Each year, new strains of influenza virus cause millions to miss work or classes. From where or what do these and other “emerging viruses” arise?
Three processes contribute to the emergence of viral diseases:
- An existing virus can evolve and cause disease in individuals who had developed immunity to the ancestral virus.>>>For example, annual or biennial influenza episodes are caused by virus that have evolved into new genetic strains. The “new model” is different enough from last year’s virus that the human immune system must develop defenses as though it were responding to the virus for the first time. Between flu outbreaks, the evolving viruses are maintained within other animal hosts, especially ducks and other water fowl.
- An existing virus can spread from one host species to another. For example, monkeypox virus, which causes pocking of the host’s skin, spread from African monkeys to Asian monkeys in the 1950s, when monkeys from those two continents were transported together under crowded conditions to laboratories that were using the animals to develop and test polio vaccines. The World Health Organization documented the first cases of monkeypox in humans in forest villages of Zaire (in Central Africa) in the 1970s. It is likely that humans did not acquire the virus directly from monkeys, but rather from squirrels and other game animals that had harbored the virus without becoming ill.
- An existing virus can disseminate from a small population to become more widespread. >>>The hantavirus that made news in 1993 was not really a new virus, even to humans. Small rodents called deer mice are the reservoirs for the virus, and it is probable that occasional infections of humans in the southwestern U.S. have occurred for decades. Indeed, to the Navajo, mice are taboo, bearers of strange illness. In 1993, the population of deer mice exploded after a wet year increased their food supply. Humans became infected when they inhaled dust containing hantavirus deposited in the urine and feces of deer mice.
Thus, emerging viruses are generally not new, but are existing viruses that expand their “host territory” by evolving, by spreading to new host species or by disseminating to a larger proportion of the host species…
