Microneutralization tests for serological typing and subtyping of foot-and-mouth disease virus strains

Serological and biochemical analysis of some recent type A foot-and-mouth disease virus isolates from the Middle East

In 1986 and 1987 foot-and-mouth disease virus (FMDV) serotype A was isolated from outbreaks of disease in Saudi Arabia and Iran. Selected virus isolates were antigenically distinct from the prototype A22 virus strain (A22/Iraq/64), but were serologically related to each other. However, polyacrylamide gel electrophoresis showed that whilst the respective Saudi Arabian structural polypeptides were homogeneous, those from an Iran isolate were distinct. Direct sequencing of part of the P-1D (VP1) gene demonstrated considerable difference in nucleotide homology between the two groups of viruses; the Saudi Arabian viruses were closely related to each other but only distantly related to both the A22 prototype virus strain and the Iranian virus isolate. The latter viruses were only slightly more closely related to each other. Thus there appeared to be at least two distinct FMDV type A variants co-circulating in the Middle East, both of which differed considerably from the classical A22 subtype.

Nonspecific immunostimulation against viruses

A trypsinized preparation from chromogenic selected strain of Mycobacterium phlei (NSI) stimulated the recipient immune system non-specifically against a variety of viruses viz. Rabies virus (RNA virus). Marek's disease (DNA virus) and Foot and Mouth disease virus (RNA virus) in phylogenetically different hosts like mice, chicks and guinea pigs respectively. Investigation into mechanisms of such nonspecific immunostimulation revealed that there was induction of strong cell mediated immune response (CMIR) against both specific as well as to nonspecific viral antigens as evinced by LMIT, lymphokine (LyIF) assay and lymphocyte transformation test etc. There was induction of appreciable quantity of "Infectivity inhibiting substance(s)" (IIS) which was not the classical antibody (Ig) in the serum of NSI inoculated animals and birds. This substance(s) neutralized FMD, IBR, rabies and Newcastle disease virus in cell culture, mice and in embryonated eggs. Electrophoretic separation of the NSI induced serum revealed an increase in beta fraction of globulin.

The differentiation of foot and mouth disease virus strains using an indirect sandwich enzyme-linked immunosorbent assay saturation model

Sigmoid saturation curves were fitted to the results of titrations of antiserum to foot and mouth disease virus against homologous and heterologous virus strains. Differentiation of strains was readily evident from the different levels of the homologous and heterologous curves. These differences could be quantified by comparison of the saturation curve parameters K and PRmax. Factors which affect variations in K and PRmax and their biological significance were investigated by varying the first phase antibody and the antigen used in the test. PRmax was found to represent an overall combining potential of the antigen with both sera used in the sandwich test. K, which was theoretically a measure of affinity, also reflected antibody titre. Relationships measured using this model were found to correlate with the reference test system--two-dimensional microneutralization.

The effect of antiserum quality on strain specificity assessment of foot and mouth disease virus by the neutralization reaction

The factors affecting the virus strain specificity of antibody to foot an mouth disease virus prepared by a variety of protocols in several species were evaluated by neutralization tests. The time at which the serum was taken, the antigen dose given, whether or not revaccination had occurred and the animal species in which the sera were prepared, did not appear to affect the strain specificity of serum prepared to inactivated antigens when measured in neutralization tests, probably because of the restricted nature of the antigenic site involved. However, variation was observed with convalescent animal sera or sera from animals which had received trypsin cleaved virus were used. For these reasons banks of reference antisera are prepared as pooled sera using one or two inoculations of inactivated antigen.

Antigenic variation in foot-and-mouth disease: studies based on the virus neutralization reaction

The neutralization reaction is the most appropriate in vitro reference test system for assessing intratypic antigenic variation as it involves the antigenic determinants responsible for virus strain specificity and evoking protective antibody. Antigenic relationships determined in different neutralization test systems were independent of the system used and were assumed to truly reflect antigenic variation. The two-dimensional microneutralization test was found to be appropriate for foot and mouth disease (FMD) virus strain differentiation. To minimize test to test variation, comparisons are performed as matched pairs. The pooled variance of the test system is used to assess the significance of the relationships obtained. Antisera from convalescent animals were less specific than those from vaccinates. Serum quality seemed less critical for the virus neutralization than the complement fixation reaction. A system for FMD virus strain differentiation based on the use of the virus neutralization reaction taking into account the statistical and biological significance of observed r values is recommended.

Evaluation of the antigenic variation within type-A foot and mouth disease virus isolates from Asia

The serological interrelationships among 17 type A FMD virus strains from eight Asian countries were studied by the two-dimensional microneutralization test. Complex direct and indirect relationships were observed. Overall, however, the virus strains studied could be classified as belonging to the A22 group on the basis of r value differentiation at P less than 0.01.

