Prevention and Reduction of Feed Contamination

Importance of salmonella in feed

It is generally accepted that Salmonella is the major biological hazard for animal feed. Industrially-produced feeds were said to be more at risk from Salmonella contamination than other feed types, and were most likely to become contaminated through their raw materials, especially oilseed meals and animal-derived protein sources.  Although usually they are heat-treated to kill Salmonella and other bacteria, re-contamination can occur afterward during cooling and subsequent handling.

Strategies used in combination to eliminate Salmonella

Dr Frank Jones from the University of Arkansas pointed out that in general 3 different strategies are used in combination to eliminate Salmonella presence in animal feeds.

- Prevent contamination of ingredients
- Prevent the growth of bacteria
- Killing pathogens

Prevent contamination

Dust control is one of the most important measures in the Salmonella control program. Moreover, it has been observed that fat/oil can protect Salmonella. Therefore, besides dust prevention, it should also be avoided that fat or oil contaminates the environment.
In general, dust samples are more sensitive than swabs or scrapings for routine screening purposes, but swabs or scrapings may be more useful when following up a suspicion of contamination in a plant in order to identify precisely the location of resident contamination.
To identify regular problems with ingredients or post-processing contamination, it continues,  sample dust from several checkpoints. The first of these is the ingredient pit auger – usually, the sample is taken from the auger system below or behind the intake pits. In storage areas, dust from within grain bins provides an optimal sample. But if the interior is inaccessible, use the dust from ledges inside the top of the bin or from sieves, or spillage from augers.
Other dust checkpoints are the main air aspiration cyclone system (unless this is not accessible, in which case the less sensitive option of vacuumed or swept dust could be used), surfaces below coolers or on associated framework, at the pellet shaker (lines producing meals would sample alternatively at finished product bins or augers) and on ledges near to feed discharge points in out-loading gantries.
Similar lists are in the guide for sampling in other places, such as storage facilities and for the frequency with which samples should be taken. However, more emphasis may be useful on the separate aspect of the person or people doing the sampling.

Prevent growth

The most important criterion for the growth of bacteria is moisture. There are some sources of moisture that is very difficult to avoid such as condensation or high humidity. Also during a good manufacturing process moisture is used sometimes as a “kill” measure, e.g. in the conditioner to increase temperature. This moisture can enable Salmonella to survive and grow, particularly when Salmonella is present in niches in the spots somewhere in the production system. Finding those spots is an important task of the mill personnel. A good sampling method and risk assessment can help to identify those growth niches.

Killing of Salmonella

In principle, there are two measures to kill bacteria in the feed mill process; heat treatment, particularly via pelleting, and chemical treatment. Normally these measures are taken alone or in combination. 

The human factor

People who work in the plant are an important factor in spreading Salmonella. Special clothing and shoes can help to reduce the risk. In addition, it is good to segment the feed mill in so-called “dirty” and “clean” areas and avoid the movements of personnel between the two areas as much as possible.

Critical places

One possible place to examine in a mill would be around bucket elevators. These conveyor devices generally create aerosols of considerable amounts of dust. Also, heat is generated, particularly around the pulleys in the head and boot sections. Each batch of feed passing through the boot section of the elevator is exposed to material that has already collected there and the heat generated may also cause moisture to concentrate in cooler areas of the boot, resulting in an increased potential for contamination.
Another risk factor occurs where raw material conveyors are located close to coolers, and therefore potentially in the flow of air drawn in to cool pellets. Dr. Jones observed that risks increase if covers on ingredient conveyors have been left open for ease of access, or windows opened that may have allowed insects and wild birds into the feed processing area.
His work has confirmed frequent other warnings that the heat of the pelleting process has only a temporary effect on microbial contamination, with microbe numbers likely to rebound quite quickly afterward. Sometimes, it seems, the problem is connected to the presence of dust or debris around the pellet mill so that heat-treated material is re-contaminated at the exit of the pelleting chamber. Feeds can also be easily contaminated in poorly designed or managed coolers, Dr. Jones agreed.

Resistant bacteria

Some types of Salmonella establish themselves on surfaces by creating a form of slime, or biofilm, and that this may then make them more resistant to certain sanitizers. It is most likely to occur on surfaces that are often damp and are not cleaned regularly. The biofilm could also be protecting the bacteria against drying.
From the work carried out at Norway’s National Veterinary Institute on feed manufacturing, several common disinfectants have been found to have a reduced effect on the particular Salmonella serovars associated with biofilm formation. Strains of serovars with this ability are known to persist in Norwegian feed mills.

