Year : 2006 | Volume
: 9 | Issue : 3 | Page : 137--144
Nipah virus encephalitis: A cause for concern for Indian neurologists?
Amit Halder, Ambar Chakravarty
Department of Neurology, Vivekananda Institute of Medical Sciences, Kolkata, India
59, Beadon Street, Kolkata - 700 006
The first and only recorded outbreak of Nipah virus (NV) encephalitis in India occurred in the winter of 2001, although the causative organism could only be identified 5 years down the line in 2006. The first ever-recorded outbreak of NV encephalitis occurred in the Malaysian peninsula in 1998-99; though between 2001 and 2005, at least four outbreaks occurred in our neighboring country of Bangladesh. The threat of further outbreaks of this dangerous disease looms large on the Indian subcontinent, given the natural reservoir of the definitive host, namely, fruit-eating bats of the genus Pteropus. This review would briefly highlight the epidemiology, clinical aspects and diagnosis of NV encephalitis to enlighten the neurological community of the country for early detection and implementation of preventive measures in the event of further outbreaks, especially those which are generally passed of as «SQ»mystery diseases«SQ» in the lay press and even by governmental agencies.
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Halder A, Chakravarty A. Nipah virus encephalitis: A cause for concern for Indian neurologists?.Ann Indian Acad Neurol 2006;9:137-144
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Halder A, Chakravarty A. Nipah virus encephalitis: A cause for concern for Indian neurologists?. Ann Indian Acad Neurol [serial online] 2006 [cited 2020 Feb 21 ];9:137-144
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Between January and February 2001, a highly infectious neurotropic viral illness (encephalitis) shattered the tranquility of the calm foothills of the Himalayas in North Bengal. The Siliguri epidemic (named after the town that bore the brunt of the disease) did not last for more than a month; but in its brief sojourn, it caused tremendous panic. The reasons for the fear were threefold. No one could identify the causative agent. The disease was highly lethal; in those with clinically documented features of encephalitis, the case fatality rate was as high as 74%. Lastly, the disease was highly infectious. Most of the victims were the medical and paramedical staff of governmental and private hospitals where affected patients were admitted, starting with a private nursing home, where the index case was first treated.
Japanese encephalitis (JE) is generally endemic in that part of the country. The Siliguri outbreak however was not caused by JE virus, as was demonstrated early and reported in the lay press and subsequently confirmed. The clinical features, the demographic profile and the epidemiological characteristics seemed to suggest a different organism. Laboratory investigations conducted at that time failed to identify this elusive agent, though positivity for measles virus antibody prompted some to guess the interplay of a 'mutated' measles virus in the genesis. A brief report on the illness was presented by Halder et al at the World Congress of Neurology, London, in September 2001 without mentioning much of the detailed neurological features or being backed by sound laboratory data.
Some 5 years down the line, a group from the National Institute of Virology, Pune, in collaboration with the Center for Disease Control, Atlanta, Georgia, USA, has dug up the dead past in a recent article published in Emerging Infectious Disease. Based on a retrospective analysis of the materials collected during the epidemic, they suggested that the Siliguri outbreak was caused by a novel infectious agent - the Nipah virus (NV).
A question arises - why should we bother about a relatively small outbreak caused by an exotic agent some 5 years back and which never yet surfaced again? The reason would be made adequately clear in the course of this review as the epidemiological and clinical aspects of this serious disease would be slowly unveiled and then more explicitly stated in the concluding section.
NV is an emergent paramyxovirus causing severe encephalitis in humans. Nipah, along with Hendra virus (HV), has recently been designated a new genus Henipavirus within the family of Paramyxoviridae and subfamily Paramyxovirinae. This genus has certain unique biological traits.,
They have a broad range of host specificity, both in vitro and in vivo. The genomes of HV and NV are 18,234 nucleotides long, making them the largest paramyxovirus genome. The increased size is due to a longer Open Reading Frame (ORF). The P gene of the virus encodes three nonstructural proteins (C, V, W) in addition to P. The P gene contains an RNA-editing site that is identical to the editing site found in measles virus. It is not surprising therefore that the Siliguri outbreak was initially thought to be caused by a mutated measles virus. The V and the W proteins appear to be the virulence factors. The L proteins of the Henipavirus have six linear domain structures, but instead of the highly conserved GDNQ sequence within domain 3, they have the GDNE sequence. The cleavage site of the fusion protein (F) of the Henipavirus contains a single basic amino acid. It does not contain R-X-R/K-R consensus sequences for furine proteases that are seen in the morbiliviruses, rubulaviruses and pneumoviruses. They are unique in that they have a leucine residue at the amino terminus instead of the phenylalanine found in most other members of Paramyxoviridae.
