PRRS in North America, Latin America, and AsiaC. Dewey
Population Medicine, University of Guelph, Guelph, Ontario, Canada N1G 2W1
Abstract - The PRRS virus is currently widespread in North America and has been diagnosed in several Latin American and Asian Countries. Columbia broke with PRRS in 1997 and affected 11 000 animals from 3 farms. The source of the outbreak was imported piglets. Serological tests found 5% of extensive units and 21% of intensive units positive. In Venezuela, clinical outbreaks of PRRS began in 1996 with either the reproductive losses or the chronic respiratory problems affecting separate herds. Serological samples in 1996/97 from 32 herds, over 11 states found 72% herds positive. Ninety percent of herds with clinical signs tested PRRS-positive but only 36% of herds without clinical signs were positive. In Japan pigs with chronic pneumonia were serologically positive for PRRS in 1993. A retrospective study found that 15% of pigs imported between 1987 and 1989 were PRRS positive. In 1993-94, 96% of areas of Japan were sero-positive. Approximately 10% of herds showed clinical problems due to PRRS. Most viruses were similar to the United States strains. In Thailand, stored serum from 1989 was positive. The prevalence in pigs increased from 8% to 55% and the proportion of positive herds increased until 1996. There are no reports of severe clinical problems due to the virus and perhaps due to the very hot climate. The isolated viruses are most closely related to the North American strains, although imported breeding stock come from both Europe and North America. In the Philippines the sero-prevalence of herds is 64%. In the Republic of Korea, surveys in 1993, `95, and `97 indicate that 18%, 22% and 72% of herds were positive respectively. There were few clinical cases in Korea, so it appeared that the field strain was mild. The virus resembles North American strains although European breeding stock has been imported. PRRS is controlled in Korea through early diagnosis and repopulation. In Canada, clinical signs of PRRS were seen in 1987 and a retrospective study of serum indicates that the virus was in Ontario in 1979. Currently approximately 80% of herds are infected. This prevalence rate is higher in very dense pig producing areas such as Ontario, Quebec and Nova Scotia, however, there are still many negative herds. This number is expected to increase as units apply management techniques to produce PRRS negative herds. In western Canada and Prince Edward Island, the herd prevalence is lower perhaps due to geographic separation. In 1992, PRRS was reported in 19 states in the United States. In 1995, a serological study was done on 286 herds, 76% of which did not use the 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. In naive herds, reproductive symptoms occur first and last 4 months. In 25% of farms, these reproductive losses become chronic. The reproductive syndrome returns when the proportion of naive animals reaches a critical level. The respiratory form of the disease includes chronic, high morbidity and mortality in the nursery and sometimes in the grower-finisher herd. Common secondary infections include Streptococcus suis, Haemophilus parasuis, Mycoplasma hyopneumoniae and Swine Influenza Virus, and Actinobacillus pleuropneumoniae . Porcine Respiratory Disease Complex (PRDC) is found in grower/finisher pigs with a sudden onset of respiratory symptoms with high morbidity and mortality. Diagnostic laboratories find PRRS alone in 33% of cases, and PRRS + M. hyopneumoniae in another 24% of cases. PRDC is associated with mixing pigs from multiple sources with a diversity of individual infections and immune status, large populations and a diversity of disease agents and serotypes of agents. Veterinarians use programs of vaccinations and pulse medications to control the losses due to PRDC. Sow Abortion and Mortality Syndrome (SAMS) is an atypical form of the reproductive portion of PRRS found in both vaccinated and unvaccinated herds. There is a sudden onset of abortions affecting all parities and stages of gestation. On average 10% of sows abort but this ranges from 10-60%, and there is a sow and boar mortality superior to 5%. These clinical signs last for 2-4 weeks. The PRRS virus most commonly enters herds with incoming breeding stock animals or via semen. The virus can be present in tonsils up to 213 days post infection. Infected boars can be viremic for 8-11 days, and the serum tests positive on PCR for 21-25 days but the semen can be positive for 9 to 31 days. Boars can shed intermittently in the semen for 59-90 days post infection. Although experimental work indicates that previously vaccinated animals do not shed the PRRS virus, this work has not been fully explored under field conditions. Virus can be transmitted from the sow to her piglets via the transplacental route, in both the whey and cellular components of colostrum and milk, and/or via oral/nasal secretions, faeces, and urine. Previous vaccination of the sow appears to prevent the shedding in the milk in sows subsequently exposed to the wild virus. Although the virus is capable of living in wild ducks, species other than pigs are not thought to be important vectors. Airborne spread of PRRS is rare. In controlled experiments where pigs have no nose-to-nose contact, the virus does not consistently spread from infected to non-infected pigs. Although the virus is stable when frozen, it is not hardy outside the pig and is susceptible to drying. In North America, various control programs have been instituted to control the clinical problems due to PRRS in affected herds. These include, depopulation of the nursery with a 14-day down time, euthanizing sickly pigs, limiting cross fostering to the first 24 hours of birth and within farrowing rooms, leaving 2-3 days down time per nursery room, eliminating feed-back programs, and reducing routine antibiotic injections and second iron injections. Pigs receiving passive immunity via the colostrum are less susceptible to challenge and experience a reduced incidence of viremia, leukopenia, and fever. Passive immunity lasts 3-6 weeks and is dependent on the herd of origin. There is an increasing number of PRRS free herds in North America due to depopulation and repopulation programs. The biosecurity measures used to maintain this status include quarantine of incoming breeding stock for 60 days and serological testing on arrival and prior to entry into the herd. These herds are repopulated with PRRS-free breeding stock OR segregated early weaning from PRRS-positive herds with stable sow units, OR embryo-transfer from PRRS-positive herds. Segregated early weaning to produce PRRS negative pigs relies on sourcing pigs from stable sow herds where there is uniform sero-positivity in the sows, uniform colostral immunity in the piglets and no transmission of the wild virus or the vaccine virus from the dam to the piglet prior to weaning. Limiting the number of occasions of new additions to the herd and the number of sources of incoming stock is essential to maintain the PRRS-free status.
Corresponding author: C. Dewey Tel.: (1) 519 824 4120, ext. 4070; fax: (1) 519 763 3117;
© INRA, EDP Sciences 2000