THE PROPOSAL OF A PROGRAM FOR CONTROL, SUPPRESSION AND ERADICATION OF BOVINE VIRAL DIARRHEA VIRUS INFECTION IN CATTLE HERDS IN THE REPUBLIC OF SERBIA

Bovine Viral Diarrhoea (BVD) is a viral infection of cattle caused by Bovine Viral Diarrhoea Virus (BVDV). Th e most important source of infection is persistently infected and sick cattle, but also other susceptible species. BVDV infection, except for cattle, occurs in sheep, pigs, goats and wild ruminants. Th ese animals can be reservoirs of the virus, and thus the source of infection. Th e greatest economic losses caused by BVDV infection in cattle farming are the direct consequences of transplacental infection, as a result of foetal deaths, congenital malformations, neonatal and postnatal mortality, including mucosal diseases and slow growth and poor performance results of the surviving animals, as well as acute infections of respiratory and alimentary infections. Economic losses due to BVDV infection can be seen in a cattle herd even several years aft er the infection. Moreover, BVDV infection is considered as the third most important disease to cattle farming industry, immediately aft er rinderpest and the foot-and-mouth disease. Th e primary task of BVDV infection control is to prevent prenatal infection. Th is procedure includes the determination and elimination of persistently infected (PI) animals from the herd. Aft er the removal of these animals, great attention must be paid to introducing new animals into the herd and preventing the occurrence of transplacental infection. On the basis of existing data and real suppositions about the distribution of BVDV infection in


INTRODUCTION
Bovine viral diarrhoea (BVD) is virus infection of cattle, which still remains a mystery for both cattle owners and clinician veterinarians and scientists even if it was fi rst described decades ago by Olafson et al., in 1946. Th e causal agent is one of the smallest RNA viruses (Bovine Viral Diarrhoea Virus -BVDV) from the genus Pestivirus, family Flaviviridae. Th e most common source of infection are persistently infected and sick cattle, but also some other susceptible species such as sheep, swine, goat and a range of wild ruminants. Th ese animal species could be also potential virus reservoirs in nature (Nettleton and Entrican, 1995).
Th e pathogenesis of BVDV in cattle is highly unique and manifests distinct features that are not observed in other infections of viral aetiology (Houe, 1999). Intrauterine infection of the foetus before 125 gestation day, because of the immaturity of foetal immune system, results in recognizing viral antigens as the own antigens, which in turn induces immune tolerance and persistent infection aft er they are born (Bronwlie et al., 1984;Bronwlie, 1991). Such persistently infected (PI) animals are usually born as clinically healthy, yet harbouring the virus that continues to replicate in their bodies and shedding it into the environment for life. In PI animals (mostly aged 6-24 months) mutation of the existing persistent noncytopathic (ncp) biotype of the virus to the cytopathic (cp) biotype occurs and consequently produces clinical form of mucosal disease (MD) with rapid fatal outcome. In view of that, the consequences of foetal infection including persistently infected (PI) animals and subsequent development of fatal mucosal disease are unique feature of BVDV infection (Bronwlie, 1991;Houe, 1999). Existence of noncytopathic and cytopathic viral strains and their complex relation toward susceptible animals, especially pregnant cows and their foetuses, as well as potential transmission to other susceptible species such as domestic and feral ruminants and pigs strongly emphasize the importance of better understanding of this viral infection of cattle.
BVD infection of cattle is characterized by a wide spectrum of clinical manifestations including transient fever, viral diarrhoea of cattle, respiratory disturbance, lower conception rate in breeding females, abortions, foetal mummifi cation, congenital malformations, immune tolerance and persistent infection progressing to MD. Moreover, subclinical infection associated with leucopoenia and immunosuppression can occur (Baker, 1987).