The G proteins of the henipaviruses also differ from the H or HN glycoproteins of other paramyxoviruses in that they have no known hemagglutinin or neuraminidase activities. The G protein binds to an unidentified cellular receptor. This receptor binding allows the F protein to mediate fusion with the host cell membrane. The replication strategy is however similar to the other paramyxovirus.
Epidemiology and Transmission
Fruit-eating bats (Pteropus species) are the natural reservoir for the henipaviruses. Humans are usually infected via the intermediate hosts. In case of NV, pigs are the usual intermediate hosts. But exposure to infected fruit bats or materials contaminated by infected bats or direct human-to-human transmission is also possible. Bats are classified in the order Chiroptera (from the Greek 'cheiros,' meaning hand; and 'pteros,' meaning wing) and it is within the genus Pteropus in the family Pteropodidae or old world fruit bats, that we find the natural hosts of HV and NV. Pteropid bats are commonly referred to as 'flying foxes.' Sixty-five Pteropus species are distributed from Madagascar through the Indian subcontinent to southeastern Asia and Australia and as far east as the Cook Islands. Some Pteropus species are among the largest of all bats, weighing as much as 1.2 kg and displaying a wingspan of up to 1.7 m. Pteropus species are unique because they lack the complex neural and behavioral mechanisms required for echolocation that characterize the vast majority of bat species. Instead, they have large eyes and they navigate visually, feeding mainly on fruits and flowers, which they locate by smell.,,
Although brought into much attention by the epidemic of NV encephalitis in Malaysia in 1998-99 (vide infra), isolated cases of Hendra virus causing encephalitic illness amongst animal handlers were being reported since as early as 1994. NV and HV having close genomic similarity were difficult to differentiate serologically earlier. In fact, even during the Malaysian epidemic of 1998-99, initial results indicated the causative organism to be Hendra virus. Later of course, with viral isolation and development of a specific serological marker, the identity of the NV was made known. The word 'Nipah' originated from the name of a village 'Sungai Nipah' in the Malaysian peninsula, one of the first villages where pig farmers developed an encephalitic disease.
The reported HV/NV encephalitic illnesses recorded so far and related milestones are depicted in a tabular form in [Table 1]. ,,,,,,,,,,,,,,,,,,,,,,
The first known human infection with NV was detected during an outbreak of severe febrile encephalitis in peninsular Malaysia and Singapore, in 1998-1999. Direct contact with pigs was the primary source of human infection. A total of 276 patients with viral encephalitis were reported in that epidemic. Most of the victims were adult males involved in pig farming or pork production. The spread of virus within the pig farms and between states of Malaysia was due to movement of pigs. Transmission between pigs in the same farm was attributed to direct contact with excretions and secretions such as urine, saliva, laryngeal and pharyngeal secretions. Iatrogenic transmission by use of same needles was also implicated.,
But what exactly led to the spillage of the virus from its natural reservoir into the pigs remains a subject of speculation. Species jumping of viruses can be due to evolutionary or ecological reasons. But NV is an old virus and has not undergone any evolutionary change. Most authorities believe that ecological factors led to their emergence. This can be due to a change in the number density and management of pigs. But more importantly, the curse of unplanned deforestation of pulpwood has taken its toll on the natural habitat of the fruit bats in the last two decades. This coupled with the EL Nino Southern Oscillation - related drought prompted migration of bats from their natural habitat in the costal forest on to the villages where the pigsties (piggeries) were located.
The southern oscillation refers to an oscillation in the surface pressure (atmospheric mass) between the southeastern tropical Pacific and the Australian-Indonesian regions. When the waters of the eastern Pacific are abnormally warm (an EI Nino event), sea level pressure drops in the eastern Pacific and rises in the west. The reduction in the pressure gradient is accompanied by a weakening of the low-latitude easterly trades. This condition results in redistribution of rains with flooding and droughts. The drought played a major role in animal migration from forestlands towards villages in several ways. With destruction of trees, there had been shortage of food in the forest, especially for the fruit-eating bats. Second, the dry weather resulted in forest fires, causing further loss of trees and lack of food supply for the fruit-eating bats. Thirdly, the forest fire caused a severe haze, resulting in poor visibility for the bats, which preferred to migrate to the cleaner 'air' of the villages closer to human and pig habitation. These EL Nino related factors coupled with 'intentional' deforestation played a major role in upsetting the ecological balance, perpetuating transmission of NV from bats to pigs and pigs to humans in Malaysia.
