By Chitra Padmanabhan
Health officials in Canada are very busy these days. They are
placing chickens at fixed points all along their border with the United
States of America. That's an enormous distance of 2,500 km.

It's not a practical joke, nor have the Canadians gone mad. They are
using these chickens to see if the deadly West Nile virus is lurking
around. The virus infects birds, so they think that the chickens have a
good chance of catching the virus. Or the virus will catch the chickens.
The Canadians are worried because the West Nile virus is
transmitted to humans by mosquitoes. It killed seven people in New York
last year.
Countries around the world are realising something
important. That it may just be possible to stop certain kinds of people
from entering their land, but it is very difficult to try and stop
viruses that travel from one end of the earth to another. When they
travel to new places, they adjust very easily to those climates and
sometimes start destroying the local plant and animal life. These
biological polluters are called smart polluters.
These smart
polluters can be carried across borders of countries unknowingly. Just
as we humans are travelling across the globe more often than earlier,
these biological polluters have also started journeying much more. They
travel in the ballast of tankers. They slither into aircraft through
their wheels. They bore their way into objects that air travellers may
be carrying from one country to another.

There are some good examples of how these polluters work. The water
hyacinth of South America is choking lakes in China and Africa. Tree
snakes from Papua New Guinea are busy eating up bird varieties in the
country of Guam, which is quite far away. Tiger mosquitoes from Asia are
busy biting Americans.
Suddenly, a lot of things become
clearer. Like why the customs officials in many foreign countries
prevent you from bringing in a small plant, or a decorative item made of
wood that is in its natural form� They tell you that its in the rules.
They
have these rules because they know that these varieties of plants that
are special to specific places have the power to spread new diseases
among native plants and animals. They are called biological polluters
and they always create problems in places where they do not belong. They
could be special varieties of plants, bugs or even animals.