Download Difference in virus excretion and transmission between dairy cows and calves in the course of an acute FMDV infection

Appendix 26
Difference in virus excretion and transmission between dairy cows and calves in the
course of an acute FMDV infection
Orsel, K1,*, Dekker, A.2 , Bouma, A1., Stegeman, J.A1., De Jong, M.C.M3,
1: University of Utrecht, Veterinary Faculty, Department of Farm Animal Health,
Utrecht, The Netherlands.
2: Central Institute for Animal Disease Control, CIDC-Lelystad (Wageningen-UR), Lelystad,
The Netherlands
3: Animal Sciences Group, Wageningen University and Research Centre, Wageningen,
The Netherlands
* PO box 80.151,3508 TD Utrecht, The Netherlands. Tel.: +31 30 2537551, Fax: +31 30 2521887
Email address: [email protected]
Abstract:
The aim of this study was to identify difference in virus excretion and virus transmission between
calves and cows infected with FMDV and observe differences in clinical expression of FMDV
infection.
Material and Methods: We performed two experiments, one with calves of 10 weeks old and one
with lactating dairy cows, both unvaccinated. In each group, half of the group was inoculated with
FMDV isolate O/NET/2001 and the other half of the group was exposed by direct contact to these
inoculated animals. Clinical inspection, virus titration and serology tests were performed. The mean
daily virus excretion (MDV), the total virus excretion as measured by the area under the curve and
the duration of virus excretion (T, in days) were determined. To quantify transmission the
reproduction ratio R was estimated with a maximum likelihood estimate using an S-I-R model.
Results: In the calves mild clinical signs were observed and not all contact exposed calves became
infected. Dairy cows showed severe clinical signs and all in-contact animals became infected. The
mean daily virus excretion differed significantly between cows and calves in both inoculated and
contact exposed animals (Mann-Whitney-U test: inoculated cows versus calves p =0.049 and
contact exposed cows versus calves P= 0.004). Total virus excretion differed significantly for status
(inoculated versus contact-exposed) and age, while duration of infectiousness was only significant
for status. The reproduction ratio for calves Rcalves was estimated 2.52 [1.13;52.1], and for dairy
cows Rcows= ∞ [1.3; ∞].Both R values were significantly above 1 (p<0.01), but did not differ
significantly between groups of calves and cows (p=0.59).
Conclusion: Adult dairy cows showed more severe clinical signs and excreted more virus in the
infectious period compared to calves. FMDV transmission did not differ significantly in groups of
calves and cows. Differences is virus excretion might lead to a differentiated control strategy during
an epidemic of FMD
Introduction:
During the foot and mouth disease outbreak of 2001 in the Netherlands, differences in clinical signs
between and within susceptible species were observed. On a farm with veal calves only 4 out of 74
were tested positive for FMDV without notification of clinical signs, whereas clinical signs in dairy
cows were very obvious and severe (Bouma et al., 2003).
Since these reported differences were based on observational studies, and the effect on virus
transmission was not quantified, we studied the course of an infection in groups of cattle that are
important in the Netherlands: calves of 10 weeks old, representing veal calves, and multiparous
dairy cows to look into more detail at some infection parameters in the course of an acute infection.
Materials and Methods:
Data used for this study came from two transmission experiments that were published previously
(Orsel et al., 2005, Orsel et al., 2006). Animals included in this study were 24 Holstein Friesian
calves and 20 Holstein Friesian dairy cows, both non-vaccinated. Half of the groups were inoculated
intra-nasally with O/NET/2001 field strain. After one day these groups were re-united hereby
exposing the other half of their group by direct contact. We checked the cattle on a daily basis for
clinical signs. From the day of inoculation (0 dpi) till 14 dpi heparinised blood and oropharyngeal
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fluid (OPF) swab samples were collected for virus and genome detection. Clotted blood for serology
was taken up till 28 days after inoculation.