The route of introduction of virus into the pigs was also facilitated by the practice of growing fruit trees adjacent to the piggeries.
In April and May 2001, a cluster of febrile neurological illnesses with nine deaths was reported in a village in Meherpur district, Bangladesh. Preliminary investigations by the Bangladesh Ministry of Health and the World Health Organization (WHO) excluded a diagnosis of Japanese encephalitis, dengue fever or malaria, but 2 of 42 serum specimens obtained from village residents in May 2001 showed reactive antibodies to Nipah virus antigen in tests performed at the US Center for Disease Control and Prevention (CDC). However, a comprehensive investigation of this outbreak was not conducted.
In January 2003, a further cluster of febrile illnesses with neurological features and eight reported deaths occurred in adjoining villages in Naogaon district, 150 km from the village in Meherpur district. Similarities in the clinical manifestations observed among patients in Naogaon and Meherpur raised the question of whether the outbreaks were caused by the same agent. But unlike the Malaysia epidemic, no intermediate amplifying host could be identified. This led to the conjecture that the virus was transmitted directly or indirectly from bats to the humans.
Two outbreaks consisting of 48 cases of NV were detected in 2004 in two adjacent districts (30 km apart) of central Bangladesh (Rajbari and Faridpur) with a case-fatality rate of nearly 75%. Because of heightened surveillance, other small clusters and isolated cases (n=19) were identified during the same period in seven other districts in central and northwest Bangladesh. Although antibodies to NV were detected in fruit bats from the affected areas in 2004, an intermediate animal host was not identified, which suggests that the virus was transmitted from bats to humans. In fact, human-to-human transmission of NV was documented during the Faridpur outbreak of 2004.
Four NV isolates from Bangladesh share 99.1% homology but exhibit more interstrain nucleotide heterogeneity than the sequences of the human isolates in Malaysia, which were nearly identical. These varying amounts of genetic variability may reflect differences in the mode of transmission of NV in the two countries.
A further outbreak in Tangail district of Bangladesh occurred in end 2004 - early 2005 and has also been reported.
Unlike the Malaysia outbreak, 'bat to pig to human' transmission was unlikely to have occurred in Bangladesh. For religious reasons, pig farming is not practiced in Bangladesh and pig population is low. Hence direct bat-to-human transmission and then human-to-human transmission seemed most likely.
Tan, in a talk delivered at the World Congress of Neurology, November 2005, at Sydney, proposed that this route might be through contaminated date palm juice, which humans consume. Collecting date palm juice in earthenware pots hung atop date palm trees (after making an incision in the bark of the trees) is a common practice in rural areas of eastern India and Bangladesh during winter months. The bats feed on the juice, thus contaminating the juice with their saliva, which is subsequently drunk by humans.
The Siliguri epidemic clearly resembles the Bangladesh epidemic. Most of the victims had nothing to do with the pigs. They were medical and paramedical staff of the nursing homes where the affected cases were brought., This, together with the demonstration of the viral genome in the urine of the affected patients, clearly supports the case for a human-to-human transmission, perhaps with a similar bat-to-human transmission through contaminated date palm juice, to start with. Unconfirmed reports suggest that the index case in the Siliguri outbreak was brought to a private nursing home from a neighboring village by an ambulance (personal communication). The patient, his accompanying person and the ambulance driver -all succumbed to the illness.
The incubation period of NV in humans is not known. It has ranged from several days to 2 months. WHO suggests that the incubation period is between 4 and 18 days in majority of instances. The virus after introduction causes a systemic multiorgan vasculitis.,, CNS is most often affected. However, endothelial affection is observed in the heart, lung, kidney and spleen. Immunohistochemistry shows intense staining of endothelial and parenchyma cells and multinucleate giant cells, characteristic of paramyxovirus infections. NV has been isolated from CSF, tracheal secretions, throat swabs, nasal swabs and urine. Pigs show extensive inflammation of the airways with interstitial pneumonia. There is a harsh nonproductive cough (mile long cough).
The clinical features of NV encephalitis have been described extensively in the Malaysian epidemic. Almost all the patients (97%) had fever at presentation. Headache was present in the majority. Cough, myalgia and vomiting were also present in a significant number.
