Did you know that disinfection as part of the clean-up process at a pig production site could even help Salmonella bacteria to survive? This is because most disinfectants have the effect of coagulating protein. An amount of protein will be present in any faecal material left in the pen after incomplete cleaning. Hardened by coagulation after contact with the disinfectant, it forms a layer that protects rather than destroys pathogens located deeper in the dirt. These bacteria then can emerge to cause more contamination when the fragments of soiling are explored by the next pigs in the pen.
The message must be, of course, that hygiene protocols against Salmonella at farm level risk being counter-productive unless every speck of organic matter is removed before disinfection begins. German veterinary specialists, summarising their experience in advising operators of pig herds with a Salmonella problem', have warned recently about this risk. They reported finding that too many producers were not taking enough care to remove all the dirt at cleaning time.
Their research also demonstrated that different cleaning routines need to be practised where Salmonella is the main target, compared with the more traditional rituals aimed at a respiratory pathogen such as mycoplasma. The important places to clean against pneumonia, for example, are the pen areas that pigs can reach – for the simple reason that transmission is almost entirely by direct contact with the pig. To control Salmonella demands attention also to surfaces capable of indirect contact, from the blades of a power fan to the roof beams and ceiling.
"There is no magic to this, it is just a question of knowing where Salmonella can hide," says Professor Thomas Blaha of the University of Veterinary Medicine in Hannover, Germany, who supervised the research for the doctoral thesis of DVM Kerstin Bode. "We mention fan blades because dust settles on them and harbours the bacteria. Most people switch off their fans to save cost after rooms are emptied, but do not directly power-wash them. The use of a pressure washer in the area beneath the fans then makes the blades damp and the dust sticks to them even better than before. But the usual practice in cleaned rooms is to switch on the heating before the next pigs arrive. This dries the blades and the layer of dirt is loosened, ready to fall down and carry the Salmonella to the pigs when the fans are switched back on.
"In a similar way we have observed that few units clean the ceiling in every room. It is not of importance for controlling all infections, but dust on the ceiling always poses a risk of Salmonella transfer if it is shaken loose by even a slight vibration of shutting a door."
The Hannover team's call for new cleaning and disinfection procedures to be adopted in the fight to beat Salmonella formed one of the highlights of the 7th Safepork conference held recently in Verona, Italy. Among the conference proceedings shortly being made available through website www.safepork2007.sistemacongressi.com will be Kerstin Bode's presentation of investigations into the reservoirs of infection lingering even at herds that practise the highest level of traditional hygiene protocols.
Behind this study was the experience of a German production enterprise on multiple sites. The sow herd for the network was apparently healthy and high-hygiene and all its piglets went through a single nursery site where no pigs from other sources entered. The complete output from the nursery was shared equally between 3 finishing units and they also did not bring in animals from outside the system. Yet each one of the finishing sites was rated as Category 3 for Salmonella status, indicating that up to 95% of the blood samples collected from their pigs were found to be positive for exposure to the bacteria.
Intensive sampling established afterwards that the infection was circulating internally, rather than having been introduced from outside such as by birds or rodents. To break the cycle required a clean-up not only of pens, fans and ceiling, but also of less obvious areas. This meant such places as the walkways used for moving pigs, the loading ramp that took them onto the slaughterhouse truck and the room where unit staff and visitors exchanged their street shoes for farm boots.
"Until now, most attention on Salmonella has looked at sources that could bring the bacteria into a herd from outside, with the obvious candidates being birds, rodents, feed and people," Professor Blaha comments. "However, our results highlight the Salmonella reservoirs within the pig operation left by incomplete cleaning and disinfection procedures. These reservoirs may play an even more important role than the constant introduction of Salmonella into a herd.
"When dealing specifically with Salmonella at an otherwise healthy unit, therefore, we can say that even the best of the current procedures for cleaning and disinfection is unlikely to be enough if it does not include the connecting service areas on the farm and those representing indirect contact. They may not have to be included in every cleaning cycle, however. In a herd that is visibly clean apart from having a high Salmonella load, doing this once should keep bacterial levels low for at least the next year."
Close to consensus on Salmonella control
Held every 2 years, the Safepork international meeting on disease agents that might pass from pork to humans has moved around at each edition. This time it was in the Italian city of Verona, in 2009 it transfers to Deventer in the Netherlands. But while the organisational cycle and objectives remain unchanged, the 2007 conference was marked out from its 6 predecessors by revealing a far greater measure of agreement among the Salmonella control professionals than has been true in the past.
An important aspect of this new consensus ends the long previous debate over the relative merits of emphasising control measures either at the slaughterhouse or on the farm. For producers, of course, it would be convenient if the slaughterers had to bear most of the work and cost. Their view seemed supported by evidence of a major multiplication of bacteria in pigs after arrival in the abattoir's lairage area and in carcases after slaughter, simply through cross-contamination. However, the multi-country general agreement at Safepork 07 was that steps to control Salmonella must be taken at every stage of the production chain. Neglecting primary production will only have the effect of destroying everything achieved at the slaughter end. In the same way, farm measures will be meaningless without action at the slaughterhouse, not least involving a cleaning/disinfection protocol for the lairage area once a Salmonella control programme has been started in the supply herds.
Also agreed: every herd has its own Salmonella pattern, so do not expect to find answers in standardised procedures for control. The measures followed will have to be farm-specific and based on common-sense. Two units apparently identical for size, housing and feeds are quite likely to show surprising differences for the rate at which the results of their blood samples come back positive. Each will need to be handled individually.
Equally, there is no so-called silver bullet or single do-it-all measure such as a vaccine or feed acid that takes care of Salmonella without help. Effective control depends on enacting a set of measures and they will fail unless each one is completed; there are numerous stories of failure because, say, rodent control was overlooked or wild birds were not kept out of feed storage silos.
Protocols for monitoring herds in regions or countries also are being harmonised, Safepork presentations showed. Their united aim is to identify the herds carrying most Salmonella bacteria into the food chain so that actions might be tried to reduce the loading of pathogens at a farm site. Related to this and relatively new, conference presenters were almost unanimous in backing the approach of finding the worst offenders and helping them to improve. They stressed that this was appropriate specifically for countries with a high prevalence of Salmonella. Clearly, somewhere with just one problem herd among thousands would not be justified in spending money on industry-wide monitoring.
So the European perspective returned to conclusions reached about country status, by an expert panel for Europe's food safety agency EFSA that had been asked to consider options for mitigating Salmonella in pigs and pork. By the judgement of this panel the Scandinavian countries of Sweden, Finland and Norway can be regarded as low-prevalence. In a middle category for prevalence there is only Denmark. This leaves all other members of the 27-national European Union as still in a higher-prevalence situation demanding an investment in monitoring to find the worst herds.
One last agreement at the Safepork meeting highlighted the need for patience. Success in controlling the pork-to-people form of the infection does not happen overnight, said delegates. At least 6-7 years may pass from the start of the monitoring process before a measurable reduction can at last be seen.
