The endangered California condor faces an "epidemic" of lead poisoning from scavenging carcasses contaminated by lead bullets despite years of costly conservation efforts, scientists said Monday.

The rare birds were reduced to a population of just 22 in 1982, and have since recovered to number about 400, with half of those still in captivity, said the study in the Proceedings of the National Academy of Sciences.

About five million dollars are spent per year on programs to boost the birds' population through captive breeding and release programs. But if those efforts were to cease, the birds would likely die off again, said the study.

Lead poisoning remains a critical danger, and efforts to limit the use of lead bullets by hunters in California in the past few years have not cut down on the number of chronic poisoning cases, said researchers.

"We will never have a self-sustaining wild condor population if we don't solve this problem," said first author Myra Finkelstein, a research toxicologist at the University of California Santa Cruz.

"Currently, California condors are tagged and monitored, trapped twice a year for blood tests, and when necessary treated for lead poisoning in veterinary hospitals, and they still die from lead poisoning on a regular basis."

Each year, nearly a third of condor blood samples showed serious lead exposure and 20 percent of free-flying condors in California are found to have blood lead levels that require treatment, according to the researchers.

Without chelation therapy to remove lead from the blood, birds can suffer paralysis, stiff joints and lose their ability to fly. At high levels, lead poisoning can kill.

The effects of chronic sublethal lead poisoning on the central nervous system are unknown and deserve further study, the authors said.

About half of all free-flying condors in California have required some treatment for lead poisoning since 1997.

Lead poisoning is believed to be one of several factors that led to the near extinction of the species decades ago.

However, efforts by conservationists to convince the US government to ban the use of lead ammunition in hunting land animals have met fierce resistance from gun rights groups, and lawsuits are ongoing.

Condors' main meals come from eating carcasses of large mammals like deer, or gut piles that are left behind by hunters. Lead bullets fragment upon impact, spreading pieces throughout the animal.

Previous research by co-author Donald Smith, professor of environmental toxicology at UC Santa Cruz, had shown that ammunition was the principal source of lead poisoning in condors.

The latest study includes five times as many cases and expands on those findings, using isotope ratios found in different sources of lead to show that condors are often poisoned by the type that comes from bullets.

The "majority of free-flying condors have a blood lead isotopic composition that is consistent with lead-based ammunition," said the study.

The state of California set a partial ban on the use of lead ammunition in condor habitat in July 2008, and that ban was later expanded. However researchers have been unable to find any corresponding drop in lead poisoning cases.

"Unfortunately, even if only a few people are still using lead ammunition, there will be enough contaminated carcasses to cause lead poisoning in a significant number of condors," Finkelstein said.

And while conservation efforts have succeeded in stabilizing the population, those measures have to be maintained in order to prevent the species from declining again, researchers said.

"Lead exposure and poisoning levels in condors continue to be epidemic," said co-author Dan Doak, a professor in Colorado University-Boulder's Environmental Studies Program.

"Despite the current efforts to help the species, the wild population will decline again toward extinction in a few decades unless these unsustainable and very expensive efforts continue in perpetuity."