Disease detectives net
a new virus
from PIG INTERNATIONAL -
OCTOBER 1998-VOLUME 28, NUMBER 10
A severe and of ten fatal condition seen during the nursery or early grower stages in an increasing number of countries over the last 2 years has become the first example of a new pig disease to be diagnosed through the Internet, according to comments by researchers at the 1998 International Pig Veterinary Society congress.
Affected pigs often appear jaundiced and are prone to coughing, but it is their typically thin and unthrifty look which has contributed most to the condition’s initial name of the post-weaning multisystemic wasting syndrome or PMWS.
The earliest encounter with this peculiar weight-loss syndrome was on a high-health hard in western Canada in 1991. Since 1996, however, similar signs have been reported with rising frequency in several parts of North America [Mexico and the USA as well as Canada] and on European until in France, Spain, Denmark and Northern Ireland.
Investigators meanwhile were starting to close in on a tiny and previously unknown virus as the probable cause. Diagnostic pathologist Dr Ted Clark from the University of Saskatchewan, Canada, described at IPVS how he had used the Internet to circulate post-mortem findings to other laboratories. This electronic appeal for comments led to a contact with US scientists who detected a resemblance to circovirus infections in poultry. More recently the suspicion has hardened that a novel type of circovirus is indeed responsible.
“A non-pathogenic porcine circovirus [PCV] has been know for many years and antibodies to it are common in the pigs of many countries,” said Dr Clark. “There is now good evidence that a second type exists which is pathogenic. It has been isolated from pigs with typical lesions, although there is not yet conclusive proof because experimental infection does not always obtain clinical signs or the same consequences as in field cases.
“ After examining over 500 pigs from more than 50 affected farms in western Canada since 1996, I have become convinced that diagnosis should not rely on one single post-mortem. You need to look at multiple animals because the type and distribution of lesions will vary with the stage the disease has reached. They might also differ in severity depending on whether a PRRS infection coexists in the herd.
“ Groups of 5-10 suspect pigs should be necropsied at 6-8 weeks old. Many of the lesions are only visible histopathologically, so it is important to submit a complete set of tissues for examination. This must include the lymphoid organs and all lung lobes as well as the stomach and intestines, tonsils, kidneys, pancreas, spleen and liver.”
The lymph nodes are consistently involved, he commented. Often they are so enlarged they can be seen protruding through the skin on the back legs. Some pigs are presented as icteric or jaundiced, indicative of liver failure. The kidneys show damage in over 50% of cases. Lungs tend to be firm and mottled, ranging in color from pink to yellow. Pneumonia affects the airways in addition to the interstitial tissue. Gastric ulcers can occur.
Also from Saskatchewan, practitioner Dr John Harding listed the 5 main clinical signs he found on farm visits. The first and most common of these is an obvious unthriftiness. Additionally the animals are likely to be coughing, to be pale and to have a rough coat. Some will show diarrhea. A particular clue to PMWS comes, however, when jaundiced pigs are observed in otherwise healthy-looking pens.
“It can be a significant cause of mortality anywhere between 15-70 kg in Canadian herds,” Dr Harding continued. “On 15 farrow-to-finish units that are PRRS negative, the first signs appeared when the pigs were about 42 days old. The onset in the nursery is usually quite subtle and early stages in a 15-20 kg pig may be no more than grounds for suspicion. Death losses tend to be high at the start of an out break and then decline, but the average duration of the outbreaks in those 15 herds was 18 months. During that time their annual mortality rate after weaning was almost 7%”
Causing concern in countries such as Spain is the extra impact of a mixed infection involving both PRRS and PCV viruses. Dr Mariano Domingo [Universitat Autonoma de Barcelona] told the congress that up to 80-90% of weaned pigs in some Spanish herds have died, principally due to pneumonia. Studies of affected pigs have found genetic material showing the presence of the coronavirus located extensively throughout their damaged tissues in almost all cases. But the large majority of the herds of origin also had PRRS and, where blood testing of live animals was possible, about half of the samples were similarly positive.
“It may be hard to discriminate between the effects of these infections,” he warned. “different lesions could occur where PRRS and PCV are present together.”
Early French cases of so-called piglet wasting disease were mistaken for PRRS when herds in Brittany first became affected at the start of 1995. The Breton version features depression, muscle wasting, diarrhea and respiratory distress, most critically in the age range 8-12 weeks. Antibiotic treatments have had no success. Over 20% of victims die within a week of showing the first signs.
“More than 100 herds have been involved so far,” says Dr Francois Madec [CNEVA, Ploufragan], “We have PRRS-negative farms with severe disease. Our view now is that the presence of the porcine coronavirus is a necessary condition for the wasting disease to occur, but several other environmental factors are also needed before the most serious consequences arise. It is not monofactorial like Aujeszky’s disease/pseudorabies, for example.
“We think a lot can be done by management to control the disease. We are working now to find which environmental influences are the most important. At present, the menu of actions for an individual unit has to be compiled from 25 possibilities!”
French investigators have successfully isolated 2 types of porcine coronavirus from enlarged lymph nodes in the same piglet, adding to international evidence that a new form of the virus has arrived to exist side by side with the widespread harmless sort. Even so the cause-effect relationship between the virus and the wasting syndrome has yet to be finally established.