Statistical analysis: A linear mixed effect model was used to analyze our data in which both fixed
and random effects were included. As dependent variable the AUC (area under the curve) was
calculated which represents the total virus excretion in oro-pharyngeal fluid per animal for the full
sampled period of 14 days. Two binary variables ‘age’ (calf or cow) and ‘status’ (inoculated or
contact-exposed) were included as fixed variables and ‘group’ (experimental unit) as a random
effect. The mean daily virus excretion (MDV) per animal is the average virus titer per day the
animal tested positive in oro-pharyngeal fluid. This was analyzed with a Mann Whitney U test
(SPSS 12.0.1). To compare duration of virus excretion between the two age-groups survival
analysis using a Cox proportional hazard model was used (S-Plus 7.0). The period was calculated
from the first day till the last day virus was present in OPF. Days that tested negative within this
period were considered positive (false negative).
A stochastic S-I-R (susceptible-infectious-recovered) model as described by De Jong and Kimman
(De Jong & Kimman, 1994) was used, in which S is the total number of susceptible, I the total
number of infectious and R the number of recovered animals. The contact-exposed animals were
all allocated to the S category, whereas cows that either showed clinical signs and/or tested
positive in one of the laboratory tests, were allocated to the I category. After recovery from
infection the animals were classified as R. The total number of recovered animals at the end of the
experiment is the final size of the infection chain. The final size of infection was used to estimate
the reproduction ratio R by means of a maximum likelihood estimator (MLE) (Bouma et al., 1996).
The null hypothesis - the reproduction ratio in non-vaccinated calves was equal to the reproduction
ratio in dairy cows (H0: Rcalves = Rcows) - was tested one-sidedly against the alternative hypothesis
that the transmission ratio in is lower in the calves compared to dairy cows (H1: Rcalves < Rcows)
(Kroese & de Jong, 2001). The epidemiological model is described in more detail by Orsel et al.
(Orsel et al., 2005).
Results:
In the groups of calves, mild clinical signs were observed and not all contact-exposed calves
became infected, whereas in the groups of dairy cows the clinical signs were very severe and all
contact-exposed cows became infected. In the linear mixed effect model, both “age”, “status” as
well as the interaction-term had a significant effect on the AUC (age: p=0.03, status: p=0.0005,
interaction: p=0.0002).
Linear mixed effect model
Random effect: Group
AUC ~ age + status + age*status
Coefficient
Age
0.484
Status
-0.226
Interaction
0.250
Confidence interval
(0.147;0.822)
(-0.341;-0.110)
(0.135;0.366)
The mean daily virus excretion differed significantly between cows and calves in both inoculated
and contact exposed animals (inoculated cows versus calves p=0.049 and contact exposed cows
versus calves p=0.004) as shown in figure 1. However, no significant difference in MDV was
observed when comparing inoculated and contact-infected cattle of the same age-group.
From the survival analyses it was shown that both status and the interaction between status and
age significantly contributed to the duration of virus excretion (status: p=0.002, with interaction
p=0.0006). The fixed variable age was not found significant (p=0.11).
Cox proportional hazard model
Random effect: Group
Coefficient
Status
0.549
Interaction
-0.655
Age
-0.484
Confidence interval
(0.200;0.898)
(-1.027;-0.283)
(-1.076;0.108)
The exact number of days animals excreted virus in OPF swab samples is visualised in figure 2.
These results showed that differences between young and adult cattle for total amount of virus
excreted and duration of virus excretion were observed. Transmission in the groups of calves
resulted in a estimate of Rcalves of 2.52 [1.13;52.1], significantly above 1 (p=0.01), and for the
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dairy cows Rcows = ∞ [1.3; ∞], also significantly above 1 (p=0.007). When testing the hypothesis
H0: Rcalves = Rcows, (H1: Rcalves < Rcows no significant difference was observed (p=0.59).