BVD virus infection, both acute and subacute, is commonly associated with rather silent clinical manifestation or has a subclinical course. Th us, the infection oft en remains unnoticed in cattle herds or is neglected (Roeder and Harkness, 1986;Paton et al., 1999). Th e situation changes when the infection occurs in pregnant animals causing transplacental infection of the foetus. Substantial health and economic losses are due to frequent failed insemination, stillborn calves with congenital malformations, absence of conception, prolonged service periods, early embryonic mortality, abortions or delivery of PI off spring (Baker, 1987). Such persistently infected progeny are apparently clinically healthy yet remain viraemic for life and shed huge amounts of the virus into the environment via their secreta and excreta. Consequently, those animals are the most important reservoirs of the virus in the herd throughout their life (Baker, 1987;Bronwlie, 1991). Th e outcome of persistent infection is always fatal and due to clinical form of mucosal disease. In addition to the aforementioned direct losses, indirect damage associated with BVDV infection is far too broader and immeasurable for both individual herds and entire cattle production in the country.
Th e greatest damage for cattle industry associated with BVD directly implicate transplacental infections and consequent foetal mortality, congenital malformations, neonatal and postnatal mortality as well as mucosal disease, poor growth and performance of survivors (Roeder and Harkness, 1986). Considerable economic losses caused by BVDV infection can endanger the herd even for years aft er infection. For example, economic damage of BVDV infection on 14 cattle herds in Th e Netherlands was calculated to some 24-161 US Dollars per cow with an average loss of 77 USD (Houe, 1995). In Great Britain, total annual economic loss from BVDV infection is estimated to some 120 million British Pounds (Bitsch and Ronsholt, 1995). Calculated losses associated with BVDV in Denmark are some 17 million USD per year, whereas economic damage attributed only to mucosal disease in eight Danish cattle herds with about 115 animals ranged between 2,380 and 2,980 USD per farm (Houe, 1995).
Th ese data clearly indicate the importance of BVD in cattle production. In the Republic of Serbia, the fi rst cases of BVD infection were identifi ed on the basis on clinical picture and serological examination in 1966 and 1968, respectively (Cvetnić, 1983). Initial research on the prevalence of infection in Serbia date back to 1973, when testing for the presence of BVDV-specifi c antibodies was performed on 224 blood sera of cattle from six farms (four from the region of Banat and two from the regions of Belgrade and Valjevo). Th e results revealed 166 (74%) seropositive animals (specifi c antibodies were detected in blood of animals that were in contact with the virus, i.e., infected).
By the beginning of 1990s, the examination of cattle from two farms in the area of Valjevo was performed. Of the 86 examined calves from a beef herd 48 (55.81%) were seropositive, whereas the rate of seropositive animals from a dairy farm with total 178 cows ranged between 30.55% and 52.24% depending on the age (Kurčubić, 1993).
A range of serological surveys conducted during last 15 years revealed infection rates in the range of 0-100%, depending on the farming system, herd size and territory. Th e percentage of seropositive animals is higher in northern and central parts of Serbia compared to the southern regions. Th e majority of big cattle herds (formerly public-owned farms) are characterized by high BVDV seroprevalence rate (the prevalence of seropositive animals ranges from 60% to even 100%). However, some of those herds are absolutely free from infection ( Th e fi rst extensive researches of isolation and genotyping of BVDV isolates in Serbia were performed in the period 1999-2004 revealing the presence of BVDV genotype 1 and subtypes 1f, 1b and 1g (Petrović et al., 2004/b;Petrović, 2006). At that time, BVDV2 genotype has not been identifi ed in Serbia; however, it was introduced in 2008 through cattle trade (unpublished information).
According to the obtained data and convincing anticipation about prevalence of BVD infection in our country, substantial economic losses as well as the current problem-solving trends in EU countries and having in mind the competitiveness of our cattle owners in meat and dairy industry and in the production of breeding material aft er accession to EU market, the need for relevant legislation pertaining to management and control of this viral disease in Republic of Serbia cannot be emphasized enough. It is proposed that the control of infection should be initially based on a BVDV voluntary eradication program at herd level and establishment of BVDV-free herds. Certifi cation of free herds would be administered by Veterinary Directorate, which would provide an online available database of BVDV-free herds containing relevant data on the herd and validity date of the certifi cate.
