Development of a preferential relationship with the mother by the newborn lamb: importance of the sucking activity

Lambs develop a preferential relationship with their mothers within 24 h after birth. In a first experiment, we attempted to determine whether neonatal sucking was a potent reinforcer in the establishment of this preference by temporarily covering the udders of ewes after parturition. Lambs were assigned to one of three treatments: they had free access to the udder (group 1) or were prevented from sucking either between birth and 6 h afterward (group 2) or between 12 and 18 h after birth (group 3). Measurements of cholecystokinin plasma levels at birth and at 6 and 18 h showed that all lambs ingested some colostrum when the udder was made accessible. When tested in a two-choice test situation at 24 h of age, lambs from group 1 spent significantly more time near their mothers than near the alien ewes, those from group 2 did not show any discrimination, and lambs from group 3 clearly preferred the alien ewes. At 2 days of age, lambs from groups 1 and 3 showed a strong preference for their mothers, whereas those from group 2 still displayed no preference. In a second experiment, we attempted to determine whether sucking also had a reinforcing value in the maintenance of this preference. Lambs were assigned to one of three treatments: they had free access to the udder (group 1) or were prevented from sucking for 6 h either at birth (group 2) or at the age of 3 days (group 3). Lambs from group 2 did not discriminate between their own and alien dams when tested at 24 h of age, unlike those of groups 1 and 3. When a second test was performed at 4 days of age, all the lambs showed a strong preference for their dams. We concluded that sucking has strong rewarding properties in the establishment of a preferential relationship with the mother by the lamb. On the other hand, the maintenance of this preference does not rely on sucking as strongly.     

Roger Brown

Salmonella abortusovis strain Rv6 (Sao Rv6) is a live attenuated vaccine used for a few years to protect ewes against abortive salmonellosis. As Salmonellae, particularly Salmonella aro mutants, have considerable potential as vehicles for the presentation of heterologous vaccine antigens, Sao Rv6 was tested in order to develop a vaccinal vehicle for small ruminants. Five vector plasmids were tested in Sao Rv6; these plasmids, which carry Maltose Binding Protein (MBP) expressed as protein, but differ in their promotors, had been previously tested in S. typhimurium strain SL3261, and were transferred into Sao Rv6. The five plasmids were stable in vitro, and the recombinant Sao Rv6 expressed MBP at various levels. Intraperitoneal infection of OF1 mice with the recombinant bacteria did not modify the characteristics of Sao Rv6; dissemination and infection levels were similar in all groups and all mice developed antibodies to Salmonella antigens as measured by ELISA. In contrast, only animals immunized with Sao Rv6 carrying the pNTE plasmid developed a serum antibody response to MBP. This plasmid was then tested in sheep; following subcutaneous immunization with Sao Rv6-pNTE, dissemination and infection levels were not modified in comparison with sheep immunized with Sao Rv6 lacking plasmid. Antibodies specific to MBP were detected in sera of sheep immunized with Sao Rv6-pNTE, purified MBP, and with S. typhimurium SL3261-pNTE as positive controls. These results demonstrate that Sao Rv6 can be used as a vehicle for heterologous antigens in sheep with pNTE as plasmid vector.     

Main cardiotoxic drugs

The potential to increase passive serum antibody titres to a polysaccharide antigen in neonates, by preparturient vaccination of the dams was investigated. Dairy cows in five private herds were vaccinated with a commercial Pasteurella haemolytica culture supernatant vaccine (Presponse, Langford Inc.), at 6 and 3 weeks before their calculated due dates. Dams' sera, colostral whey, and post-colostral calf sera were assayed for antibodies of the IgG1 isotype binding purified capsular polysaccharide of P. haemolytica A1, using an enzyme immunoassay. Antibody titres were analyzed using the General Linear Model procedure (Statistical Analysis Systems Institute Inc.). Vaccinated dams had a significant increase in serum antibody titre after vaccination compared with non-vaccinates (P <0.01), and their antibody titres in colostral whey were significantly higher (P <0.05). Calves of vaccinated dams had significantly higher passive antibody titres than those of non-vaccinates (P <0.01) in all herds.     

