The US efforts to find out the source of the country's first home-made case of mad cow disease in a Texas-born beef cow have failed so far as the investigation closed Tuesday.

The investigators traced 413 herd mates and offspring of the infected 12-year-old Brahma cross cow and found 147 might had been slaughtered for food, feed or other use while 21 could not be traced.

Government officials still believe the Texas cow, which tested positive in June, ate contaminated feed before the 1997 US ban on the use of animal remains in feed.

The mad cow disease is known to spread out only through eating infected animal brain and other nerve tissue.

The Texas cow is the second case of the brain-wasting animal disease confirmed in the United States. The first case was reported in Dec. 2003 in a cow imported from Canada.

Source: Xinhua