2003 Pommier Iserpd Rome

4 th International Symposium on Emerging and Re-emerging Pig Diseases – Rome June 29th – July 2nd, 2003

EPIDEMIOLOGICAL STUDY OF PORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME VIRUS INFECTION IN FRENCH FARROWING-TO-FINISHING HERDS 1

P. Pommier 1, E. Pagot 1, A. Keïta 1 , V. Auvigne 2, T. Nell 3, B. Ridremont

4

Centre Technique des Productions Animales (CTPA), Ploufragan, France 2 Ekipaj , St Grégoire, France 3 Intervet International b.v., Boxmeer, The Netherlands 4 Intervet France S.A., Beaucouzé, France Key words: PRRS – epidemiology – virology - serology

% Sero +

100 80

8

60

6

40

4

20

2

0

0 6

Material and methods Serological and virological surveys were performed in ten farrowing-to-finishing herds infected with PRRSV. All the herds had serological or virological evidence of PRRSV infection prior to the survey. Their mean size was 209 sows, and they were all located within a radius of 50 km. In five herds the gilts were vaccinated with an inactivated PRRS vaccine (3 injections prior to first farrowing). No other PRRSV vaccination was carried out in these herds. Two rounds of testing were performed in each herd with a 4-month interval between (Spring and Summer 2001). For each round, 25 sows and 60 growing pigs were bled. The sows were selected according to their parity (five gilts, five primiparous sows, five second litter sows, five third litter sows and five sows of parity 4 or greater) and the growing pigs were sampled according to their age (6, 9, 12, 15, 18 and 22 weeks). The individuals sampled in the first and second round of testing were not the same. Both virological and serological screenings were carried out at the R&D laboratory of Intervet International on all samples, using methods previously described (4). For the virological isolation, wild-type virus was grown on alveolar macrophages (PAM); isolation of PRRSV on porcine primary lung macrophages was carried out by CPE detection. For the serological analysis, all sera were analysed by a immunofluorescence test (IFT). The specific anti-PRRS titres were determined as the reverse of the highest serum dilution (log2) at which a specific fluorescence was still visible. Titres equal to or greater than 4 were considered positive. The statistical analysis was performed using the age group as the statistical unit, the percentage of seropositive pigs in an age group as outcome variable and farm, age and period as explanatory variables (SAS, Proc GLM). A correlation analysis, at farm level, between the results of the two rounds of testing was also performed (a = 0.05). Moreover, a qualitative analysis of the serological results was performed: the 40 seroprofiles were classified blindly into nine types (five for the sows, four for the growing pigs).

10

% Viro +

viroprevalence (%)

Figure 1: PRRS sero and viroprevalence of the growing pigs, according to age Seroprevalence (%)

Introduction The Porcine Reproductive and Respiratory Syndrome virus (PRRSV) was first detected in Europe in 1990 and in France in 1991 (3). The infection has since become widely prevalent, contaminated herds usually becoming persistently infected. Nevertheless, the pattern of infection can vary from one infected herd to another. Understanding this epidemiological variation is important in evaluating the consequences of PRRS infection in the field and to managing control and/or eradication programmes. Thus, the object of this paper is to study the evolution of PRRS infection in farrowing-to-finishing herds.

9

12

15

18

22

Age (weeks)

Serology 28% of the 500 sows, and 49% of the 1170 growing pigs sampled were seropositive. There was at least one seropositive animal in all the seroprofiles of the growing pigs, and in all but two of the sows. The seroprevalence and the mean titre of the positive sows were higher for the youngest sows (parity 0 and 1, Fig 2). The seroprevalence was also strongly age-dependent in the growing pigs (Fig 1). As with the virology results there was a seasonal effect. But, contrary to the virology results, the seroprevalence was greater at the second testing round for the sows (34 vs. 24 %, p=0.01) and the growing pigs (55% vs. 43 %, p=0.001). This increase was particularly evident in the youngest growing pigs (16% vs. 1%, Fig 3).

Seroprevalence

50

Mean Titre

40

8.5

20

8

10

7.5

0

7 1

2

Parity

62

9

30

0

Results Virology The PRRS virus was isolated in none of the sows but in 3.5 % of the growing pigs, the age groups between 9 and 15 weeks being particularly affected, representing 97 % of all virus isolations (Fig. 1). The viroprevalence (i.e. the percentage of positive animals by virological isolation) was higher at the first round of testing: 6% of the growing pigs from 6 herds tested positive at the Spring round and 1.2 % from 3 herds at the Summer round. Virus was isolated at both rounds in only one herd.

