Spatiotemporal patterns and risks of herd breakdowns in pigs with postweaning multisystemic wasting syndrome

Abstract

A retrospective cohort study of 116 British pig farms was undertaken to investigate the epidemiological risk factors associated with herd breakdowns with postweaning multisystemic wasting syndrome (PMWS). Farmers reported the PMWS status of their herd (case definition 1) and, where applicable, when the disease was first suspected and what they observed; they described a prolonged increase in mortality in six to 16-week-old pigs that was not attributable to any disease known to be on their farm. There was over 90 per cent agreement on the farmers' PMWS status between the farmers and their veterinarians. Approximately 70 per cent of the breakdowns were confirmed at the laboratory (case definition 2) except during the outbreak of foot-and-mouth disease (FMD) in 2001 when it was reduced to 30 per cent. Porcine circovirus type 2 antigen was detected in pigs examined postmortem (case definition 3) in approximately 90 per cent of the farms with increased mortality. The breakdowns occurred initially in the south of England and spread west and north, as well as locally in a radial pattern from the affected farms, and there was strong statistical evidence that there was non-random space-time clustering. The risk of herd breakdowns with PMWS was not constant; therefore, for each case definition, three survival models were developed with outcome variable time to breakdown of between January 2000 and January 2001, February 2001 to September 2001 (during FMD) or October 2001 to December 2003. Exposures with a bivariable significance of P<0.20 were tested in three multivariable Cox proportional hazard models. From January 2000 to January 2001 the risk of a herd breakdown with PMWS for definitions 1, 2 and 3 was greater for farms with 600 or more breeding sows, and for definitions 1 and 3 there was an increased risk associated with the purchase of replacement gilts rather than using homebred replacements. For definitions 1 and 3 the farms where the nearest pig farm had no breeding pigs were at greater risk of a breakdown than those where the nearest farm had breeding stock, as were the farms where visitors were not requested to avoid pigs for more than three days before visiting the farm during the FMD outbreak. From October 2001, the associated risks were identical for all three case definitions; farms were at greater risk when they had 600 or more breeding sows, if visitors had not avoided contact with pigs for more than three days before visiting the farm, and when there was a farm with PMWS less than five miles away. The affected farms were more likely to have disease associated with porcine parvovirus, porcine reproduction and respiratory syndrome virus, erysipelas, Escherichia coli and salmonella. These exposures were positively associated with large herds and the farm being close to other pig farms, but did not remain in the final models for breakdown with PMWS, indicating that such farms may be at greater risk of many infectious diseases.

Coreplication of the Major Genotype Group Members of Porcine Circovirus Type 2 as a Prerequisite to Coevolution May Explain the Variable Disease Manifestations

“…Therefore, we favor a model with a constantly evolving genetic PCV2 pool in cells, in individual animals, and in pig herds to explain the disease etiology. This would further explain the population dynamic of germination centers for PMWS spread and outbreaks in Switzerland and Great Britain, as if a new infectious agent coemerged with PCV2 (42). Under this assumption, the slow progression of the disease in individual animals during maturation involves harmful virus mutation and recombination that lead to increased lymphopenia, which finally triggers disease.…”

A cross-sectional study of the prevalence and associated risk factors for capped hock and the associations with bursitis in weaner, grower and finisher pigs from 93 commercial farms in England

“…The pathology of capped hocks was identical to that of bursitis (Gillman et al, submitted). The pig farms in this study were representative of the English pig farm population in size, geographical location and ratio of indoor and outdoor farms; in total approximately 2% of the GB pig holdings with breeding sows were visited (Woodbine et al, 2007). Therefore this study should provide a more accurate measure of the prevalence of capped hock in England in herds with at least 100 breeding sows than previous studies which have used a smaller number of farms from a limited geographical region (Penny and Hill 1974;Mouttotou et al,.…”

Insights into the Evolutionary History of an Emerging Livestock Pathogen: Porcine Circovirus 2

“…The high levels of swine trafficking that occur due to the globalization of the pig/pork industry, along with the high prevalence of asymptomatic PCV2 infections, combine to create an ideal circumstance for the emergence of a highly virulent strain of PCV2 (55). Epidemiological links between infected farms have been determined for outbreaks in the United Kingdom and New Zealand, among others, highlighting the potential for PCV2 transmission through the pig trade (41,71). Microevolution of PCV2.…”