Oct 16, 2003 (CIDRAP News) The recent finding of bovine spongiform encephalopathy (BSE) in a 23-month-old bull in Japan suggests that Japan may have more cases of the disease than previously suspected, according to a University of Minnesota expert on the disease.
"For me the Japanese case suggests that in fact they had a much larger epidemic than most people realize, because this animal had a massive exposure to develop the disease this early," said Will Hueston, DVM, PhD, director of the university's Center for Animal Health and Food Safety in St. Paul.
The occurrence of BSE, or mad cow disease, in an animal less than 24 months old is rare and implies that the animal was exposed to a large dose of infective material, Hueston explained. "With this disease, the larger the exposure, the shorter the incubation time," he said.
Japanese officials announced last week that tissue from the young bull, which was slaughtered Sep 29, had tested positive for BSE, or mad cow disease. It was Japan's eighth case of BSE. The case was detected in Japan's regular BSE surveillance, which involves testing of all cattle after slaughter. Eating meat from BSE-infected cattle is believed to be the cause of variant Creutzfeldt-Jakob disease in humans.
The bull had not shown any signs of the disease, said Hueston, who served on a panel of four experts that evaluated Canada's response to its own single case of BSE this year. "We don't know exactly when it would've shown clinical signs," he said. "Most likely it would've been within the next 6 months or so."
In the roughly 180,000 worldwide cases of BSE documented so far, only about 1 in every 15,000 has been in an animal younger than 24 months, said Hueston. Most of those cases have been in the United Kingdom. There, the Department for Environment, Food and Rural Affairs reports that BSE has been identified in only two animals younger than 24 months.
BSE is believed to spread through livestock feed containing protein derived from infected animals. The United States and Canada in 1997 banned the feeding of meat-and-bone meal from cattle and other ruminant (cud-chewing) animals back to ruminants. But Japan didn't ban the use of such feed until its first case of BSE showed up in 2001, according to Hueston.
Hueston said Japan's BSE cases probably stem from cattle and feed imports back in the 1980s. "Basically they've acknowledged that they imported cattle from Great Britain during the BSE time period in the 1980s, and they imported feed up through 1990 from Italy," he said. Some of that feed probably contained cattle-derived protein that was infective, he said.
BSE was probably a "silent" presence in Japan in the 1990s, with cases escaping discovery and the BSE agent being recycled in feed, Hueston said. Even when ruminant protein officially wasn't being fed to ruminants, there may have been cross-contamination at feed mills, he suggested. "It's very possible that the bull had contaminated feed," he said. "Essentially that's what they're suggesting."
Japan's program of testing all cattlelaunched after BSE first appeared in Japanis the most aggressive BSE surveillance effort in the world, Hueston noted. He said the fact that only a few cases have turned up in Japan since the testing began probably has to do with several factors: the BSE incubation period of about 3 to 5 years; the effect of exposure dose, where a smaller dose means a longer incubation time; and the limitations of the screening test for BSE. An animal will not test positive until the brain is affected, which is generally not until the disease has been progressing for 2 to 3 years, he explained.
Hueston said he expects that the Japanese will find more cases, "but these may be spread out over time." He added, "I need to find out more about how they're responding to BSE, whether they are sacrificing the whole herd when an individual cow is found positive or whether they're simply taking the individual case. That may affect the course of the number of cases they see."
He said the core underlying question is the extent of exposure: "what range of doses to how many cows over what period of time."
While the Japanese are testing all cattle carcasses for BSE, the American BSE surveillance program focuses on cattle that die on the farm or show signs of neurologic disease, Hueston said. The latest case in Japan, along with Canada's single BSE case last May and other recent developments, will probably create pressure to increase US surveillance, he said.
Japanese officials, in reporting the BSE case last week, said tests suggested that the form of prion protein (BSE agent) found in the young bull was atypical. Investigators plan to inoculate the agent into laboratory animals to test its transmissibility and other characteristics, according to the report from Dr. Masako Kurimoto of the Ministry of Agriculture, Forestry, and Fisheries.
Hueston said it's unclear what the finding of an atypical prion protein means. "Some pathologists we've talked to have said they think the findings could just describe an early case of BSE," he said. He speculated that the protein pattern from a brain tissue sample from an early, asymptomatic case could differ from what would be seen in a full clinical case.