Researchers have described the Nipah virus as an unpredictable emerging pathogen, saying there is potential for cases to emerge in new areas in the foreseeable future.
The Nipah virus, centered in South East Asia, is closely related to another bat-borne pathogen, the Hendra virus, in Australia.
Nipah infections can be severe and lead to death.
The natural host for the virus is fruit bats, also known as megabats, especially those of the Pteropus genus.
Natural infections in domestic animals have been described in farmed pigs, horses, domestic and feral dogs, and cats.
Natural Nipah transmission is possible within a species, for example, pig-to-pig, and between species, such as bat-to-human, pig-to-human or horse-to-human.
In a just-published Nipah review in the journal Animals, Luigi Bruno and his fellow researchers said humans or animals could spread the virus in different ways.
It is peculiar, they said, how the viral transmission modes among different hosts can change depending on the geographical area for various reasons, including different breeding methods and eating habits. Some animals, for example, can be infected by eating fruit that an infected bat has partially eaten.
While infected bats are generally symptom-free, the disease can typically cause fever, respiratory issues, and neurological problems associated with brain inflammation in other animals.
Outbreaks have been described in Malaysia, Singapore, Bangladesh, India and the Philippines with, in some cases, severe respiratory and neurological disease and high mortality in both humans and pigs.
The cornerstones for control of the disease are biosecurity and potential vaccines, which are still under development.
The virus was named after an outbreak in the village of Sungai Nipah, Malaysia, where it was first isolated from a human patient in 1998.
The disease is now widely described in pigs, with clinical signs involving the respiratory and nervous systems. When outbreaks occur in swine, the only possible measures are isolation, blocking movements, and killing infected animals.
The review team said that the emergence of the virus and the potential of transmission to other animals and humans seem to be related to losses in the bats’ habitat.
The primary outbreaks in reported species have so far occurred in the geographical area of the carrier bats – that is, in Malaysia, Singapore, Bangladesh and India, but cases have also been described in the Philippines.
“Nipah virus infection is an emerging and potentially dangerous disease whose spread must be curbed,” they said.
A “One Health” approach, considering humans and animals as well as the environment, is required to control the disease effectively.
Bat-to-human transmission is possible by inhaling aerosol Nipah virions or ingesting Nipah-contaminated palm fruits or sap. Nipah-contaminated palm fruits also cause infection in pigs and horses fed with or upon accidental food ingestion of contaminated fruit or palm sap.
Pig-to-human transmission is well recognized in pig farms and among pig slaughterhouse workers in at-risk areas.
“An outbreak with a mortality rate of 82% was reported in the Philippines in 2014,” the authors noted. “Of particular interest is the fact that ten patients had a history of direct contact with horses and horse meat consumption.
“In addition, the deaths of 10 equines were reported during the same period, nine of which exhibited neurological symptoms. Unfortunately, the deceased animals were not subjected to molecular biology tests to ascertain the presence of the Nipah virus genome.”
Five human patients, including two healthcare workers, contracted the disease through direct contact with infected people. This viral strain was genetically correlated with the Malaysian strain for which human-to-human transmission has been described as a minor source of contagion.
“This suggests the possibility of a coevolution of different variants of the Nipah virus in bats and the consequent increase in the likelihood of mutations and spillover events.”
The authors said climate and human-driven changes could have important roles in the spread of viral infections to animals and humans.
Fruit bat geographical distribution is highly dependent on food sources, a combination of nectar, pollen and fruit, especially those produced by Eucalyptus trees in woodlands and open forests.
“Therefore, a lack of food resources due to deforestation and climate change may disperse flying foxes outside their typical habitats with consequently increased levels of stress.
“Studies using urine cortisol concentrations to measure the physiological stress in Pteropid bats have pointed out that lower winter temperatures increase cortisol concentrations, which are associated with Hendra virus excretion in flying foxes (fruit bats).
“A similar condition may occur with the Nipah virus.”
Another aspect is body condition related to nutritional status. Poor body condition is associated with increased seroconversion and Hendra risk in flying foxes.
“This increases the urinary excretion of viable virus, with an increased risk of transmission to horses.”
The authors caution that, due to the worldwide distribution of bats, potential new reports and spillovers are foreseeable.
The Nipah virus may pose a potential risk of spread to humans and animal husbandry in areas of the world that receive economic migrants from Nipah virus outbreak locations, especially when they come in contact with Nipah-permissive livestock such as pigs.
“Accurate monitoring of migrant entries from areas of Nipah virus outbreaks or borders should be adopted as a health measure in compliance with specific national or international health security regulations and procedures to cope with the spread of the Nipah virus into uncontaminated world areas.”
They said the high mortality rate upon infection and its acute course makes the disease difficult to diagnose, especially in countries that do not have equipped and easily accessible diagnostic facilities. Diagnostic kits are still expensive, the authors noted.
“Clear action and surveillance plans are essential, especially in Southeast Asia,” they said.
Help from the World Health Organization by way of economic resources, such as equipped laboratories and medical and veterinary professionals, as well as Nipah-virus-oriented training, would be appropriate, they said.
The drafting of new surveillance plans and the organization of training internships for doctors and veterinarians are concrete answers to the challenges that lie ahead.
“Wildlife expert veterinarians will certainly play a decisive role in the epidemiological scenarios of Nipah virus outbreaks and in those countries that have direct/indirect relationships with those of Southeast Asia at risk.”
The authors say particular attention should be given to climate and human-induced changes known to play important roles in the spread of viral infections to animals and humans, as well as in the maintenance of wildlife and biodiversity, especially when wildlife habitat is subjected to significant losses.
“The compromising of wildlife habitats can lead to increased spillover events of zoonotic pathogens such as the Nipah virus spreading from wildlife to domestic animals and humans, potentially leading to the emergence of new zoonotic outbreaks.”
The review team comprised Bruno, Maria Anna Nappo, with a local health authority in Italy; and Luca Ferrari, Rosanna Di Lecce, Chiara Guarnieri, Anna Maria Cantoni and Attilio Corradi, with the Department of Veterinary Science, part of the University of Parma in Italy.
Bruno, L.; Nappo, M.A.; Ferrari, L.; Di Lecce, R.; Guarnieri, C.; Cantoni, A.M.; Corradi, A. Nipah Virus Disease: Epidemiological, Clinical, Diagnostic and Legislative Aspects of This Unpredictable Emerging Zoonosis. Animals 2023, 13, 159. https://doi.org/10.3390/ani13010159
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