Many viral diseases are endemic in cattle populations worldwide.The ability of many viruses to cross the placenta and cause abortions and fetal malformations is well understood.There is also significant evidence that viral infections have additional actions in dairy cows,which are reflected in reduced conception rates.These effects are,however,highly dependent on the time at which an individual animal first contracts the disease and are less easy to quantify.This paper reviews the evidence relating to five viruses that can affect fertility,together with their potential mechanisms of action.Acute infection with non-cytopathic bovine viral diarrhea virus (BVDV) in mid-gestation increases abortion rates or causes the birth of persistently infected calves.BVDV infections closer to the time of breeding can have direct effects on the ovaries and uterine endometrium,which cause estrous cycle irregularities and early embryo mortality.Fertility may also be reduced by BVDV-induced immunosuppression,which increases the susceptibility to bacterial infections.Bovine herpesvirus (BHV)-1 is most common in pre-pubertal heifers,and can slow their growth,delay breeding,and increase the age at first calving.Previously infected animals subsequently show reduced fertility.Although this may be associated with lung damage,ovarian lesions have also been reported.Both BHV-1 and BHV-4 remain latent in the host following initial infection and may be reactivated later by stress,for example associated with calving and early lactation.While BHV-4 infection alone may not reduce fertility,it appears to act as a co-factor with established bacterial pathogens such as Escherichia coli and Trueperella pyogenes to promote the development of endometritis and delay uterine repair mechanisms after calving.Both Schmallenberg virus (SBV) and bluetongue virus (BTV) are transmitted by insect vectors and lead to increased abortion rates and congenital malformations.BTV-8 also impairs the development of hatched blastocysts;furthermore,infection around the time of breeding with either virus appears to reduce conception rates.Although the reductions in conception rates are often difficult to quantify,they are nevertheless sufficient to cause economic losses,which help to justify the benefits of vaccination and eradication schemes.