BVD is mainly transmitted via persistently infected animals (PI animals). They are the most important source in the spreading of the virus (transmission source). A PI animal is created through the infection of an unborn bovine foetus by the mother between the 40th and 120th day of pregnancy. At this point of time, the immune system of the foetus is not completely developed, leading to lifelong viremia. The calf does not recognise the pathogen as such and will carry the virus throughout the rest of its life (PI animal).
Infections with the BVD virus results in the formation of antibodies (detectable in the serum, milk and bulk milk)
Direct detection of the virus (serological and molecular biological methods from blood, tissue, secretion and organ samples)
Detection from Ear Tissue Samples
An innovative and efficient programme to detect PI animals has been in place in Tyrol since Spring 2005. It was the first time that the testing of ear tissue samples from all newly born calves had been carried out on a province-wide basis. The system was improved in 2008 by the taking of tissue samples from ears during the statutory animal tagging process (tamper-proof sample taking!). Subsequently, the antigen can be detected in the laboratory using an ELISA test.
One of the most important objectives in BVD diagnostics is the timely identification of a PI animal. BVD viruses can be concealed by maternal antibodies (antibodies passed on by the mother to the calf via colostrum), making an early detection impossible. This so-called "diagnostic loophole" plays no role in the examination of tissue samples. Thus, it is possible to identify and cull PI animals within the first week. An additional problem is the temporally delayed increase in antibodies following a BVD infection. BVD antibodies can only be detected post-infection from about the 7th day onwards. This delay is even bigger when testing bulk milk.
The majority of infections with the BVD virus proceed without symptoms, though possible symptoms include diarrhoea, fever, coughing, mucosal erosions, loss of appetite, reduced milk production, fertility problems and weakened immune systems (decreased performance). Pregnant animals may miscarry or give birth to deformed and very weak calves.
A special variant of the disease, Mucosal Disease (=MD), can occur as a double infection of the individual animal. This happens when a PI animal is infected additionally with a cytopathogenic virus strain. MD is characterised by a serious progression of the disease and always ends fatally. Symptoms of MD include bloody diarrhoea, high fever, mucosal erosions and ulcers in muzzle and nose.
Preventing the birth of animals carrying the virus (PI animals)
Culling existing virus-carrying animals in the livestock to protect the herd from new infections
Monitoring activities with the help of antibody tests for serum and milk
Phylogenetic analyses of the virus strains could yield valuable results in terms of infection routes as part of epidemiologic issues. Pathogen analyses are extremely helpful when retracing infection sources.
AGES carried out a comprehensive study to collect data on BVDV and BDV strains circulating in Western Austria in 2005 and 2006 (Hornberg, A.; Revilla-Fernández, S.; Vogl, C.; Vilcek, St.; Matt, M.; Fink, M.; Köfer, J.; Schöpf, K. (2008): Genetic diversity of pestivirus isolates in cattle from Western Austria. Vet Microbiol. 2009 Mar 30;135(3-4):205-13)
Blood samples from Tyrol and Vorarlberg from 2005/2006 with unique Ag positive results in the Erns ELISA were frozen and archived on a continuous basis. These 353 BVDV antigen-positive blood samples were examined using microbiological methods at the AGES Institute for Veterinary Examinations in Mödling. The positive field samples were tested using Real-time (RT) – PCR and the BVDV strains were subsequently characterised using nucleotide sequencing, followed by phylogenetic analysis.
The study conducted at the veterinary institutes in Mödling and Innsbruck was based on the genetic heterogenity of the BVDV strains. This was determined using molecular biological sequence analysis. The classification of the sample types showed a very high genetic diversity in the BVD strains in Tyrol and Vorarlberg. A total of 8 different BVDV 1 subtypes were found, three of which were isolated for the first time in Austria. The subtype BVDV 1h was the most common (143 isolates), but also subtype 1f (79 isolates) was also widespread. In addition, two BVDV 2 strains were found and a Border Disease Virus (BDV) isolate could be classified in cattle for the first time. The BVDV 2 isolates were a 99% match with the German subtypes 104-98 from Lower Saxony, described by Tajima et al. in 2001. Only little geographic correlation was found in the incidence of the various strains at farm level.
BVD Red Deer
Cattle and red deer (Cervus elaphus) can get in contact with one another directly or indirectly on pastures, making the transmission of the virus between these two types of animals possible. Evidence of the transmission of Pestviruses between cattle and wild ruminants could be found in several European countries and the USA.
The AGES Institute for Veterinary Examinations in Innsbruck has collected data on the prevalence of BVDV in the Austrian red deer population, using virus tests from ear tissue samples as part of a study.
Ear tissue samples were taken in addition to brain samples from a total of 567 Austrian red deer living in the wild and from 133 animals from game reserves and holding estates, as part of an officially commissioned Chronic Waste Disease monitoring programme. These ear tissue samples were tested for BVDV using the HerdCheck BVDV Antigen ELISA and commercial Real time RT-PCR. No evidence of BVDV was found in any of the 700 tissue samples examined using the ELISA and Real- time RT-PCR.
Serologic studies on the spreading of BVDV in wild animals showed a prevalence of about 5 % in red deer in Germany and Italy. These results suggest that there are also infections in the Austrian red deer population. A study carried out in 2004 found serological evidence of a BVDV infection in one red deer in Carinthia.
Knowledge of the dynamics of complex infections in reservoirs outside the target population is key for effective control programmes. The diagnostic data suggest that persistent viremia in Austrian red deer is extremely rare to non-existent. This leads to the conclusion that the virus does not exist in a self-circulating form in the red deer population and that Austrian red deer are no virus reservoir for the BVD virus. The chances of a possible transmission of the virus from red deer to cattle can be regarded as very low.
Schöpf, Karl; Revilla-Fernández, Sandra; Steinrigl, Adolf; Fuchs, Reinhard; Sailer, Andreas; Weikel, Joachim; Schmoll, Friedrich (2016): Retrospective epidemiological evaluation of molecular and animal husbandry data within the Bovine Viral Diarrhoea (BVDV) control programme in Western Austria during 2009–2014. Berl Münch Tierärztl Wochenschr 129:196-201. doi 10.2376/0005-9366-129-15102, http://vetline.de/open-access/158/3216/