Nipah Virus: An Emerging Health Concern

Nipah virus is a relatively rare zoonotic disease (spread from animals to people) with a high fatality rate — up to 75%. It was first identified in 1999 in Malaysia, where human disease resulted from the proximity of humans, domestic pig farms, fruit trees, and fruit bats, the animal reservoir. Small annual outbreaks are common in Bangladesh and may be seen in some parts of India, Singapore, and other nearby countries.

However, 2023 has seen an escalation of cases, reminding us that disease can take hold at any time and that the interactions of people, animals, and the environment could lead to disease developing in non-endemic (non-natural) areas. We must be vigilant and know what to look for to prevent future spread.

Nipah virus: Transmission and risks

While it isn’t a concern in the U.S., Nipah virus (NiV) causes seasonal outbreaks in Bangladesh, usually affecting less than 100 people. However, as of September 26, 2023, an outbreak larger than commonly experienced is ongoing in India in the state of Kerela and several other Asian countries. Cases have reached over 300, with more than 100 deaths. This causes concern, and controlling the outbreak, understanding transmission, and preventing disease spread are ongoing.

Similar to measles and mumps

NiV is part of the Paramyxoviridae family of viruses. A potentially rapidly fatal illness, NiV shares a genetic family with well-known and potentially dangerous viruses. Measles and mumps, two common childhood maladies once prevalent in the U.S. but made almost obsolete through childhood vaccinations, fall into this family. Additionally, parainfluenza virus and respiratory syncytial virus (RSV), a common cause of severe disease in young children and older adults, are part of this group.

Nipah virus: A global health threat

The following NiV characteristics give the public health world concerns. This disease:

• Has no vaccine
• Has the risk of human-human transmission
• Is zoonotic (spread from animals to humans)
• Has high death rates (mortality rates) in people

Because of these factors, the World Health Organization (WHO) and others recognize this as an emerging global health threat. Thankfully, NiV isn’t present in the U.S. However, given the presence of other viruses in this family, the contagious nature of the disease, and the risks when bats, pigs, and human activities intertwine, this emerging zoonotic threat is a virus worth knowing about.

Animal reservoir: Fruit bats

An animal reservoir represents the wildlife host of a disease that remains symptom-free but sheds and spreads the virus. For NiV, fruit bats (Pteropus species) have been identified as the key species of concern. This species commonly resides in India, Singapore, Malaysia, and Bangladesh, where periodic outbreaks occur.

However, another possible reservoir, the flying foxes (Pteropoid species), may also pose a risk, known to reside in Thailand, Indonesia, Cambodia, the Philippines, and Madagascar. Thus, outbreaks may develop in these areas, leading to the spread of the disease. For clarification, flying foxes are a type of fruit bat, but not all fruit bats are flying foxes; hence, the two different genera of bats are noted as possible animal reservoirs.

Additionally, domestic pigs become accidentally infected and act as intermediate hosts. They can provide a means to pass the virus to humans without direct contact with bats. Intermediate hosts may show clinical signs or remain symptom-free.

Disease transmission and risks

In the Malaysian outbreak in 1998–1999, when NiV was first identified, a combination of factors permitted disease emergence. The fruit trees near where pigs were raised attracted the reservoir species (fruit bats). This exposed the pigs to bat waste containing the virus. Infected pigs and their waste then provided exposure for the humans. Subsequent outbreaks, however, haven’t always had this specific combination of bats, fruit trees, and pigs, suggesting that the presence of bats in a region can create exposure risk.

Those at high risk of developing NiV infection include:

• Abattoir workers (slaughterhouse employees)
• Veterinarians or farmers working with pigs and or those in close proximity to fruit bats and related species that have been shown to harbor the virus
• Caregivers of infected, sick individuals with NiV
• Those who consume fruits or other food products contaminated by bat waste or drink raw date palm sap
• Those climbing trees contaminated with bat waste
• Those exposed to secretions from infected pigs, bats, or humans
• Human-human transmission

Thus, by limiting animal-to-human, bat-human, and human-human transmission, outbreaks can hopefully be prevented but at least more easily contained.

Affected species

In addition to human infections, several domestic species can become infected by NiV, including horses, goats, cats, and dogs.