Hygiene in delivery

All the mills should arrange formal training on biosecurity policies and procedures for its feed delivery drivers and production personnel, with repeat courses every six months.
The vehicle cab should be equipped with plastic boot covers for the driver’s use on each visit and also disposable floor mats, that are similarly discarded into a designated container when the delivery is completed.
Better scheduling of feed deliveries is also urged, with priority given to the farms with the highest health rating either to receive a designated separate transport or to be first in a sequence of visits by the same truck. Most often this would mean starting with genetic or seedstock enterprises, ahead of farms with commercial breeding animals and then those that grow or finish for market. Strict washing, disinfection and drying of trucks would be essential at any time that this pyramid flow of feed deliveries was changed.

Swabs of trucks

Trucks, after completing their delivery, should be given a thorough clean-up on the farm or at a commercial truck wash, before delivering to another customer or returning to the mill. Not only the trucks that deliver the final feed should be sanitized, also the trucks that deliver the feed ingredients should follow a strict hygiene protocol. The recommended routine starts with pressure-washing to clean off mud from the wheels, wheel wells and undercarriage, followed by disinfection. What is more, the steering wheel/assembly, foot pedals and gear shifts should be sprayed with disinfectant.
As for those disposable overalls, boot covers and caps supplied to the driver, the same source points out that it is not always possible to discard them appropriately at the farm so an alternative has to be available. It proposes that these items could, for example, be placed in a sealed plastic bag that is sprayed with an approved disinfectant and left in the truck cab for safe disposal once back at the factory.
An insecticide is among the truck hygiene items that should be furnished to feed delivery drivers. Flies or other insects need to be prevented from entering the vehicle, it comments. In the event that they do enter, the driver should use the spray to kill them before the truck leaves the farm premises.

What is your biosecurity policy for dealing with visitors to the feed mill?

Each mill should have a clearly stated visitor policy. This notice should be posted at the entrance to the property and company buildings.
A logbook of visitors should be maintained.
Visitors in the feed processing and handling areas should be limited only to individuals essential to the operation of the mill.
Visitors to the mill facility should follow a predetermined route in order to control exposure to feed products.
Foot baths or dip mats should be placed wherever truck drivers will be entering or exiting their trucks and at each entry/exit point into company buildings.
Mills that have retail feed stores on site should keep retail customer traffic completely isolated from the mill facility, vehicles, mill production employees and truck drivers. Mills that do not have a retail feed store on premises should discontinue all other retail feed sales from the mill.
Mills that permit retail feed store owners or other wholesale customers pick up loads at the facility should not allow the drivers to leave the cab of their vehicle, and the vehicle should be cleaned and disinfected prior to entering the mill property. If the drivers must get out of their vehicle, then their feet should be disinfected and access to the facility should be controlled to prevent the risk of spreading disease. Traffic flow patterns for these vehicles should be designed to prevent them from contaminating the feed mill.
Service personnel (vendors, electricians, machine repairmen, meter readers, etc.) and regulatory inspectors should be restricted in their access to production and transportation areas within the facility. Visitors that may go to other mills or locations where birds are present should have their feet and hands sanitized as well as wear protective coveralls, boot coverings and hair coverings supplied by the mill. A company contact person should communicate biosecurity concerns to each outside vendor of potential concern.

Managing the air to the cooler

Salmonella control procedures in the feed mill must include management of the air to the pellet cooler to avoid re-contamination. These remarks are extracted from ‘Biosecurity programmes for Salmonella control’ by S.G. McIlroy MRCVS, Northern Ireland.
Heating the feed at standard pelleting temperatures (65-70°C) is highly unlikely to achieve total decontamination. Subsequent multiplication of any residual Salmonella can occur. Furthermore, the pellet cooler can be a frequent and effective method of re-contaminating finished feed by drawing in large volumes of air that may carry Salmonella particles of dust.
Often, the dust particles will move from the raw material end of the mill to the finished feed area. Engineering procedures which minimize the flow of dust particles across a mill, and facilitate a “clean” air supply to coolers, will impact on the level of contamination in the finished feed product.
A specialized biosecure milling plant in Northern Ireland prevents re-contamination of decontaminated feed by enclosing the entire manufacturing process within a complete hygiene barrier. Positive air pressure is maintained throughout the facility. All the air supply is filtered through a comprehensive ventilation system, down to 5 microns, to remove any dust particles which could potentially be contaminated.