Serological Evaluation of a Foot-And-Mouth Vaccination Campaign In
Young Cattle In Buenos Aires Province, Argentina
León EA1, Stevenson MA2, Fernández D3, Robiolo B4, Aznar MN1, Duffy SJ1, Späth EJA5, La Torre J4
1. INTA, CICVyA, Inst. Patobiología, Área Epidemiología, Castelar, Argentina (eleon@cnia.inta.gov.ar)
2. EpiCentre, IVABS, Massey University, Palmerston North, New Zealand
3. COPROSA Buenos Aires, La Plata, Argentina
4. CEVAN-CONICET, Buenos Aires, Argentina
5. INTA, EEA Balcarce, Balcarce, Argentina.
ABSTRACT
A survey to evaluate the effectiveness of vaccination campaigns against foot-and-mouth disease was
performed in Buenos Aires province, Argentina, with two aims: a. to estimate the proportion of young cattle
having antibody levels sufficient to protect them from natural infection; b. to estimate the contribution of
district, farm, and animal to the total variation in the probability of an individual being adequately protected.
The study population was cattle six to twelve months of age that should have already a primary FMD vaccine
four to five months before sampling. Thirty nine districts were included in the study. A two-stage cluster
sampling design was used to select farms and animals within farms. Sera were analyzed by liquid phase
ELISA test for FMD serotypes A2001 and O1Campos. Immunity levels were classified as either high (≥
log101.77) or low based on antibody titre. A total of 13,832 sera were analysed.
A total of 77.4% of sera presented a high immunity level. Differences among districts were observed: median
percentage: 76.8%, the range was from 59.1% to 94.0%. Farms presenting ≥35% of samples with a high
immunity level were 1,286 out of 1,367 (94.1%). An intercept only, mixed-effects logistic regression model
was fitted to assess the contribution of district, farm, and animal on the probability of being adequately
protected against FMD. The proportions of variance occurring at the district, herd, and animal level were
6.7%, 28.1%, and 65.2%, respectively.
These results provide evidence that Argentina’s FMD vaccination strategy is effective in providing immunity
to FMD even in calves that were vaccinated once. Farm was a significant contributor to the variation in
immunity, suggesting that improving vaccination strategies at the farm level might improve overall campaign
effectiveness. District was a minor contributor to the variation in immunity.
KEYWORDS
Foot-and-mouth, vaccination, serology, herd immunity, Argentina
INTRODUCTION
Vaccination of cattle against foot-and-mouth disease (FMD) in Argentina is compulsory and is systematically
carried out twice a year. Due to the size (2.8 million km2) and the heterogeneity of the country and the large
cattle population (56 million head), FMD vaccination campaigns encounter a number of operative challenges.
For this reason, surveys to evaluate the effectiveness of campaigns are necessary. We conducted a crosssectional study of young cattle that had been vaccinated on at least one occasion against FMD. Our first aim
was to estimate the proportion of the study population that had antibody levels sufficient to protect them from
natural infection. Our second aim was to estimate the contribution of district, farm, and animal to the total
variation in the probability of an individual being adequately protected.
MATERIALS AND METHODS
Sampling frame
The province of Buenos Aires has an area of 300,000 km2. Its territory is divided into 126 administrative
districts, of which 105 have cattle farms. The total number of cattle in the province is around 20 million head,
distributed among approximately 60,000 livestock farms. The target population for this study was cattle 6 to
12 months of age that had already received a primary FMD vaccine 4 to 5 months before the date of
sampling.
Sampling design
A two-stage cluster sampling design was used. The number of farms to be included in the study, per district,
was calculated based in the following parameters: a. level of confidence: 95%; b. precision: 10%; c. expected
prevalence of animals presenting serological level of antibodies compatible with protection: 65%, based on
previous data (León et al., 1997); d. number of samples to be taken per farm: 10; rate of homogeneity: low.
Thirty seven of the 105 cattle producing districts in Buenos Aires province participated in the study. Samples
were collected at the time booster FMD vaccinations were given during March, April and May 2004.
Diagnostic tests
Sera were analyzed by single dilution liquid phase blocking ELISA (slpELISA) for FMD serotypes A2001 and
O1Campos.
The assay was carried out as previously described by Robiolo et al. (1995) for vaccine potency studies,
except that in this work we have used only a single dilution of each serum. The absorbance value of each
serum sample at a dilution 1:64 was interpolated using a standard curve that correlated antibody titres of five
control sera with their absorbance values at the stated dilution.
Since both viruses induced a similar response, only the results for O1Campos are presented.
Interpretation
Each sample was classified as high or low immunity level based on antibody titre. A titre ≥ log101.77 for O1C
serotype was classified as high.
