Cleaning the feed chain from Salmonella, a daunting job

Salmonella in feed keeps making the headlines. Every year recalls take place due to infections with this clever bug. In many places strict control programs are put in place, but the danger of recontamination after cleaning is always present. And the bacteria are more resistant to treatments. Indeed, it is a daunting job to clean the feed chain from Salmonella.

By Dick Ziggers, editor All About Feed


Salmonella is a major cause of food poisoning outbreaks around the world, leading to product recalls, plant suspensions, and other costly measures. In the US the Food and Drug Administration (FDA) and the Food Safety & Inspection Service (FSIS) have food safety programs in place to protect the human food supply, just as the European Food Safety Authority (EFSA) and similar government agencies around the world. Qualified as a zoonosis Salmonella bacteria can migrate from animal to human and make them sick. FDA and EFSA have a zoonosis order and both organisations recognise that control of Salmonella in animal feeds and feed ingredients is a major component in HACCP programs in pre-consumption processes of food. In the industrialised world zoonotic Salmonella infections come from food animals and their products. In developing countries, where animal protein consumption is lower, main sources of Salmonella outbreaks are contaminated vegetables, water, and human-to-human transmission, which are believed to be causing a comparatively larger proportion of the human cases than those in industrialised countries.

‘Science and risk-based approach’

In the US a FDA Salmonella policy takes a science- and risk-based approach to regulating Salmonella in animal feed, according to the National Grain and Feed Association. Under the FDA’s draft policy, the agency identifies eight Salmonella serotypes that are of regulatory concern to animal health:
  1. poultry feed with S. pullorum,S. gallinarum or S. enteritidis;
  2. swine feed with S. choleraesuis;
  3. sheep feed with S. abortusovis;
  4. horse feed with S. abortusequi; and
  5. dairy and beef feeds with S. newport or S. dublin.

Previously, the FDA had applied regulatory oversight to all types of Salmonella that might be present in animal feed, although it exercised enforcement discretion. The policy also mentions several types of commercial heat treatments, such as pelleting, extrusion and rendering, as well as irradiation, as effective in killing Salmonella.

Jones and Richardson (2004) in the US collected 886 samples (68 feed ingredient samples, 189 dust samples, and 629 feed samples) from three feed mills each of which produced between 100,000 and 400,000 tonnes of feed a year. Samples were collected on three days (Monday, Wednesday, and Friday), during two seasons (early spring and summer), and between 07.00 h and 17.00 h approximately once per hour. Samples were collected from five locations within each mill: ingredient receiving, at the mixer, at the pellet mill, from pellet coolers, and at load-out.

They concluded that feed ingredients remain a source of feed contamination (Table 1). However, dust must also be considered a major contamination source. Indeed, it would appear that dust accumulation around the pellet mill could effectively negate the sanitising effects of pelleting. The researchers also indicate that Salmonella contamination rates were related to mill management practices. More recently Jones (2011, Journal of Applied Poultry Research, 20:102-113) released a review of practical Salmonella control measures in animal feed. In this review he clearly indicates that thermal treatment (usually pelleting) alone is not enough to eliminate Salmonella. Pelleting reduces, but may not completely eliminate Salmonella contamination because of limitations of the process or recontamination after thermal processing. Chemical additions to control Salmonella in feed primarily involve the use of products containing organic acid, formaldehyde, or a combination of such compounds.

Pelleting and extrusion


The use of high temperatures to accomplish pasteurisation during processing is based on the destructive effects of time and temperature on microorganisms. However, equal numbers of bacteria placed in a process are not always destroyed with the same ease by heat, states the American Feed Industry Association AFIA in its Salmonella control guidelines (www.afia.org). Microbiologists have identified at least 11 factors or parameters of microorganisms and their environment that can affect heat destruction. These factors include moisture or water activity, fat levels, presence of salts, presence of carbohydrates, pH, protein content, number of organisms, age of organisms, inhibitory compounds, and time and temperature history. Despite the influence these factors can have on the resistance of microorganisms to heat, thermal destruction during the processing steps of pelleting or extrusion is the most critical control step for destruction or reduction of Salmonella and other pathogenic microorganisms.

Temperatures reached in pelleting and extrusion are critical in Salmonella control.

While heat treatment is the most effective control method, sometimes it is not appropriate and other options are available. However, the use of chemical treatments (organic acids and formaldehyde) should be approached with caution as recent research has suggested that such treatments interfere with Salmonella detection methods rather than killing the organism. Currently, US Food and Drug Administration (FDA) regulations allow the irradiation of lab animal feed to 50 kGy to control microorganisms, and poultry feed to 25 kGy for the control of Salmonella.