Th is article gives an overview of procedures for control and eradication of BVDV infection in the world. Also, a draft of the Program for the control, suppression and eradication of BVDV infection in cattle herds in the Republic of Serbia is presented, that is, preparatory procedures and methods for laboratory and epizootiological testing and monitoring aimed at elimination of PI animals as major infection source as well as obtaining and maintaining of BVDV-free status of the herd.

CURRENT STATUS AND PERSPECTIVES OF BVD IN SERBIA AND WORLDWIDE
BVD infection is widespread among cattle population worldwide. When speaking from an epizootiological point of view, the presence of infection is evident in almost all countries. Th e rate of infected animals, that is, that were in contact with the virus ranges between 60 and 90% in countries with developed cattle industry. In the territory of former Yugoslavia, the infection has been fi rst described by Đuričković et al., in 1966 on the basis of clinical picture and pathomorphological fi nding. Serological confi rmation was realized by Cvetnić et al., in 1968(Cvetnić S., 1983. Th e researches on the incidence of the infection in the territory of Serbia performed during past 20 years strongly indicate its signifi cant prevalence throughout the country. Th e strategy for control and eradication of BVDV infection has initially been launched in Scandinavian countries. Th e program was based on identifi cation and elimination of PI animals from the herd without vaccination. Norway was the fi rst country that understood the actual problem of BVDV infection and initiated the eradication program in 1993. Bulk milk samples originating from 26,430 dairy herds were examined using indirect ELISA. Other Scandinavian countries followed the procedure. Essentially, the program relies on the following steps: 1) identifi cation of BVDV free herds, 2) prevention of the infection in these herds and 3) reduction of the number of infected herds (Waage et al., 1996). Reduction of the number of infected herds was accomplished through identifi cation and removal of PI animals from the herd as well as prevention of acute infection outbreak (Oirshot et al., 1999; Bitsch and Ronsholt, 1995;Waage et al., 1996). If the repeated laboratory test has confi rmed the presence of antibodies in the herd, bulk milk samples from 3-5 fi rst-calf heifers were tested. In case of positive fi nding, further testing was performed using blood sera of fi ve young animals aged 8-12 months. Herds suspected to have one or more PI animals underwent rigorous ban on trade with livestock or grazing cattle on common land until repeated testing confi rms their BVDV-free status. Declaring of a herd as BVDV-free is based on two subsequent repeated serological demonstration of the absence of antibodies in bulk milk sample from the milk tank on the farm, bulk milk sample from fi rst-calf heifers and in bulk sample of blood serum from young animals examined at least four months later (Waage et al., 1996).
National voluntary program for control and eradication of BVDV without vaccination started in Sweden aft er 10-year experience in BVDV control at local and regional levels. Eradication involved identifi cation and elimination of PI animals resulting in getting BVDV-free population (Alenius et al., 1996). Aft er elimination of PI animals, the primary action of the program was control of virus introduction into the herd by direct contact of cattle with PI animal or via newly purchased animal carrying PI foetuses as well as prevention of infection spread by indirect routes such as transportation vehicles, auctions or visitors. Th e measures pertaining to infected herds encompassed prevention of contacts with cattle and sheep from herds other than those certifi ed as BVDVfree. All animals intended for trade were provided with individual certifi cate to be free from BVDV infection and to originate from a BVDV-free herd. Owners of BVDV-free herds were advised to purchase new animals exclusively for certifi ed herds. Pregnant seropositive animals were introduced into other herd only if they were seropositive (confi rmed presence of antibodies) before conception. Th e measures taken in infected herds encompassed prevention of contact with animals from other herds, ban on selling animals and purchasing new animals in the period of program duration. Th roughout three years, 11,120 (70%) dairy herds and 3,075 (20%) beef herds participated in the control program. In this period, 7,585 (47%) dairy herds and 1,456 (10%) beef herds were certifi ed as BVDV-free. Vaccination was absolutely excluded (Alenius et al., 1996).
