Herd Health: PCV2 takes centre stage at the Krakow symposium
Tuesday, December 4, 2007
A summary of some findings from the numerous studies presented 5th International Symposium on Emerging and Re-emerging Pig Diseases
by S. ERNEST SANFORD
The 5th International Symposium on Emerging and Re-emerging Pig Diseases was held in the beautiful and historic city of Krakow, Poland last June 24-27. This symposium has grown significantly in stature and number of attendees since the first one held in St. Paul, Minn., in 1991.
The first three symposia were devoted exclusively to PRRS and Aujezsky's (pseudorabies) diseases. The name of the symposium was changed to its current form in 2003 when PCV2/ PMWS was added and Aujezsky's Disease (AD) was removed, as more and more of the major pig-producing countries successfully eradicated the disease. It should be noted that Canada is one country that has never had AD.
With over 1,200 attendees, this was easily the largest of the five symposia and had nearly twice as many as at the previous highest attendance in Rome in 2003. This symposium focused on PCV2, PRRS and Swine Influenza Virus. PCV2 took centre stage, however, surpassing PRRS in both the number of submitted papers (Table 1) and in capturing the attention of the delegates.
This article is devoted to brief reviews of selected papers on porcine circovirus disease presented at Krakow.
PCV2 Transmission via pigs from PCVD-active herds to pigs from PCVD-free herds
- (Dupont K. et al, Pg 90)
Pigs from four PCVD-active herds were co-mingled with pigs from two PCVD-free herds in four separate rooms for 48 hours allowing for: (i) same-pen direct contact; (ii) neighbouring pen, nose-to-nose contact; (iii) across the aisle or no direct or nose-to-nose contact. A fourth non-contact control group was placed in a fifth room.
During the study, 45 of 324 pigs developed PCVD. Of these 45 pigs, 30 were from PCVD-active herds and 15 from PCVD-free herds. Of the latter, 11 had direct (same pen) contact, three had nose-to-nose (neighbouring pen) contact and one pig was across the aisle. No control pigs developed PCVD.
DNA sequencing of the 45 PCVD pigs identified eight different PCV2 "strains." Characteristically, two to three different strains emerged as specific for each room. The PCV2 strain with which pigs from the PCVD-active herds came into the study was the strain found in them when they, in turn, developed PCVD. The pigs from PCVD-free herds which developed PCVD during the study were found with a different strain from the one they had come into the study with, or the PCV2 titre was below the detection limit of the assay.
The PCV2 strain in these pigs when they developed PCVD was the same one present in the pigs from the PCVD-active herd(s) placed in the same or neighbouring pen.
Take-home message. PCV2 infects pigs primarily via direct contact (same pen) and less frequently via indirect contact (neighbouring pen) or by air. Feces and nose-to-nose contact are the most important means for transmission and spread of PCV2. Also, PCV2 is transmitted by pigs from PCVD-active herds to pigs in PCVD-free herds. A more pathogenic PCV2 strain was not a factor in this study.
PCV2 Dynamics in a PCVD-active herd
- Hjulsager C.K. et al, Pg 46
Serum samples from sows in 10 PCVD-affected herds, and serum, nasal and fecal samples from their piglets, were tested by Polymerase Chain Reaction (PCR) to determine PCV2 antigen levels. Serum from the sows was tested about one week post-farrowing, and samples from the piglets were tested at one, three, six, nine and 12 weeks of age.
PCV2 was not detected or detected in only low titres in serum of sows. Most piglets developed PCV2 infection over time, irrespective of whether they developed PCVD or not. The level of PCV2 was no different in nasal secretions, serum or feces, whether the pigs developed PCVD or not, suggesting that PCV2 replicated to high levels in all pigs in a PCVD-active herd but that some pigs are better able to cope with the infection and become subclinically infected without developing clinical signs.
The authors stressed that PCV2 load cannot be used as a diagnostic tool to predict which pigs within a litter would develop PCVD in a PCVD-active herd.
