A survey of wild swine in the United States for evidence of hog cholera

The results of surveillance for hog cholera (HC) in wild swine (Sus scrofa) collected from throughout the United States from 1979 to 1987 are presented. Sera collected from 1,218 wild swine and tissues from 637 were evaluated for HC antibodies and virus, respectively. Included within this surveillance were samples from Santa Cruz and Santa Rosa Islands, California, where HC virus had been deliberately introduced into wild swine during the 1950's in attempts to eradicate these animals. All evaluations were considered negative for HC. It appears that the HC virus does not maintain itself in dispersed swine populations and that wild swine have not remained a reservoir of HC since its eradication in domestic swine in the United States.

A rapid serum neutralization test in microplates for the detection of antibodies to hog cholera virus

The fluorescent antibody serum neutralization (FASN) test for the detection of antibodies to hog cholera virus was developed utilizing 96-well and Terasaki microplates. This microtechnique, especially when performed in Terasaki plates, offers some advantage if compared with conventional FASN in coverslip cell cultures, being easier and more rapid, saving of reagents and allowing simple microscopic observation.

A study on the susceptibility of minipig kidney (MPK) and rabbit kidney (RK13) cell line cultures to the lapinized Chinese strain of hog cholera virus

The susceptibility of two established cell lines of pig (MPK = minipig kidney) and rabbit (RK13 = rabbit kidney) origin to the lapinized Chinese (LC) strain of hog cholera virus (HCV) was studied. Spleen cells from rabbits infected with the virus under study were inoculated to cell cultures of either MPK and RK13 cells and subsequent passages were made by culturing the trypsinized infected cells with the normal cells. Only the MPK cell line appeared to be susceptible to virus replication. Since no cytopathic effects (CPE) were observed, the presence of the viral antigen in the inoculated cultures was detected by immunofluorescence tests. The virulence of the virus for rabbits was enhanced after its cultivation in MPK cell cultures. When the MPK cell culture system adapted virus was tested in neutralization trials in the presence of an HCV reference immune serum it was found that the virus did not modify its antigenic structure in any extent. Finally, the culture adapted virus appeared to be more immunogenic for rabbits than the original rabbits adapted virus. Based on these results, it seems reasonable to suggest the use of MPK cell line for the propagation of the LC strain of HCV as an alternative to the use of rabbits for the preparation of HCV vaccine.

The application of monoclonal antibody panels to characterize pestivirus isolates from ruminants in Great Britain

Monoclonal antibodies were prepared against bovine virus diarrhoea virus and hog cholera virus. They were used to test 101 field isolates of ruminant pestivirus in a simple binding assay using an indirect immunoperoxidase label on fixed cell cultures. The monoclonals were divided into three panels: (1) pestivirus group specific, (2) hog cholera specific, (3) selectively reactive with ruminant pestiviruses. The reaction patterns with panel 3 were analyzed by a computer spreadsheet to determine the percentage match with seven reference patterns. Field viruses could be divided into two main groups. Group A consisted of 73 (87%) of the 84 bovine isolates and three (19%) of the 16 ovine, and was reactive with 40% or more of panel 3. Group B showed only limited reactivity with panel 3 and comprised 13/16 (81%) of the ovine (border disease) isolates together with the remaining 11 (13%) bovine viruses.

An enzyme immunoassay employing monoclonal antibodies and detecting specifically antibodies to classical swine fever virus

An enzyme immunoassay (complex-trapping-blocking ELISA, CTB ELISA) for the detection of antibodies against classical swine fever virus (SFV) has been developed. The CTB ELISA employs two monoclonal antibodies directed against different antigenic sites of SFV. A set of 2545 pig sera was tested in the CTB ELISA and in the neutralizing peroxidase-linked assay (NPLA) for neutralizing antibody to SFV. The CTB ELISA and the NPLA confirmed each other in 97% of the sera. The CTB ELISA detects low-level antibodies that can be found early after infection with low-virulent SFV strains or in postvaccination sera or sera with maternal antibodies. The CTB ELISA scored no false-positive results, whereas the NPLA scored 9 sera positive for SFV on a set of 81 pig sera that had antibodies against bovine viral diarrhoea virus.

