CAN VACCINATION PREVENT TRANSPLACENTAL TRANSMISSION OF BTV-8?

CAN VACCINATION PREVENT TRANSPLACENTAL

TRANSMISSION OF BTV-8?

S. Galleau,

Senior Veterinary Scientist, EU Clinical R&D

Merial S.A.S. – Centre de Recherche de Saint-Vulbas

TM BTVPUR ALSAP are tradenames of Merial

Introduction

Bluetongue

OIE listed disease

Vector-borne infectious disease of ruminants (bites of Culicoides midges)

Ability of BTV to « overwinter » recognized for decades ⇒ mechanisms?

© IAH 2008; reproduced from Wilson, Darpel & Mellor (2008), PLoS Biology 6(8):e210. Published under a Creative Commons Attribution License (CCAL2.5).

Strong evidence with the field strain of BTV-8 currently

circulating in northern Europe (Menzies et al., 2008; De

Clercq et al., 2008)

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Study contextControlled safety study conducted in pregnant cattle of a BTV-free dairy farm, beginning at the end of March 2008

BTV-8

BTV-1 Source: DGAL

Farm located in French Prealps, in a BTV-8

free area

3

96 pregnant cows or heifers randomly allocated to two study groups according to their gestation stages:

Parameters monitored in both groups:

- Calving data and health status of the calves until 15 days old

- BTV-8 serology (SN) in dams on D0, D28 and D49 and at calving

- BTV-8 serology (SN) in progeny at 2 days old +/- 1 day and 15 days old +/- 2 days

Study context

- General reactions and rectal temperature following vaccination

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Vaccinates Controls

48 animals 48 animals

Inactivated experimental bivalent vaccine containing purified BTV-4 and BTV-8 antigens, adjuvanted with aluminium hydroxide and saponin

Remained unvaccinated

Subcutaneous injection (1 mL) twice 28 days apart (D0 and D28)

31-Mar 08

1st inclusion (86 cows)

2nd inclusion (10 heifers)

19-May 08 16-Dec 08

Last calving recorded

BTV-8 SN results following vaccination - 1st inclusion

0.0

0.5

1.0

1.5

2.0

2.5

23-Mar 2-Apr 12-Apr 22-Apr 2-May 12-May 22-May

Date

Ab

tit

res (

log

10

PD

50

%)

Controls

Vaccinates

Vaccination

BTV-8 SN results following vaccination - 2nd inclusion

0.0

0.5

1.0

1.5

2.0

2.5

12-May 22-May 1-Jun 11-Jun 21-Jun 1-Jul 11-Jul

Date

Ab

tit

res (

log

10

PD

50

%)

Controls

Vaccinates

Vaccination

⇒ BTV-8 free status of the herd confirmed on the period 31-Mar to 07-Jul 08

Study context

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BTV-8 infection in the farm

1st inclusion 2nd inclusion

31-Mar 08 19-May 08 16-Dec 08

Last calving recorded

Beginning of September: clinical suspicion of BTV-

8 infection in a non vaccinated cow of the

farm confirmed by laboratory analyses

(serology by ELISA and viraemia by RT-PCR)

12-Sept 08

All cows remaining in the study: BTV serology by ELISA and viraemia by RT-PCR (Local State

Diagnosis Laboratory)

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BTV-8 infection in the farm

All the vaccinated cows protected against BTV-8 viraemia

Almost all control cows ELISA-positive and tested positive for BTV viral RNA in the blood by RT-PCR ⇒ BTV-8 infection had spread in the herd

freq

uen

cy

Dams - PCR results on 12-Sept 08

PCR resultsnegpos

0

4

8

12

16

20

24

Controls Vaccinates

freq

uen

cy

Dams - ELISA results on 12-Sept 08

ELISA resultsdtnegpos

0

4

8

12

16

20

24

Controls Vaccinates

7

BTV-8 infection in the farm

When did BTV-8 infection occur?

12-Sept

Almost all unvaccinated controls

tested positive by ELISA and RT-PCR

BTV-8 infection confirmed in one non vaccinated

cow of the farm

31-Mar 19-May 16-Dec 2008

Last calving recorded

1st inclusion

2nd inclusion

11-Jul

First unvaccinated control tested positive by BTV-8

SN test at calving

22-Aug

All controls serologically

negative to BTV-8 at calving

⇒ spread of BTV-8 into the farm probably during the first 10 days of August

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BTV-8

BTV-1

BTV-8 epizootic in France in 2008

The BTV-8 infection in the farm was perfectly correlated with the evolution of the BTV-8 epizootic in France at the same period

BTV-8 situation in France at the end of 2007, no evolution until March 2008

Source: DGAL

Location of the farm where the

study took place

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BTV-8

BTV-8 epizootic in France in 2008

BTV-8 situation in France on 18-Nov 08

Source: DGAL

The BTV-8 infection in the farm was perfectly correlated with the evolution of the BTV-8 epizootic in France at the same period