Examination of bulk milk samples within the framework of a program for control of BVDV infection in Denmark encompassed 16,113 dairy herds, and presence of PI animals was established in 39% of herds. In addition to identifi cation and elimination of viraemic PI animals, the program included the following prophylactic measures: 1) only tested and non-PI confi rmed animals could be introduced into the herd; 2) any seropositive pregnant heifer that was to be introduced into the herd was quarantined until calving and could not enter the herd before the calf was tested PI negative; 3) to prevent occurrence of acute infection in the herd all newly purchase animals were isolated during three weeks; 4) animals were not grazed in areas close to herds suspected to contain PI cattle; 5) common pastures had to be free from PI animals; 6) all animals participating on exhibitions or auctions had to be tested and certifi ed as BVDV-free and non-PI; 7) on the infected farms, the testing encompass also small ruminants; 8) fi eld veterinarians strictly followed relevant biosecurity measures to prevent potential transmission between the herds. Th e herds were considered free from infection in case of absence of virus specifi c antibodies in blood serum of three calves older than eight months tested at least 12 months aft er removal of all PI animals from the herd (Bitsch and Ronsholt, 1995).
In the framework of an experimental program of BVDV infection control in Italy (the Province of Rome), which was based on identifi cation and culling of PI animals blood sera from all animals older than one year from 174 mainly dairy herds were examined (Ferrari et al., 1999). According to the results of serological testing the herds were categorized as negative if none of the animals was tested seropositive. Th e herd was claimed positive if one or more animals were seropositive. Negative herds underwent repeated testing aft er 30 days. If the second examination revealed negative result, the herd was declared free from BVDV. To the purpose of maintaining the free status, the tests were repeated every six months including examination of bulk milk sample from the milk tanks from dairy farms or examination of blood samples on beef farms. In seropositive herds, all animals aged 6-12 months were tested for the presence of specifi c antibodies in serum samples. According to the results, the herds could be categorized as "with or without" recent BVDV infection. In herds without recent infection, seronegative status of 6-12 months old animals was checked at 6-month intervals. If the tested animals (6-12 months of age) have revealed permanent negative results throughout 18 months (three consequent testing at 6-month intervals), the herd could be certifi ed as BVDV-free. In herds characterized by recent infection (one or more seropositive calves aged 6-12 months) all animals were tested for the presence of virus specifi c antibodies. All seronegative animals were tested for the presence of BVDV virus. Th e animals were categorized as PI if the virus was detected during two subsequent tests at 30-day interval. Seronegative animals were tested every six months to exclude or confi rm new cases of infection. Th e herd was claimed BVDV-free if all 6-12 months old animals born 12 months aft er elimination of the last PI cattle were seronegative as well as if there were no seropositive fi ndings in the group of seronegative animals (Ferrari et al., 1999).
More extensive program for the control and eradication of BVDV in Italy considered the potential application of programmed vaccination. In the herds with high seroprevalence, vaccination of seronegative animals before conception was practiced (Cancelloti and Carlotto, 1985). Adequate information about the status of BVDV infection in the herd was the prerequisite for implementing an adequate vaccination program. Serological and virological examination was performed with an aim of confi rming and excluding of PI animals. At the same time, relevant prophylactic measures were applied, including "all in -all out" approach, frequent and thorough disinfection, quarantine and isolation as well as rigorous hygienic measures at calving. Th e control of BVDV infection in Italy involved attenuated vaccine, which was considered safe and eff ective prophylactic tool, especially in calves. Th e heifers on dairy farms were vaccinated at age of 12-14 months. During the period of program implementation, it was established that proper determination/identifi cation of herd status before vaccination was of crucial importance and that vaccination cannot be considered a long-term option. Administration of attenuated and/ or killed vaccine for an indefi nite period and without precisely knowing of infectious status of the herd is highly hazardous and economically unacceptable.