Outbreaks of classical swine fever in Great Britain in 1986

Classical swine fever was confirmed in 10 herds in Britain between April 10 and June 25, 1986 and typical acute disease was seen in nine of them. Serological evidence of exposure to classical swine fever virus was found in a further seven herds which, together with another nine, were slaughtered as dangerous contacts. Altogether 7781 pigs in 26 herds were slaughtered at a cost of 450,101 pounds for compensation alone. In order to detect subclinical disease, the majority of traced herds were blood sampled as well as inspected. A total of 119,169 pigs were inspected in 506 herds and 8302 blood samples were collected. Three primary outbreaks were identified, all attributed to the feeding of unprocessed waste food containing imported pig meat products. There was no spread of disease from two of these primary outbreaks.

Classical swine fever: virulence and tissue distribution of a 1986 English isolate in pigs

Following the recurrence of classical swine fever in the United Kingdom in 1986, a virus isolated from a single outbreak was studied. A major factor in the spread of this disease is considered to be the presence of infectious virus in tissues taken from animals at certain stages of infection, although their condition may escape detection by routine inspection either before or after slaughter. Intranasal inoculation of the isolate into eight-week-old pigs reproduced the acute form of the disease. The pigs were killed or died between seven and 25 days after inoculation. The virus concentration was determined in a wide range of tissues taken at different stages of infection. Infectious virus was present at high concentrations in all the tissues taken and at all stages of infection. Any porcine tissue is therefore a potential source of infection even when it is taken either before the animal displays detectable signs of disease of after it develops serum neutralising antibodies.

Identification of hog cholera viral isolates by use of monoclonal antibodies to pestiviruses

A collection of 90 field isolates of hog cholera virus (HCV) was used to test the specificity of four hybridoma cell lines secreting monoclonal antibodies against pestiviruses. Reaction of virus isolates and monoclonal antibodies was controlled by an indirect immunofluorescence assay (IFA). Two monoclonal antibodies which had been generated against HC virus strain "Alfort 187" were reactive only with HCV field isolates and an HCV reference strain but not with bovine viral diarrhoea virus (BVDV) reference strains. Two other monoclonal antibodies (generated against BVDV, strain NADL) reacted only with BVDV reference strains but not with HCV field isolates, although with 3 of these strains focal reactions involving only a few cells were detected. The ability to discriminate between both viruses is a diagnostic need which may be fulfilled by these monoclonal antibodies.

The protective value of vaccine-induced neutralising antibody titres in swine fever

The relationship between vaccine-induced antibody titres against swine fever virus (SFV), as measured by the neutralisation peroxidase-linked assay (NPLA), and protection against virus multiplication, excretion and transmission, disease and death was studied in 46 pigs. The pigs were housed individually and challenged intranasally with 100 pig ID50 of the virulent Brescia strain of SFV. In order to detect virus transmission, a swine fever (SF)-susceptible sentinel pig was placed in contact with the vaccinated animal 2 days after challenge. All 11 pigs with pre-challenge NPLA titres less than 12.5 responded to the challenge with fever, 8 out of 10 showed leucopenia, 7 transmitted virus to their contact and 3 died. Of the 9 animals with titres greater than or equal to 12.5 and less than 25, 8 developed fever, 6 out of 7 had leucopenia, 2 excreted and/or transmitted virus and all survived. Of the 12 pigs with pre-challenge titres greater than or equal to 25 and less than 50, 5 responded with fever, 6 out of 10 had leucopenia, 4 excreted virus and none died. Although all pigs with prechallenge titres greater than or equal to 50 showed a booster response, virus transmission was not observed, indicating that in the case of exposure such animals would not contribute towards the spread of field virus. From an epidemiological point of view, titres less than 32 were found inadequate.

