The case of mad cow disease that surfaced in California this week reignited a long-running debate about what has been described as a weak link in the U.S. beef supply: the lack of a mandatory system to trace the path a cow takes from farm to fork.

The infected cow’s path ended at a California dairy farm, where the animal died without posing any safety risks to the nation’s food supply, regulators said. The government has not revealed much else, except that the cow had a rare form of the disease generally not linked to contaminated feed.

Feed was the source of the first U.S. case of mad cow disease, in 2003, and the mad cow epidemic that devastated English cattle herds in the 1990s; steps have since been taken to ban sources of infection in cattle food.

The United States is one of the few beef-producing countries that does not have a mandatory animal identification system that enables it to trace a cow from birth through the slaughterhouse and beyond, though a proposal has been in the works for years.

“If we discover that this case was part of a larger outbreak, we might not be able to find all the animals in that cohort that were exposed to the same feed,” said Sarah Klein of the Center for Science in the Public Interest. “If the feed is not the problem, then this is yet another warning shot. The question becomes: Do you wait for the big outbreak before you can justify the need for a system to track these animals?”

The United States has had three cattle cases before, in December 2003, June 2005 and more recently in March 2006, when a cow on an Alabama farm tested positive for the disease. In the Alabama case, the government could not identify the cow’s herd of origin or determine its exact age.

The fourth incident, in California, “clearly highlights the need for a comprehensive national animal identification system,” Rep. Rosa DeLauro (D-Conn.) said in a statement Wednesday. “We were lucky to identify this case, but we must invest in a strong national identification system that has the potential to improve animal traceability.”

A study by Kansas State University last year found that six of the eight largest exporters of beef have adopted a mandatory national cattle identification system — but not the United States. Even the tiny country of Botswana tracks its cattle from 3 months of age with a unique number lodged in a microchip, the World Organization for Animal Health reported. In New Zealand, consumers can learn everything about the animal a piece of steak came from through a barcode on its packaging, Klein said.

Several attempts to come up with a national identification system for livestock in this country have failed due to strong resistance by segments of the cattle industry, which viewed it as a costly government intrusion, the Congressional Research Service reported in 2010.

In its most recent proposal, the Agriculture Department would allow each state to come up with its own identification system for livestock. If the cattle cross state lines, however, the USDA would then require tagging of the animal.

The plan, which affects all livestock, should be finalized “very, very soon,” Agriculture Secretary Tom Vilsack told Bloomberg Television on Wednesday.

Bill Bullard, who heads a group that represents small cattle farmers and ranchers, said many producers already voluntarily identify their cattle using ear tags. It’s profitable for them to do so because there is consumer demand for it.

“But if the government were to mandate it, the profits producers are now receiving for voluntarily identification of their animals would evaporate,” said Bullard, chief executive of R-Calf. “We’re against government mandating a cost on our industry.”

Dairy cows like the one in California make up just a small segment of the cattle population, but they are easier to track, at least as long as they are producing milk. Dairy farmers generally keep good records on the genetics of their animals to keep track of which ones produce good-quality milk and lots of it.

Nearly all dairy cattle have ear tags. Some carry the year of the animal’s birth, others only an identification number that allows that and other information to be traced from a farm’s records. In animals whose birth records are unknown, age can be estimated from teeth.

An exact age is ultimately available “in a high percentage of dairy animals,” said Linda Detwiler, a veterinarian on the faculty of Mississippi State University’s College of Veterinary Medicine, who previously worked at the USDA. “We should be able to tell the year of birth of this animal.”

The age is crucial in this case because it will shed light on what feed the cow might have been exposed to in its first year, which is when animals with mad cow disease contract the infection, researchers believe.

A 15-year-old dairy cow is rare, experts said, so it is unlikely this animal was born before 1997. That is the year that flesh and bones from mammals were banned from use as supplements in feed going to cattle, sheep and goats. However, all three of the United States’ earlier cases of mad cow disease, or bovine spongiform encephalopathy (BSE), were in animals born before 1997. They almost certainly got the infection from contaminated feed.

The feed ban was strengthened in 2008 to prohibit the use of certain cattle tissues in all animal feed, including that going to chickens, pigs and pets. No cases of BSE in either the United States or Canada have been found in animals born after that date.

However, at least eight of Canada’s 18 BSE cases were in animals born between the original feed ban and its strengthening — suggesting that some infected material might somehow have still been getting into cattle during that period. Experts believe that may have occurred when the animals were given chicken feed containing flesh from rendered cows, as was legal at the time.

If the newly identified American cow was born after the more stringent feed ban, it would be evidence that infected material was still occasionally getting through — and would be an especially worrisome finding.

It is possible, however, that the latest BSE case didn’t arise from feed at all. That’s because it is “atypical BSE,” a slightly different form from the “classical BSE” that infected millions of cattle in Britain in the 1980s and 1990s.

Atypical BSE is seen mostly in older animals, some of which have no symptoms. The number of cases in Europe is consistent from year to year — about 2 cases per one million animals 8 years or older at the time of testing. Those two facts — old age and a constant rate of disease — have led some experts to believe that atypical BSE may be arising spontaneously. But there is no proof that’s what’s happening.