A serological and biochemical study of new field isolates of foot-and-mouth disease virus type A in Peru, 1975 to 1981

Three foot-and-mouth disease virus type A isolates recovered from field outbreaks in the Department of San Martin, Peru, during the period 1975 to 1981 were compared with each other, and the South American vaccine strains A24 and A27, by complement fixation (CF), virus neutralization (VN) and polyacrylamide gel electrophoresis (PAGE). Complement fixation and VN tests gave comparable results distinguishing the field isolates from each other and from the vaccine strains. Analysis of the structural polypeptides by PAGE also showed clear differences between all the viruses examined. Samples from tissue culture passaged and mouse adapted strains of one of the field isolates gave identical patterns in PAGE, but differences were observed in the polypeptide pattern of the A24/BRA/55 strain and the Peru vaccine strain, which were serologically indistinguishable. Results illustrate a continued antigenic variation in an endemic area where vaccination has been used; however, asymmetric serological reactions between the A24 vaccine strain and the most recent field isolate indicated that a vaccine incorporating A24 should still give adequate protection.

A serological evaluation of 1979-1982 Kenyan foot-and-mouth disease type SAT 2 viruses

Serological evaluations of foot-and-mouth disease type SAT 2 viruses isolated in Kenya between 1979 and 1982 were performed using the two-dimensional microneutralization test. Nine field isolates of epizootiological significance were compared with four vaccine viruses. The results obtained identified Tan 5/68 as the most appropriate reference vaccine virus strain since it had the broadest serological spectrum. Potent Tan 5/68 vaccines would be expected to provide adequate protection against the contemporary SAT 2 field viruses. In the case of K183/74, which also was shown to have a broad spectrum with viruses isolated in Kenya, the results show that the 1982 isolate from central Kenya was significantly divergent (r less than 1.00 at P = 0.01) and warranted tactical revaccination for its control. The study highlighted the fact that strain R1215 which had been isolated from the oesophageal-pharyngeal swabs of asymptomatic carrier cattle had a narrow serological spectrum suggesting that such viruses could be unsuitable as vaccine for the national campaign.

Guanidine and heat sensitivity of foot-and-mouth disease virus (FMDV) strains

A study of the ability of 49 strains of FMD virus to replicate in BHK-21 monolayer cells maintained under a standard agar overlay containing 5.2 mM guanidine hydrochloride and to withstand heat inactivation at 54 degrees C for 1 h showed that strains belonging to serotypes C, O and Asia 1 were generally more resistant to guanidine and heat stable than the SAT 1, 2 and 3 serotypes. The type A viruses as a whole occupied an intermediate position between these two groups. In vitro passage in BHK-21 cells influenced the guanidine sensitivity of 3(O, C and SAT 3) of the 7 FMD serotypes suggesting that this is not a stable genetic marker. Heat stability of the FMD viruses, however, did not change on passage, suggesting that this is a stable characteristic inherent in any homogeneous FMD virus population.

Foot-and-mouth disease typing and serology in Zimbabwe

A small foot-and-mouth disease diagnostic unit was established in Zimbabwe. Twenty-six out of 27 field specimens were successfully typed using the complement fixation technique. The antibody response in cattle to the SAT II vaccine was determined using a microtitre neutralisation method. Techniques are described which should be within the capacity of laboratories in developing countries.

The application of a single radial haemolysis technique to foot-and-mouth disease virus-antibody study

Experiments are described for the evaluation of the single radial haemolysis (SRH) technique applied to FMD virus, using a method involving the coupling of either antigen or antibody to sheep erythrocytes. The antigen coupling method detected specific antibody but the technique was found to be impractical as it gave reproducible results only with purified virions. However, the antibody coupling technique gave clear zones of lysis using tissue culture harvest virus as well as purified virus, and was also shown to be type specific for detecting antigen. A comparison of the SRH test with the single radial immunodiffusion (SRID) and complement fixation (CF) tests indicated that the SRH was a more sensitive test for assaying FMD antigen than either the SRID or CF tests. Although the SRH and SRID test results correlated well, those obtained in the SRH and CF tests did not. The test was not found to be superior to the CF test in assessing the quality of antigen since, like the CF test, it reacted both with the immunizing 140S antigen and the non-immunogenic 12S protein subunits.

Stability and immunogenicity of empty particles of foot-and-mouth disease virus

Three strains of foot-and-mouth disease virus were shown to contain significant amounts of naturally occurring 75S, empty particles as well as the infectious, 140S full particles. One of these strains--A Pando (1970)--was studied in detail. The empty particles from this virus strain were shown to have an observed sedimentation coefficient of 67S in 0.04 M phosphate buffer; they were labile in SDS, non-infectious and probably RNA-free and, on heating, they broke down to 12S subunits as did the 140S particles. The empty particles differed from the full particles in their polypeptide composition since they contained VP0, but there was no evidence for a diminished content of VP4. The 75S particles were shown to be present in significant amounts and to be stable to AEI inactivation. At 4 degrees C they were stable for at least two years. In guinea pigs they were as immunogenic as the 140S particles. The antisera raised against the 75S particles had the same serological specificity in neutralization tests as sera prepared against the 140S particle. It was concluded that the 75S particles from the A Pando (1970) strain of FMD virus may provide as important a contribution as 140S particles to the immunogenicity of inactivated vaccines prepared from this virus strain.