Incidence of classical swine fever (CSF) in wild boar in a densely populated area indicating CSF virus persistence as a mechanism for virus perpetuation

A virological survey was carried out to establish the distribution of classical swine fever (CSF) virus among wild boar in the Federal State of Brandenburg, Germany. Organ materials and blood samples were collected from 11,670 wild boar shot or found dead during the period March 1995 to December 1997. In total 211 (1.8%) wild boar were positive for CSF virus or antigen. The incidence of CSF-positive animals decreased continuously from 4.6% at the beginning of the epidemic in 1995 to 0.7% in 1997. The highest incidence of positive animals (22%) was found in wild-boar piglets younger than 3 months of age in 1995. The findings were indicative for the decisive role which young wild boar play in the epidemiology of CSF. Following intrauterine transfer some of the wild-boar piglets were probably persistently infected with CSF virus as experienced experimentally. Such piglets can be held responsible for CSF virus perpetuation within the wild-boar population. No CSF virus was isolated from adult wild boar weighing more than 75 kg. During 3 years of monitoring a sufficient number of susceptible wild boar, in particular young animals, was available to maintain the infection chain in that area. It was concluded that persistently infected piglets and the high population density of wild boar in the Brandenburg region offered optimal conditions for the establishment of an CSF epidemic.     

Classical swine fever (CSF) in wild boar: the role of the transplacental infection in the perpetuation of CSF

Thirty-four pregnant wild sows and their unborn progeny derived from an endemically infected population in the district of Nordvorpommern (Mecklenburg-Western Pomerania) were investigated for classical swine fever virus (CSFV) and antibodies. During the last 2.5 years of the epidemic, 20 out of 34 pregnant wild sows investigated were serologically positive. No CSFV or viral RNA was detected in organs derived from these animals and their progeny. This indicates that young wild boars persistently infected by transplacental virus transmission do not play a crucial role in the perpetuation of CSFV in wild boar. Other factors seem to be more important for the establishment of CSF as well as for virus perpetuation in the population.     

Simulating the spread of classical swine fever virus between a hypothetical wild-boar population and domestic pig herds in Denmark

Denmark has no free-range wild-boar population. However, Danish wildlife organizations have suggested that wild boar should be reintroduced into the wild to broaden national biodiversity. Danish pig farmers fear that this would lead to a higher risk of introduction of classical swine fever virus (CSFV), which could have enormous consequences in terms of loss of pork exports. We conducted a risk assessment to address the additional risk of introducing and spreading CSFV due to the reintroduction of wild boar. In this paper, we present the part of the risk assessment that deals with the spread of CSFV between the hypothetical wild-boar population and the domestic population. Furthermore, the economic impact is assessed taking the perspective of the Danish national budget and the Danish pig industry. We used InterSpreadPlus to model the differential classical swine fever (CSF) risk due to wild boar. Nine scenarios were run to elucidate the effect of: (a) presence of wild boar (yes/no), (b) locations for the index case (domestic pig herd/wild-boar group), (c) type of control strategy for wild boar (hunting/vaccination) and (d) presence of free-range domestic pigs. The presence of free-range wild boar was simulated in two large forests using data from wildlife studies and Danish habitat data. For each scenario, we estimated (1) the control costs borne by the veterinary authorities, (2) the control-related costs to farmers and (3) the loss of exports associated with an epidemic. Our simulations predict that CSFV will be transmitted from the domestic pig population to wild boar if the infected domestic pig herd is located close to an area with wild boar (<5 km). If an outbreak begins in the wild-boar population, the epidemic will last longer and will occasionally lead to several epidemics because of periodic transfer of virus from groups of infected wild boar to domestic pig herds. The size and duration of the epidemic will be reduced if there are no free-range domestic pig herds in the area with CSF-infected wild boar. The economic calculations showed that the total national costs for Denmark (i.e. the direct costs to the national budget and the costs to the pig industry) related to an outbreak of CSF in Denmark will be highly driven by the reactions of the export markets and in particular of the non-EU markets. Unfortunately, there is a substantial amount of uncertainty surrounding this issue. If hunting is used as a control measure, the average expenses related to a CSF outbreak will be 40% higher if wild boar are present compared with not present. However, a vaccination strategy for wild boar will double the total costs compared with a hunting strategy.     

