IPVS: Cracks found in disease control concepts
Continuing our coverage of highlights at the 19th biennial congress of the International Pig Veterinary Society, held this year in Denmark.
It was the place to be for anyone wanting to hear the newest information on the diseases PRRS and PMWS. While that aspect of the 2006 International Pig Veterinary Society congress could have been forecast, however, less predictable were the warnings of the return of parasites as a significant risk in commercial pork production. Equally unexpected, even for some veterinary practitioners attending the IPVS meeting, were the questions raised about certain fundamental concepts of modern health management.
Take the widely advocated routine of vaccinating gilts against Mycoplasma hyopneumoniae before they are brought into the breeding herd. A University of Minnesota report from the USA suggested that this may be less valuable as a management tool than previously thought. Animals infected 80 days earlier were able to transmit the agent to vaccinated pigs as well as to unvaccinated stock after an exposure period of 14 days. So the vaccination used did not prevent infection and that any vaccinated herd-replacement gilts were considered likely to shed the organism. On this evidence, breeding herd operators and their advisers will have to think again about the most appropriate strategy to use for protection after new gilts are introduced.
Another American report had discussed the common recommendation by US veterinarians to employ all-in/all-out in the management line-up for controlling PMWS. For the herds studied, all-in/all-out pig flow was one of the factors reckoned to add to the risk of a positive PMWS diagnosis in the herd. The observation that continuous-flow production was associated with fewer positive diagnoses was said to be consistent with anecdotal data from some veterinary advisers.
Other risk factors in this instance were a history of ascarid infestation and housing pigs on solid floors. The heightened PMWS risk here seemed due to the migration of ascarid worm larvae within the pigs, thought perhaps to be triggering clinical signs of the wasting disease through an influence on their immune system. Worm parasites go together with solid-floored pens more often than with slats, but there were additional remarks from other countries that the parasite problem had also increased in line with the uptake of bedded accommodation.
All-in/all-out performed better in a Danish test of actions to combat Actinobacillus pleuropneumoniae (APP). The Danes demonstrated that fewer batches of pigs were infected when the finishing units were managed all-in/out by site than by room. Their results indicated transmission between rooms to be a significant source of infection with APP, although it must be mentioned that some APP infection also was found in places using all-in/all-out by site. All batches of grow-finish pigs from APP-infected sow herds should probably be assumed to include some animals carrying the infection after picking it up in the farrowing unit. Detection is an issue because, as the research in Denmark observed, APP can be found in the tonsils of serologically negative pigs.
To return to Mycoplasma hyopneumoniae, Swedish work has shown that seronegative pigs at 10 weeks old may carry low doses of the mycoplasma. Examination of lung lesions at slaughter before, during and after employing vaccination suggested a route in which M. hyopneumoniae established itself in the finishing period, despite batch management and good hygiene in the intervals between batches. With carrier pigs possibly contributing to this pattern, one of the best ways to control the disease may be by strategic vaccinations. There was support, too, for the assumption that early vaccination of piglets may assist M. hyo control even in herds where the sows and gilts are seropositive by natural infection. In Italy, sow vaccination was consistent with a good antibody response to natural infection by their vaccinated piglets.
Minnesota researchers from the USA had earlier described how a sow herd lost its stability for Mycoplasma hyopneumoniae following a period of high replacement rates as part of a genetic improvement programme. Apparently the sudden influx of non-infected animals, even though they had been vaccinated, destabilised the herd's health status and gave rise to an appearance of clinical signs. The same group reported separately finding a direct relationship correlation between the prevalence of M. hyopneumoniae in the nasal tracts of pigs at weaning and the extent of lung lesions in these animals at slaughter. Control strategies focused on reducing the mycoplasma in the sow herd should therefore help cut back on the severity of the subsequent disease. According to the investigators, this could have real promise in leading to a significant reduction in the use of antibiotics and vaccines in growing pigs.