A majority of patients had impaired level of consciousness (55%). Patients with reduced level of consciousness had prominent signs of brain-stem dysfunction, including abnormal doll's eye reflex, pinpoint pupils with variable reactivity and prominent vasomotor changes consisting of hypertension and tachycardia, which suggested involvement of the medullary vasomotor center. Seizures occurred in 23% of all patients and all but one of these patients had generalized tonic-clonic seizures; the patient had focal motor seizures with secondary generalization. No patient had status epilepticus. Segmental myoclonus, characterized by focal, rhythmic jerking of the muscles, was present in 32%. Eight patients had cerebellar signs. Absent or reduced tendon reflexes with hypotonia were seen in 56% of patients. This condition was more common in patients with a reduced level of consciousness than in those with a normal level (81 vs. 26%). In 10 comatose patients, a distinct pattern of recovery was observed, in which flaccid tetraplegia and areflexia persisted while cranial motor function and cognition improved, mimicking the locked-in syndrome. In 8 patients, limb power subsequently recovered and reflexes returned. Four were able to walk independently.
Data regarding the Siliguri outbreak is scanty; however, 100% of the patients had fever. Altered sensorium was present in 97%. Respiratory symptoms were present in 51%. Involuntary movements and convulsions occurred in 43%. Haldar et al, in their report on the same outbreak, noted a slightly higher incidence (52%) of seizures and 42% had ataxia. No neck rigidity was found. While the group from National Institute of Virology, Pune, did not find any cranial nerve involvement, Haldar et al documented lower cranial nerve involvement in their study. Abnormal plantar responses were recorded in 11 of the 16 patients studied. The clinical features were thus in essence similar to the Malaysian epidemic.
The higher incidence of seizures and ataxia in the Siliguri epidemic might be due to their presentation at a later stage. Indeed, 42% of the Malaysian patients with decreased level of consciousness went on to have seizures. Slurring of speech and lower cranial nerve involvement was also documented.
The mortality rate in the Siliguri epidemic was as high as 74% in those with features of encephalitis. This is closer to the mortality data from Bangladesh. In the South Asian epidemic, the case-fatality ratio was much lower - 34%. This difference could be ascribed to the difference in standards of health care between the two places.
It is important to recognize the salient clinical features of NV encephalitis that distinguish it from JE, which include early brain stem signs, early ataxia, segmental myoclonus and terminal autonomic dysfunction. In areas where JE is endemic, presence of such clinical features would alert a clinician to think of NV encephalitis rather than JE.
Laboratory diagnosis of NV is challenging in more than one way. NV and HV have been classified internationally as biosecurity level 4 (BSL4) agents. This implies that the propagation of the viruses should be conducted under physical containment level 4 (BSL4) conditions. The strict applications of these guidelines hamper virus isolation.
NV grows well in Vero cell. Tissues could be used after processing for inoculation in cell cultures. Virus isolates are then identified using immunostaining, neutralization, Polymerase chain reaction (PCR) of culture supernatants, electron microscopy and immune electron microscopy.
In the Malaysian outbreak, viral isolation was attempted. Syncytial cell formation on Vero cell cultures (American Type Culture Collection and certified cell line 81) several days after inoculation indicated the presence of the virus. CDC, quickly identified the virus as 'Hendra-like', but electron microscopy showed that they lacked the characteristic double fringe of the HV. Brain, lung, kidney and spleen were the usual organs submitted for study. A cytopathic effect develops within 3 days, but two 5-day passages are recommended before judging the attempt unsuccessful.
It is very useful and allows for retrospective study of formalin fixed tissues. Polyclonal and monoclonal antisera are now available for study.
NV grown in cultured cells can be visualized by negative contrast electron microscopy. Virus antibody interactions can also be seen by immunoelectron microscopy.
Polymerase chain reaction
PCR is now a routine at CDC for M gene amplification. Reverse transcriptase (RT) PCR can be used for detection of viral sequences in both fixed and fresh tissue or CSF. PCR based on N gene coding is also in vogue. PCR has allowed for phylogenetic-based study. NV and HV were shown to group together in close relationship with the members of the genus Morbillivirus. The sequence analysis also allows comparison of the strains responsible for the various outbreaks. It may in future throw further light into the routes of propagation.
Serum neutralization tests (SNT)
SNT is the accepted reference standard in serology. Both HV and NV have been quantified using plaque assay procedure.
Serum and cerebrospinal fluid samples are tested with an IgM-capture enzyme-linked immunoabsorbent assay (ELISA) and indirect IgG ELISA for antibodies against Hendra virus antigens. These tests were found to be useful during the outbreak at Malaysia, pending the development of antibody tests that used NV antigens.
In the initial investigation of an outbreak and for ongoing surveillance, one needs serological tests that are safe and quick. ELISA is ideal in this situation. However, nonspecific reactions have been encountered. CDC now employs indirect ELISA for detection of IgG. This is supplemented by the use of capture ELISA.