Studies on the virulence of two field isolates of the classical Swine Fever virus genotype 2.3 rostock in wild boars of different age groups

The virulence of two isolates of the classical swine fever virus (CSFV) was studied in experimentally infected wild boars of different ages. The isolates, originating from wild boars shot in Mecklenburg-Western Pomerania (isolate '1829-NVP') and in Rhineland-Palatinate (isolate '11722-WIL'), belong to the genetic subgroup 2.3 Rostock. Clinical picture, transient viraemia, virus excretion and gross lesions at necropsy as well as a failure of virus detection at the end of the experiment revealed that this virus subtype was only moderately virulent. Whereas one subadult wild boar and both 7-week-old wild boar piglets infected intranasally became sick and died, only one of three 8-week-old animals which survived after contact infection remained CSFV positive until the end of the experiment [34 days post infection (dpi)], although neutralizing antibodies were present. This underlines the role of young boars in CSF epidemics. The isolate '11722-WIL' was shed by an infected adult wild boar and was transmitted to susceptible piglets. Interestingly, all animals which became sick and died also were found to be infected with a secondary pathogen. Therefore, we assume that after infection with moderately virulent CSFV simultaneous infections with other pathogens may be important for the clinical course and the outcome of the disease as well as for a spread of the virus in field.     

Classical swine fever in wild boar in the Lombardy region of Italy from 1997 to 2002

In 1997, outbreaks of classical swine fever occurred in wild boar and domestic pigs in the Italian province of Varese (Lombardy region). Statutory animal health measures were imposed, and a control plan aimed at preventing the virus from spreading outside the infected hunting zone of 108 km2 was implemented. However, in 1998, virologically positive wild boar were found outside this zone, including within Swiss territory. The infected zone was then enlarged to include the three other hunting areas of the northern province of Varese and the neighbouring Swiss territory. Italian and Swiss veterinary services initiated a joint policy to control the disease by hunting young wild boar and conserving older, immune animals. The percentage of virologically positive animals gradually decreased, the last being found in July 2000. The seroprevalence initially increased, but in 2000 and 2001 it decreased from 42.2 per cent to 8.8 per cent, indicating that the wild boar population had had no further contact with the virus.     

Oral immunization against classical swine fever (CSF) in the Federal State of Brandenburg from 1995 to 1997

The first findings of classical swine fever in wild boar occurred in the rural district Ostprignitz-Ruppin of Brandenburg in March 1995. Up to this time the area was considered to be free of swine fever in domestic and wild boars. The swine fever cases were marked particularly by the occurence of dead games and clinically decreased individuals as well as by the involvement of young wild boars. (< 10 kg carcass weight). In Brandenburg, the eradication programme of classical swine fever was based on oral immunization with an attenuated type C vaccine (so called Dessauer wild boar lure) carried out within the frame of a field trial since 1995. The success of oral immunization was examined by the development of swine fever positive-findings within the swine fever endangered district and by the seroconversion in the entire vaccination area. Due to the oral immunization the classical swine fever prevalence decreased from 4.65% in March 1995 to 0.58% in December 1997. After the third immunization the seroconversion reached on average rates between 30% and 35%. The seroconversion increased with carcass weight and with number of subsequent immunization. During the investigation period the rate of seroconversion in wild boars up to 17 kg carcass weight was 13.2%, in wild boars from 18 to 28 kg carcass weight 23.1%, sub-adult wild boars from 29 to 56 kg carcass weight 33.2%, and the adult wild boars above 57 kg carcass weight 41.5%.     

