This is the VOA Special English Agriculture Report.

Exotic Newcastle disease is a bird virus found in many parts of the world. Many kinds of birds carry it. But it especially affects chickens and a few others.

It spreads very fast and there is no treatment. Many birds die without appearing sick. The infection does not present a serious health threat to humans. But economic losses can be huge as birds are destroyed and trade is restricted to contain outbreaks.

Exotic Newcastle disease, also known as END, is the most severe form of Newcastle disease.

Experts say the easiest way to prevent the virus is to import birds from flocks that are disease-free. Vaccines are also used, although experts say the virus may sometimes cause deaths even in vaccinated flocks.

In the United States, the Agriculture Department says poultry birds are rarely vaccinated against the virus unless an outbreak happens. The most recent outbreak began in 2002 in California. State officials said it cost more than one hundred sixty million dollars to fight. California was declared disease-free the next year, after the killing of more than three million birds.

The Global Invasive Species Database says signs of the disease may appear from two to fifteen days after a bird is infected. An infected hen lays fewer or no eggs, or eggs with thin shells. A sick bird may develop breathing and intestinal problems and twist its head and neck. It may run around in circles or not move at all.

Exotic Newcastle disease spreads fastest among birds kept close together. The virus is spread through bird droppings and fluid from the nose, mouth and eyes.

To control outbreaks, experts advise quick destruction of infected flocks. They also advise limiting entry to farms and disinfecting vehicles as they come and go.

People can also transport the virus on their shoes and clothes. The virus can survive several weeks in a warm, moist environment. And there seems to be no limit if it is frozen. But ultraviolet rays in sunlight can kill it.

To reduce infection risks, the Organization for World Animal Health warns against keeping any pet birds on a farm. It even advises against hiring pet bird owners as farm workers. The Agriculture Department says Amazon parrots, for example, can spread the virus for more than a year but not get sick.

And that's the VOA Special English Agriculture Report, written by Jerilyn Watson. I'm Steve Ember.