Barriers against a bacterial transfer
Presenting our next report from the 2008 congress of the International Pig Veterinary Society, held recently in South Africa
Salmonella remains in the foreground of official thinking on veterinary health because of its known associations to food-related illness in humans. Among the signs of its continued importance this year has been the publication of a European survey into incidence in animals and humans, as well as the occurrence of food poisoning' outbreaks such as one in Denmark that was suggested to have links to the importation of contaminated meat.
Presentations and comments to the 2008 IPVS Congress made clear that Salmonella infections similarly still receive close attention in pig research around the world, with most studies focused on the bacteria as a potential source of a zoonosis. Unfortunately, according to specialists attending the meeting, the clear message from the trials reported in Durban was that researchers are simply repeating the work of the past. .
For a refresher course on Salmonella control in pig production, they added, there is no need to look further than the sessions of the IPVS held in the year 2000. The main farm-level remark from the Melbourne meeting had been from Europe and said that acidifying either the feed or the drinking water is helpful as an intervention strategy. Plus, units with a low rate of Salmonella infection could preserve this status for a long period despite the many challenges posed by the environment, whereas it can be difficult to reduce the bacterial level in a highly infected herd.
But the over-riding consideration based on practical experience stays the same in 2008 as in 2000: it is illogical to even start a campaign of monitoring and intervention at farms until the conditions in the slaughterhouse are completely satisfactory. We know beyond doubt that apparently low-Salmonella pigs can become high-risk by the time of their slaughter, after picking up the bacteria in the lairage holding pen. It has also been demonstrated that batches of pigs from the same source can emerge with a different Salmonella status once transformed into carcases. In other words, the slaughter point is the bottleneck where infection is shared and problems are intensified.
This does not mean leaving the farm situation totally unmonitored. The extent of infection should be determined on a regular basis. Nevertheless, the first place to start in fighting Salmonella is the slaughterhouse and not the farm.
In one respect, the congress reports even offered a ray of hope arising from modern techniques of Salmonella control. An approach applied for this purpose in Europe depends on checking samples of so-called meat juices extracted from carcases. Now it seems the meat juice sampling could give an inexpensive way of surveying herds against other diseases.
"The use of meat juice samples in Salmonella surveillance programmes is widespread inside the European Union," commented Irish veterinary practitioner Dr Noel Kavanagh. "These samples have provided a simple and low-cost alternative to blood serum for serological monitoring of Salmonella antibodies in pig herds. Reports at this congress show that a number of studies have been conducted to establish if meat juice samples could also represent a suitable alternative to serum for serological detection of a range of pig diseases in pig herds."
In a study he reported concerning units in Ireland, meat juice and serum were compared as samples for checking by a commercial Elisa assay that is used in testing for Aujeszky's disease/pseudorabies virus. Normally this test is done with serum samples, but the study wanted to investigate whether it could be modified to give comparable results on meat juice.
Modification of the Idexx AD Elisa test involved a sample of meat juice at a concentration of 25 percent with overnight incubation only at 2-7°C. The specificity of the modified procedure (defined as the percentage of negative serum samples also giving a negative result on meat juice) gave a rating of 98.7 percent in the overall study and 100 percent in the AD negative herds in which no false positives were recorded. The meat juice version correctly identified the Aujeszky's serological status of all herds tested.
Results from a small-scale study in Germany have demonstrated the suitability of meat juice as a means of detecting the viruses of PRRS and PCV2 in pig herds. The report by Jan Boeohmer of IVD innovative Veterinary Diagnostics, with co-workers from the animal health department of the Niedersachsen chamber of agriculture, has opened up new diagnostic applications for the meat juice samples which are allocated in Germany's mandatory Salmonella surveillance scheme. In this study, detection of antibodies for PRRS was carried out using a modified Idexx Herdchek PRRS 2XR antibody Elisa, with meat juice rather than serum samples. For PCV2, a RT-PCR process targeted the circovirus capsid gene in both meat juice and serum samples. Antibody Elisa values of blood serum and meat juice samples resulted in the correct identification of PRRS status of both negative and positive herds, including individual animals. Similar results were achieved for PCV2.
These IPVS presentations build on a fund of knowledge that already exists about meat juice testing for zoonotic disease agents. For example, there was a reminder at the congress of 1998 work which had recorded the antibody response after experimental infection of pigs with Trichinella spiralis, T. britovi and T. native. High antibody responses were seen in all experimental groups, including pigs in which no larvae had been recovered from carcase muscle. Since then it has become generally agreed that the strong and consistent antibody response found with meat juice indicates the usefulness of this material where a blood sample is not available, or where it is more convenient to conduct the assays on meat juice from slaughterhouse materials rather than blood samples. Work conducted about the same time infected pigs experimentally with Toxoplasma gondii and analysed meat juice samples and serum. The Elisa test results on meat juice were then compared with those on serum. Meat juice samples were deemed to be a suitable alternative to serum for the serological detection of Toxoplasma infection in pigs. More recently, a 2001 study monitored PRRS virus infection status in pig herds by conducting Elisa tests on meat juice samples and then comparing with blood samples collected from the same herds. A total of 18 herds were classified as PRRS negative by both test systems and 29 as PRRS sero-positive, an acceptable level of herd classification accuracy was achieved using this test. A year later, a meat juice Elisa procedure was employed to conduct a serological study of Swedish pigs for Toxoplasma antibodies and found a 5.2 percent prevalence of sero-positives.
Success in stamping out
There was plenty to celebrate in the reports to the 2008 IPVS congress, not least the strong message that eradicating important disease agents from production systems, or even from complete countries, is a viable option for the management of pig health in the 21st century.
We heard, for example, how Chile contains the largest sow herd known so far to have successfully eradicated the PRRS virus. As previous Pig International reports have already indicated, Chile can claim the distinction of being the first country to eliminate the PRRS virus from its territory.
The Chilean success was followed soon afterwards by the announcement that a European nation had achieved the same, with the removal of the virus from Sweden to restore its PRRS-free status.
Elsewhere in Latin America, it seems from congress comments, that the Mexican state of Sonora is looking closely at following the Chilean example.
But the PRRS virus continues to show its ability to change. In remarks to a symposium arranged by congress-sponsor Intervet, Prof. Irene Greiser-Wilke from the Institute of Virology at the University of Hannover in Germany provided a reminder that Europe has started to see new variants. At least two genotypes have long been known in PRRS infections, she commented. They are characterised scientifically by their structure (see Figure 1), but are known generally as North American and European even though neither is restricted to a single continent. The important distinction between them is that they differ both genetically and antigenically. What is more, they can be present simultaneously in the same herd and even in the same pig.
Within the past two or three years, however, up to three new genotypes have been identified in Eastern Europe (see Figure 2). As other congress speakers commented, they are regarded as examples of the virtually continuous process of mutation of this RNA virus as it shows how it can adapt to changes in its environment. The existence of further genotypes only adds to the challenge of achieving effective control of PRRS by means of vaccination and it underlines the case for trying to eradicate wherever possible.