Download 37. Experimental transmission of FMDV in pigs

Appendix 37
Quantification of experimental transmission of FMDV O Taiwan in pigs
P.L. Eblé1*, A.A. de Koeijer2, A. Dekker1
: Central Institute of Animal Disease Control (CIDC-Lelystad), P.O. Box 2004, 8203 AA Lelystad, The
Netherlands
2
: Department Quantitative Veterinary Epidemiology, Division of Infectious Diseases, Animal Sciences
Group Wageningen UR, P.O. Box 65, 8200 AB Lelystad, The Netherlands
1
Introduction
To quantify reduction of virus transmission, transmission experiments in which an SIR model is used
to estimate reproduction ratio R with a final size method, have shown to be very suitable [1,2]. R is
the average number of secondary cases per infectious individual during its entire infectious period [3].
This implies that an infection will fade out in a population when R < 1, but can spread on a large scale
when R > 1. Besides R also the transmission rate β is an important parameter, because it can be used
in models to optimise control strategies. A GLM method [4] in which information on the time span of
the infectious process is used, allows for an accurate calculation of β. Moreover, point estimates of R
can be made, using the results of β and the infectious period T [5]. In this study, we quantified the
reproduction ratio R and transmission rate β of FMDV after several vaccination strategies.
Materials and Methods
We performed ten transmission experiments with groups of ten pigs. In each group, five randomly
selected pigs were challenged by intra-dermal inoculation in the bulb of the heel of the left hind-foot
with 0.1 ml of challenge virus containing 105 TCID50/ml FMDV O Taiwan. The remaining 5 pigs of a
group were contact exposed.
Four of the groups were non-vaccinated, two groups were vaccinated at 7 days before inoculation (7dpi), one group was vaccinated with a 4-fold vaccine dose (4-FD) at –7dpi, two groups were
vaccinated at –14dpi and one group was vaccinated with an (intra-type) heterologous vaccine at –
14dpi. For vaccination we used a commercially available double-oil-in-water emulsion [DOE]
containing 3µg of FMDV O Taiwan or O Manisa (heterologous vaccine) 146S antigen per 2 ml dose.
Contact infections were determined by inspection of clinical signs, by virus isolation of oropharyngeal
fluid (OPF) and plasma samples, (boost of) neutralising antibody titre (VN-titre) and response in an
NSP-ELISA. OPF was collected daily after challenge, heparinised blood samples and serum samples
were collected at days -14, -11, -7, -4, 0, 3, 7, 10 and 14 dpi.
OPF samples and plasma samples were assayed for the presence of virus by plaque titration on
monolayers of secondary pig-kidney cells [6]. VN-titres to FMDV O Taiwan and VN-titres to FMDV O
Manisa in serum samples were measured using a neutralisation assay [6]. Serum samples were also
tested in a commercially available ELISA (UBI® FMDV NS EIA Swine) for detection of antibodies
against non-structural proteins of FMDV (NSP-ELISA). Contact pigs were considered infected if FMDV
was isolated in OPF or plasma samples or a statistically significant rise in VN-titre was detected or a
response in the NSP-ELISA was detected.
In the final size method, the estimation of R was based on the final size of the outbreak observed in
the experiments. For calculation of R, a stochastic susceptible-infectious-removed (SIR) model was
used [7] in which R was calculated using a maximum likelihood estimator [1,8]. For the calculation of
R, all inoculated pigs were considered infectious. To determine whether transmission differed
significantly between groups, we tested the null hypothesis that there was no difference in
transmission between the non-vaccinated and the vaccinated group.
In the GLM method, the information of the VI of OPF was used. Data for similar experiments were
pooled and the results per day were entered in a GLM (Generalized Linear Model) analysis to estimate
the transmission rate using the methodology of Velthuis et al. [4]. We assumed that animals were
infectious only at he days that the VI was positive. If euthanasia was applied, the calculation was
adjusted by reducing the total group size with one. In cases where the first contact animal became
VI-positive simultaneously with the inoculated animals, we assumed that the inoculated group had
become infectious half a day earlier.
To quantify the infectious period T we used a survival analyses for censored data, using a twoparameter Weibull distribution. Subsequently, R was calculated with the results of β and T as in
Klinkenberg et al. [5]. For all three parameters (β, T and R) differences between the vaccinated and
non-vaccinated groups were analysed with a T-test.
Results
In the four non-vaccinated groups, all contact pigs became infected. In the –7dpi vaccinated groups,
also all contact pigs became infected. In the group that was vaccinated with a 4-fold vaccine dose at
–7dpi, 3 of the 5 contact pigs became infected. In the –14dpi vaccinated groups, both homologous
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and heterologous vaccinated, none of the contact pigs became infected. The results of the virus
isolation of the OPF samples are shown in Table 1. In the -14dpi homologous and heterologous
vaccinated groups, no pigs were determined that shed virus. The results of the clinical signs, virus
isolation of plasma samples and antibody detection tests are not shown, but did not detect other
contact infected pigs than the virus isolation test of OPF.
