Table 11.

Risk factors for the introduction of BVDV and their need for control (Lindberg and Alenius, 1999)

Risk	Perceived need for control	Plausible ways through which BVDV is introduced into a non‐infected herd	Comments	Proposed control
Livestock trade	Imperative	Purchase of: A PI animal A dam carrying a PI calf A seronegative animal in early pregnancy, infected during trade Other animals which has attained transient infection during trade and transmit virus to newly pregnant non‐immune animals in the destination herd	Effect on disease spread by PIs in the market will be multiplied if contacts with seronegative animals in early pregnancy can occur Prevalence of dams carrying PIs likely to be higher than prevalence of PI animals. The latter has been estimated to 1 ± 2% in an endemic situation Houe (1995) Transiently infected animals are regarded as low impact transmitters Niskanen et al. (1996)	Test for virus and antibodies in herd of origin Stop viraemic animals and pregnant animals with high titres from being traded (control of 1, 2) Recommend quarantine with re‐test after 4 weeks (control of 3, 4) Create a framework for trade between non‐infected herds, based on herd samples to prove freedom from disease (certification system)
Exhibitions	Yes	Seronegative animals in early pregnancy becomes infected at the exhibition An animal which has attained a transient infection and succeeds in infecting newly pregnant non‐immune animals after returning home	PIs present at exhibitions will constitute a severe risk for farmers bringing seronegative animals in early pregnancy Transiently infected animals are regarded as low impact transmitters	Test for virus and antibodies in herd of origin, before exhibition After exhibition: Four weeks quarantine and retest if seronegative prior to exhibition. or Arrange exhibitions for animals from certified BVD‐free herds only Freedom from disease should be reinsured by recently performed herd level retests
Animal contacts on pasture or over fences	Yes	Seronegative animals in early pregnancy become infected on pasture Some other animal attains transient infection and subsequently transmits the infection to other, newly‐pregnant non‐immune animals in the herd	Not controlling for the release of PIs on common pastures will constitute a severe risk for farmers pasturing seronegative animals in early pregnancy PI carrying dams may spread disease if they abort or calve on pasture From a disease point of view, and in terms of herd incidence, over‐fence contacts will be less important than common pasturing	Intentional contacts: Same principle as for exhibitions Unintentional contacts: Follow‐up testing for antibodies (paired serum samples) As an alternative, the animals with which contact has occurred could be tested for antibodies and virus
Live vaccines	In the context of BVDV control, the use of live BVDV vaccines should be banned until safe	At least one susceptible animal in early pregnancy becomes infected due to usage of live vaccine contaminated with non‐cytopathic BVDV strains in the production process, or disease emerge as a result of recombinations between vaccines and field strains Ridpath and Bolin (1995) and Desport et al. (1996)	Risk of introducing strains new to the cattle population in question	No vaccination or use of inactivated vaccines only
Semen and embryos	Yes	At least one susceptible animal in early pregnancy becomes infected by other dams transiently infected due to AI with semen from PI bull or transiently infected bull, or persistent foetal infection develops in dam receiving AI with semen from PI bull or transiently infected bull	Risk of introducing strains new to the cattle population in question A case has been reported with a seropositive bull constantly shedding virus in semen in the absence of general persistent infection Voges et al. (1998) Although this phenomenon is probably of low frequency occurrence, it should be noted that such bulls could only be detected by testing semen	Test for antibody and virus on all bulls entering AI stations Regular testing for antibodies on seronegative bulls during study period. (Test of semen from antibody positive bulls) Embryo donors should come from herds free from BVDV and embryos should be protected from BVDV contamination during the transfer process
Visitors, including vets, AI technicians and herdsmen in the replacement system	Unlikely to be of major importance and impact, but preventative measures are appropriate in scheme rules	At least one susceptible animal in early pregnancy becomes infected due to contact with inadequately cleaned and/or disinfected boots, instruments and similar	Risk for transmission will depend upon: Time interval between visit in infected/non‐infected herd (Prevalence of infection in the area) Type of vehicle (faeces, clothes, instruments Gunn (1993), contaminated injectables) and amount of virus transmitted Houe (1999) Pregnancy and immune status of in‐contact animal(s) in the herd	Normal hygienic measures should be taken by professionals with ambulatory services to farmers as well as other visitors For veterinarians: use knowledge about BVDV status of herds to plan routes or to call for change of clothes
On‐farm collection of slaughter animals or brokered calves by professional transportation staff	Preventative measures are appropriate in scheme regulations	At least one susceptible animal in early pregnancy becomes infected due to contact with a persistently infected sheep/pig/goat/pig/deer/elk	No evidence exists that wild ungulates, swine or goats have transmitted the infection to cattle, even though interspecies transmission is possible Nettleton (1990). Strains proven to be involved in transmission from sheep to cattle have been of bovine origin Paton et al. (1995). BVD control was not compromised by sheep when implemented on the Shetland Islands Synge et al. (1999)	Check prevalence of Border disease in the area and judge whether problem exists If so, require sheep from herds with a previous history of Border disease and sheep in close contact with BVDV‐infected cattle herds to be tested free from BVD/BVDV before introduction into non‐infected herds. Exception can be made for sheep certified BVDV‐free farms
Vectors (ticks, mosquitoes, flies)	No, at least not in the temperate climate zones	At least one susceptible animal in early pregnancy becomes infected due to contact with virus‐carrying vector	Insects, such as biting flies have been shown to be capable of carrying BVDV under experimental conditions Tarry et al. (1991). Vector‐borne transmission has never been described under natural conditions