A fly-borne flu has invaded West Marin stables, causing horses to cough in Woodacre, Nicasio, and Bolinas.

Last month, Marin's annual pony club show in Bolinas had to be postponed a week because of the bug, which has buzzed around the county since the summer.

"The flu seems to be more of an annoyance than a life-or-death kind of thing," said Grace Tolson, owner of Dickson Ranch in Woodacre, where several horses have been stricken.

The flu can trigger serious illnesses in horses that haven't been vaccinated regularly, and Tolson worries that horse owners may not recognize the flu's subtle symptoms.

Symptoms include swollen back legs, a sudden high fever followed by a deep, rasping cough and sometimes colic, said veterinarian Bruce Daniels of Point Reyes Station.

Daniels said he's treated about 100 West Marin horses that had mild cases of the flu. Much of the malaise could have been avoided, he said, if the horses had been vaccinated every three months.

Tolson has decided to outrace the flies carrying the flu around West Marin by calling all of her equestrian friends, warning them to get their horses vaccinated.

"Many local stables didn't even know about the flu," she said, adding that West Marin "seems to be a nursery for it."