During the past few years, the strategies for systematic eradication of BVDV infection were launched in many EU countries. Th e strategies were initiated either by the governments and relevant authorities or by farmer associations under government's support. Th e programs are similar to the aforementioned ones, with some minor modifi cations.
In the Republic of Serbia, the situation is vastly unfavourable -the problem and the control of BVD infection in cattle herds are absolutely neglected. Cattle owners and producers are generally left to their own devices to cope with this problem. During the past several years, there have been some attempts to establish infection control at herd level; however, such isolated attempts received any support from neither governmental authorities nor farm owners, which is partly due to frequent changes of the ownership of the farms during transition period. Th ere were some attempts of applying the Scandinavian system relying on elimination of PI animals and improvement of biosecurity measures on the farm, as well as diverse vaccine preparations. However, neither of eff orts proved successful. In that respect, this economical problem that substantially aff ects the competitiveness of Serbian cattle production was recognized by professionals in veterinary medicine and the management of the Veterinary Directorate of the Ministry of Agriculture and Environmental Protection. Th ey created the initiative for establishing an initially voluntary program for the control of BVD infection in the territory of the Republic of Serbia.
Researchers from the Scientifi c Veterinary Institute "Novi Sad" (NIV-NS) have been permanently participating in the programs for monitoring of infectious animal diseases and zoonoses in Serbia. Th us, Serbian Government placed confi dence on the scientifi c and professional competences of the researchers from NIV-NS and colleagues participating in research project No TR31084 as well as professionals from the sector of animal health protection, and initiated creation of a draft of the Program for the control, suppression and eradication of BVDV infection in Republic of Serbia. Th e program would be initially implemented on voluntary basis at individual farms and cattle herds, depending on the interest of the owners (Petrović et al., 2015). Considering that the Program for the control of BVDV infection combines successful methodologies developed and applied in several countries (Scandinavian, German, Italian and Slovenian control program), and to the introduced "up to date" diagnostic methods into the laboratories that will be included in animals testing (Petrović et al., 2010), it is expected to be successful in cattle herds that will voluntarily conduct the control of BVDV infection. Th e Program would be subsequently implemented at national level as a governmental program for the control of BVDV infection in the Republic of Serbia.

METHODOLOGIES PROPOSED FOR CONTROL, SUPPRESSION AND ERADICATION OF BVDV INFECTION
Primary goal in the control of BVDV is the prevention of prenatal infection. Th e procedure involves identifi cation and elimination of persistently infected (PI) cattle from the herd (Roeder and Harkness, 1986;Baker, 1987). Aft er elimination of PI animals, particular attention should be paid on introduction of new animals into the herd, avoiding contact infections and preventing the occurrence of transplancental infection (Baker, 1987). Th e central aspect of this volontary Program for control, suppression and eradication of BVDV infection relies on the establishment of herd free from BVDV infection. Certifi cation of the herds would be coordinated and validated by the Veterinary Directorate that would also be responsible for providing an online database of certifi ed herds with all relevant data pertaining to the herd and certifi cation validity date.