模猪场猪瘟野毒感染情况调查

利用ELISA方法对广西6个规模猪场送检的1626份猪血清进行猪瘟野毒感染情况调查。结果6个规模猪场均存在猪瘟野毒感染,其中母猪感染率为7.86%~29.21%,平均为17.52%,种公猪感染率为0%~23.52%,平均为11.83%,育肥猪感染率为5%~22.45%,平均为15.5%,断奶仔猪感染率为8.24%~18.57%,平均为12.69%。此检测结果与猪场临床发病情况基本一致,病猪多表现为繁殖障碍型、温和型的非典型猪瘟。     

A serological survey on classical swine fever (CSF), Aujeszky's disease (AD) and porcine reproductive and respiratory syndrome (PRRS) virus infections in French wild boars from 1991 to 1998

In early 1992, a CSF epizootic was clinically recognised in a wild boar population of approximately 1300 animals within an area of 250km(2) located in the east of France. In order to check the CSF situation in wild boars outside this area, a serological survey was carried out in the rest of France, for 8 consecutive years (1991-1998). This paper reports on the results obtained during this survey which included wild boars shot during the hunting period but also boars reared within fences. Around 1000-2700 sera a year were tested for the presence of antibodies to classical swine fever virus (CSFV) and also to Aujeszky's disease virus (ADV). Out of 12025 sera tested over the whole period, 80 wild boars were found positive for CSF antibodies. Sixty of them were collected on wild boars shot during the years 1992-1994 in the epizootic area located in east of France and 10 were collected in Corsica during the years 1994-1996. The last four positive samples were single reactors coming from areas or farms, which were thereafter confirmed to be serologically negative. These results together with the fact that no disease has been reported so far illustrate that the French wild boar population is probably not concerned by CSF infection (excepted in the east of France where the disease has now become enzootic). Two hundred and forty nine sera were initially detected as CSF positive but confirmed secondarily as positive for border disease (BD) antibodies. This finding shows that wild boars are also susceptible to infection by ruminant pestiviruses. Four hundred and twenty three wild boars have been found positive for ADV antibodies. In addition, from 1993 to 1995, 909 samples were tested for the presence of antibodies to porcine reproductive and respiratory syndrome virus (PRRSV). Thirty three of them were positive. The results on AD and PRRS antibody detection show that wild boars may constitute a reservoir for various infectious diseases of pigs.     

Case report: the significance of genotyping for the epidemiological tracing of classical swine fever (CSF)

In Germany, eleven outbreaks of CSF in domestic pig holdings were reported in 2002. They occurred exclusively in regions where CSF virus circulated in the wild boar population. In ten cases the phylogenetic analysis revealed that the isolates from domestic pigs and wild boar had identical sequences in the 5' non-translated region (5'NTR). However, in one case a subtype was isolated which was slightly different from the virus subtype found in the wild boar population of that region. This case is decribed in detail. The epidemiological significance of different diagnostic methods is discussed, in particular the genetic typing of CSF virus isolates.     

规模猪场猪瘟野毒感染情况调查

利用ELISA方法对广西6个规模猪场送检的1626份猪血清进行猪瘟野毒感染情况调查.结果6个规模猪场均存在猪瘟野毒感染,其中母猪感染率为7.86%～29.21%,平均为17.52%,种公猪感染率为0%～23.52%,平均为11.83%,育肥猪感染率为5%～22.45%,平均为15.5%,断奶仔猪感染率为8.24%～18.57%,平均为12.69%.此检测结果与猪场临床发病情况基本一致,病猪多表现为繁殖障碍型、温和型的非典型猪瘟.     

Long-term studies on maternal immunity for Aujeszky's disease and classical swine fever in wild boar piglets

The aim of the studies was to fathom the duration and the role of maternal immunity for Aujeszky's disease (AD) and classical swine fever (CSF) in wild boar offspring. In one experiment, two wild boar sows were infected with a low pathogenic pseudorabies virus (PRV) in 1999. A total of 51 offspring was born between 1999 and 2002 and was monitored for PRV maternal antibodies. In a second experiment, the maternal immunity for CSF was analysed. Therefore, a sow was orally vaccinated against CSF using vaccine baits containing the live-attenuated C-strain vaccine. The vaccination took place in January 1999. The sow gave birth to four piglets in 2001 and to two piglets in 2002. With respect to maternal immunity for AD, some piglets reacted positive in the ELISA up to 27-week post-partum while in the neutralization test antibodies were detected up to 15-week post-partum. The calculated half-life of neutralizing antibodies was 21 days. Regarding CSF, the neutralization titres of maternal antibodies dropped continuously reaching values of < or =10 ND50 20-week post-partum. After the 12th week post-partum, most of the sera reacted negative in the ELISA. However, after the third month, low levels of neutralization titres were still detectable. The results are discussed with respect to the epidemiology and control of both diseases in wild boar populations.     

