Although not yet widely discussed in the scientific literature or commonly diagnosed by veterinarians, PMWS is impacting the herd health of many farms. First described in Canada in 1996, the syndrome has since been diagnosed in several countries including the United States.
PMWS is a disease affecting primarily 5- to 6-week-old pigs. However, it has been diagnosed in younger pigs and in finishing herds as well. Clinically, PMWS-infected pigs fail to thrive. Unfortunately, it appears the best animals are often affected.
Those pigs will have poor appetite, be lethargic and often have labored breathing. The pigs may develop a rough hair coat. Producers often report pale or yellow pigs. PMWS can affect several animals in a pen, but does not seem to spread through a whole pen at once. This leaves a wide weight range of pigs within pens that were initially homogenous.
The disease is often found in herds that are relatively free of major respiratory diseases such as Mycoplasma pneumonia, porcine reproductive and respiratory syndrome and swine influenza. In an acute outbreak, mortality can be as high as 15 percent to 20 percent of a weaning pig group, but with time the rate will decrease.
Poor performance results from the disease and contributing factors that complicate PMWS. (See accompanying box.) Determining the cause of poor performance is the critical first step toward solving the problem and developing a control program.
Circovirus is suspected to cause PMWS. Although circovirus is not yet proven to be the sole cause, the virus has been found in association with the syndrome. Much of the research on this emerging syndrome has focused on defining transmission between animals and the mechanisms of disease.
Diagnostic efforts should be directed at detecting the presence of circovirus as well as ruling out other possible causes. Here are some starting points:
1. Assess nursery performance. Putting numbers to the diseaseÆs impact will help determine the extent of the problem and provide guidelines to measure the success of a control program.
2. Be prepared to have multiple pigs necropsied. Your veterinarian and the diagnostician will be looking for PMWS lesions as well as ruling out other diseases.
3. Assess other contributing factors. Serology of the growing pigs and sow herd will help rule out PRRS and SIV, since we donÆt yet have a blood test that can confirm PMWS. Reviewing the herdÆs environmental and nutritional programs also is important.
Developing a control program for this disease can be difficult because of limited understanding of the causes, transmission and disease mechanisms. There are management practices that have been helpful in controlling other diseases that may be useful based on what we do know. They are:
1. Manage growing pigs on an all-in/all-out schedule. While we donÆt know the transmission routes of PMWS, this practice seems to have a positive effect on the severity of outbreaks.
2. Control other diseases in the herd.
3. Provide pigs with a proper environment. The more environmental stress these pigs face, the more severe the disease.
4. Sort off PMWS suspect pigs into a separate pen. DonÆt allow chronic pigs to stay in the nursery. Do not move pigs from room to room.
5. Manage sow herd immunity. Sound acclimatization of replacement gilts may be useful in combating this disease. Planned exposure of laying hens has proven effective in the control of a similar disease in poultry. The fact that PMWS severity decreases in a herd over time suggests that herd immunity may be important.
6. Minimize crossfostering in the farrowing room. With an unknown mode of transmission, this technique has been an important component of controlling PRRS.
PMWS is real even though the details are still being investigated. Systematic diagnostic approaches to this disease will prevent other causes of poor performance from being lumped into this syndrome and thereby preventing appropriate control measures.
Good diagnostics and open dialog between veterinarians, producers and researchers should help us develop an understanding that will allow us to effectively tackle PMWS together.
It Can Be Confusing
Postweaning multisystemic wasting syndrome can be confused with, or may be complicated by, diseases or conditions that cause poor performance in the growing pig. These might include:
Porcine reproductive and respiratory syndrome
Water quality and availability
Perry Harms is a veterinarian with the Iowa State University diagnostic laboratory in Ames, Iowa.