Discussion:
In our studies huge differences were observed in severity of clinical signs between cows and
calves. Cows remained recumbent and this resulted in three cows that needed euthanasia before
the end of the experiment, whereas calves only showed mild clinical signs. No evidence of inverse
age resistance for FMDV is described in cattle in literature before. Age difference was only
suggested in literature in the observations of the 1967-1968 outbreaks of FMDV in pigs in the UK
(Hugh-Jones, 1976).
However, we cannot quantify the contribution of lactation to the observed differences.
Since severe clinical signs will most likely contribute to higher infectivity due to more virus
excretion of the infected animal, this may result in higher virus transmission. Although the survival
analysis on the duration of virus excretion did not show significant differences between calves and
cows, significant differences between mean daily virus excretion and total amount of virus excreted
(AUC) occurred. These differences might contribute to higher virus transmission. The quantification
of R, however, did not result in significant differences between calves and cows, probably due to a
low power. Nevertheless, the point-estimates of R within groups give reason to speculate that it is
more likely that small outbreaks occur among calves than among cattle. This is in consistence with
observations in the field during the outbreak of 2001.
Conclusions:
•
Differences in infectivity and/or susceptibility for FMD infection and transmission were
observed between adult lactating cows and calves.
•
The impact on the epidemic size might differ with affected age-groups and whether or not
an animal is lactating.
•
This finding might imply that a differentiated control strategy might be applicable..
Acknowledgements:
•Financial support from the ministry of agriculture in the Netherlands (BOP 7-428)
•Financial support from the European Union (EU SSPE-CT-2003-503603)
•Technicians from the FMD-laboratory in Lelystad
•Animal technicians in Lelystad
References:
Bouma, A., De Jong, M. C. & Kimman, T. G. (1996). Transmission of two pseudorabies virus
strains that differ in virulence and virus excretion in groups of vaccinated pigs. Am J Vet
Res 57, 43-7.
Bouma, A., Elbers, A. R., Dekker, A., de Koeijer, A., Bartels, C., Vellema, P., van der Wal,
P., van Rooij, E. M., Pluimers, F. H. & de Jong, M. C. (2003). The foot-and-mouth
disease epidemic in The Netherlands in 2001. Prev Vet Med 57, 155-66.
De Jong, M. C. & Kimman, T. G. (1994). Experimental quantification of vaccine-induced
reduction in virus transmission. Vaccine 12, 761-6.
Hugh-Jones, M. E. (1976). Epidemiological studies on the 1967-1968 foot-and-mouth disease
epidemic: the reporting of suspected disease. J Hyg (Lond) 77, 299-306.
Kroese, A. & de Jong, M. (2001). Design and analysis of transmission experiments. Society for
veterinary epidemiology and preventive medicine Proceedings Noordwijkerhout, 21-36.
Orsel, K., Dekker, A., Bouma, A., Stegeman, J. A. & Jong, M. C. (2005). Vaccination against
foot and mouth disease reduces virus transmission in groups of calves. Vaccine 23, 488794.
Orsel, K., Jong, M. C., Bouma, A., Stegeman, A. & Dekker, A. (2006). The effect of
vaccination on Foot and Mouth Disease virus transmission among dairy cows. Vaccine: in
press.
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Figure 1: Mean daily virus excretion for several groups
5
5
calf inoculated
calf inoculated
cow inoculated
4
calf contact-exposed
MDV in OPF (10 log)
MDV in OPF (10 log)
4
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Calves: sorted by mean daily virus excretion
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cow inoculated
cow contact exposed
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MDV in OPF (10 log)
MDV in OPF (10 log)
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Cattle: sorted by mean daily virus excretion
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calf contact-exposed
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Cows: sorted by mean daily virus excretion
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Cattle: sorted by mean daily virus excretion
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Figure 2: Duration (days) of virus excretion in inoculated and contact-exposed calves and cows
Inoculated calves
Contact exposed calves
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Contact exposed cows
Inoculated cow s
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