Herd Health: Dual PCV2a/b infection may have triggered 2004 PCVAD epidemic

Wednesday, February 4, 2009

That's a possible conclusion from one of the presentations at the 20th IPVS Congress in Durban, South Africa, in June 2008. (See Better Pork, October 2008, for earlier coverage of this conference.)

by S. ERNEST SANFORD

In this article, I'll discuss a few more PCV2 papers from the Durban conference and introduce several presented on Torque Tenovirus (TTV), a virus very similar to PCV2.

Induction of PCVAD in gnotobiotic pigs by dual PCV2a/2b infection
– Harding J.C. et al., Pg. 21.
The researchers dually infected gnotobiotic pigs with PCV2a and PCV2b isolates. Pigs were challenged with either PCV2a followed by PCV2b (PCV2a/b), PCV2a followed by PCV2a (PCV2 a/a) or with PCV2b followed by PCV2a (PCV2b/a). Pigs were also challenged with PCV2a and PCV2b isolates separately.
Results. Pigs infected with PCV2a or PCV2b alone did not develop clinical porcine circovirus associated disease (PCVAD).  PCV2a/b infection produced gross and histological lesions significantly greater than PCV2a/a or PCV2b/a infections. Overt Post-Weaning Multisystemic Wasting Syndrome PMWS developed in four out of nine (44 per cent) PCV2a/b infected pigs.
Take home. Dual PCV2a/b infection may have been the trigger that sparked the PCVAD epidemic across North America which started in Quebec and Ontario in the fall of 2004.

Pigs >140 days old fail to transfer PCVAD
– Geiger JO, et al., Pg. 17.
Field observations indicate that pigs 140 days or older, which survive a PCVAD outbreak, do not transfer PCV2 to other pigs. The researchers tested this observation by inserting sentinels from a herd unaffected by PCVAD into a barn with PCVAD-positive, recovered pigs (barrows) >140 days old for a period of 45 days.
Results. None of the introduced sentinels developed PCVAD. 
Take home.  This study supports field observations that PCVAD is not transferred by PCVAD survivors 140 days of age or older. Thus, late-stage finishers that had recovered from PCVAD would not pose a threat to PCVAD-negative pigs. This has important implications for breeding stock suppliers, since it implies that select age gilts (usually 180-200 days old) will not transfer PCVAD.
Critique. One thing undermining the results of this study is that the 125-day-old pigs used as "sentinels" from a subclinical PCVAD herd were almost certainly not PCV2-naïve pigs and were likely already immune to PCV2. Hence, they would have been unlikely candidates for re-infection by the virus when they were placed in contact with the PCVAD survivors. So, since they may not have been true sentinels, we cannot definitely conclude that the recovered 140-day-old pigs would not have transferred PCVAD, had the sentinel pigs been totally naïve to PCV2.

Torque Tenovirus (TTV) infection in Spain
– Segales J. et al., Pg. 41.
Torque Tenovirus (TTV) is a small, non-enveloped, single-stranded, circular DNA virus that is ubiquitous in human and animal populations, including pigs, such that they are considered "normal flora" in the infected species. TTVs are classified in the Circoviridae as Anellovirus and are in many respects very similar to circoviruses. In humans, five genotypes of TTV have been identified and two in pigs (TTV1 and TTV2). TTVs have not been linked to any disease in humans or pigs.
Serum banks (162 sera from 99 unrelated Spanish herds) were tested to determine the historical presence of TTVs (TTV1 & TTV2) in Spain.
Results. Both TTV1 and TTV2 were identified from the very first year of stored sera and in almost all subsequent years thereafter. Sows had a slightly higher percentage of TTV1 than growing pigs (34.2 per cent versus 30.9 per cent), whereas TTV2 was higher in growing pigs than in sows (62.8 per cent versus 46.6 per cent). The percentage of pigs co-infected with both TTV1 & TTV2 was similar in both groups (19.8 per cent in sows and 24.5 per cent in growing pigs).
Take home. This is further confirmation that TTVs are ubiquitous in pig populations and probably have been present in pigs, unknown and undiscovered, for a long time.

TTV potentiates PCV2 and PRRSV infections
– Krakowka S. et al., Pg. 99.
Groups of two- to four-day-old gnotobiotic pigs were inoculated with TTV1, followed one week later by PCV2 or porcine reproductive and respiratory syndrome virus (PRRSV), to determine if TTV infection affects the pathogenesis of either PCV2 or PRRSV infections. The reverse order of infections (PCV2 or PRRSV followed by TTV) was also done.
Results. With TTV1 followed by PCV2 one week later, 50 per cent of challenged pigs (six of 12) developed acute PMWS. No diseases developed with PCV2 first followed by TTV1. PRRSV-infected pigs developed anemia, subcutaneous hemorrhages, edema and icterus with a 23 per cent mortality and PDNS lesions in skin and kidneys.
Take home. TTV1 followed by PCV2 infection precipitated PCVAD (PMWS). The reverse (PCV2 followed by TTV1) did not. Co-infections of TTV1 with PRRSV produced porcine dermatitis and neuropathy syndrome (PDNS) lesions.

Vertical transmission of TTV
– Martinez-Guino L. et al., Pg. 97.
Colostrum collected from 61 sows from three Spanish herds and serum from 11 sows and 30 stillborn pigs, collected at farrowing or by Caesarean section, were tested for TTV1 and TTV2 via polymerase chain reaction PCR
Results. PCR confirmed 45 of 61 colostral samples and 30 stillborns from 11 sows were positive for TTV. Of the sows, seven (64 per cent) and four (36 per cent) tested positive for TTV1 & TTV2, respectively. In stillborns, prevalence of TTV1 & TTV2 was 50 per cent (15 of 30) and seven per cent (two of 30), respectively. Stillborns always had the same TTV as their dams. Although 37 per cent of the sows were co-infected with both TTVs, none of the stillborns was co-infected.
Take home. Vertical transmission of TTVs occurs in utero and via colostrums.

In utero transmission of TTV in gnotobiotes
– Pozzuto T. et al., Pg. 98.
A retrospective search for TTVs was made on tissues (serum, liver and/or kidney) from previous uninfected gnotobiotic pigs never exposed to any infectious agents.
Results. TTV 1 and/or TTV2 were confirmed by PCR in at least one of the tissues of up to 47 per cent (11 of 23) of the pigs.
Take home. These gnotobiotes had to have been infected in utero with TTV. BP

S. Ernest Sanford, DVM, Dip. Path., Diplomate ACVP, is a swine specialist with Boehringer Ingelheim Vetmedica (Canada)
in Burlington. Email: ernest.sanford@boehringer-ingelheim.com