*Dr. Gil Patterson is with the Swine Vet Center in St. Peter, Minn.
THE following is a review of swine health topics presented during the Allen D. Leman Swine Conference in September.
The conference represented one of the first major gatherings of swine industry experts since the initial diagnosis of porcine epidemic diarrhea virus (PEDV) in the U.S. Producers and veterinarians shared their experiences on infected farms and discussed viral transmission and pig survivability. Research generated information since this meeting has established the following:
* Fecal-oral spread is the most common route of transmission of the virus to naive herds.
* Proximity to a positive site seems to increase the risk of infection (University of Minnesota Swine Disease Eradication Center). A statistical analysis of PEDV spread in North Carolina showed the following odds of being infected given the distance to the nearest known positive site:
1. Within 1 mile = 8.4x;
2. Within 2 miles = 6.3x, and
3. Within 3 miles = no greater odds.
* PEDV is stable in cold, wet environments. Infectious virus survives for at least 28 days in frozen fecal slurry and between 14 and 28 days in fecal slurry at room temperature.
* Virus survival was at least 28 days in wet feed held at room temperature and up to two weeks in dry feed held at room temperature.
* Work at the University of Minnesota showed that the virus loses infectivity when heated to 140 degrees F for 30 minutes.
* PEDV is susceptible to a number of common disinfectants, including: Synergize, Virkon S, Clorox, 1 Stroke Environ and Tek-Trol (University of Minnesota Swine Disease Eradication Center).
By far, PEDV's most devastating attribute is its ability to cause diarrhea and dehydration in young pigs. On affected sow farms, preweaning mortality has been high — in most cases, approaching 100% — for the first four weeks. Herd closure, in combination with entire herd exposure to PEDV, has been the most common approach to establishing whole-herd immunity.
It is important that each animal on the farm be exposed so those showing signs like vomiting, diarrhea or inappetence are identified to ensure development of herd-wide immunity. Natural immunity will develop over two to three weeks, resulting in a reduction of viral shedding by sows and antibody-generated immunity that is passed to piglets through colostrum after birth. Be sure to emphasize day 1 piglet care to ensure that all piglets receive colostrum.
PEDV discussions at the Leman conference brought a renewed approach to biosecurity adherence throughout all phases of production.
Studies led by Dr. Jim Lowe, members of the American Association of Swine Veterinarians and Dr. Matt Turner established that cull trailers and market trucks can become contaminated during offload at a plant. This has prompted producers to build new or renovate existing truck washes with an emphasis on the ability to dry trailers.
Work done by Dr. Derald Holtcamp at Iowa State University showed that it may be possible to inactivate PEDV in the presence of manure (trailers only scraped, with no washing or disinfecting done) when heating trailers to 160 degrees F for 10 minutes or by maintaining them at room temperature for at least seven days and monitoring them using a bioassay (feeding negative pigs with material).
Consistently, though, the best decontamination of trailers for PEDV has been when all organic matter is removed and the trailer is washed, disinfected and completely dried. For greater confidence when evaluating the effectiveness of trailer decontamination, swiffer samples can easily be collected from trailers, tractor cabs, etc., to test for the presence of PEDV.
If anything, PEDV has prompted a level of emphasis on biosecurity within the industry like never before, which may, in turn, help to reduce the spread of other swine pathogens.
Another major topic of discussion at the Leman conference that is no stranger to the industry was porcine reproductive and respiratory syndrome (PRRS) virus.
Information was presented regarding methods of sow herd stabilization on positive sow farms. Two common methods were described, including live virus inoculation (LVI) or whole herd injection with a modified-live vaccine (MLV).
Work at the University of Minnesota showed that herds that utilize MLV instead of LVI will, on average, remain PRRS positive longer (33 weeks with MLV versus 26 weeks with LVI). Additional analysis showed that, on average, MLV herds could expect a quicker return to baseline production (24 weeks), defined as the average herd pigs per sow per year (PSY) during the 12 months prior to PRRS infection. An economic analysis of these additional weaned pigs showed an advantage compared to the longer time taken to achieve negative status. As a result, evaluation of MLV in a PRRS virus stabilization program should be considered (Linhares, 2013; Johnson, 2013).
