدر:
Textbook of Medical Virology
الجزء والصفحة:
2nd Edition , p244-246
2025-12-02
27
Nipah virus (NiV) was first discovered during an outbreak of encephalitis in the pig farming villages in Malaysia and amongst the abattoir workers in Singapore during 1998–1999. Initially the outbreak was thought to be due to Japanese encephalitis virus (JEV) due to pig involvement. But some of the epidemiological features like adult cases instead of children, symptoms in pigs and clustering of cases were different from Japanese encephalitis which raised the possibility of a causative agent other than JEV. Finally, a new virus was detected from various samples and was named Nipah virus after the name of the village Sungei Nipah, from where the virus was first detected.
Outbreaks: The first NiV outbreak in Malaysia occurred amongst the pig farmers who were in close contact with the pigs. The outbreak affected a total of 265 cases of which 105 died with near 68% mortality. Encephalitis was the predominant symptoms in humans, whereas pigs had respiratory ailments and neurological manifestations. It was thought that the disease has been transmitted from the infected pigs to its close contacts. Pigs were probably acquired the infection from bats who were later found to be the reservoir of the NiV.
The virus then spread from Malaysia to Singapore through trade of infected pigs and caused outbreak in 1999 among the abattoir workers.
After 1999, no NiV outbreak has been reported from these countries.
NiV outbreaks in India: In 2001, a major outbreak of encephalitis occurred in Siliguri district of West Bengal. The outbreak was confirmed to be due to NiV. Pigs were not involved in this outbreak. Person-to-person mode of transmission was thought to be responsible for the outbreak.
In 2007, NiV outbreak was reported in Nadia, West Bengal infecting five people, all of them died.
During May 2018, Nipah virus outbreak occurred in Kerala. Twenty-three cases were detected as NiV positive with a case fatality ratio of 91%. The outbreak started in Kozhikode district. Index case was a 27-year- old and was admitted with fever, myalgia and altered sensorium later succumbed to death. Nosocomial spread was observed affecting 22 cases who contracted the infection from the index case. Of the 23 NiV positive cases, 21 patients died. All cases were confirmed by detection of NiV RNA by RT-PCR and IgM antibody. The lineage found in this outbreak was similar to Bangladesh lineage. The source of infection in index case was assumed to be close contact with bats.
NiV outbreaks in Bangladesh: In 2001, Bangladesh reported its first NiV outbreak in Meherpur, where the transmission was thought to be directly or indirectly from the reservoir bat. After that, Bangladesh has reported NiV outbreaks almost every year from different parts of the country. The number of infected cases reported in various outbreaks varies from 5 to 45 with a mortality rate of 43 to 100%. Most of these outbreaks have been associated with the drinking of raw date palm sap contaminated with bat saliva or excreta.
VIRUS
Nipah virus was found to be similar to another member of Paramyxovirus; hendra virus, that causes infection mainly in horses. Both Nipah virus and hendra virus are members of the genus Henipavirus and family Paramyxoviridae. The genome of Henipavirus is the largest among the paramyxoviruses and has near 18,250 nucleotides. Unlike other para myxoviruses, NiV does not have hemagglutinin or neuraminidase surface proteins.
EPIDEMIOLOGY
Reservoir: Fruit bats or flying foxes of genus Pteropus are the natural reservoir of NiV. Virus has been isolated in bat saliva and urine. Serum of several fruit bat species has been found positive for antibody to NiV. Several species of fruit bats are present all over Southeast Asia. The migratory nature of these fruit bats possesses risk of emergence of NiV infection in new geographic area.
Amplifying host: Pig plays the role of amplifying host when infected. In Malaysian outbreak, pigs were possibly acquired the infection from bats through eating fruits contaminated with bat saliva or excreta. Unlike Japanese encephalitis, pigs also suffered from disease with respiratory and neurological symptoms. Man acquired the infection from infected pigs.
Serological evidences of NiV infection have been shown in several animals in the outbreak affected areas. NiV has been found from symptomatic dogs in natural condition. In experimentally infected cats, virus has been found to be excreted from several sites including the nasopharyngeal secretion and urine during the viremic phase. However, no animal other than pig has so far been associated with the transmission to human host.
NiV transmission cycle: NiV is transmitted mainly through bat excreta or bat saliva. Fruit or sap when contaminated with bat excreta gets infected with NiV. When these contaminated sap/fruits are taken by the human or pig they acquire the infection.
Once pigs or humans are infected, they develop neurological and respiratory symptoms. Possibly through respiratory secretion the virus gets transmitted from person-to-person or from pig to human (Fig. 1). Person-to person transmission has occurred in intra familial setting as well as in hospital setting amongst the health care workers.
Fig1. Nipah virus transmission
Molecular Epidemiology
Phylogenetic analysis of the NiV strains has shown that the virus entered to Southeast Asia in 1947. The strains have two lineages. The Indian and Bangladesh strains belong to one lineage: NiV-B and Malaysian lineage as NiV M.
The Indian strains have been shown to be >99% nucleotide similarity with Bangladesh strains. Both Bangladesh and Indian strains are clustered and have been diverged from Malaysian strains. Indian and Bangladesh lineage has shown higher case fatality rate as compared to Malaysian lineage.
The strains of Malaysia and Cambodia belong to the same lineage.
Several studies have shown that multiple virus strains co-circulate in one geographic locality and co-evolve with the reservoir.
Modes of infection: In different NiV outbreaks, the virus has been found to be transmitted either through (i) respiratory route (from pig to human or from diseased person to its close contacts) or through (ii) ingestion of contaminated fruit or sap.
CLINICAL FEATURES
The incubation period of NiV is 4–15 days.
In pigs, respiratory and neurological symptoms are predominant features. The disease is also known as porcine respiratory and neurologic syndrome and barking pig syndrome.
In humans, fever, headache, dizziness, vomiting and myalgia are common initial symptoms in various outbreaks. The symptoms then progress to develop decrease level of consciousness and altered sensorium leading to coma and death. There may be acute or late onset of encephalitis. The duration of illness usually lasts for about 7–10 days. Indian out breaks including the recent Kerala outbreak, respiratory symptoms including severe pneumonia and acute respiratory distress syndrome also have been noted as one of the predominant symptoms.
LAB DIAGNOSIS
Blood, CSF and urine are the main clinical samples required for diagnosis. Tracheal secretions can be collected in patients with respiratory symptoms. In postmortem cases, tissue samples can be taken.
NiV is a biosafety level-4 pathogen. As per the WHO guideline, the samples can be processed in biosafety level-2 after inactivation.
Serology, RT-PCR and virus isolation are the mainstay of diagnosis. Diagnosis of acute infection is done by detection of IgM antibody by capture ELISA or viral RNA by RT-PCR. IgG antibody detection in absence of IgM, however, indicates past infection.
Viral antigen can be demonstrated in the infected tissue by immunohistochemistry. Isolation of virus can be done in Vero cell line and the growth of NiV in the cell line is indicated by syncytium production.
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