Colorado health authorities are doling out rabies shot regimens one at a time and denying preventive vaccines to veterinarians and wildlife workers because of a severe national vaccine shortage.

The strict limitations come as a new strain of rabies found in skunks has emerged on the state's eastern plains.

Rabies is fatal once symptoms show up, but it is avoidable if treated with a five-shot regimen known as rabies post-exposure prophylaxis, or PEP. That leads to situations where people get treated with a series of shots, even if it's possible but unlikely that they have been exposed to rabies.

But manufacturing problems with the two producers of rabies PEP have taken away that luxury. No one at reasonable risk has been denied the vaccine, but the lack of supply has forced health officials and animal control officers to heavily scrutinize each case to determine such risk.

The El Paso County Department of Health and Environment has been working with local hospitals to educate them on when rabies shots can be avoided, and they've been evaluating patient records to see if the use of them could be tightened, said Dr. Bernadette Albanese, the health department's medical director.

Under normal circumstances, a dog bite might lead to a shot regimen as a precaution, even though the chances of getting rabies from a domestic animal are statistically low in Colorado, Albanese said. Now, animal control officers are sometimes being asked to quarantine dogs for 10 days to rule out such a possibility and avoid unnecessary use of PEP.

Health care providers such as doctor's offices and hospitals can no longer order the rabies treatments in advance. Each case requires a call to the Colorado Department of Public Health and Environment, which issues a code for the doctor to give to the pharmaceutical company.

And people in high-risk jobs such as veterinary medicine can no longer get vaccinated for rabies in advance, as they have in the past.

Human rabies exposures in Colorado are mostly from bats, and the virus is not as much of a threat here as many other states. The last human to contract rabies in Colorado was in 1931.

Still, a new strain of rabies transmitted by skunks, which was detected last year in eastern Colorado, has health officials concerned. There have been 15 positive skunks in six counties this year. In July, a cat and a raccoon contracted rabies from a skunk, both of which are indicators of an increased risk of its spread.

John Pape, an epidemiologist with the state health department, said the threat of rabies could escalate dramatically if skunk rabies reaches the populous Front Range.

The state has received a few requests a week for rabies treatments, which consist of five shots administered over a month's time.

At Memorial Health System, doctors have been using rabies vaccines that were already on the shelves, said John Ives, director of pharmacy.

The vaccine shortage began in June 2007 after one of the two pharmaceutical companies, Sanofi Pasteur, halted production at a plant in France because of compliance issues with Food and Drug Administration standards and France's laws, according to the Centers for Disease Control and Prevention.

The other company, Novartis, was unable to keep up with demand after that occurred.