Th e Program design encompasses several steps, i.e. stages of disease control, depending on the actual status of BVDV infection in a herd. 1. Stage 1: Affi rming status "free from BVDV infection"; 2. Stage 2: Obtaining status "free from BVDV infection; 3. Stage 3: Maintenance of BVDV-free status of the herd and 4. Stage 4: Loss of the status "free from BVDV infection" and procedure for restoring and reclaiming the status of BVDV-free herd Th e goals of this Program Proposal are: -Reduction of economic losses in cattle herds; -Obtaining of certifi ed breeding material to facilitate and promote its access to international market; -Better market price of such breeding material

Stage 1: Affi rming the status "free from BVDV infection"
To affi rm the BVDV-free status, the herd needs to fulfi l the following conditions: 1. If infection in the herd was not detected throughout past 12 months; 2. If none of the animals manifest clinical signs that would suggest suspect BVDV infection; 3. If the holding (farm, holding yard, pasture, etc.) where the cattle herd is located is protected by either natural or artifi cial barriers to prevent any undesirable contact between in-farm cattle with animals, people, equipment, tools or vehicles from other farms of poor or unknown/uncontrolled status with respect to BVDV infection (prevent direct and indirect contacts between animals of diff erent health status); 4. If newly purchased animals were introduced into the herd, they should originate from certifi ed BVDV-free herds, or have been isolated quarantined for 30 days and proved negative aft er relevant testing for the presence of BVD virus and BVDV-specifi c antibodies (examination performed minimum 15 days aft er putting into quarantine) 5. If none of the animals from the herd was in contact with cattle originating from herds of unknown BVDV status or herds of poorer BVDV status; 6. If artifi cial insemination is performed using only semen from the Centres for artifi cial insemination certifi ed as BVDV-free or from bulls confi rmed as free from BVDV infection or if natural breeding is practiced, only the bulls originating from BVDV-free herds or proved BVDV-free bulls are used; 7. If two consequent serological testings for the presence of BVDV-specifi c antibodies performed at 6-month interval in all animals aged 7-13 months were resulted with negative fi nding (if there are no animals of such age, the test includes all for six months older animals -animals aged 14-20 months, etc). Th e period aft er last previous test must not exceed 6 months.
Request for certifi cation is submitted to the Veterinary Directorate of the Ministry of Agriculture and Environmental Protection by the owner -farmer. Th e Request should be previously approved / verifi ed by relevant veterinary inspector according to health status examination (declaration of the relevant veterinary service responsible for herd status monitoring in line with aforementioned terms 1-6) and laboratory analysis pursuant to aforementioned term 7. According to the submitted request and relevant accompanying documentation, Veterinary Directorate of the Ministry of Agriculture and Environmental Protection awards the Certifi cate on BVDV-free status to the herd with a validity period of 12 months and enters the herd into the database (Register) of herds free from BVDV infection, which would be publicly available from the web-page of the Veterinary Directorate and is automatically updated.

Stage 2: Obtaining of status "free from BVDV infection"
If for any reason the herd cannot be declared as BVDV-free according to rules described for the Stage 1 of BVDV control or clinical manifestations or laboratory examination suggest the presence of BVDV infection in the herd, the following steps should be taken towards obtaining of BVD-free status: 1. Examine all animals in the herd for the presence of BVD virus and promptly eliminate (slaughter) all persistently infected animals (PI) from the herd; 2. During one-year period aft er elimination of all PI animals from the herd, examine every single newborn calf for the presence of BVDV during fi rst week of life and immediately eliminate all virus-positive animals; 3. One year aft er elimination of the last PI cattle, i.e., BVD-positive animal from the herd, examine all animals aged 7-13 months for the presence of BVDV-specifi c antibodies and : a. If testing results are negative, repeat testing aft er 6 months including all animals aged 7-13 months. If the results of repeated testing for the presence of BVDV-specifi c antibodies are negative and conditions of the Stage 1 of BVDV control / terms 1-6 are fulfi lled, the herd is declared as free from BVDV infection; b. In case of positive result in repeated testing, the procedure of suppression and eradication of BVDV infection should started from the beginning, that is, from Term 1 of this stage of BVDV control (Stage 2); 3. If there are no animals of such age in the herd at the moment of testing, the test includes all animals from the upper 6-month age category, that is, cattle aged 14-20 months Th e status of a BVDV-free herd is declared if all aforementioned measures and procedures are successfully implemented, resulting in negative fi nding of two consequent serological testings for the presence of BVDV-specifi c antibodies performed at 6-month interval in all animals aged 7-13 months (if there are no animals of such age, the test includes all animals from the upper 6-month age category). Request for certifi cation is submitted to the Veterinary Directorate of the Ministry of Agriculture and Environmental Protection by the owner -farmer in line with conditions stipulated under the Stage 1 of BVDV control.