Take-home message. In this study, PCV2 appeared in nasal secretions before serum and feces, and the virus accumulated to similar levels in pigs with and without clinically-active PCVD. Also in this study, PCV2 levels could not be used as a predictor for pigs that will develop PCVD.
These results must be viewed with caution, however, since finding the same level of PCV2 virus in blood and tissues of affected and non-affected pigs is directly opposite to the findings of a multitude of other investigators.
Long-term PCV2 infection and re-infection
- Opriessnig T. et al, Pg 45
Groups of pigs were infected once or four times with the same (homologous) PCV2 isolate, and a third group was infected four times, including three times with different (hererologous) isolates of PCV2. Serum was checked weekly for PCV2 ELISA titres and for PCV2 levels by quantitative PCR. Quantitative PCR is also known as real time PCR. Various organs were checked at different intervals and at termination of the study when pigs were 31 weeks of age.
Challenged pigs seroconverted to PCV2 by 21 days after the initial inoculation (PII). Irrespective of the number of times inoculated with the same (homologous) or the different (heterologous) PCV2 isolates, they remained seropositive to the end of the study at 140 days PII.
All challenged pigs were viremic by 21 days PII. A few were nonviremic by day 49 PII, but the majority remained viremic till the end of the study at 140 days PII. There were no statistical differences in the level of viremia or ELISA titres among the various inoculated groups. There was no increase in the amount of PCV2 DNA in the serum after reinfection with the homologous or heterologous isolates, compared with pigs infected with just a single dose of PCV2. Pigs infected multiple times with homologous or heterologous PCV2 isolates did not develop PDNS. No pigs got sick.
Take-home message. Pigs infected with PCV2 can remain viremic for a long time, even in the presence of high levels of PCV2 antibodies. Recurrent infection with homologous or heterologous PCV2 did not result in an exacerbation of clinical disease. Multiple infections also did not precipitate PDNS (porcine dermatitis and nephropathy syndrome). Multiple re-infections with PCV2 is one of the theories explaining the pathogenesis of PDNS. So, in this study, the multi re-infection PCV2 theory to produce PDNS could not be validated.
PCVD transmission
- Kristensen C.S. et al, Pg 73
The authors reported on two separate studies on pigs housed in pens in two identical container buildings placed one metre apart and connected by pipes which allowed air to flow from one container over to the other. In the first study, pigs from a PCVD-active herd, themselves affected with PCVD, were mixed with pigs from a PCVD-free herd in container A. Air from that container was allowed to flow over to container B, which contained pigs from the PCVD-free herd. In the first study, both herds were positive to European PRRSV.
The same protocol was used in the second study, except that the PCVD-active herd was co-infected with a U.S. strain of PRRSV and the PCVD-free herd with a European strain.
Only pigs from the PCVD-active herd, housed in container A, developed PCVD in the first study. No pigs mixed in the same pens in container A with pigs from the PCVD-active herd developed PCVD, nor did any pigs in container B receiving air from container A develop PCVD.
In the second study, pigs from the PCVD-free herd co-mingled with pigs from the PCVD-active herd in container A developed PCVD. Pigs from the PCVD-free herd, housed in container B and connected by airflow, also developed PCVD. Most pigs from the PCVD-active herd had seroconverted to US PRRSV during the study period.
Take-home message. These studies show that PCVD can be transmitted by direct contact when pigs from PCVD-active herds are co-mingled with pigs from PCVD-free herds and also by aerosol over at least short distances. There is also the indication that PRRS virus co-infection in PCVD-active pigs can act as a co-factor to encourage the transmission of PCVD from PCVD-active pigs to PCVD-free pigs. BP
S. Ernest Sanford, DVM, Dip. Path., Diplomate ACVP, is a swine specialist with Boehringer Ingelheim Vetmedica (Canada) in Burlington.