Inhibition of virus replication in the tonsils of pigs previously vaccinated with a Chinese strain vaccine and challenged oronasally with a virulent strain of classical swine fever virus

After an oronasal (O.N.) infection with classical swine fever (C.S.F.) virus, virus multiplication can be detected in the tonsils from Day 2 post infection (p.i.) till death. The course of viral replication during the first 10 days after O.N. challenge exposure of pigs, previously vaccinated with a Chinese strain vaccine in the presence or absence of maternal antibodies, was studied using direct immunofluorescence techniques on cryostat sections and virus isolations. When piglets were challenged O.N. in the presence of maternal antibodies, virus replication in the tonsils still occurred. The multiplication period and the localization of the virus, however, were directly correlated to the maternal antibody levels. The maternal antibody level also seems responsible for the efficacy of the vaccination to prevent challenge virus replication in the tonsils: vaccination in the presence of low maternal antibody titers completely inhibited virus replication; vaccination in the presence of high maternal antibody titers only reduced the multiplication period of the O.N.-administered virulent virus. In both cases, animals were challenged 1 week post vaccination. Vaccination of seronegative animals resulted in an almost complete inhibition of the virus replication in the tonsils during a full fattening period: cryostat sections revealed a limited virus replication in three out of 20 animals. In one of these animals, virus replication was probably so negligible that virus isolation remained negative.

Influence of the vaccination regime on the herd immune response for swine fever

Surveys in three areas with emergency vaccination programmes revealed that 86-94% of the pigs vaccinated in the initial mass campaign had developed an adequate antibody titre against swine fever as compared with 29-55% of the pigs involved in supplementary vaccination campaigns. The serological response of piglets born from immunised sows rose with the age of vaccination from 11% at 5-6 weeks to 42% at 7-8 weeks and 77% at 9-10 weeks. In piglets born from immunised sows, a 50% 'take' of the vaccine was not obtained before the age of 8-9 weeks. Re-vaccination of gilts born from immunised dams improved the immune status to almost the level observed after mass campaigns. To strengthen herd immunity in vaccination areas, the age of supplementary vaccination has been raised to 7-9 weeks, while breeding gilts are re-vaccinated when 6-7 months old.

Preliminary serological characterization of bovine viral diarrhoea virus strains using monoclonal antibodies

Five monoclonal antibodies against the bovine viral diarrhoea (BVD) viral strain NADL were isolated and characterized by an indirect immunofluorescence assay. Extensive cross-reactions were detected when the antibodies were tested with 12 heterologous BVD and four hog cholera (HC) viral strains. One antibody reacted with all strains tested. Two antibodies were specific for cytopathogenic BVD viruses, but failed to react with HC virus. The other antibodies reacted to varying degrees with BVD and HC viral strains.

Production of monoclonal antibodies against swine fever virus and their use in laboratory diagnosis

Thirteen hybridoma cell lines which secrete monoclonal antibodies (MCAs) against swine fever virus (SFV) strain Brescia were produced. The hybrid cells resulted from fusion of P3X63-Ag8.653 myeloma cells with splenocytes of Balb/c mice which had been immunized with purified SFV. Screening of supernatant fluids was performed by an indirect enzyme-linked immunosorbent assay (ELISA) and an immunoperoxidase monolayer assay (IPMA). The IPMA, and an immunoperoxidase test (IPT) performed on cryostat sections, were used to characterize these MCAs on several pestivirus strains. All MCAs reacted to a varying degree with all but one of the SFV strains tested. None of the MCAs reacted with the bovine viral diarrhoea virus (BVDV) strains. Two MCAs are now used routinely in the differential diagnosis between infections with field strains of SFV, and the Chinese vaccine strain and BVDV strains.

A micromethod for measuring swine fever antibody by neutralisation and immunofluorescence

A micromethod employing the neutralisation and direct immunofluorescence technique for the detection of antibodies against swine fever in pig serum samples is described. The micromethod is simple and reproducible when compared with the macromethod. A total of 80 blood samples were randomly collected for comparison. The results are promising and reveal a 100 per cent correlation with the macrotechnique.