In the Siliguri outbreak, NV-specific IgM and IgG were detected in 9 of the 17 serum samples. RT PCR detected RNA from N gene of NV in 4 urine samples from NV antibody negative patients. RNA from M gene was detected in 3 of the 5 samples. Sequence analysis confirmed that the PCR products were derived from NV RNA. Comparison of the N sequence showed an overall 97.5% nucleotide homology with the Bangladesh and Malaysian epidemics. The sequences were closer to the Bangladesh isolate.
Hematology: Thrombocytopenia and leucopenia occurs in less than 50% of the patients.
Biochemistry: Usually normal except for mildly raised enzymes.
Cerebrospinal fluid: Usually abnormal. Three-quarters of the patients with encephalitis had elevated WBC count and/or elevated protein level. There was no correlation between abnormal CSF and disease severity; virus-specific antibodies were present in the CSF in only one-third of the acute cases.,
EEG: There was presence of diffuse slow waves with focal sharp waves. Those deeply comatose or those who died had bitemporal periodic complexes of sharp and slow waves appearing every 1-2 s. There is no correlation between EEG pattern and segmental myoclonus.
Chest radiographs may be abnormal in 6-10% of the patients. The main abnormality recorded was interstitial shadowing.
CT brain is generally normal.
MRI of brain may show widespread discrete focal hyperintensities that measure 2-7 mm. These are present mainly in the subcortical and deep white matter. Some asymptomatic patients also had abnormal MRI. The lesions were attributed to widespread micro infarction due to underlying vasculitis and thrombosis of small blood vessels. These features are very distinct from JE, where thalamic/basal ganglionic involvement is common.
The MRI in the relapse and late-onset cases showed diffuse confluent involvement of cortical gray matter in the cerebral hemispheres.
Treatment of NV encephalitis is essentially supportive. Chong et al, in a study in Malaya, suggested that ribavirin is able to reduce the mortality of acute Nipah encephalitis. But their study was not randomized and has not been universally accepted. Animal experiments have shown that both ribavirin and 6-aza-uridine were able to delay but not prevent NV induced mortality. In a recent study on hamsters, Poly (I)-poly(C (12) U), an interferon inducer, was found to be effective. The molecule when used at 3 mg/kg of body weight daily from the day of infection to 10 days post infection, prevented mortality in five of six infected animals. The same group from France has conducted animal experiments to justify the use of monoclonal antibody (against F and G glycoproteins) for prophylaxis and treatment.
In absence of any widely acceptable treatment modality, the importance of prevention of spread and early recognition of NV encephalitis need not be overemphasized. Singapore controlled the outbreak in 1998 by stopping pig export from Malaysia. The Malaysian outbreak was contained by mass culling of over 1 million pigs and since 1999 no other NV outbreak occurred in that country. In view of the rather different mode of transmission (proposed) in Bangladesh (and possibly India), such measures are not applicable. As human-to-human transmission definitely occurred, very strict hygienic measures (especially by medical personnel) by way of wearing gloves and masks (not used during the early stages of the Siliguri outbreak) seem mandatory. In India, where pig farming is practiced, these should be located at a distance from human habitats and periodic serological check-up may be conducted but would be expensive. During any further outbreaks, serological screening of contacts and isolation of those tested positive seem very essential, in view of human-to-human transmission noted in Siliguri and Bangladesh.
The very purpose of writing this review had been to appraise Indian physicians and neurologists about the possible future hazard from similar outbreaks (as occurred in Siliguri) across the country (especially eastern, northeastern and northern parts) in keeping with the natural habitat of the fruit-eating bats. The clinical distinctions from JE had already been pointed out and include presence of early brain stem and cerebellar signs, segmental myoclonus and autonomic dysfunction. Considering the series of outbreaks in our neighboring country Bangladesh, the threat to Indian population is real as the Pteropus bats are capable of flying long distances. The situation had been well summarized by Chadha et al.: 'Given the distribution of the locally abundant P. Giganteus, the apparent natural reservoir of NV in this area, outbreak of NV will likely continue to occur in Bangladesh and northern India. Establishing appropriate surveillance systems in these areas will be necessary so that NV outbreaks could be detected quickly and appropriate control measures initiated.'
Although mechanisms of transmission from bat to livestock and human have been postulated, there are few data on which we can rely to develop models of viral transmission or risk management strategies for control of diseases caused by henipaviruses in the future. The high virulence of henipaviruses, the absence of therapeutic intervention strategies and vaccines and their classification as BSL4 pathogens have undoubtedly impeded the rate at which information has been generated on the biology and pathogenesis of diseases caused by Hendra and Nipah viruses.
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