Babesial antibody dynamics after cattle immunisation with live vaccines, measured with an indirect immunofluorescence test

The efficacy of vaccination of Argentinean cattle against babesiosis and anaplasmosis using live immunogens was tested to detect specific antibodies in samples obtained about 60 days after vaccination. Under these conditions a higher than expected proportion of cattle failed to show antibodies against Babesia bigemina. Therefore, a study was designed to evaluate if this failure was due to insensitivity of the routine test to detect antibodies to B. bigemina or to lack of infectivity of the live vaccine. Four groups (G) of cattle were each inoculated subcutaneously with 10 million Babesia bovis (vaccinal strain R1A), 10 million B. bigemina (vaccinal strain S1A) and 10 million Anaplasma centrale (strain M1). G1 and G2 consisted of ten Angus bulls 20-24 months old and ten Angus bulls 15-18 months old, respectively; G3 and G4 consisted of ten and 16 Holstein 1-month-old male calves, respectively. Blood samples were obtained on days 0, 10, 20, 30, 40, 50 and 60 after vaccination and the sera were analysed with an indirect immunofluorescent (IFA) test to detect antibodies to B. bovis (baseline dilution for a positive result 1:60) and B. bigemina (baseline dilution 1:120). Positive IFA titres were considered as evidence of the infectivity of the Babesia vaccinal strains contained in the vaccine. All Angus bulls were found positive to antibodies against both Babesia species, by day 20 (B. bovis) and day 30 (B. bigemina), whereas 10-25% of Holstein calves were negative throughout. The partial lack of vaccine infectivity in the calves was considered to be a consequence of innate resistance of young calves to Babesia. Antibody titres to B. bovis and B. bigemina declined by day 60 after vaccination. However, all cattle that were positive to B. bovis antibodies on day 50 were still positive to the IFA test 10 days later while 10%, 30% and 12% of cattle of G1, G2 and G3 that were positive to B. bigemina antibodies on day 50 after vaccination were found negative to the IFA test on day 60. In future, samples taken on days 40-50 will be used for detection of B. bigemina antibodies in vaccinated cattle, on day 60 for A. centrale and on either occasion for B. bovis. The reaction to the inoculation of B. bigemina S1A strain appears to lag behind the reaction to B. bovis R1A strain. It is not certain if this is a normal reaction to this B. bigemina strain or the result of interaction with the B. bovis strain.     

Vaccination with glutaraldehyde-fixed bovine respiratory syncytial virus (BRSV)-infected cells stimulates a better immune response in lambs than vaccination with heat-inactivated cell-free BRSV

The lamb model was used to investigate the possible protective effects of vaccination with inactivated viral antigens against experimental infection with bovine respiratory syncytial virus. Two groups of eight lambs were vaccinated with either glutaraldehyde-inactivated cell-associated virus or heat-inactivated cell-free virus and subsequently challenged with live virus, along with a group of naive lambs. The virus was shed for significantly longer periods, and the virus titres in nasal secretions were significantly higher in the group of naive lambs than in the two groups of vaccinated lambs. The period of virus-shedding in nasal secretions and virus titres was significantly lower (p < 0.01) in the group of lambs immunized with the cell-associated preparation. The same antigen stimulated better cellular immune responses as measured by virus-specific cytotoxicity or by virus-specific lymphocyte proliferation. However, priming with inactivated vaccines had no significant effect on lymphocyte responses to phytohaemagglutinin, which was found to be significantly reduced (p < 0.01) following challenge with live virus.     

Acute phase toxoplasma abortions in sheep

Within 13 days of the experimental infection of 15 oestrus-synchronised ewes with 2000 sporulated oocysts of Toxoplasma gondii at 80 to 90 days of gestation 11 had aborted. The infection induced pyrexia and specific antibody in all the ewes. One ewe resorbed its fetus, 11 ewes aborted and three delivered, at full term, live congenitally infected lambs whose pre-colostral serum was antibody-positive. Tissues from the aborted fetuses and placentae from the live lambs were examined for toxoplasma infection by polymerase chain reaction (PCR) amplification of the B1 gene and by mouse inoculation. The live lambs were all shown to be infected by both methods, but there was no evidence of infection in any of the tissues from the acute phase abortions, suggesting that these fatalities occurred before the placenta or the fetus had been invaded by T gondii. Such toxoplasma-induced, acute-phase abortions are likely to be impossible to diagnose from fetal tissues. These results have implications not only for the diagnosis of naturally occurring ovine abortions but also for the understanding of the pathogenesis of toxoplasma-induced abortion.     

