Process and Testing
Once a cow tests positive for bovine spongiform
encephalopathy (BSE), veterinarians at the U.S.
Department of Agriculture’s (USDA) Animal and Plant
Health Inspection Service (APHIS) immediately begin
an epidemiologic investigation. Its purpose is to
identify and locate cattle that may have been
exposed to the same source of BSE prions,
presumably in contaminated feed, as was the
BSE-positive or index cow. It is unlikely that these
animals have BSE, but by association, they are now
part of the targeted population for testing.
The epidemiologic process requires we deter-
mine the origin of the index cow and locate any prog-
eny or birth cohorts to verify there is not more than
one infected animal from the same herd.
Experience in countries that have previously
detected BSE indicates that it is uncommon to have
more than one infected animal in the same herd.
However, BSE epidemiology efforts focus on birth
cohorts—all cattle born on the positive animal’s birth
premises within 1 year, before or after, the date of
birth of the BSE-positive animal—in order to deter-
mine a common feed source as the possible mode of
infectivity. (Note: If the precise date of birth of the
index animal is unclear, a potential age range is
used, with 1 year added to each end of that range.)
In most cases, some of the birth-cohort animals
will have moved off the birth premises; others will
have gone to slaughter or died. Investigators can
sometimes pinpoint the current locations of these ani-
mals or where they have been in the past by thor-
oughly examining herd records.
Cattle of Interest
When birth cohorts cannot be definitively identi-
fied, a herd inventory and analysis of herd records
are used to identify a group of cattle that includes all
birth cohorts as well as any other cattle that cannot
be ruled out as birth cohorts. All of these animals—
birth cohorts and any additional cattle—will be
defined as “cattle of interest.” Any prog