Antibodies, adjuvants, biosecurity – and the list goes on and on for factors relating to Postweaning Multisystemic Wasting Syndrome (PMWS). Without a straightforward cure or vaccine, research into this disputed diagnosis continues.
There is a glimmer of hope on the horizon, however. Iowa State University Veterinary Diagnostic Laboratory (ISU-VDL) reports a leveling off of diagnosed PMWS cases and there are whispers of a new vaccine.
The overall trend of PCV2 associated diseases including pneumonia and PMWS increased dramatically during the five-year period from 1998-2002 but may have peaked, based on submissions to the ISU-VDL.
One of the contributors to wasting pigs, whether as fall-behinds or true PMWS cases associated with PCV2 is ‘management by the averages’, according to veterinary researchers.
The numbers may also be part of the question of which came first the “chicken or egg.”
“Many cases of PMWS are diagnosed in the field today,” notes Tanja Opriessnig, DVM,
Veterinary Diagnostic and Production Animal Medicine at Iowa State University, “and those samples aren’t routinely sent into diagnostic labs.”
Like Porcine Reproductive and Respiratory Syndrome Virus (PRRSV), there is no simple answer for predicting outbreaks, transmission methods or prevention strategies. The key to reducing mortality and morbidity is in early identification of PMWS, according to Locke A. Karriker, DVM, Veterinary Diagnostic and Production Animal Medicine, Iowa State University.
“Identifying clinical signs early is crucial both at the individual hog level and the herd level,” Karriker points out. “Early identification of affected pigs and movement to a hospital pen facilitates closer routine evaluation and treatment of secondary disease problems such as bacterial pneumonia. Early identification at the herd level allows for adjustments of management such as vaccination timing to avoid the onset of clinical disease.”
Failure to thrive
Reduced feed and water consumption
Presence of hallmark PMWS lesions characterized with granulomatous inflammation and lymphoid depletion in the lymphoid tissues.
Presence of PCV2 antigen or nucleic acids associated with the characteristic lesions.
Karriker urges veterinarians and producers to send in samples of suspected PCV2 diseases to a qualified testing laboratory.
“Numerous respiratory and systemic diseases in swine look like PMWS,” he says, “but many of these diseases can be prevented with vaccination. Clients shouldn’t assume their herd’s symptoms are PMWS until they know for sure. Discontinuing a vaccination could result in a large scale outbreak of a preventable disease.”
The need for large groups of evenly sized pigs at weaning in today’s production systems results in cross-fostering, holding back “light” pigs and increasing weaning times for small pigs.
According to Karriker, these practices have diminished size differences in groups but at the expense of increasing age variation. “One of the contributors to wasting pigs, whether as fall-behinds or true PMWS cases associated with PCV2, is ‘management by the averages’,” he adds.
“Studies indicate that the timing of PCV2 immune response and the decline of maternal antibodies is relatively consistent from hog to hog,” points out Karriker. “This means that variation in age creates a variation in immune status among hogs in the group. When pigs of variable immune status are exposed to any disease, circulation of the disease becomes prolonged.”
Managing environment “by the average” can be costly as well.
With larger facilities, groups of hogs in nurseries and finishers have increased in size as well. “Yet most facilities have a single ventilation setting or single ration for the entire group that is correct for the average weight of the pigs in the group,” notes Karriker. “Addressing the average may not provide the optimal conditions for the extremes in size such as the largest or smallest pigs.”
Karriker argues that the solution to managing “by the average” is difficult but must emphasize stockmanship and the value of individual pigs.
When asked about their biosecurity programs, most swine producers will go to great lengths to explain the protocols for adding new livestock to the herd, washing of trucks and trailers used for transport, and disease testing of semen.
Iowa State University veterinary researcher Tanja Opriessnig has been looking into the use of vaccine to help control PMWS in swine.
“These measures focus on preventing disease entry into the farm from outside sources,” points out Karriker, “however, opportunity to improve pig health still exists by controlling disease circulation within farms and production groups.
He suggests, among other things:
Avoid continuous pig flow in the farrowing rooms – stick to all in/all out.
Don’t hold back “light” finishers – you are exposing incoming pigs to the least healthy
animals in the herd. Consider moving “lights” off-site for finishing.