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Adaptation of the study design

Despite the BTV-8 infection in the farm, the safety study was pursued

To assess the possible transmission of BTV-8 from the dams to the progeny: in addition to the serological monitoring of the calves, virological monitoring (RT-PCR) following birth

freq

uen

cy

Calves - Age at blood sampling after birth for PCR

Age (days)1234

0

4

8

12

16

20

24

Controls Vaccinates

11

RT-PCR results in the calves

BTV-8 status of the control calves according to the date of birth

30-Aug 14-Sep 29-Sep 14-Oct 29-Oct 13-Nov 28-Nov 13-Dec

Date (2008)

Negative

Positive

All the calves born from vaccinated cows were RT-PCR negative

10/24 calves born from unvaccinated cows were RT-PCR positive

freq

uen

cy

Calves - RT-PCR results after birth

PCR resultsnegpos

0

4

8

12

16

20

24

Controls Vaccinates

Fisher’s exact test, p = 0.001

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Origin of viraemia in the control calves

Several hypotheses for BTV-8 infection of the control calves:

Transmission through bites of Culicoides midges

Contact transmission from infected dams to the calves (suggested in literature (Menzies et al., 2008))

Transplacental transmission

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Origin of viraemia in the control calves

Was BTV-8 infection of calves due to a transmission by midges?

⇒ Very unlikely because:

- Viral RNA was detected very early after birth, which is not compatible with the normal replication cycle of the virus

- The RT-PCR results are not compatible with this hypothesis: positive cases with low Ct values observed very early after birth

Age at blood sampling (days)

freq

uen

cy

Control calves - RT-PCR results after birth

PCR resultsnegpos

0

2

4

6

8

1 2 3 4

RT-PCR results in positive control calves

Ct

valu

e

Age at blood sampling (days)

1 2 418

22

26

30

34

38

42

Positive threshold 14

Origin of viraemia in the control calves

- Equivalent susceptibility of the calves (“vaccinates” vs. “controls”) to infection after birth

Calves - BTV-8 SN results after birth

Ab t

itre

s (l

og10 P

D50%

)

Controls Vaccinates0

1

2

3

4

Following colostrum uptake, load of specific BTV-8 antibodies even higher in calves born from controls than those born from vaccinated cows

Was BTV-8 infection of calves due to a transmission by midges? (ctd)

⇒ Very unlikely because:

- However, only control calves were RT-PCR positive

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Origin of viraemia in the control calves

Was BTV-8 infection of calves due to a contact transmission?

⇒ Very unlikely for the same reasons as previously mentioned

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RT-PCR results in positive control calves

Ct

valu

e

Age at blood sampling (days)

1 2 418

22

26

30

34

38

42

Positive threshold

Origin of viraemia in the control calves

⇒ The most likely origin because:

The low Ct values (i.e. high level of viral RNA) measured very early after birth is only compatible with a transplacental infection of the calves

Was BTV-8 infection of calves due to a transplacental transmission?

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The field experience reported here provides an additional evidence of the ability of BTV-8 currently circulating in northern Europe to be placentally transmitted to the progeny

At the time of BTV-8 spread in the herd, the dams were in their second or third part of pregnancy. Under these conditions, the vertical transmission of BTV-8 was a very efficient processsince out of the 23 BTV-8 positive controls, 10 gave birth to a BTV-8 positive calf, i.e. 43%.

Transplacental transmission of BTV-8

Gestation stage of the cows on 08-Aug 08

Ges

tation s

tage

(day

s)Controls Vaccinates

160

180

200

220

240

260

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Vaccination can prevent transplacental transmission of BTV-8

Immunization of pregnant dams with the experimental Merial vaccineresulted in a full protection of the progeny since none of the calves born from the 23 vaccinates were BTV-8 positive

freq

uen

cy

Calves - RT-PCR results after birth

PCR resultsnegpos

0

4

8

12

16

20

24

Controls Vaccinates

This is an additional and striking demonstration of complete prevention of viraemia in the animals vaccinated with BTVPUR AlSap vaccine of Merial

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Conclusion

Although the initial experimental design of our study did not intend to assess the vertical transmission of BTV-8 in cattle, the observation of 48 naïve pregnant cows from March 08 to December 08, while a field BTV-8 contamination occurred, allowed to demonstrate that BTV-8 infection can result in 43% of transplacental contamination

Under the conditions of the study, immunization of pregnant cattle with the inactivated Merial vaccine provided a full protection against vertical transmission of BTV-8

BTV-8 vertical transmission at such high frequency should be taken into account for the sanitary management of the disease

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Acknowledgments

C. Hamers(1), A. Blosse(1), A. Bolon(2), M. Blanchet(1), S. Goutebroze(1)

(1) Merial S.A.S. – Centre de Recherche de Saint-Vulbas, France(2) Independent Investigator, C/O Merial

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