Diseases From Animals: A Primer
A is for animals, Z is for zoonoses.
Equine Encephalitis and West Nile Viruses
The equine encephalitis and West Nile viruses are transmitted
from wild birds to humans -- and to horses -- by mosquitoes. We read a lot
about West Nile virus, because of its recent arrival in the U.S. and its rapid
spread across the nation. It can cause a very dangerous infection of the brain
and spinal cord. So can equine encephalitis, which has long been firmly rooted
in the U.S. In fact, eastern equine encephalitis is considered a much more
serious disease. About 30% of people who get it die, and another 30% have
lasting nerve damage.
Most years there are very few cases of equine encephalitis. But
some years are much worse than others -- and there's no way to predict in
advance when there will be an outbreak. It's too soon to know whether West Nile
virus will be the same every year or come in cycles in the U.S.
It's hard to think of a more horrible disease than Ebola
hemorrhagic fever. Ebola virus is spread by contact with the blood or body
fluids of an infected person. Does it come from animals? Probably. Monkeys and
great apes get it -- and people can get it from them when they butcher them for
food. But monkeys die of Ebola, so they can't be the ultimate host. Most
researchers think there's an animal out there harboring the virus. They just
haven't found it yet.
That SARS emerged in China's Guangdong province seems sure.
What's not sure is where it came from. SARS is a coronavirus, but it's not like
any other member of the coronavirus family. Some researchers think it may have
come from an endangered animal known as a masked palm civet -- like most exotic
animals, a culinary delicacy in parts of China. Others find the evidence weak.
Whether SARS evolved in animals or humans remains a matter of debate.
One disease that's definitely evolving in animals is influenza.
And one place it's evolving is none other than Guangdong, China, where animals
are kept in close proximity to one another. Flu viruses tend to arise in ducks
and geese. They spread to chickens and to pigs. Pigs can also get infected with
human flu viruses, so they make a good mixing pot for new flu. When an animal
or a person is infected with two different flu viruses, the viruses like to
swap parts. Voilà! A new virus emerges.
Infectious disease specialists don't wonder whether there will
be a new worldwide flu epidemic. They only wonder when it will happen. There
have been two recent close calls.
In 1997, lethal bird flu arose in the poultry markets of Hong
Kong. People got infected and died, but the slaughter of millions of chickens
stopped the virus before it learned how to spread from person to person. In
2001 and 2002, similarly bird flu viruses evolved in Hong Kong chickens.
Fortunately, they didn't spread to humans.