OUTBREAK NEWS: NIPAH VIRUS
Nipah virus is a newly recognized zoonotic virus. The virus
was discovered in 1999. It has caused disease both in animals and in humans,
through contact with infectious animals. The virus is named after the
location where it was first detected in Malaysia. Nipah is closely related
to another newly recognized zoonotic virus (1994), called Hendra virus,
named after the town where it first appeared in Australia. Both Nipah
and Hendra are members of the virus family Paramyxoviridae. Although
members of this group of viruses have only caused a few focal outbreaks,
the biologic property of these viruses to infect a wide range of hosts
and to produce a disease causing significant mortality in humans has made
this emerging viral infection a public heath concern.
Natural host
It is currently believed that certain species of fruit
bats are the natural hosts of both Nipah and Hendra viruses. They are
distributed across an area encompassing northern, eastern and south-eastern
areas of Australia, Indonesia, Malaysia, the Philippines and some of the
Pacific Islands. The bats appear to be susceptible to infection with these
viruses, but do not themselves become ill. It is not known how the virus
is transmitted from bats to animals.
Transmission
The mode of transmission from animal to animal, and from
animal to human is uncertain, but appears to require close contact with
contaminated tissue or body fluids from infected animals. Nipah antibodies
have been detected in pigs, other domestic and wild animals. The role
of species other than pigs in transmitting infection to other animals
has not yet been determined.
It is unlikely that Nipah virus is easily transmitted to
man although previous outbreak reports suggest that Nipah virus is transmitted
from animals to humans more readily than Hendra virus. Despite frequent
contact between fruit bats and humans there is no serological evidence
of human infection among bat carers. Pigs were the apparent source of
infection among most human cases in the Malaysian outbreak of Nipah, but
other sources, such as infected dogs and cats, cannot be excluded. Human-to-human
transmission of Nipah virus has not been reported.
Clinical features
The incubation period is between 4 and 18 days. In many
cases, the infection is mild or inapparent (sub-clinical). In symptomatic
cases, the onset is usually with influenza like symptoms, with high fever
and muscle pains (myalgia). The disease may progress to inflammation of
the brain (encephalitis) with drowsiness, disorientation, convulsions
and coma. Fifty percent of clinically apparent cases die.
Treatment
No drug therapies have yet been proven to be effective
in treating Nipah infection. Treatment relies on providing intensive supportive
care. There is some evidence that early treatment with the antiviral drug,
ribavirin, can reduce both the duration of feverish illness and the severity
of disease. However, the efficacy of this treatment in curing disease
or improving survival is still uncertain.
Protection of health care professionals
The risk of transmission of Nipah virus from sick animals
to humans is thought to be low, and transmission from person-to- person
has not yet been documented, even in the context of a large outbreak.
Therefore, the risk of transmission of Nipah virus to health care workers
is thought to be low. However, transmission without percutaneous exposure
(through a break in the skin barrier) is theoretically possible, as respiratory
secretions contain the virus. This is why it has been categorized as a
biohazardous agent that should be managed in a high-level biosecurity
laboratory. It is recommended that close contact with body fluids and
infected tissues be avoided if Nipah infection is suspected.
Outbreaks of Nipah and Hendra viruses
From September 1998 to April 1999, there was a large outbreak
of encephalitis in Malaysia. During the investigation of this outbreak,
Nipah virus, a previously unrecognized virus, was identified as the causal
agent. A total of 265 people were infected, of whom 105 died. Ninety-three
percent of cases had occupational exposure to pigs. An associated outbreak
among abattoir workers in Singapore during March 1999 led to 11 cases,
with one death. These workers had been handling pigs that had been imported
from the outbreak areas in Malaysia.
There have been three recognized outbreaks of Hendra virus
in Australia in 1994 and 1999. Three human cases, leading to two deaths,
were recorded in the 1994 and 1995 outbreaks. In 1995
a horse was infected, with associated human cases. The precise mode of
virus transmission to the three Australian patients is not fully understood.
All three individuals appear to have acquired their infection as a result
of close contact with horses which were ill and later died.
Source: WHO Weekly Epidemiological
Report, Vol 77, No 36, 2002.
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