The more the governments of the world try to control the spread of PRRS (Porcine Reproductive and Respiratory Syndrome), the more the deadly virus seems to spread.
Adoption of national and international rules controlling the reporting of disease and the movement, quarantine, and slaughter of affected pigs hasn't stopped PRRS from becoming "the pandemic of the 1990s," reports Cate Dewey, DVM, University of Guelph, Ontario, Canada.
PRRS truly has become pandemic in that it has infected pigs across a wide region of the world and in large numbers.
PRRS has spread rapidly around the world by the sale of pigs, semen and airborne spread of the virus, she says. Control measures have done little to stop this spread, other than perhaps decreasing the impact of epidemics.
In response, some countries have adopted routine serological testing for lab submissions and/or surveys.
Countries voluntarily reporting PRRS disease status have paid the price. Sales of breeding stock and semen have been restricted.
PRRS is not a reportable disease in some countries. Dewey suggests either they are not testing for the disease, or indeed their herds are negative.
PRRS was added to the International Office of Epizootics (OIE) list of reportable diseases in 1992. Each member country is supposed to list the disease status and control measures at the end of the calendar year.
A brief rundown of PRRS status was presented by Dewey at the Leman Swine Conference this fall:
North America PRRS was reported in 19 states in the U.S. as early as 1992. A USDA National Animal Health Monitoring System survey looked at serum from 286 herds, 76% of which did not use a PRRS vaccine. Overall, 69% of farms had at least one serologically positive animal and 59% of the unvaccinated herds had at least one positive animal, relates Dewey.
PRRS was observed in Canada as early as 1987. A retrospective study of stored serum indicates its presence in Ontario as early as 1979. Today an estimated 60% of herds are infected.
Boehringer Ingelheim's PRRS vaccine was first sold for use in pigs age 3-18 weeks for respiratory problems with PRRS. They sold 16 million doses the first year. It has been used in the U.S. and Canada to reduce respiratory problems in nursery and grow-finish, to provide immunity to incoming breeding stock and widespread immunity in the breeding herd.
More than 90 million doses of the vaccine were sold in 11 countries. It was approved for use in non-pregnant sows in 1996, Dewey says. Table 1 on page 40 offers more history of vaccine use.
Mexico Our southern neighbor recently announced that it is officially PRRS positive though no serological surveys have been conducted. Mexico's regulations still require that imported pigs must come from herds that have been free of clinical signs of PRRS for the previous two years.
South America Brazil reports no clinical cases of PRRS, and claims to be officially PRRS negative, even though it imports breeding stock from the U.S., Mexico and Europe. A serological survey in 1996-97 of pigs from large integrators and seedstock firms turned up only negative results.
Chile is also officially PRRS free, despite reports of pigs showing clinical signs typically seen in PRRS outbreaks.
Europe The winter of 1990-91, PRRS spread rapidly across Germany, the Netherlands and Belgium. Some 80% of herds in the Netherlands showed clinical signs.
Between March 1991 and October 1992, the European Union (then known as the European Community) decided to start reporting outbreaks on a monthly basis. A serological survey found 35% of 1,940 herds positive for PRRS. Samples came from France, Belgium, the Netherlands, Cyprus and the United Kingdom (UK).
Also during this time, a control program was started. Exports from towns in Europe were banned for eight weeks after the end of clinical signs in an area. Cleanuprequirements were bolstered. Dewey says measures may have slowed spread of disease, but some previously PRRS-negative countries became positive during the control program.
There were 133 outbreaks of PRRS in the UK during 1991-92. By 1995 it was estimated that 40-50% of herds were positive; clinical problems were variable.
According to Dewey, British veterinarian Michael Muirhead reports herds have been very successful in fighting respiratory problems caused by PRRS. They implement a vaccination program for Mycoplasmal pneumonia at 10 days of age and again at weaning. British producers are also using partial depopulation followed by all-in, all-out programs.
>From 169 breeding herds affected from 1991-92 in northern France's Brittany, PRRS infection has apparently spread over many portions of the country, Dewey says.
Spain became infected with PRRS in 1991 following importation of feeder pigs from Germany. The two infected herds were slaughtered and officials report the country is free of PRRS. But reports suggest there are PRRS positive herds and the virus is widespread.
Switzerland tested free of PRRS after identifying signs of classical forms of the disease in the early 1990s.
Airborne spread from Germany was blamed for 40 herds in Denmark coming down with PRRS in 1992. A 1994 survey suggested that 15-20% of Danish herds were infected with the virus. Officials approved use of a vaccine in July '96. By October '96, a number of pork producers reported having problems with the reproductive form of PRRS. The vaccine was isolated from stillborn pigs and scientists were able to show that the vaccine virus had spread from vaccinated to unvaccinated pigs. By November '96, use of the vaccine was discontinued.
Russia Early tests for PRRS proved negative. However, there have been outbreaks of PRRS reported over the last four years.
Asia Chronic pneumonia in Japanese herds in 1993 turned out to be PRRS. A retrospective study found that 15% of pigs imported between 1987 and 1989 were PRRS positive. In 1993-94, herds in 96% of areas of the country appeared to be positive for PRRS. About 10% of herds tested showed active infection. Most strains identified proved similar to those isolated in the U.S.
A 1993 survey found that 18% of herds in both the Phillipines and the Republic of Korea were PRRS positive.
China has been officially declared PRRS positive. Law requires PRRS-positive animals to be slaughtered. But only animals testing positive are being killed, not whole herds, points out Dewey. China requires imported breeding stock to be PRRS negative.
New Control Methods
Because it can cross the placenta and shed in semen, embryo transfer, cesarean-derived pigs and artificial insemination are not foolproof ways to prevent spread of the PRRS virus, says Dewey. However, semen can be tested for presence of the virus.
A new method is being used to reduce the odds of infection from new herd introductions. Double quarantine refers to quarantine and serological testing of the source herd and purchased stock.
According to Dewey, weaned pigs and young gilts will test PRRS positive if they come from a herd with an active infection. But if the herd is chronically affected, and particularly if all age groups are not housed together, it's possible for the animals to test negative on a serological test. Animals will develop a titer by two weeks after infection.
"Repeating serological tests, 60 days apart, will provide the most reliable measure of disease status, particularly if the incoming animals are mixed with susceptible pigs from the purchasing herd," comments Dewey.
She says a biosecurity program of testing semen and quarantining and testing incoming animals has worked to keep some herds free of PRRS, even in some pig-dense areas.
In the UK, where PRRS vaccine is not used, producers are advised to expose replacement breeding stock to infection before breeding. Gilts enter the breeding barn at 175 days of age, become acclimated to conditions for two weeks, then are exposed to 6- to-10-week-old nursery pigs (and their manure) for three weeks. Gilts stay two more weeks for immunity to develop and are br ed at age 220 days.
New pig barns are also built in less pig-dense areas.
The British believe that together these management steps reduce clinical signs and viral shedding, says Dewey.