Classical swine fever virus strain "C" protects the offspring by oral immunisation of pregnant sows.

The aim of this study was to evaluate if oral immunisation of wild sows protects the fetuses from transplacental infection. Two experiments were carried out with gilts vaccinated orally with C-strain virus approximately 5 weeks after insemination. They were challenged at mid-gestation with highly virulent classical swine fever virus (CSFV) or moderately virulent field virus. The results revealed that oral vaccination has no negative impact on the pregnancy, and all vaccinated sows developed neutralising antibodies. After infection no symptoms were detected in the six vaccinated-infected sows. Challenge virus could neither be found in blood, nasal and fecal swabs or saliva nor in organs sampled at necropsy. Likewise, all fetuses originating from vaccinated sows were virologically and serologically negative. In contrast, the controls developed a short viremia and as a result of the transplacental infection all fetuses were CSFV positive. In addition, 22 serologically positive wild sows of an endemically infected area, where oral vaccination had also been carried out, and their offspring were free from CSFV or viral RNA. Our results confirm that oral immunisation of pregnant wild sows with C-strain vaccine may protect the fetuses against CSF.

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.

Does multiple oral vaccination of wild boar against classical swine fever (CSF) have a positive influence on the immunity?

We studied the efficacy of multiple vaccinations of wild boar against classical swine fever (CSF) using a C-strain vaccine. The study consisted of two experiments. In the first experiment, 7 to 8 months old animals were vaccinated either three or four times at an interval of 7 days or twice at an interval of 14 or 28 days. In the second experiment, the efficacy of oral immunisation in young boars (3 months old) was examined after fivefold vaccination at intervals of 14 or 28 days. Independently of the immunisation scheme all wild boar developed neutralising antibodies. An evaluation of the antibody titres 28 days after the initial vaccine application showed that single vaccination and triple immunisation at an interval of 7 days induced the highest antibody titres (X > or = 1/80). In multiple vaccinated young boars (vaccinated at intervals of 14 or 28 days) the third vaccination led to a slight reduction or to an only moderate increase of the antibody titre. In a challenge study after the fifth vaccination all wild boar were protected (no viraemia, no virus excretion, no post-mortem virus detection in organs). This was confirmed by the fact that sentinel animals were not affected. Although other immunisation schemes also were effective, booster vaccination at an interval of 28 days is recommended as basic procedure for eradication of CSF in wild boar. Triple vaccination might also be used at the beginning of the control measures.

Role of birds in transmission of classical swine fever virus.

Active transmission of classical swine fever virus (CSFV) was studied in six birds (five ravens, one hooded crow) and two laying hens. Cloacal swabs, blood and organs of birds and hens as well as blood and organ samples of pigs which had been fed with faeces derived from CSFV infected birds or which had come in contact with faeces of infected hens were negative for CSFV. None of the animals seroconverted during the study. This result demonstrates that active virus transmission by these animals is unlikely. Dissemination of CSFV from wild boar to domestic pigs is discussed.

Oral immunization of pigs against classical swine fever. Course of the disease and virus transmission after simultaneous vaccination and infection.

The efficacy of simultaneous vaccination of pigs against classical swine fever (CSF) and challenge was evaluated. In this study, domestic weanling pigs were vaccinated orally with a conventional live virus vaccine based on CSF virus (CSFV) C strain and were challenged simultaneously with CSFV of different virulence. All the animals vaccinated and challenged with a high dose of highly virulent Koslov strain died while three of five animals challenged with a low dose of highly virulent Alfort 187 strain survived, shed the virus in nasal secretions, developed antibodies, and four of them showed a transient viremia. All the animals vaccinated and challenged with the low virulent field isolate MV 140/Riems survived, showed a short viremia and developed antibodies. No CSFV or CSFV RNA could be detected in the animals surviving the infection. This study demonstrates that oral vaccination of wild boars in an infected area bears no risk for the development of a persistent CSF infection.

[Comparison of laboratory diagnostic methods for the detection of infection with the virus of classical swine fever in the early inspection phase: an experimental study]. / Vergleich labordiagnostischer Methoden zum Nachweis einer Infektion mit dem Virus der Klassischen Schweinepest (KSPV) in der frühen Infektionsphase: experimentelle Studie.

Virus isolation in the PK-15 cell culture, two commercial antigen ELISAs, reverse transcriptional-polymerase chain reaction (RT-PCR), and flow cytometry have been evaluated to detect viremic pigs in the early period of classical swine fever virus (CSFV) infection. Domestic pigs were experimentally inoculated with the virulent CSFV strain 'Alfort 187' and two field isolates. CSFV isolation and RT-PCR were found to be the most sensitive methods for the detection of highly virulent CSFV in the early period of infection which is characterized by the absence of clinical symptoms. Using antigen ELISAs and flow cytometry CSFV could be detected in infected animals after the onset of clinical signs. After infection with a less virulent CSFV field isolate originating from wild boar, viremic pigs could be identified by direct virus isolation. The reasons for the negative results of the RT-PCR still remain unknown. In conclusion we recommend to modify the procedure (antigen ELISA) for the detection of clinically healthy domestic pigs in accordance with the decision 98/413/EC.

Detection of low-virulent classical swine fever virus in blood of experimentally infected animals: comparison of different methods.

The effectiveness of virus isolation, commercial antigen enzyme-linked immunosorbent assay (ELISA), reverse transcriptase-polymerase chain reaction (RT-PCR), and flow cytometry in detection of a low-virulent classical swine fever virus (CSFV) in blood in the early period of infection was evaluated. Domestic pigs at the age of 6-8 weeks and young wild boars were inoculated with a low-virulent field isolate of CSFV originating from a wild boar. This virus induced serious clinical reaction in only one pig which was naturally infected with Pasteurella multocida. Nine of 13 infected domestic pigs showed viremia. All infected weanling pigs were found viraemic by virus isolation on day 6 post infection (p.i.) but virus-free by RT-PCR. The flow cytometry was apparently not as sensitive as the virus isolation. Two young wild boars infected with the virus were viremic only for the first 2 days p.i. Virus isolation and RT-PCR were of similar sensitivity. Three different commercial antigen ELISAs used were not able to detect viral antigen in any animal.