New discoveries reported at the Minnesota swine conference

Tuesday, June 5, 2012

The 38th Leman Swine Conference delivered much new information on control and elimination of PRRS virus being used in PRRS ARC&E projects and compelling new insights
into peri-weaning failure to thrive syndrome (PFTS)

by ERNEST SANFORD

The 38th Allan D. Leman Swine Conference was held in St. Paul, Minn., last September and a wide array of topics was presented, with engaging new discoveries reported on air filtration for PRRS virus (PRRSV), peri-weaning failure to thrive syndrome (PFTS) and the announcement of the validation of an ELISA standard for oral fluids diagnostic testing. Here are a few notes I captured from the proceedings of the conference.

PRRS antibody detection in oral fluid samples: The first generation PRRS oral fluid ELISA is ready for prime time
– Kittowornrat A, et al., page 45.

Commercial PRRS serum antibody (Ab) ELISA (Herd Chek PRRS 3X ELISA) was adapted for oral fluid samples. Oral fluid ELISA is not supported by IDEXX, but Iowa State University has made necessary adaptations that now allow for running PRRS ELISA on oral fluid samples. Changes include:
•    Incubation time and temperature (16 hours at 4C)
•    Sample dilution (1:2)
•    Sample volume (250 ul)

Adjustments in kit controls and conjugate concentration now allow the IDEXX PRRS ELISA kits to be used for oral fluid sample ELISA analysis. Sensitivity was determined to be 94.7 per cent and specificity was 100 per cent.

Evaluation of intervention for reducing the risk of PRRS introduction into filtered farms via retrograde air movement (back-drafting) through idle fans
– Alonso C, et al., page 47.

Retrograde movement of PRRSV-contaminated bioaeorosols (back drafting) through non-filtered points (e.g. inactive fans) represents a major risk to filtered barns.

This study attempted to determine the risk of back drafting, at what air speeds, and to validate commercially available solutions to prevent this risk. A total of 10 replicates of five interventions were studied using PRRSV concentrations from one log to seven logs.

Results. Air speeds of 0.76 metres per second created backdrafting.  Nylon socks, aluminum shutters and windsocks and double shutter systems prevented PRRSV backdrafting. Standard plastic shutters and shutters and canvas covers did not prevent backdrafting at any concentration of PRRSV logs.

Evaluation of PRRS MLV vaccine on viral shedding
– Linhares D, et al., page 49.

Results. PRRS MLV vaccine post-infection led to reduction of PRRSV shedding and dissemination.
Cumulative PRRSV RNA detected in aerosol samples was significantly lower in challenged-vaccinated populations than in unvaccinated controls. There was also significantly less PRRSV-positive oral fluid samples in the PRRS MLV-vaccinated and challenged populations versus the unvaccinated controls.

New protein identified in PRRSV
– Murtaugh M, et al., page 51.

Hypothesis: Additional ORFs present in PRRSV genome may contribute to its biological properties. Researchers screened purified virions of VR2332, prototype type 2 PRRSV for novel polypeptides.
Results. A 51 amino acid (aa) polypeptide was discovered in an alternative upstream ORF5a of subgenomic mRNA (sgmRNA) encoding major envelope glycoprotein into virions. A similar ORF is present as an alternative reading frame in all PRRSV sgmRNA5 and in all other arteriviruses. This suggests that ORF5a protein plays a significant role in arterivirology. This discovery also provides new potential targets for immunological and pharmacological intervention in PRRS.

Take home. A previously unknown protein (55 aa polypeptide) in ORF5a was discovered in PRRS virus. The function is still unknown, but ORF5a antibodies from immune pigs did not neutralize PRRSV infection of permissible cells. Immunization with a recombinant of this polypeptide does not protect pigs against infection.

A conserved motif in ORF5a appears to control variation in glycosylation of GP5, indicating this region of GP5 is not under immunological selection to escape neutralizing Abs.

Advanced characterization and analyses of Lawsonia spp. and Brachyspira spp.
– Gebhart C., pages 97-99.

Lawsonia spp. is present in many different animals and is now a bona fide emerging pathogen in horses.

There are different species of Lawsonia spp. in different species of animals.
Experimentally, researchers identified 79 nontypable Brachyspira spp. isolated from clinical cases from five states between 2009-2011. NADH oxidase (nox) gene analyses were performed.

All isolates were negative for B. hyodysenteriae and B. pilosicoli. In all, 56 were identified as B. murdochii and 19 were identified as Serpulina spp. These Serpulina spp were β-hemolytic and from pigs with bloody scours clinically.