A comparative study on the diagnosis of maedi-visna infection in serum and colostrum samples using agar gel immunodiffusion (AGID) technique

Serum/colostrum pairs were collected from 245 ewes in 6 sheep herds which had been determined previously to be infected with MV virus and were tested against maedi-visna infection using AGID test. Positive rates were detected as 3.8-41.2% in tested flocks. Serum and colostrum samples obtained from 53 sheep were positive for MV virus specific antibodies by AGID test. 16 colostrum samples were negative although serum samples obtained from the same animals were found to be positive for MV antibodies. Of the 245 sera and colostrum pairs tested, there was total agreement of results (+ or -) in 229 and disagreement in the results with the other 16 serum/colostrum pairs. Of the latter, all serum samples were positive and all colostrum samples were negative for MV antibodies. This study compared colostrum and serum samples for the determination of MV antibodies using AGID test under field conditions on naturally infected animals and on healthy animals. The results show that colostrum antibodies can be detected using AGID test and that colostrum is a reliable material to determine anti-MV virus antibodies. The procedure can be used for herd diagnosis.     

Human immune responses to influenza virus vaccines administered by systemic or mucosal routes

Healthy adult volunteers were immunized by parenteral or oral routes with trivalent inactivated influenza vaccine (A/Chile/1/83 (H1N1), A/Mississippi/1/85 (H3N2), and B/Ann Arbor/1/86), or intranasally with live attenuated, cold-adapted influenza type A/Texas/1/85 (H1N1) reassortant virus. In all volunteers, cells spontaneously secreting IgA, IgG or IgM antibodies specific to influenza virus were detected in peripheral blood on days 6-13 after immunization, and specific IgA, IgG and IgM antibodies to influenza vaccine were measured in sera and external secretions (saliva and nasal lavage). Following systemic immunization, a raise in specific antibodies of all isotypes was observed in sera beginning on day 13. Although small variations in IgA and IgM antibodies in saliva and nasal lavages were detected, antigen-specific IgG significantly increased between days 13 and 27. Intranasal administration of attenuated virus induced IgA and IgG antibodies in serum as well as in secretions. Serum antibodies were not substantially influenced by oral immunization, only a small increase in all isotypes was observed in volunteers' sera 21 days after ingestion of vaccine. However, in secretions, antigen-specific IgA and IgG responses were detected one week after immunization and reached a peak response on day 20. These studies show that different routes of immunization can be effective for the induction of specific antibodies, and support the concept of the common mucosal immune system in humans by demonstrating that the oral or intranasal administration of antigen-induced specific antibodies of IgA isotype in external secretions, preceded by the transient appearance in peripheral blood of specific antibody-producing cells.     

Effective immunity to dental caries: passive transfer to rats to antibodies to Streptococcus mutans elicits protection

Rat dams, given intravenous injections of heat-killed Streptococcus mutans 6715, mutant C211 demonstrated significant agglutinin activity to the homologous S. mutans in colostrum, milk, and serum. This antibody activity was associated with the immunoglobulin G (IgG) class. High titers of anti-S. mutans antibody associated with the IgG class were also exhibited in the sera and saliva of the offspring that suckled these dams. After challenge with the homologous, live S. mutans, these offspring developed significantly fewer caries on all molar surfaces than did nonimmunized infected controls. A secretory immune response (manifested by the presence of specific IgA antibody to S. mutans in colostrum and milk) was elicited (i) in rat dams locally injected, in the region of the mammary gland, with heat-killed S. mutans antigen, and (ii) in other rat dams that were provided formalin-killed S. mutans in their drinking water. Offspring suckling these dams were challenged with virulent S. mutans before weaning and developed significantly fewer caries than did their infected controls. These findings clearly suggest that passively derived IgG or IgA antibodies to S. mutans are protective against dental caries.     

Immunological properties of plaque purified strains of live attenuated respiratory syncytial virus (RSV) for human vaccine

Respiratory syncytial virus (RSV) causes severe lower respiratory tract disease in infants, young children, and the elderly. Efforts to develop satisfactory live or inactivated vaccines have not yet been proven successful. Our research focuses on the development of four purified live attenuated RSV sub-type A human vaccine clones. Temperature sensitive (ts) and attenuated purified clones of either cold-adapted (ca) RSV or high-passage (hp) RSV were administered intra-nasally (i.n.) to BALB/c mice and tested for immunogenicity. All four clones produced significant anti-RSV F IgG2a and IgG1 titres in the sera of mice, RSV-specific neutralizing titres higher than those produced by their wild-type progenitor viruses, cytotoxic T-lymphocyte (CTL) activity, and total protection against wild-type (wt) viral challenge. These purified vaccine candidates await testing in humans to determine which contain the required balance between immunogenicity and attenuation.     