Try to avoid holding back small pigs at weaning – by moving them back with younger pigs you are creating a continuous pig flow.
Avoid commingling highly variable sources in age and health status.
Increase weaning age to reduce the number of pigs that waste due to maladjustment.
Severely restrict cross-fostering to avoid variations in immune status in the nursery.
Control secondary infections – especially pneumonia.
Separate gilt offspring to better evaluate immunity levels and determine vaccination programs.
Disinfect with adequate products – research shows 1-Stroke Environ® Disinfectant and Cleaner (Steris Corp., Mentor, Ohio) and Roccal® D Plus (Pfizer Animal Health) to be very effective at inactivating PCV’s infectious potential.
Swine practitioners and their clients should reevaluate any vaccine practice greater than one year old. Sentinel animals or groups of unvaccinated animals can help confirm disease exposure within a herd. Awareness of disease patterns within an established geographical radius also helps determine the need to vaccinate for a specific disease. Evaluation of not only antigens but also adjuvants should be considered, experts say.
“In situations of confirmed PMWS associated with PCV2, where vaccination cannot be avoided, changing to a vaccine with a less stimulatory adjuvant may be warranted,” says Karriker. Oil-in-water adjuvants appear to be the most likely to influence a PCV2 infection, followed by aqueous and aluminum hydroxide.
“Research suggests that at the early stages of infection 21 days post inoculation (DPI) oil-in-water adjuvants, aqueous and aluminum hydroxide increased the severity of lymphoid depletion associated with PCV2,” reports Opriessnig.
Type of adjuvant, timing of vaccination, age of pig, and disease exposure play important roles. “Trials show that pigs vaccinated for M. hyopneumoniae indicated no or minimal PCV2- associated lesions when pigs are vaccinated two to four weeks prior to expected PCV2 exposure,” notes Opriessnig.
Editor’s note: The author is a freelance writer working from Mankato, Minn.
Is there a genetic influence on PMWS
A limited study conducted by Patrick Halbur, DVM, PhD, and Tanja Opriessnig, DVM, at Iowa State University’s Veterinary Diagnostic and Production Animal Medicine Laboratory, compared the influence of PCV2 infection on Duroc, Landrace and Large White genetics.
All the pigs were inoculated with PCV2 at 5-7 weeks of age. According to Opriessnig, although all the pigs seroconverted to PCV2, only pigs from the Landrace group developed PMWS and its characteristic lesions.
“The incidence of PMWS based on gross and microscopic lesions was 0 percent in Durocs, 15.8 percent in Landrace and 0 percent in Large White,” reports Opriessnig.
She acknowledges that the study was small and more work needs to be done. “This was a small study but it appears this subject will warrant further investigation both here and in Europe.”
Types of PCV2 disease
Trends in types of PCV2-associated diseases in field cases submitted to the ISU veterinary diagnostic laboratory
Vaccination for PMWS
Research using chimeric clones composed of PCV1 and PCV2 viruses caused an immune response to wild-type PCV2. These results are a result of work conducted by Patrick Halbur, DVM, PhD, and Tanja Opriessnig, DVM, at Iowa State University’s Veterinary Diagnostic and Production Animal Medicine Laboratory, and Dr. Xiang-Jin Meng, MD, PhD, Virginia Polytechnic Institute and State University,
“By using different clone combinations we were able to produce an attenuated chimeric PCV1-2 live virus as well as the chimeric PCV1-2 infectious DNA clone,” explains Opriessnig. “Both induce effective immunity against PCV2 infection and could potentially serve as an effective vaccine.”
“This work by Halbur and Opriessnig suggests that vaccination can be protective for PCV2,” agrees Locke Karriker, DVM, Veterinary Dagnostic and Pro-duction Animal Medicine, Iowa State University. “I expect that we will see commercial vaccines within the next year or two,” he remarks.
While swine practitioners and their clients faced with recurrent outbreaks of PMWS will no doubt welcome a vaccine, Karriker points out that PMWS, like PRRSV, has reminded us of the continued important value of stockmanship, biosecurity, sanitation and pig flow in controlling this and other diseases.