CHUR, Switzerland -- Swiss officials started culling 280 cows on Monday after a rare tropical blood illness hit a farm in the southeast of the Alpine country, bringing back memories of the battle against mad cow disease.

Josef Schmidt of the Federal Veterinary Office said the animals suffered from the blood illness anaplasmosis, whose symptoms include fever, jaundice and anemia and which can kill or severely weaken animals that then become carriers for life.

"Because the symptoms can appear after between two to six weeks after infection, there was no other solution than to kill the entire herd," Schmidt told Reuters. The decision was taken after 28 cows died on the same farm in just a few days.

"The animals are already very weak and would have given hardly any milk," Schmidt said, adding that the disease, which is spread via ticks, lice, ants and flies, poses no danger to humans.

It was not immediately known how the herd contracted the disease.

The first cows died last week and numbers quickly ran up during the weekend while scientists worked both in Chur, capital of the southeastern Graubunden mountain area, and in Zurich to find the reason for the deaths.

Only 16 percent of the originally 300-strong herd has shown no signs of the illness yet. The owner of the herd lost his entire livestock three years ago due to mad cow disease.

No signs of the illness have been found beyond the herd in Chur, but the Swiss are keen to prevent the disease from depressing the milk market in the way mad cow disease did with the meat market.

Since the first discovery of mad cow disease -- bovine spongiform encephalopathy (BSE) -- in Switzerland in 1990, there have been 420 cases altogether, with around 12 cases reported so far in 2002.