Transient classical swine fever virus infection in wild boar piglets partially protected by maternal antibodies

An experimental study was conducted to investigate the clinical course of classical swine fever (CSF) in wild boar piglets partially protected by maternal antibodies. Five healthy wild boar piglets with a low serum titre of colostral antibodies against CSF virus were challenged with virulent CSF virus at the age of three months. Apart of reduced food intake and diarrhoea no major clinical symptoms were noticed after challenge. These signs were seen during the second and third week of infection, afterwards the piglets recovered completely. CSF virus could be re-isolated from blood samples taken on day 12 and day 19 post challenge. From blood samples taken later on and from the organ material taken at post mortem examinations no CSF virus could be isolated anymore. It can be concluded that the presence of maternal antibodies influences the clinical course of CSF in terms that the outcome is rather transient than lethal. Such wild boar could play a crucial role in the spread of CSF virus and might contribute to the maintenance of long lasting epizootics.     

Classical swine fever virus: clinical, virological, serological and hematological findings after infection of domestic pigs and wild boars with the field isolate "Spante" originating from wild boar

A classical swine fever virus (CSFV) field isolate originating from wild boar was investigated on its virulence in domestic pigs and wild boar. Three weaner pigs and two wild boars (yearlings) were intranasally inoculated with the isolate "Spante" and tested for clinical, virological, hematological and serological findings until day 31 after infection (p. i.). One day p. i. the piglets were put in contact to three sentinel pigs. During a period of 31 d neither the domestic pigs nor the wild boars showed clinical signs specific for CSF. Two infected weaner pigs became transiently viraemic, transmitted CSFV in nasal secretions, showed a slight leukopenia and reacted serologically positive. The contact infection resulted in a viraemia in two sentinel piglets on day 30. Only one contact animal developed antibodies. None of the wild boars became viraemic, excreted CSFV in nasal secretions or developed antibodies. The CSFV isolate "Spante" represents a low virulent virus. Referring to a significant higher percentage of virologically positive tissue samples after nested PCR compared with the virus isolation, persistence of CSFV is discussed.     

Long‐Term Studies on Maternal Immunity for Aujeszky's Disease and Classical Swine Fever in Wild Boar Piglets

The aim of the studies was to fathom the duration and the role of maternal immunity for Aujeszky's disease (AD) and classical swine fever (CSF) in wild boar offspring. In one experiment, two wild boar sows were infected with a low pathogenic pseudorabies virus (PRV) in 1999. A total of 51 offspring was born between 1999 and 2002 and was monitored for PRV maternal antibodies. In a second experiment, the maternal immunity for CSF was analysed. Therefore, a sow was orally vaccinated against CSF using vaccine baits containing the live‐attenuated C‐strain vaccine. The vaccination took place in January 1999. The sow gave birth to four piglets in 2001 and to two piglets in 2002. With respect to maternal immunity for AD, some piglets reacted positive in the ELISA up to 27‐week post‐partum while in the neutralization test antibodies were detected up to 15‐week post‐partum. The calculated half‐life of neutralizing antibodies was 21 days. Regarding CSF, the neutralization titres of maternal antibodies dropped continuously reaching values of ≤10 ND₅₀ 20‐week post‐partum. After the 12th week post‐partum, most of the sera reacted negative in the ELISA. However, after the third month, low levels of neutralization titres were still detectable. The results are discussed with respect to the epidemiology and control of both diseases in wild boar populations.     