A new species was identified and is now called – "novel, strongly β-hemolytic (NSN) Brachyspira spp."
 
Hemagglutinating encephalomyelitis virus (HEV) infection in young pigs
– Rossow K., pages 107-108.

There has been a change in clinical presentation of HEV, a change in the pattern of disease consisting of marked difference in the number of HEV-positive pigs in different pig flows (probably due to the introduction at laboratories of highly sensitive PCR technology versus previous bacterial isolation technology).

If you test enough pigs, you'll find tonsils of post-weaned pigs are HEV-positive in groups with clinical disease.

HEV in non-tonsillar tissues (e.g. stomach, lung, salivary glands, nasal turbinates) correlates with clinical disease.

When pigs have post-wean disease, like PFTS, you will find pre-wean pigs with HEV in multiple tissues from pigs with clinical disease issues ranging from inappetence to scours.

Use of RT-PCR Ct values to assess trends in magnitude of PRRS virus viremia over time
– Angulo J, et al., pages 253-108

Real-time PCR (RT-PCR) cycle threshold (Ct) values were used to quantify the values of PRRSV viremia in a large production system undergoing a PRRSV control program. Pigs were vaccinated at weaning with a PRRS MLV vaccine. When the system became full of vaccinated pigs, late nursery and mid-finsher pigs were blood-sampled monthly, sera pooled 5:1 and tested by PRRSV rt RT-PCR, and Ct values were reported.  A higher Ct value means a lower viral load in the sample. The Ct values for each pool were summarized over two-month intervals and analyzed within and across production phases for each period.

Results. Nursery Ct values rose over time for both wild type and PRRS MLV vaccine PCR values, indicating decreasing viral load. More importantly, only wild type virus was detected in finishers. Ct values rose significantly after full system vaccination. Ct values rose over time across all production phases indicating decreasing PRRS viral load over the collective population.

The rise in Ct values over time, after PRRS MLV vaccination, suggests that vaccination decreased viral load across the entire system. Since this is the first known use of Ct as an indicator of viral loads, these results need more corroboration and should be interpreted with caution.

Control and elimination of a PRRS virus field isolate from a continuous flow, single-site nursery-finisher unit using Ingelvac PRRS MLV vaccination
– Garbes N, et al., page 254

PRRS field virus (RFLP 1-18-4) was eliminated from a continuous-flow, single-site, nursery-finisher complex housing 2,800 pigs per group with 9-10 weeks age difference between each group by using double-mass vaccination with a PRRS MLV vaccine of all pigs on site. The herd was undergoing PRRS area regional control and elimination (ARC&E).

A short herd closure period between groups decreased shedding and limited horizontal transmission of PRRSV.

Elimination of both field and MLV PRRSVs was confirmed by entering PRRSV-naïve weaned pigs into the complex between days 125 and 135 after mass vaccination. These latter pigs did not develop PRRSV antibodies, indicating no exposure to either field on MLV PRRSVs.

It must be pointed out that customized herd plans like this are necessary for PRRS ARC&E programs.

The site consisted of two nursery buildings with four and six nursery rooms per building, respectively. There are two finisher sites with four buildings each. Each finisher building has a dedicated hallway, but exhaust air is shared among the four buildings on the site.

Pigs from the oldest group were marketed two weeks (day 14 to day 0) prior to the first vaccination (day 0).

On day 0 all pigs from the three groups housed on-farm (3-4, 12-13 and 12-22 weeks of age) were vaccinated IM with PRRS MLV vaccine, but 200 pigs to be marketed in the next 21 days were not vaccinated.

On day 21, all vaccinated pigs were revaccinated with a second dose of PRRS MLV vaccine, excluding pigs marketed (220 pigs) or to be marketed in the next 21 days (~1,000 head).
Between day 42 and 52, a new group of PRRSV-naïve weaned pigs from a PRRS-positive stable herd undergoing PRRSV elimination was entered. All pigs in this group were vaccinated with PRRS MLV vaccine.

Between day 125 and 135, another PRRSV-naïve wean group was entered into the nursery; these pigs were not vaccinated.

The internal biosecurity was elevated prior to and during the control and elimination process. A random subset of the above group was bled on day 184 (9 weeks post placement) and was PRRS ELISA negative.

In all, the 38th Leman Swine Conference delivered considerable new information on control and elimination of PRRS virus being used in PRRS ARC&E projects and compelling new insights into peri-weaning failure to thrive syndrome (PFTS). It was also the first conference in recent history that had a dedicated session on swine dysentery, a re-emerging disease in swine in North America. 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