From rats to bats and beyond

Nipah virus, the disease which was 'discovered' in 1999 in Malaysia after an encephalitis-like infection spread through pigs and humans, is now known to be carried by certain species of fruit bats (see Bats take the blame ). Although the epidemic was relatively short-lived, the economic and social effects will continue to be felt for some time and scientists are eager to learn from the experiences and expertise gained from the international collaboration which helped to effectively control the disease in such a short time.

Fruit bats, also known as flying foxes, are the natural host of Nipah virus as well as Hendra virus, a similar disease recognized in 1994 after it killed two people and a number of racehorses in Australia, and Menangle virus, which affected pigs in New South Wales in 1997. All three viruses are members of the Paramyxoviridae virus family, which includes measles and mumps in humans and transboundary diseases, such as rinderpest, peste des petits ruminants and Newcastle Disease in livestock. It seems unlikely that Hendra and Nipah viruses are easily transmitted to humans. Nevertheless the ability of these viruses to infect a number of hosts and to result in a disease which can cause significant mortality in people and disrupt livestock commerce and trade, has made these emerging viral infections an increasing concern. The Government of Malaysia was effective in its rapid response to the Nipah outbreak and the culling of pigs prevented the further spread of the epidemic. However, of the 257 people infected with the disease, more than 100 died and over one million pigs were slaughtered. Millions of dollars in lost revenue will never be regained and pig farming in Malaysia is now only allowed in 'identified pig farming areas'.

An emerging pattern

According to CSIRO, fifteen new paramyxoviruses have been discovered during the last forty years in animal hosts ranging from rats, bats, dolphins, seals, snakes, horses to humans. Four of the fifteen have recently emerged in Australia, including Mossman virus which was first detected in Queensland in 1970 in a rat and was later also found in wild mice. CSIRO's Australian Animal Health Laboratory (AAHL) is now the only institute in the world holding a collection of all major newly discovered paramyxoviruses. Dr Wang of AAHL feels that it is almost inevitable that other viruses will be discovered; fruit bats are found in a wide range of countries across south-east Asia and they are known to travel over considerable distances.

It is not yet known how paramyxoviruses are transmitted or 'jump' from bats to other animals. The mode of transmission from animal to animal, and from animal to human is also uncertain, but appears to result from close contact with contaminated tissue or body fluids. With Nipah virus, for instance, there seems to be no evidence of human infection among bat keepers and in the Malaysian epidemic, it appeared that pigs were the only effective carrier, although other sources such as infected roaming dogs and cats, could not be ruled out. In August 2002, an international research programme involving virologists, ecologists, medical researchers, modellers and wildlife disease biologists from the US, UK, Malaysia and Australia was awarded a four-year grant to further investigate the transmission of paramyxoviruses. In addition, the team hope to develop a mathematical model which will help determine the factors that could lead to an outbreak and to predict areas where novel and existing viruses are likely to occur.

It is hoped that another international collaborative effort between AAHL and the Canadian Center for Foreign Animal Health Disease will also lead to the development of a rapid diagnostic capability for Nipah virus and the investigation of various methods of vaccine production. There are currently no drug therapies proven to be effective in treating Nipah infection, although there is some evidence that early treatment with the antiviral drug, ribavirin, can reduce the severity of the disease.

Much has been gleaned from the Malaysian Nipah epidemic but there is still much to learn. Models, diagnostic tests and vaccines may, as yet, not be available but advance planning* for emergency management of disease outbreaks is a pre-requisite if future outbreaks of existing or emerging viral diseases are to be effectively controlled. This requires legislative, management and operational procedures to be in place, and countries not just in south-east Asia but also in South America, where similar viruses have been discovered in bats, have to ask themselves whether they are sufficiently prepared. If not, another virus may well emerge bearing the name of the town or suburb where the next epidemic occurs.

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