Oral immunisation of wild boar against classical swine fever: concluding analysis of the recent field trials in Germany

The recent oral immunisation trials in wild boar against classical swine fever (CSF) in Germany are described and evaluated in summary. After the first field study in Lower Saxony from 1993-1995 further immunisation trials started in Mecklenburg-Western Pomerania, Brandenburg, Lower Saxony, Baden-Württemberg and Saxony-Anhalt. The immunisation strategies and the size of the vaccination zones were different in the individual federal states. In principle, the bait vaccine based on the CSF virus strain "C" were laid out by hand. Later also the aerial distribution was carried out in selected areas of Mecklenburg-Western Pomerania. The application of baits by plane was introduced at the beginning of the immunisation measures in Saxony-Anhalt apart from the manual distribution. Up to now, the field trials show that the oral immunisation can be an additional tool for CSF control by increasing of herd immunity and reduction of the CSFV prevalence. However, the immunisation was not sufficient enough for young boars in the most field studies. Based on the evaluation of the immunisation experiments an improved immunisation procedure is recommended.     

Efficacy of a live attenuated vaccine in classical swine fever virus postnatally persistently infected pigs

Classical swine fever (CSF) causes major losses in pig farming, with various degrees of disease severity. Efficient live attenuated vaccines against classical swine fever virus (CSFV) are used routinely in endemic countries. However, despite intensive vaccination programs in these areas for more than 20 years, CSF has not been eradicated. Molecular epidemiology studies in these regions suggests that the virus circulating in the field has evolved under the positive selection pressure exerted by the immune response to the vaccine, leading to new attenuated viral variants. Recent work by our group demonstrated that a high proportion of persistently infected piglets can be generated by early postnatal infection with low and moderately virulent CSFV strains. Here, we studied the immune response to a hog cholera lapinised virus vaccine (HCLV), C-strain, in six-week-old persistently infected pigs following post-natal infection. CSFV-negative pigs were vaccinated as controls. The humoral and interferon gamma responses as well as the CSFV RNA loads were monitored for 21 days post-vaccination. No vaccine viral RNA was detected in the serum samples and tonsils from CSFV postnatally persistently infected pigs for 21 days post-vaccination. Furthermore, no E2-specific antibody response or neutralising antibody titres were shown in CSFV persistently infected vaccinated animals. Likewise, no of IFN-gamma producing cell response against CSFV or PHA was observed. To our knowledge, this is the first report demonstrating the absence of a response to vaccination in CSFV persistently infected pigs.     

Evaluation of the oral immunisation of wild boar against classical swine fever in Baden-Württemberg

The oral immunisation of wild boar against classical swine fever (CSF) in Baden-Württemberg is described and evaluated. The bait vaccine based on the CSF virus (CSFV) strain "C" proved to be safe in wild boar of all age classes. The modified immunisation procedure consisting of three double vaccinations per year was very effective. CSFV was not detected beyond the second immunisation campaign. The average rate of seropositive wild boar diagnosed over all immunisation periods was 49.2%. The seroprevalence rate increased significantly during the first year of immunisation and reached its maximum after the third vaccination period with 72% antibody positive animals. The higher percentage of seropositive young boars in this field trial compared to the seroprevalence rates in this age class in other field trials in Germany may be attributed to the new vaccination scheme. Factors that may be responsible for the decreased herd immunity after the fourth or sixth immunisation period are discussed.     

Canine distemper virus infection among wildlife before and after the epidemic

In 2007–2008, a canine distemper virus (CDV) epidemic occurred among wild animals in Wakayama Prefecture, Japan, and many mammals, including the wild boar and deer, were infected. In this study, CDV prevalence among wild animals was surveyed before and after the epidemic. At first, an enzyme-linked immunosorbent assay (ELISA) with horseradish peroxidase-conjugated protein A/G was established to detect CDV antibodies in many mammalian species. This established ELISA was available for testing dogs, raccoons and raccoon dogs as well as virus-neutralization test. Next, a serological survey of wild mammalians was conducted, and it was indicated that many wild mammalians, particularly raccoons, were infected with CDV during the epidemic, but few were infected before and after the epidemic. On the other hand, many raccoon dogs died during the epidemic, but CDV remained prevalent in the remaining population, and a small epidemic occurred in raccoon dogs in 2012–2013. These results indicated that the epidemic of 2007–2008 may have been intensified by transmission to raccoons.