Farms presenting ≥35% of samples with high immunity level were considered as adequately protected. This
cut off was taken because it is the lowest limit of the 95% CI of 65% (expected proportion of animals with
high immunity level) for 10 samples (sample size per farm), meaning that farms presenting <35% of samples
with high immunity level are significantly different from those presenting 65%.
Statistical analyses
Our aims were to estimate the contribution of the various levels of organisation (animal within farm within
district) to the total variation in the probability of an animal being adequately protected against FMD. We
specified a binary response variable Yijk = 1 if the kth animal on the jth farm in the ith district was adequately
protected against FMD and zero otherwise. An intercept only, mixed-effects logistic regression model was
fitted using Markov chain Monte Carlo simulation in WinBUGS (Lunn et al. 2000). Uniform prior distributions
were assumed for the intercept and the district- and farm-level random effect terms. In WinBUGS we ran the
Gibbs sampler for 50,000 iterations after discarding the first 2000 burn-in samples. Posterior sample sizes
were determined by running sufficient iterations to ensure that the Monte Carlo standard error of the mean
was at least one order of magnitude smaller than the posterior standard deviation for each parameter of
interest.
RESULTS
Samples
A total of 13,832 serum samples were analysed, from 1,367 farms in 39 districts. Of the total number of
samples 10,699 presented a high immunity level (77.4%; 95% CI 76.7% to 78.0%). The distribution of
animals with a high immunity level varied among districts (Table 1).
Table 1. Distribution of animals with high immunity level, per district
Number of samples per district
Distribution of animals with high immunity level1
(minimum, maximum and quartile values)
n min
n max
Average
Min
25
50
75
Max
107
420
355
59.1%
71.9%
76.8
82.9
94.0%
1. Titre ≥ log10 1.77 (slpELISA)
A total of 1,286 out of 1,367 farms (94.1%) were adequately protected. The median percentage of animals
with high immunity level per farms was 80%. The distribution of farms adequately protected per district is
shown in Table 2.
Table 2. Distribution of farms properly protected, per district
Distribution of farms properly protected1
(minimum, maximum and quartile values)
Number of farms per district
n min
n max
Average
Min
25
50
75
Max
11
42
35
78.0%
90.9%
94.7
100.0%
100.0%
1. Farms presenting ≥35% of samples with high immunity level
Study of variation
Within the model the level 1 (animal level) variance was constrained to unity. As a result of the variance
being measured on the binomial rather than logit scale the estimates of the proportion of variance at the
district, farm, and animal level were computed by assuming the level 1 variance on the logit scale was π2/3.
This is based on interpreting the outcome (adequately protected against FMD: yes or no) as the result of an
underlying latent process with a continuous logistic distribution (Snijders and Bosker 1999).
The variance estimates at the district and farm level were 0.336 (95% Bayesian credible interval 0.234 0.455) and 1.420 (95% Bayesian credible interval 1.254 - 1.603), respectively giving rise to the total variance
in the data as 5.046 (0.336 + 1.420 + π2/3). The proportions of variance occurring at the district, farm, and
individual animal level were 6.7% (0.336 ÷ 5.046), 28.1% (1.420 ÷ 5.046), and 65.2% (π2/3 ÷ 5.046),
respectively.
CONCLUSIONS
Our results provide evidence that Argentina’s FMD vaccination strategy is effective in providing immunity to
FMD even in calves that were vaccinated only once. Farm was the main contributor to the variation in
animal-level immunity. This suggests that improving strategies at the farm level, for example delaying
vaccination events to ensure that passive immunity levels in the bulk of the calf population have declined,
might improve the effectiveness of campaigns and hence herd immunity. Although district was a minor
contributor to the variation in animal-level immunity, re-iteration of the importance of vaccination technique to
district-level vaccination teams is advised. It is still important since FMD vaccination campaigns in Argentina
are organised at this level.
REFERENCES
León, E.A.; Duffy, S.J.; Späth, E.J.A.; Funes G.; Cosentino, B.; Dotta, J.; Robiolo, B.; Periolo, O.; La Torre, J.
(2004) Foot-and-mouth disease control in Argentina by vaccination. OIE Conference on Control of Infectious
Animal Diseases by Vaccination. Page 58.
Lunn, D; Thomas, A; Best, N; Spiegelhalter, D (2000) WinBUGS - A Bayesian modelling framework:
Concepts, structure, and extensibility. Statistics and Computing 10, 325 – 337.
Snijders, T.; Bosker, R. (1999) Multilevel Analysis: An Introduction to Basic and Advanced Multilevel
Modelling. Sage, London.
Robiolo, B.; Grigera, P.R.; Periolo, O.H.; Seki, C.; Bianchi, T.; Maradei, E. (1995) Assessment of foot and
mouth disease vaccine potency by liquid-phase blocking ELISA: a proposal for an alternative to the
challenge procedure in Argentina. Vaccine, 13:1346-52.