Stage 3: Maintenance of status "free from BVDV infection"
Maintenance of the BVDV-free herd status relies on implementation of rigorous biosecurity measures and monitoring of herd's health status by relevant veterinary service. Th e status is verifi ed at 12-month intervals by serological examination of all animals aged 7-13 months for the presence of BVDV-specifi c antibodies. Based on the obtained data and negative serological fi nding the herd certifi cate is extended for the period of next 12 months. Besides the abovementioned, the herd must comply with all terms stipulated in the Stage 1 of BVDV control, terms 1-6.
If laboratory examination aimed at obtaining disease-free status of uncertifi ed herd or in a certifi ed herd reveals positive serological fi nding in animals aged 7-13 months, the following steps should be taken: 1. Test all animals in the herd for the presence of BVD virus and promptly eliminate (slaughter) all persistently infected animals (PI) from the herd; 2. During subsequent one-year period, examine every single newborn calf for the presence of BVDV during fi rst week of life (immediately eliminate virus-positive animals); 3. One year aft er elimination of the last PI cattle, i.e., BVD-positive animal from the herd, examine all animals aged 7-13 months for the presence of BVDV-specifi c antibodies according to the procedure described under the Stage 1 of BVDV control and if: a. test results are negative, repeat testing for the presence of BVDVspecifi c antibodies aft er 6 months including all animals aged 7-13 months. If the results of repeated testing are still negative and conditions from the Stage 1 of BVDV control / terms 1-6 are fulfi lled, the herd is declared free from BVDV infection and the certifi cate on BVDV-free status is issued; b. the result in repeated testing is positive, the procedure of suppression and eradication of BVDV infection is started from the beginning, that is, from Term 1 of this Stage of BVDV control. 3. If there are no animals of such age in the herd at the moment of testing, the test includes all animals from the upper 6-month age category, that is, cattle aged 14-20 months, etc.
Th e monitoring of herd's health status is performed by relevant veterinary service or institution, which should conduct a detailed and comprehensive inspection at least twice yearly at minimum 4-month intervals. Th e inspection must include clinical examination of the cattle as well as control of the compliance of farm practices with the terms 1-5 stipulated in the Stage 1 of BVDV control. Aft er the inspection, the relevant institution or veterinary service completes the appropriate Protocol on herd status, that is, whether the conditions for declaring the herd as free from BVDV infection are fulfi lled or not. Th e Protocol is made in three copies submitted to 1) farm owner, 2) veterinary inspector and 3) responsible fi eld veterinarian.
Th e owner is obligated to report any evident disturbance of health status of his animals that suggests potential BVDV infection as well as each case of abortion to the responsible fi eld veterinarian, who has to perform sampling of materials for virological and serological examination and submit them to relevant laboratory for further testing for the presence of BVDV infection.
Th e status of a BVDV-free herd is declared if all above stated measures and procedures are completed, and if two consequent serological examinations for the presence of antibodies against BVDV performed at 6-month interval in all animals aged 7-13 months resulted with negative fi nding (if there are no animals of such age, the test includes all animals from the upper 6-month age category). Request for certifi cation is submitted to the Veterinary Directorate of the Ministry of Agriculture and Environmental Protection by the owner / farmer in line with conditions stipulated under the Stage 1 of BVDV control.

Stage 4: Loss of the status "free from BVDV infection" and procedure for restoring and reclaiming the BVDV-free herd
If it is established that the requirements for declaring the herd as free from BVDV infection are not fulfi lled, the owner is issued a Decision on withdrawal of the certifi cate on BVDV-free status (or the validity of the certifi cate is not extended to the further period) by the Veterinary Directorate of the Ministry of Agriculture and Environmental Protection.
Th e data on the herd that has lost its status of BVDV-free herd are deleted from the Registry of herds free from BVDV infection by the Veterinary Directorate.
To restore the lost BVDV-free status of the herd, the owner is obligated to fulfi l all conditions and procedures for obtaining of the status "free from BVDV" infection described in the Stage 2 of BVDV control.