Cases of postweaning multisystemic wasting syndrome (PMWS) are on the upswing after declining for several years.
Throughout the early-to-mid '90s, the total number of PMWS cases reported from Iowa State University's (ISU) Veterinary Diagnostic Laboratory (VDL) were barely a blip on the radar screen.
Then they jumped into the hundreds, before peaking at 1,116 in 2002.
Following two years of decline, cases are on their way back to near-peak numbers for 2005 (Figure 1).
Data was compiled by Patrick Halbur, DVM, and Tanja Opriessnig, DVM, ISU's VDL.
Halbur presented an update on PMWS at ISU's 13th annual Swine Disease Conference for Swine Practitioners in Ames, IA.
Disease Signs
One thing that has stayed the same over time, says Halbur, is that the two most commonly diagnosed porcine circovirus Type 2 (PCV2)-associated production problems continue to be due to respiratory disease as part of the porcine respiratory disease complex (PRDC) and PMWS. In 2005, it appears that the number of PCV2-associated cases of PRDC and PMWS will be about equal.
Much less commonly diagnosed diseases include PCV-2-associated enteritis, abortions and porcine dermatitis and nephropathy syndrome (PDNS).
Halbur reports that the group at ISU's VDL recently investigated a couple of outbreaks of respiratory disease and infertility in boar studs naïve for porcine reproductive and respiratory syndrome (PRRS). The group concluded that PCV2 played a key role, along with Mycoplasmal pneumonia and opportunistic bacteria, in these cases.
PMWS is a chronic wasting syndrome. Current knowledge suggests that for PMWS to cause disease, three things must occur:
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Circovirus must be present;
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PCV2 must be associated with lymphoid depletion lesions; and
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Secondary disease agents must be present to trigger the onset of PMWS.
Most common respiratory disease co-infections that trigger PMWS include PRRS virus, mycoplasma and swine influenza virus.
PMWS typically strikes pigs 2-4 weeks after placement in finishers, or in most cases at 12-14 weeks of age, points out Halbur.
“In most herds that have a problem with circovirus-associated PMWS, we can attribute 5-10% mortality to this syndrome,” says Halbur.
“PCV2 causes sporadic abortions and increased mummified fetuses in the breeding herd. PCV2 infection of suckling and nursery pigs may also cause sporadic cases of heart failure in young growing pigs,” explains Halbur.
It still remains puzzling why a high percentage of pigs are infected with PCV2 but remain clinically healthy.
Strain Differences
Halbur says research has verified major differences in virulence among strains that appear very similar genetically.
“You look at the genome and say there is very little difference, and they shouldn't react differently in pigs. But when we have taken isolates like that and put them in pigs, we have shown there are marked differences in how some of these strains act in the pig.
“So right now we know there are differences among strains, but we don't know how many strains there are, and we don't know how to predict if a strain is virulent or non-virulent,” adds Halbur.
Host Susceptibility
Research at ISU's VDL has also shown clear differences in host susceptibility. Certain lines of pigs within breeds are more susceptible than others to PCV2-associated PMWS, he says.
Additional research into host susceptibility is being pursued, says Halbur.
Management Steps
Follow these steps to treat PMWS:
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Start with a diagnostic checkup to confirm the problem is PCV2-associated PMWS, PRDC or another manifestation of PCV2-associated disease.
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Once there is confirmation, identify the co-infecting agents and consider appropriate treatment and control options for those “triggers.”
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Vaccinating for mycoplasma when it has been identified as part of the syndrome has proven particularly effective in controlling signs of PMWS or PRDC, reports Halbur. Make sure vaccine timing is correct and use a good-quality commercial vaccine.
In some cases, in addition to vaccination, he also suggests pulse medicating with therapeutic levels of an antibiotic such as chlortetracycline.
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Focus on pig comfort. Provide appropriate pig density, good ventilation and reduce stress levels.
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Minimize mixing and moving of pigs. Flow production all-in, all-out.
Vaccine Update
Commercial vaccine trials in Europe have shown effectiveness in reducing both the PCV2 circulation in sows and in improving pig health.
Similarly, a number of U.S. companies have shown promising results in field trials on PCV2 vaccines.
