Hendra Virus: Vaccines Available but Underused

By Chris Healey

Horse owners in Australia are reluctant to vaccinate their horses against an emerging viral illness capable of sickening humans.

Hendra virus, an emerging infectious disease of horses and humans, has been responsible for the death of 4 people and dozens of horses in Australia since its discovery during a 1994 outbreak of an acute respiratory illness among horses and stable workers in Queensland, Australia. Laboratory tests performed during that outbreak confirmed horses and humans became sick from identical viral agents.

An epidemiologic investigation revealed flying foxes of the Pteropus genus act as Hendra virus reservoirs. Health officials have hypothesized that horses contract the illness through inadvertent consumption of infected bat urine. Hendra virus spreads to humans who come into contact with body fluids, tissues, or excretions of infected horses. Those who work closely with horses, such as equine veterinarians and stable hands, are most at risk of Hendra virus infection.

Early on, researchers discovered Hendra virus glycoproteins could be exploited as an immunization strategy. Following a human Hendra virus death in 2009, and an exposure in 2010, a vaccine for horses was released in 2012 by Zoetis, Inc. As an animal vaccine, developers were spared arduous human pharmaceutical testing protocols and quickly released the product.

The vaccine, called Equivax HeV, is unprecedented in preventative medicine. Not only is it the first vaccine licensed and commercially available to prevent illness from a BSL-4 agent, a pathogen requiring the highest laboratory safety protocols, but it is also the first veterinary vaccine used to transitively prevent illness in humans.

Similar to how smallpox and measles vaccination prevents spread of their respective illnesses, Hendra-vaccinated horses are less likely to transmit Hendra virus to humans by reducing viral shedding. Equivax HeV provides Hendra protection for the horse and the people who interact with it.

Despite vaccine advantages, horse owners say they cannot afford it. A single administration can cost upwards of $200, and booster administration is needed every 6 months for the life of the horse to maintain immunity. Many are unwilling to pay. As a result, only 11% of horses in Australia are estimated to have received the vaccine.

Health authorities are working to approve guidelines recommending yearly booster administrations, cutting immunity maintenance costs in half.  Veterinarians say more horse owners will choose to vaccinate as attitudes toward occupational safety change. Greater awareness of the danger posed to equine veterinarians and stable hands working with unvaccinated horses is expected to place a stigma on non-vaccinating establishments.


(Image Credit: Fainmen)

Nipah Virus in Bangladesh: A Cautionary Tale

By Chris Healey

Since December 2013, an estimated 11 people have died from a Nipah virus outbreak in Bangladesh’s capital city, Dhaka, according to information from the Institute of Epidemiology, Disease Control, and Research. That outbreak is part of an almost yearly occurrence of Nipah virus in Bangladesh linked to deforestation and the resulting displacement of indigenous fruit bats.

Nipah virus is a member of the Henipavirus genus within the parmyxoviridae family. The virus shares its genus with the Hendra virus, a similar emerging infectious illness of horses and humans in Australia.

Nipah virus was discovered to be the causative agent of a 1998 outbreak of a respiratory illness with encephalitis in Malaysia. By May 1999, 276 cases of Nipah virus were reported during that outbreak. Approximately 106 of those cases were fatal. Health officials believe the virus was first transmitted from bats to pigs, then from pigs to humans. Nearly 70% of cases during that outbreak were reported in individuals who worked closely with pigs.

An extensive epidemiologic investigation of the 1998 Malaysian outbreak traced Nipah virus to Indian flying foxes, fruit bats indigenous to India and surrounding countries. Nipah has not appeared in Malaysia since 1999 after the culling of over one million pigs in response to that outbreak. However, a more severe form of Nipah has occurred every year in Bangladesh since 2001, with exceptions in 2002 and 2006. The case fatality rates in Bangladesh have ranged from 69% to 92%, compared to 38% in the Malaysian outbreak. There is no evidence of swine involvement; health officials believe bats are transmitting the illness directly to humans.

Nipah virus is a concern to the international health community because of its effective manipulation of the host immune system and broad host range. Its ability to infect pigs, bats, and humans stems from exploitation of a highly-conserved protein receptor common among cells of mammalian species. Nipah virus possesses a glycoprotein on its surface that interacts with those mammalian protein receptors to allow cell entry.

Health officials believe that the Bangladeshi cases of Nipah virus originate from bats displaced by deforestation in the country. The spread of Nipah virus parallels the resurgence of yellow fever in Africa, where deforestation efforts have disturbed mosquitos that typically feed on primates high in treetops where a sylvantic cycle between mosquito and primate is maintained with little human participation. However, destruction of African rainforests brings treetop-feeding mosquitos to the forest floor where they feed on, and transmit yellow fever to, loggers and villagers in nearby communities. Similarly, bats that once remained sequestered in the forests of Bangladesh are being forced into populated areas due to habitat loss.

Nipah virus is an example of human vulnerability to animal illnesses, also known as zoonoses. Animals displaced into human communities carry their illnesses with them. As people alter and populate previously undisturbed parts of the world, we must prepare to encounter those animals and their associated illnesses.


(Image Credit: Rusty Clark)