Relationships among ewe milk production and ewe and lamb forage intake in Suffolk and Targhee ewes nursing single or twin lambs

Suffolk and Targhee ewes (30 each) with single or twin lambs were used in four periods beginning in late gestation and continuing through weaning to evaluate breed differences in milk production, lamb BW, and DMI by ewes and lambs. In Periods 1 (late gestation) and 2 (early lactation), ewes (Period 1) and ewes with lambs (Period 2) were individually penned, fed .45 kg of barley x ewe(-1) x d(-1) and allowed ad libitum access to chopped alfalfa. Ewes and lambs grazed native range in Periods 3 and 4. Grazed forage DMI was estimated using chromic oxide. Estimates of milk production were obtained by handmilking. Average lamb age was 4, 45, and 73 d at the beginning of Periods 2, 3, and 4, respectively. Milk production tended (P = .20) to be greater for Suffolk than for Targhee ewes. Targhee ewes produced 85% more (P = .001) wool than Suffolk ewes. From 33 d prepartum to 89 d postpartum, Suffolk ewes consistently weighed more (P = .001) than Targhee ewes. Suffolk ewe BW loss (-.15 kg/d) was greater (P = .01) than Targhee ewe BW loss (-.02 kg/d) from 33 d prepartum to 6 d postpartum. From 6 to 89 d postpartum BW gain did not differ (P = .69; .05 kg/d) between breeds. From birth to 89 d postpartum, Suffolk lambs consistently weighed more than Targhee lambs (P = .003). From birth to 89 d postpartum, ADG was greater for Suffolk than for Targhee lambs (P = .006). Targhee ewes consumed 25% more (P = .01) feed over the course of the study than did Suffolk ewes. Grazed forage DMI by Targhee lambs was 26% greater (P = .01) than DMI by Suffolk lambs. When meat production is the primary income from sheep, one potential advantage of Suffolks compared with Targhees is more rapid gain with less feed intake.     

Diagnosis of toxoplasma abortion in ewes by polymerase chain reaction

Eighteen oestrus-synchronised ewes were infected experimentally with 1500 sporulated oocysts of Toxoplasma gondii between 80 and 90 days of gestation. The infection induced pyrexia and specific antibody in all the ewes. One ewe resorbed its fetus, five ewes aborted and 12 delivered live, congenitally-infected lambs whose pre-colostral serum was antibody-positive. Tissues from the aborted fetuses and placentae from the live lambs were examined for toxoplasma infection by polymerase chain reaction (PCR) amplification of the B1 gene and by mouse inoculation. Using a simple protocol of tissue preparation without DNA extraction and a nested format, PCR was as sensitive as mouse inoculation. Placental cotyledon gave a higher sensitivity of detection than brain, lung or liver, and 16 of 19 placentae were positive by PCR compared with 13 of 18 by mouse inoculation. In mock-infected tissues, as few as 10 tachyzoites could be detected. The PCR could be applied to tissues unfit for mouse inoculation.     

Serologic responses of young colostrum fed dairy calves to antigens of Pasteurella haemolytica A1

