To the Editor: When a disease emerges, the trend is to assume that another important and spreading infection is about to devastate humans or animals. Some qualification of the term "emergence" is needed to put emerging diseases into a context for each target species. There may be a cause for alarm and further action or, alternatively, no real change except in knowledge. In Australia, for example, an old disease "emerged" in a new area, while in another, a disease new to the continent emerged. The two disease agents were Ross River virus (which causes fever and polyarthritis in humans) and bluetongue virus (which often causes fatal disease in sheep). Both causative viruses are insect-borne.

Ross River virus is probably of very ancient lineage as an infection transmitted between marsupials and indigenous mosquitoes and was on the Australian continent long before humans first entered (some tens of thousands of years ago). In 1975, the infection was not known to occur in Tasmania, the state separated from the Australian mainland by a wide stretch of sea. In that year, my group detected a clear-cut seroconversion to Ross River virus in sentinel cows in northern Tasmania (1). Cooperative investigations found antibody-positive sera first in marsupials and then in persons who had never left Tasmania. The existing clinical condition of polyarthritis was linked to Ross River virus only after the causative virus was recognized indirectly (2). The marsupial populations of mainland Australia and Tasmania were continuous until the seas rose at the end of the last ice age. Ross River disease had emerged in Tasmania, but only from obscurity.

Bluetongue viruses occur widely in southern and eastern Asia (3). This general picture has been established only since the discovery of bluetongue virus in Australia. Overt disease occurs in sheep on the fringes of the endemic-disease region and in susceptible sheep imported into various countries within the region (3). In contrast, at least eight members of the bluetongue group of viruses have entered Australia and some of these the Pacific countries; they must have arrived after ruminant populations were introduced in New Guinea, Australia, and the Pacific islands. They spread through the ruminant-dependent Culicoides species vectors, also introduced (4). Emergence of bluetongue in Australia has so far meant one sheep dead of bluetongue disease in 1989 (in a population of 100 million sheep) since the discovery of the presence of bluetongue virus in Australia in 1977 (4). However, the potential for a major epidemic remains and the discovery caused major trade difficulties. In both these situations the disease was emergent but its potential was very different.

Since the latter part of 1994, two newly recognized zoonotic viruses have been reported from Australia: paramyxovirus (equine morbillivirus), which caused deaths in horses and humans (5), and lyssavirus (closely related to rabies virus), which has also caused a human death in Australia (6). In both instances, strong evidence indicates that bats are the maintenance hosts. Bats are probably the oldest form of placental mammal in Australia, with fossil evidence from the Middle Miocene era, circa 15 million years ago (7). Some species of bats migrate between various countries of southern Asia and the Pacific; probably they migrated more in the ice ages when sea distances were shorter. This past continuity of the bat populations and the inadequate study of the Microchiroptera in Asia for rabies viruses led me to forecast in 1989 that rabies (or a rabieslike virus) was established in bats in Australia (8). These recently recognized disease agents are both emergent but have not become important for humans or animals.

As a hypothetical example, Ebola virus has been shown by Swanepoel and others to multiply well in bats of three species (9). Two of the species used experimentally belong to the genus Tadarida, which is well represented in Africa, Asia, and Australia (10). Evidence that bats are the reservoir hosts of Ebola virus (Reston), which is known to infect monkeys in the Philippines should be sought in the Philippines and for its silent presence in bats in Australia. If such evidence were found, Ebola would be labeled as emerging in Australia and other countries between Australia and the Philippines, although no cases of disease might ever occur east of the line where monkeys are indigenous.

The scientist who looks for evidence of a disease agent in a country or region and the regulator who has to deal with the public health or economic consequences of a new or newly recognized disease have conflicting interests. The national and international reaction to the discovery of many agents in countries where they had not been found does not take into account the measure of risk for the disease. The term "emergent" disease needs some qualifiers to diminish fear and overreaction.

One way would be to rate the risk for the disease on a scale of 1 to 5; another would be to provide a context that notes the capacity of the agent to spread and cause illness and death.

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