Infections with bovine virus diarrhoea virus (BVDV) are widespread througho
ut the world. Although the prevalence of infection varies among surveys, th
e infection tends to be endemic in many populations, reaching a maximum lev
el of 1-2% of the cattle being persistently infected (PI) and 60-85% of the
cattle being antibody positive. Persistently infected cattle are the main
source for transmission of the virus. However, acutely infected cattle as w
ell as other ruminants, either acutely or persistently infected, may transm
it the virus. Transmission is most efficient by direct contact. However, as
infections have been observed in closed, non-pasturing herds, other transm
ission routes seem likely to have some practical importance. Differences in
BVDV prevalence among regions or introduction of virus in herds previously
free of BVDV are often associated with particular epidemiological determin
ants such as cattle population density, animal trade and pasturing practice
s. However, on a few occasions there have been no obvious explanations for
infection of individual herds. Estimates of economic losses due to BVDV inf
ection vary depending on the immune status of the population and the pathog
enicity of the infecting virus strains. Introduction of the infection into
a totally susceptible population invariably causes extensive losses until a
state of equilibrium is reached. Infection with highly virulent BVDV strai
ns causing severe clinical signs and death after acute infection gives rise
to substantial economical losses. At an estimated annual incidence of acut
e infections of 34%, the total annual losses were estimated as US$ 20 milli
on per million calvings when modeling the losses due to a low-virulent BVDV
strain. At the same incidence of infection, the losses due to a high-virul
ent BVDV strain were estimated as US$ 57 million per million calvings. Low-
virulent BVDV infections caused maximum losses at an incidence of 45%, wher
eas high-virulent BVDV infections caused maximum losses at an incidence of
65%. Thus, cost-benefit analyses of control programs are highly dependent o
n the risks of new infections under different circumstances and on the stra
ins of the virus involved. (C) 1999 Elsevier Science B.V. All rights reserv
ed.