The potential and limitations of early calfhood vaccination to induce active immunity to Pasteurella haemolytica A1 in conventional colostrum fed calves were investigated. Holstein dairy calves (n = 29) were vaccinated at 2 and 4 weeks of age, or at 6 and 8 weeks of age with a commercial culture supernatant vaccine (Presponse, Langford Inc., Guelph, Ont., Canada), or remained unvaccinated as controls. Serum antibody titres were measured using an indirect bacterial agglutination assay, a leukotoxin neutralization assay, and enzyme immunoassays for antibodies of the IgM, IgG1, and IgG2 isotypes binding purified capsular polysaccharide of P. haemolytica A1. Seroconversion (fourfold or greater increase in serum antibody titre) rates were compared using Fisher's exact test. The effects of passive antibody titres and age on response to vaccination were assessed by linear modelling. Vaccination at 2 and 4 weeks of age was associated with 40%, and 0% of calves seroconverting on the basis of agglutinating antibody titres, and leukotoxin neutralizing titres respectively, and 50%, 0%, and 0% seroconverting on the basis of IgM, IgG1 and IgG2 antibodies to capsular polysaccharide, respectively. Agglutinating antibody responses were not related to prevaccination antibody titres, or to age at vaccination. Higher responses (p = 0.08) to leukotoxin were observed in older calves (after taking differences in prevaccination titres into account). Statistical analyses of responses to capsular polysaccharide among calves with comparable prevaccination IgG1 antibody titres revealed significantly higher IgM, IgG1 and IgG2 responses in older calves. Rising titres of IgM antibodies in nonvaccinated calves after 5 weeks of age suggest natural exposure to P. haemolytica A1 or antigens which result in serologic cross-reactions as a means of priming immune responses.     

Influenza virus neutralizing antibodies and IgG isotype profiles after immunization of mice with influenza A subunit vaccine using various adjuvants

The influence of various adjuvants on the development of influenza virus neutralizing antibodies and distribution of anti-influenza virus IgG isotypes after immunization of mice with influenza A (H3N2) subunit vaccine was investigated. Serum titres of influenza virus neutralizing antibodies and titres of influenza specific IgG isotypes were determined by a neutralization enzyme immunoassay (N-EIA) and a cell-associated antigen enzyme immunoassay (CA-EIA), respectively. Serum antibody titres as measured by the two tests correlated highly (r = 0.82; P < 0.001). N-EIA titres were enhanced by 38- and 34-fold, when L180.5/RaLPS and FCA, respectively, were administered with 1 microgram of vaccine. The adjuvants Q-VAC, L180.5 [W/O/W], L180.5 alone and Montanide ISA 740 were only moderately or not effective in enhancing the immune response to the 1 microgram dose of vaccine. The Q-VAC and L180.5/RaLPS adjuvants favoured IgG2a and IgG2b isotype responses to influenza compared to the other adjuvants. We suggest that N-EIA and CA-EIA may be valuable tools to monitor the effects of adjuvants on the neutralizing antibody and antibody isotype responses after influenza vaccination.     

Risk factors for perinatal and postnatal mortality in lambs

Factors associated with preweaning mortality in lambs were identified by developing risk profiles with logistic regressions for perinatal and postnatal mortality. Compared with heavy lambs, lambs of low birth weight had almost twice the risk of perinatal mortality (odds ratio [OR] = 1.9) and lambs of average weight had a slightly lower risk (OR = 0.7). Two of four lambing location categories affected perinatal mortality, with lambs born at unmonitored areas at greatest risk (OR = 2.7). Multiple births increased the risk of perinatal mortality (OR = 1.5), especially among Targhee lambs (OR = 4.0). Breed variations in perinatal mortality were significant in Suffolk lambs (OR = 1.9) and Booroola Rambouillet lambs (OR = 2.1). Lambs born weak had an increased risk of postnatal mortality while strong lambs had a decreased risk (OR = 3.7 and 0.6, respectively) if the dam had an adequate milk supply. Poor milk supply increased the risk of postnatal mortality for lambs of average vigour (OR = 3.3), but did not change the risk for weak or strong lambs. Male lambs castrated at 30 days of age were at less risk of postnatal mortality (OR = 0.3) than females. There were slight increases in the risk of postnatal mortality for intact males (OR = 1.3), low birth weight lambs (OR = 1.6), and lambs born in sheds (OR = 1.3). Suffolk lambs (OR = 1.8) and Targhee lambs (OR = 1.6) had a higher risk of postweaning mortality.     

Protection of mice against gastric colonization by Helicobacter pylori by single oral dose immunization with attenuated Salmonella typhimurium producing urease subunits A and B