Animals may show no clinical signs, and many pigs often remain asymptomatic. However, if signs develop, they are similar to humans with a fever, trouble breathing, a barking-like cough, and/or neurological changes such as twitching and trembling. The disease in pigs spreads rapidly, though it is not usually fatal, unlike in people.

Incubation period

The time it takes from exposure to the first overt symptom, or the incubation period, is often confined to a set period with many viruses. It is generally 4–14 days with NiV. However, one nuance and challenge with NiV is that there can also be a prolonged incubation of up to 45 days and possibly longer. This makes it more difficult to control the spread if individuals do not know they are exposed or do not manifest clinical signs for a long time after exposure.

Symptoms of Nipah virus

Symptoms initially are often non-specific and could be consistent with numerous different conditions. However, it can rapidly progress to neurological disease, which can be fatal. Clinical signs may include:

• Fever
• Headache
• Lethargy
• Mental confusion
• Disorientation
• Encephalitis
• Some people also develop severe respiratory illness (e.g., atypical pneumonia, cough, breathing trouble)

Because symptoms start as non-specific, diagnosis can be difficult, and it may mimic various diseases. However, it should always be considered in areas known to be endemic (exist in nature).

Treatment options

Treatment for NiV is symptomatic (supportive care), as no current therapies or vaccinations are available to manage this disease. Supportive care includes treating individual symptoms as warranted and providing sufficient hydration and rest. Even with hospitalization and symptomatic care, the disease is fatal in 40–75% of infected individuals (case fatality rate, CFR). Though over the 20+ years since the first known outbreak in Malaysia, there have been less than 2000 known cases, it is one of the most lethal viral diseases known to date.

Caregivers and those with close contact with infected individuals must take appropriate precautions to prevent exposure and transmission.

According to the Centers for Disease Control and Prevention (CDC), as of 2020, a monoclonal antibody (a protein used to stimulate the immune system), m102.4, completed phase 1 clinical trials and was used on a compassionate basis. This means that researchers/medical professionals can offer a possible therapy with a drug or treatment option that is not yet approved in hopes that it will improve the patient’s health outcomes. It is used in patients without clinical trials or other viable treatment options.

Antiviral drugs such as remdesivir, given post-exposure to non-human primates, prevented disease transmission and may be a viable option in humans in future outbreaks.

Long-term complications

In addition to a late-onset (long incubation period), patients with NiV may seem to recover from neurological disease (encephalitis) only to have it develop later on (months to years) after the initial infection. Others may have ongoing neurological compromises such as memory issues, trouble with activities of daily living, and even behavioral and mental changes such as depression.

Thus, people who have traveled to or reside in areas where the disease is present should advise healthcare providers of travel and exposure if they develop symptoms consistent with NiV.

NiV prevention

For people at risk of NiV or traveling to areas where it may exist in nature, it is important to take preventative steps to lessen their risk of disease. Measures include the following:

• Practicing appropriate hand hygiene
• Avoiding interacting with any sick pigs or bats
• Avoiding bat roosting (nesting) areas
• Not eating or drinking items that could be contaminated with bat waste
• Using protective barriers, e.g., gloves and face masks, when interacting with people infected with NiV
• Avoiding areas where known reservoirs live (fruit bats) and where another possible reservoir, the flying foxes, reside

Practicing safe animal interactions and food handling practices can keep people healthy.

Nipah virus: A One Health disease

So, why should we care about NiV if we don’t live in areas where it is endemic? While, thankfully, NiV isn’t present in the U.S. or outside of Asia, bats in Africa have been identified with the virus. Further, it is a reportable disease per the World Animal Health Organisation (WAHO/OIE). It is also on the WHO’s list of infectious diseases for which urgent research is warranted.

Diseases potentially spreading from a wildlife reservoir (fruit bats) to domestic animals, including pigs with relatively easy transmission to humans, pose a great threat to global health and demonstrate the importance of looking at diseases from a One Health viewpoint. NiV helps provide a good example of how environmental, animal, and human health are intimately intertwined.

Though, to date, NiV has only been identified in specific regions of the world, its spread to other areas should be of concern. Therefore, people working closely with animal species known to harbor the infection or those traveling to areas where the disease has been identified should be aware and cautious.