Additionally, one large midwestern system showed that there are advantages in the development of gilts when using MLV over LVI. This included a lower mortality rate, improved average daily gain, better feed conversion, increased selection rate, fewer culls, faster time to heat-no-service and higher farrowing rates and total number of pigs born.
Taking these numbers into consideration, the estimated added cost of LVI for this system was $66 per gilt (Williams, 2013).
Another challenging virus discussed in depth at the Leman conference was swine influenza virus (SIV).
A recent active SIV surveillance project in growing pig herds showed that 91% of herds test positive at least once over a one- to two-year period. Estimates regarding the economic impact of SIV are a $3.23 per head loss in grow-finish populations. This number is amplified to a $10.41 per head loss with a co-infection of influenza and PRRS.
Additional surveillance studies have been conducted by Dr. Matt Allerson focusing on sampling weaned pigs. It was found that up to 84% of farms that test positive for SIV continued to test positive at least once on subsequent monthly tests. Viruses were found to be more than 98.8% genetically similar within farms that continued to test positive. Some farms continued to shed for as many 165 days after the start of the sampling periods.
These results highlight the role endemic infections on sow farms play in the distribution of virus to grow-finish populations (Allerson, 2013).
Vaccinating pigs to obtain protective immunity from influenza viruses is a challenge. Viral strains are diverse and undergo genetic changes as they move through populations of pigs. Additionally, the viruses are capable of recombining with other influenza viruses to form new strains.
For these reasons, it is important to practice routine surveillance for SIV to determine if there is a match between a vaccine and the virus circulating on a farm. Multipliers should consider SIV testing as part of standard monthly testing in order to help confirm the status of incoming gilts onto commercial farms. Commercial sow farms can test for SIV by taking nasal swabs from weaned pigs. Hanging ropes is a great tool for SIV testing in grow-finish barns and can be done for gilts in isolation.
In the case of acute outbreaks, mass vaccination of the sow herd and gilt developer units, repeated twice, four weeks apart, with a commercial or autogenous vaccine has been shown to reduce viral shedding in weaned pigs (Classen, 2013).
A comparison of closeout performance in a mycoplasma-negative commercial grow-finish flow both prior to and following infection with Mycoplasma hyopneumoniae (M. hyo) was summarized. An economic analysis revealed a loss of $7.92 per market pig during the acute phase of the outbreak. Much of this loss was due to slower growth and additional days on feed (7.2 days longer).
Biosecurity programs can use this information to emphasize the importance of keeping major pathogens such as M. hyo out of naive herds. This is especially important now that an increasing number of sow herds have eradicated M. hyo and are weaning negative pigs (Gillespie, 2013).
Allerson, M. 2013. Influenza virus prevalence and risk factors in weaning-age pigs. Allen D. Leman Swine Conference, Sept. 14-17, St. Paul, Minn.
Classen, D. 2013. Management and challenges of dealing with swine influenza. Allen D. Leman Swine Conference, Sept. 14-17, St. Paul, Minn.
Gillespie, T. 2013. Mycoplasma infection costs in a naïve swine population. Allen D. Leman Swine Conference, Sept. 14-17, St. Paul, Minn.
Johnson, C. 2013. PRRS control and eradication options for breed to wean farms. Allen D. Leman Swine Conference, Sept. 14-17, St. Paul, Minn.
Linhares, D. 2013. What have we learned using load, close, expose to produce PRRSv-negative pigs from positive breeding herds? Allen D. Leman Swine Conference, Sept. 14-17, St. Paul, Minn.
University of Minnesota Swine Disease Eradication Center. Accessed at www.cvm.umn.edu/sdec/SwineDiseases/PEDV/index.htm.
Williams, N. 2013. Optimizing production efficiency. Allen D. Leman Swine Conference, Sept. 14-17, St. Paul, Minn.