9.5

3

>3

Mean titre of seropositive sows

Seroprevalence (%)

Figure 2: Seroprevalence and mean titre of the sows, according to their parity

4 th International Symposium on Emerging and Re-emerging Pig Diseases – Rome June 29th – July 2nd, 2003

% Seroprevalence (%)

Figure 3: Mean titre of the seropositive growing pigs, according to their age 100 80 60 40 20 0

Period 1

9

12

15

18

22

Age (weeks)

However, there was no significant relationship between the two successive controls in the same herds, neither for the sows (r test, p=0.37), nor for the growing pigs (r test, p=0.12). Moreover, there was no significant relationship between the seroprevalence of the sows and the growing pigs during the same testing round (R2 = 0.08, p=0,24). Qualitative analysis of the seroprofiles The seroprofiles for the growing piglets were classified into four types: “Late” : Seroconversion at 18 weeks and later (n=4) “Mid” : Seroconversion at 15 weeks (n=6) “Early” : Seroconversion at 12 weeks or earlier (n=6) “Atypical” : Young positive – older negative (n=4) The seroprofiles for the sows were classified into 5 types: “Active +++” : High prevalence for all parity numbers (n=2) “Active ++” : High prevalence for the young sows (n=7) “Active +” : No parity number with a high prevalence or only the oldest sows (n=8) “Negative” : All sows seronegative (n=2) “Atypical” : Sows Parity 0 negative, sows parity 1, 2 and 3 positive (n=1) Results according to this classification are given in Table 2. For the growing piglets, only two farms had the same profile after the 4-month interval (Farm 5 and 7). They were both “Atypical”. Five of the six profiles with an early seroconversion were observed at the summer testing. For the sows, only one herd had the same profile in both periods (Farm 10, Active ++). For seven out of ten sow-herds, the viral circulation is more active at the second testing round. These observations are consistent with the global increase in seroprevalence at the second round of testing.

Spring testing round

5

Atypical

Atypical

Negative

Active +

7

Atypical

Atypical

Active +

Active ++

2

Late

Mid

Active ++

Active +

6

Late

Late

Active +

Active ++

8

Late

Early

Active +

Negative

3

Mid

Early

Active +

Atypical

4

Mid

Early

Active +

Active ++

9

Mid

Early

Active ++

Active +++

Early

Active +

Active +++

10

Early

Mid

Active ++

Active ++

Acknowledgements The authors wish to thank the farmers and their veterinarians for their technical assistance.

References 1. Auvigne A., Cambriels L., Robert F., Seegers H., 1994, Impact of the Porcine Respiratory and Reproductive Syndrome one year after contamination, Journées Rech. Porcine en France, 26, 19-24 2. Horter D., Pogranichniy M., Chang C., Evans R., Yoon K-J., Zimmerman J., 2002, Characterization of the carrier state in porcine reproductive and respiratory syndrome virus infection, Veterinary Microbiology, 86, 213-228 3. Madec F., Albina E., Berthelot S., Pansart J.F. 1994, PRRS in Brittany (France) a sero-epidemiological study of the first wave (November 1991 – February 1993), Journées Rech. Porcine en France, 26, 13-28 4. Van Woensel P.A.M., Liefkens K., Demaret S., 1998, Effect on viraemia of an American an a European serotype PRRSV vaccine after challenge with European wild-type strains of the virus. Veterinary Record 142, 510-512.

Table 2: Typology of the seroprofiles according to testing period Growing pigs Spring Summer testing testing Farm round round

Mid

Discussion In this survey, three factors showed a clear link with PRRS viral circulation: the herd unit, the age and the period during which the testing round was carried out. The viral circulation was higher in the growing pigs than in the sows. Virus could only be isolated in the growing pigs, which also had a higher seroprevalence, and a higher mean titre in seropositive animals The age effect was observed in both the sows and growing pigs. The seroprevalence was higher for parity 0 and 1.This tendency has not been observed in previous studies, for instance one year after the virus contamination of herds (1). For the growing pigs, viral circulation was limited to a 6-week period, between 9 and 15 weeks of age. Nevertheless, the viroprevalence in these pigs was lower than that following experimental challenge (2). Viroprevalence was higher during the first testing round (Spring), while seroprevalence was higher during the second (Summer). One possible explanation is that a “wave” of viral circulation affected the geographical area during the Spring, resulting in a higher seroprevalence three months later. The two main hypotheses to explain this “wave” are area-spread and simultaneous resurgence. Large area-spread of the PRRSV was never observed in Brittany (3); considering that all the herds were infected prior to the survey, simultaneous resurgence, perhaps due to climatic conditions, seems the more likely. In our study, which concerned infected herds only, it was not possible to differentiate herds with an early infection in growing pigs, from those with a later infection, since the onset of PRRS infection frequently changed from one round of testing to the next (see Table 2). Consequently, the objective of a PRRS prophylactic programme in growing pigs should be to protect piglets during the whole risk period, i.e. from 9 to 15 weeks of age. Further studies are needed to confirm this in other areas or management systems.

Period 2

6

1

Sows Summer testing round

63