The results of the Maximum Likelihood Estimation of R with the Final Size Method are shown in Table
2. When testing H0: Rvac >=1, p significantly < 0.01 for the –14dpi homologous vaccinated group.
When testing H0: Rnon-vac<= Rvac, p significantly < 0.01 for the –14dpi homologous vaccinated, the –
14dpi heterologous vaccinated and the –7dpi vaccinated 4-fold vaccine dose groups.
The results of the estimation of the transmission rate β, the estimation of the infectious period T and
the estimation of the reproduction ratio R with the GLM method are shown in Table 2. For all
vaccinated groups, all three calculated parameters (β, T and R) differed significantly from the nonvaccinated group (p<0.01).
Conclusions
We showed that vaccination significantly reduced transmission of FMDV. Using the final size method,
we showed that homologous and heterologous vaccination at -14dpi and vaccination –7dpi with a 4fold vaccine dose reduced R significantly, as compared to the non-vaccinated group. With the GLM
method, we showed that vaccination significantly reduced β, T and R, also after vaccination at -7 dpi,
even though R not < 1. The estimates we made can be of importance in models to optimise control
strategies.
References
Bouma, A., De Smit, A.J., De Jong, M.C.M., De Kluijver, E.P. & Moormann, R.J.M. 2000.
Determination of the onset of the herd-immunity induced by the E2 sub-unit vaccine against classical
swine fever virus. Vaccine, 18: 1374-1381.
De Jong, M.C.M. & Kimman, T.G. 1994. Experimental quantification of vaccine-induced reduction
in virus transmission. Vaccine, 12: 761-766.
Diekmann, O., Heesterbeek, J.A.P. & Metz, J.A.J. 1990. On the definition of and computation of
the basic reproduction ratio R0 in models for infectious diseases in heterogeneous populations. J.
Math. Biol., 28: 365-382.
Velthuis, A.G.J., de jong, M.C.M., Kamp, E.M., Stockhofe & N., Verheijden, J.H.M. 2003.
Design and analysis of an Actinobacillus pleuropneumoniae transmission experiment. Prev. Vet. Med.,
60(1): 53-68.
Klinkenberg, D., de Bree, J., Laevens, H. & de Jong, M.C.M. 2002. Within- and between-pen
transmission of Classical Swine Fever Virus: a new method to estimate the basic reproduction ratio
from transmission experiments. Epidemiol and Infect, 128(2): 293-299.
De Leeuw, P.W., Tiessink, J.W.A. & Frenkel, S. 1979. Vaccination of pigs with formaldehydeinactivated
aluminium
hydroxide
foot-and-mouth
disease
vaccines,
potentiated
with
diethylaminoethyldextran (DEAE-D). Zentralbl Veterinarmed B, 26: 85-97.
Becker, N.G. 1989. Analysis of infectious data. London: Chapman and Hall.
Kroese, A.H. & de Jong, M.C.M. 2001. Design and analysis of transmission experiments. Society
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xxi-xxxvii.
233
Table 1: Results of the virus isolation of OPF samples as used in the GLM
Exp. 1
I
C
1
0 dpi
1 dpi
0
5
0
3
2 dpi
5
5
2
non-vaccinated
Exp. 2
Exp. 3
I
C
I
C
Exp. 4
I
C
vac -7dpi
Exp. 1
Exp. 2
I
C
I
C
0
5
0
4
0
2
0
0
0
4
0
2
5
5
5
5
5
5
2
0
1
0
0
5
5
2
2
2
0
0
0
0
0
2
3
2
3
4
3 dpi
#
#
3*
4 dpi
5 dpi
4
##
2
6 dpi
vac -7dpi, 4-FD
Exp. 1
I
C
4
4
3*
3
#2
3
#
0
##
2*
1
0
0
1
####
0
0
8 dpi
0*
9-14 dpi
VI pos:
0
###
5
5
5
5
5
5
5
5
5
5
3
####
0
5
1
###
7 dpi
5
1
##
###
5
##
###
1*
0
#
##3
3
1
= number of pigs found positive in virus isolation
= end of input GLM (all contact pigs infected)
3
= end of input GLM (all infectious pigs stopped shedding virus)
*= euthanasia pig, therefore number of pigs minus 1
#
= earlier infectious pig scores negative in VI
2
Table 2: Results Final Size Method and GLM Method
GLM Method
Final Size Method
R (CI)
β (CI)
R (CI)
T (CI)
Groups
∞ (2.1-∞)
non-vac (4x)
3.5 (2.1-5.9) 6.5 (5.7-7.3)
23 (14-39)
∞ (1.3-∞)
vac -7dpi (2x)
1.6 (0.8-3.0) 5.3 (4.7-6.0) 8.4 (4.3-16)
vac -7dpi , 4-FD (1x)
1.2 (0.2-5.4)
0.4 (0.1-1.3) 2.3 (1.0-5.4) 0.95 (0.2-4.0)
vac -14dpi (2x)
0 (0-0.9)
*
0
0
vac -14dpi, het vac (1x)
0 (0-2.2)
*
0
0
*= none of the inoculated animals shed virus, therefore, the transmission rate cannot be quantified.
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