Zero tolerance in Sweden

Despite much research and many national and international attempts to implement control strategies, the incidence of human salmonellosis in most countries remains high. One notable exception is Sweden, which remains essentially free from the Salmonella problems typical for most other industrialised countries. The background for the Swedish success started as early as 1961 with governmental regulation. Earlier a voluntary programme for the control of fowl typhoid (S. Gallinarium/Pullorum) had been running since 1941. Severe Salmonella epidemics during 1953-54 demonstrated the need for a more comprehensive control programme. The control and its applications are continuously being revised. The objective of the Salmonella control is that animal products delivered for human consumption shall be free from Salmonella. The concept is that animals delivered for slaughter shall be free from Salmonella. The control program is as rigorous as effective. Apart from an eradication policy and no use of antibiotics in treating infected flocks, also an intensive prevention program is in place. For feed this is in place since 1960. All Swedish factories participate in the control. The current strategy for the control is to detect, as early as possible in the production chain, when Salmonella contaminated raw material has entered the factory. All samples are examined at the National Veterinary Institute. Swedish legislation prescribes to heat materials >75°C for poultry feed. When this is done for 30 seconds a thousand fold decrease in bacteria is established. In a long term conditioner the retention time should be longer than two minutes. Salmonella has been isolated after heat treatment only on rare occasions. The primary objective is that Salmonella contaminated feed must not leave the factory. Actions are taken when Salmonella is isolated in different parts of the production chain. The costs for the Salmonella control program have to be paid by the producers. Even when factories have to be closed in order to control a Salmonella contamination, the factory owner pays despite the fact that the decision was made in agreement between the owners and the authorities. It is almost impossible for other countries to apply the Swedish model of Salmonella control, which requires near freedom from Salmonella in domestic food animal production from the onset.

Does it pay?

In Finland researchers tried to quantify the costs and benefits of two Salmonella control policies for broiler production. The control options were the Zoonosis Directive 92/117/EC and the more intense strategy, the Finnish Salmonella Control Programme (FSCP). The comparison included the Salmonella control costs in primary and secondary production and the direct and indirect losses due to Salmonella infections in humans in 2000. The total annual costs of the FSCP were calculated to be €990,400 (0.02 €/kg broiler meat). The average control costs in the broiler production chain were seven times higher with the FSCP than with the Zoonosis Directive alone. However, the public health costs were 33 times higher with the Zoonosis Directive alone. The value of one prevented loss of life per year exceeded the annual control costs of the FSCP. Thus the conclusion was that due to significant savings in public health costs compared to costs of FSCP, the FSCP was found to be economically feasible.

Pig feed more difficult

Because pigs rarely show clinical signs of salmonellosis, undetected carriers can more easy enter the food production chain. A study carried out in Switzerland (2005) aimed to estimate the probability and the level of Salmonella contamination in batches of feed for finishing pigs in Swiss mills and to assess the efficacy of specific process steps for reducing the level of contamination with Salmonella. The scientists gathered and combined data on the various parameters having an influence on the final contamination of feed and used a model to predict contamination. The simulation showed that
  • depending on the production pathway
  • the probability that a batch of feed for finishing pigs contains Salmonella ranged from 34% (no specific decontaminating step) to 0% (organic acids and heat treatment). Most of the Swiss production was shown to undergo some kind of decontaminating step. A heat treatment, in combination with the use of organic acids, was found as a solution of choice for the control of Salmonella in feed.

Effect of grinding

The influence of grinding (fine vs. coarse) of mash feed and for pelleted feed on Salmonella occurrence in finishing pigs was investigated in Denmark in several trials. In coarsely ground mash feed Salmonella detection was significantly reduced. And the use of coarsely ground meal for pellet feed production also led to a reduction in detection. However, it did not reach the same low level that was found in coarse ground mash feed.

Also at the Institute of Animal

Nutrition at the University of Veterinary Medicine in Hanover (Germany), several dissertations recently focused on the influence of feed structure (fine or coarsely ground) and acidification of feed and Salmonella occurrence in piglets and fatteners. In these works it was established that under experimental conditions and from field studies coarse grinding of feed in combination with the use of organic acids (or their salts) an adequate reduction of Salmonella contamination in piglet rearing and at finisher phase could be achieved.

When feed is ground coarser the following effects will occur: reduced passage rate in the stomach dry matter rate of stomach contents increases growth of lactic acid bacteria is promoted volume of dissociated lactic acid increases pH is reduced. This leads to a significantly reduced ability of survival of the orally ingested Salmonella from the environment. Jorgensen et al. (2000) also determined in experiments with pigs fed on mash that the concentration of acetic, butyric and propionic acid in the stomach was significantly higher than in pelleted feed - an additional “barrier effect” against Salmonella and other gram-negative bacteria.

Present research

In Belgium the Flemish Institute for Agricultural and Fisheries Research is presently carrying out a study on the reduction of Salmonella contamination of pork through the use of innovative feed additives. This project, which started in March 2010 and ends in March 2012 will further optimise the use of feed additives on the basis of organic acids and essential oils to reduce the shedding of Salmonella by fattening pigs. A previous project showed the effectiveness of these additives. Their effectiveness will be further improved by adjusting the composition (active substance and dose). Only new additives are tested, which have indicated in the literature as having potential to reduce Salmonella shedding. Apart from controlled trials, fields trials are carried out to validate the potential of the additives and to estimate the effect of the reduction of Salmonella contamination in the slaughterhouse.