Reducing risks of Salmonella infections in poultry

There are more than 2500 different Salmonella serotypes and only a few are causing problems in poultry. Some are vertically transmittable, where others infect other birds horizontally. What can be done to reduce the risk of releasing contaminated poultry products to the consumer market?

By: Ricardo A. Soncini MVD, Brazil

Salmonellas are gram negative enterobacteria which affect mammals and domestic fowl around the world. In poultry exclusively two serotypes are recognised to produce clinical Salmonellosis:  S. gallinarum (S.g) and S. pullorum (S.p), (named “Fowl Typhoid” and “Pullorum Disease” respectively). Many other serotypes, known as the paratyphoid group (of which more than 2.500 are described) may infect or contaminate flocks without causing disease symptoms. However these Salmonella’s may contaminate poultry products provoking food poisoning in human beings.

Both groups are related to important economic losses in the poultry industry: S.g and S.p due to mortality and bird performance, while paratyphoid may infringe the safety of poultry products for human consumption.

Food-borne diseases

The transmission of the typhoid group between birds occurs horizontally, by direct contact from infected to susceptible birds, or vertically by eggs. Several members of the paratyphoid group adapt well to birds and may invade and colonise in organs without causing disease or disorders. This is frequently the case with serotypes like S. enteritidis, S. typhimurium; S.hadar or S. heidelberg. Unfortunately they are the most prevalent and often related to outbreaks of food-borne diseases in humans. Paratyphoid Salmonellae enter flocks in many ways (Figure 1). Some serotypes (especially members of the groups B and D) usually use the transovarial route (vertical) and produce offspring that is infected as of day one with the serotype prevalent in the breeder hen. Other serotypes (such as C, E, G) are less invasive and persist in the digestive tract. They usually enter a flock through mechanical means (fomites, feed, pests and vehicles) or breaking the on-farm biosecurity barriers.


Spreading and contamination

Because of the wide variety of serotypes involved and the diverse sources of infection, strategies are necessary to prevent the risk of contaminating flocks. As the eradication of Salmonellas in poultry is an utopia, some serotypes need to be eliminated due to their economic damage (S.g and S.p) or their impact on public health (S. enteritidis, S. typhimurium, S. heidelberg). As these specific serotypes are strongly related to the reproductive tract of female birds it is necessary to rigorously break the cycle within these hens. Serotypes that do not use the transovarial route usually remain in the intestine and their eradication requires strict control measures focusing on environment factors (rodents, flies, feed, pets, etc.)

Strategies to prevent spreading:

Hygiene and disinfection: Before the introduction of a new flock, all buildings and equipment have to be cleaned and disinfected. This is needed more than ever when the previous flock was Salmonella positive. The efficiency of the sanitation process needs to be checked by a rigorous bacteriologic and quantitative test to ensure that enterobacteria counts are below the accepted < 103 cells per 25 cm2.
Pest control: The Alphitobius diaperinus, or litter beetle, is a bad bug since it is the host and vector of several avian pathogens, including Salmonellas. That is why disinfection should be performed immediately after depopulation of a house to prevent them from migration or hiding till the next flock comes in.

Also necessary is a good rodent control program. This includes toxic baits and measures to prevent rodents and wild birds from entering the buildings.

People: Farm workers are the key factor in an effective Salmonella control program. They must all know and meet all biosecurity standards established for the farm. They have to obey all rules regarding hygiene in the bathroom, clean hands, clothing and footwear operations and refrain from contact with domestic birds outside working hours. In this sense it is very useful to implement the Good Manufacturing Practices for farms since most failures are caused by human failings.

Feed: Feed is a rather important source of contamination in flocks. Grains, meat meal as well as environmental dust are common sources of feed contamination. Although pelletising feedstuffs will destroy almost all bacteria present in feed, recontamination may take place in the cooler environment or by dust and insects present in the feed mill. Studies demonstrate that Salmonella contaminations within feed mills are clearly associated with poor management and environmental conditions. Cleaning and disinfection of the production lines and surroundings should be a normal practice whereby products like organic acid and/or formaldehyde should be used. By keeping HACCP principles in mind one should segregate contaminated and clean areas (before and after the pelletise zone) to secure the microbiological quality of feed.

Hatchery: The hatchery is not only an incubator for eggs but also is an efficient multiplier and spreader of Salmonellas. This is especially the case when there are positive and negative eggs or chicks inside the hatchery. Cross contamination may then take place through direct contact, dust, handling during sexing and vaccination, and during storage and transportation to the farm. In just a few hours, a carrier chick can spread large amounts of Salmonellas by faeces and consequently contaminate many other birds inside the hatchery. Important measures to reduce the spread of infection are: egg disinfection, avoid mixing of positive and negative eggs and use different incubators or days for brooding. Most important is to always start working with eggs from Salmonella free flocks and clean and disinfect all systems when contaminated eggs and chicks have been processed. Keep at any time eggs and chicks of positive and negative origin separated, also during transport.

Use of vaccines: Vaccination proves to be a useful tool to prevent and/or control Salmonella infections. There are two groups of vaccines: inactivated (bacterins) and attenuated live vaccines. Both are serotype specific, so they have a limited protective effect to the same or Salmonella serotypes within their own group (eg. D group S. enteritidis and S. gallinarum). There are live vaccines for biological control available in the market against S.gallinarum (9R); S.enteritidis (bacterins and live vector or modified strains) and S. typhimurium (killed and live attenuated).

A useful vaccination program for breeders or layers should include a combination of live and killed vaccines to achieve efficient protection by the combination of cellular and antibody activation. Vaccination in broilers is limited and often restricted to live and serotype specific vaccines. Nevertheless for reasons of food safety the European Community (Regulation (CE) 1177/2006) encourages the use of vaccines in birds for meat production too.

Probiotics and other additives: The use of probiotics is becoming more and more popular and proves to be a useful tool in the fight against Salmonella infections. In recent research work we (Avila L.F et all 2011; Soncini R. et all 2011; Herich R. et all 2010) showed the beneficial use of this option. Besides probiotics, the continued use of organic acids remains to be another alternative in a prevention program.

What to do with positive flocks
In the past it was quite common to use antibiotics in Salmonella positive flocks, but since this proved to be not totally effective and for human health reasons a prudent use of antibiotics is advised and a more rigid strategy is followed. Therefore it is suggested to eliminate S. gallinarum and S.pullorum infected flocks.

Flocks infected with serotypes which cause a threat to public health (such as S.E.; S.thipymurium; S.heidelberg) should be treated and ultimately culled, while their products should be processed in isolated batches and undergo thermal treatment before releasing to the consumer market.

Prevention is the best tool to control Salmonella infections. Nevertheless it is important to apply a regular bacteriological monitoring to know and verify the results of the Salmonella prevention program. Critical points are breeders during production, hatcheries, feed mill and in broilers a few days before date of slaughter. In case of positive outcomes one can take correctives measures.

Concluding remarks:

  • Salmonella bacteria are strongly adapted to poultry, and generally go without provoked health problems.
  • The most important manner of contamination in poultry is by vertical transmission (especially by serotype of the B and D group) and by the environment (for the other groups). Control strategies are dependent on the serotype present.
  • The poultry industry invests a lot of money and energy to minimise the risk of Salmonella contamination and has initiated several “farm to fork” biosecurity programs to protect the consumers.