The cow confirmed earlier this month as being Japan's eighth with mad cow disease did not show any mutations in its prion genes, according to research findings.

The results of the study suggest that external factors may have caused the cow's infection.

"It is quite unlikely that genes were the reason" for the cow's infection with the disease, formally known as bovine spongiform encephalopathy, said Yoshio Yamakawa of the National Institute of Infectious Diseases.

The Holstein -- infected with an atypical form of mad cow disease -- had a different prion structure that the seven earlier cows.

The 23-month-old cow, from Ibaraki Prefecture, was unusually young to be diagnosed with the disease, which in the past has turned up in cows 24 months or older.

After the Health, Labor and Welfare Ministry confirmed the case on Oct. 6, the Tokyo-based institute analyzed the cow's brain and found no abnormality in the prion genes.

The cow was taken to a slaughterhouse last month.

Mad cow is believed to be caused by the consumption of meat-and-bone meal contaminated with prions -- protein particles lacking nucleic acid that have been linked to nervous system illnesses such as Creutzfeldt-Jakob disease, a rare, brain-wasting illness in humans.

Experts say CJD can be contracted in several ways, such as through transmission from cattle to humans through infected food, by surgical procedures, by an inherited abnormal gene, or sporadically, with no apparent cause.

Although the institute's findings show the latest infection was probably not caused by genetic factors, Yamakawa said it is possible for a sporadic form of CJD to break out even if the genes are normal.

Any further analysis requires the results of animal testing, he added.