Henipaviruses belong to the family of paramyxoviruses. Two species have been identified to be zoonotic, causing disease in animals. These are the Hendra virus (HeV) and the Nipah virus (NiV). They produce severe and often fatal illness in humans and horses.
HeV infection in horses, and then in humans, was first reported in 1994 in Australia. In contrast, NiV infection was first observed in pigs and subsequently in humans in 1998, in Malaysia. In Bangladesh, henipavirus infection was traced independently of contact with infected animals.
Fruit bats (Pteropus species, also called ‘flying foxes’) are the natural hosts of henipaviruses. The Hendra virus is probably transmitted to horses, which are the main intermediate hosts, through the ingestion of food contaminated with the droppings, urine or other excretions of infected fruit bats. The bats themselves do not show any clinical signs of illness. Transmission of HeV to man occurs through close contact with infected horses, probably through their respiratory secretions and urine.
NiV is carried from pigs, which are the main intermediate hosts, to humans, via aerosols, or direct contact with infected respiratory secretions, saliva or urine, or surfaces contaminated by these secretions. Pigs may have acquired the infection via fruits half-eaten by bats, and contaminated by the virus. Other possible intermediate hosts are dogs, cats, horses and goats.
In some cases NiV transmission has been independent of direct contact with livestock, through the ingestion of sap from the date palm, which was contaminated by bat excretions. In other cases the NiV infection was transmitted from the infected excretions of pigs to abattoir and farm workers in contact with the animals. A high rate of human to human spread by NiV was seen in an outbreak in Bangladesh, unlike earlier outbreaks in Malaysia.
The disease manifests in 4-20 days or 5-12 days, for HeV and NiV infection respectively. It presents as fever with acute encephalitis, or as an acute influenza-like illness leading to severe respiratory illness, or as meningitis. The mortality ranges from 40-70% for NiV infection, and 50% with HeV. It is highest among those with acute encephalitis. Survivors have severe residual disabilities, such as incoordination, muscular weakness and difficulty with thought processing and mental functions. Others show paralysis of the eye muscles, with resultant visual problems.
Encephalitis in HeV infection presents as motor weakness, confusion and disorientation, or seizures. In NiV infection, there may be fever, vomiting, headache, dizziness and loss of consciousness. Rising heart rates or blood pressure, kidney impairment, bleeding from the gastrointestinal tract, septicemia, and convulsions, are also seen.
Why are henipaviruses so deadly? The answer lies in their ability to encode several proteins which block the innate immune response in infected animals and humans. These inhibit the cell’s response to viral infection, and allow viral replication. These thus act as virulence factors, blocking the interferon-stimulated antiviral defense mechanisms from kicking in inside the infected cells. The virus causes destruction of small blood vessels in many major organs, such as the brain, liver and kidney, causing organ failure. This is associated with microinfarction, infection, and organ failure.
The emergence of henipavirus disease is likely to be the result of increased interaction between the humans or other animals and fruit bats, as a result of habitat loss, and a higher rate of hunting forays into territory hitherto left to the bats.
Diagnosis and treatment
Diagnosis is by the detection of specific antibodies to Hendra and Nipah viruses, in blood and cerebrospinal fluid. Antibodies develop in 2-3 weeks and persist for up to 3 months.
Treatment is symptomatic, and no vaccine or antiviral drug has been developed so far to treat the disease. Prevention is by ensuring HeV vaccination of all horses in risk situations, as well as minimal human contact with fruit bats, isolation of sick animals, precautions against direct contact with infected secretions and excretions and extensive culling of animals confirmed to have the infection.