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)