Helicobacter pylori is a Gram-negative bacterial pathogen associated with gastritis, peptic ulceration, and gastric carcinoma. The bacteria express a strong urease activity which is known to be essential for colonization of gnotobiotic pigs and nude mice. UreA and UreB, two structural subunits of the active enzyme, were expressed in the attenuated Salmonella typhimurium live vaccine SL3261 strain. Evaluation of protection against H. pylori was performed in Balb/c mice by oral immunization with a single dose of the vaccine strain. Five weeks after immunization, mice were challenged orally three times with a mouse-adapted H. pylori wild type strain and, six weeks later, mice were sacrificed to determine H. pylori infection by detection of urease activity from the antral region of the mouse stomachs. In several independent experiments, we observed 100% infection with H. pylori in the non-immunized mice and no infection (100% protection) in the mice immunized with S. typhimurium expressing recombinant UreA and UreB. Specific humoral and mucosal antibody responses against UreA and UreB were observed in mice immunized as indicated by western blots and ELISA assays. These data shows that oral immunization of mice with urease subunits delivered by an attenuated Salmonella strain induced a specific immune response and protected mice against H. pylori colonization. Single oral dose immunization with UreA and UreB delivered by a live Salmonella vaccine vector appears to be an attractive candidate for human vaccination against H. pylori infection. In addition, this model will aid to elucidate the effective protection mechanisms against H. pylori in the gastric mucosa.     

Passive protection of neonatal calves against bovine coronavirus-induced diarrhea by administration of egg yolk or colostrum antibody powder

The protective effect of egg yolk and colostrum powders prepared from hens and cows vaccinated with inactivated bovine coronavirus (BCV) antigen was evaluated in a challenge model with a virulent BCV strain. Twenty three calves from BCV-free herds were randomly divided into control and several treatment groups. All calves were orally challenged with 1 x 10(9) TCID50 of the virulent Kakegawa strain of BCV at 24 to 36 h after birth. Calves in treatment groups received either egg yolk powder or cow colostrum containing BCV specific antibodies. Daily treatment with these antibody preparations started 6 h until 7 days post-challenge. Control calves which received no antibody had severe diarrhea and all died within 6 days after infection. In contrast, calves fed milk containing egg yolk or colostrum with neutralization titers of 1:2560 or 1:10,240 respectively all survived and had positive weight gain unlike the other treatment groups. These results indicate that the orally administered egg yolk and colostrum powders protected against BCV-induced diarrhea in neonatal calves and that the egg yolk used provided a higher degree of protection compared to colostrum powder on a titer basis. Treatment with whole egg yolk from immunized hens therefore provides a more efficacious alternative to the existing methods of specific passive protection against BCV.     

Seroconversion of a trivalent measles, mumps, and rubella vaccine in children aged 9 and 15 months

The serological response to MMR vaccine was evaluated in 109 9-month-old infants having no history of measles vaccination, and in 98 15-month-old children who had received monocomponent measles immunisation at 9 months. The combined vaccine contained Schwarz, Urabe Am9, and Wistar RA 27/3 live attenuated virus strains. Preimmunisation antibody levels were extremely low for the 9-month-old children, indicating that maternally-transmitted antibodies do not persist at this age. In the case of mumps, preimmunisation antibody levels were significantly higher in the 15-month-old than in the 9-month-old group. A difference between groups in terms of postimmunisation antibody titres was observed only for rubella, with titres being significantly higher in the older group. Seroconversion rates were high in both groups and no serious events attributable to vaccination were observed. The MMR vaccine can thus be administered to children as young as 9 months of age. Evidence for the efficacy of a two-dose schedule, i.e. at 9 and 15 months, is presented.     

Assessment using ELISA of the herd immunity levels induced in cattle by foot-and-mouth disease oil vaccines

The development of a liquid-phase blocking sandwich ELISA (LPBE) to measure antibodies (Ab) produced in cattle with the O, A and C foot-and-mouth disease virus (FMDV) types of commercial vaccines used in Argentina is described. The test was specific: 99% of na?ve cattle sera (n = 130) gave titres below log10 = 1.2, and none had a titre above log10 = 1.5. Comparative studies with serum neutralization test (SNT) using sera from cattle which received one or more vaccine doses is reported. The overall rank correlation coefficient (Spearman's rho, rs) between SNT and LPBE were highly significant (rs > 0.67, P < 0.0001) for all vaccine strains. LBPE Ab titres on sera collected 90 days post vaccination were compared with results of cattle protection tests by applying a logistic regression. The minimum Ab titres at which 85% and 75% of the cattle were protected for each FMDV type were determined in order to interpret field Ab data in terms of protection. Application of this method allows large scale serological examinations to monitor antibody levels in vaccinated animals as an indirect indicator of the FMD control program status in the field. Its use in the evaluation of commercial batches of FMD vaccine is discussed.