Salmonellas and the Poultry Industry – Part I
By Martine Boulianne
Salmonellas are bacteria well-known by the poultry industry. First and foremost considered a risk to human health
By Martine Boulianne
Salmonellas are bacteria well-known by the poultry industry. First and
foremost considered a risk to human health, we tend to forget that some
of these salmonellas are also deadly to poultry.
Salmonellas are bacteria well-known by the poultry industry. First and foremost considered a risk to human health, we tend to forget that some of these salmonellas are also deadly to poultry. Luckily, these latter are absent in Canada. In this article, I will discuss various salmonellas and their means of transmission. In Part II next month, I will cover salmonella survival in the environment, methods of detection and the various means of prevention and control.
Pullorum Disease and Fowl Typhoid
There are two types of salmonellas; those that are host-specific and can only infect a single species or closely related ones, and those not host specific i.e. they can affect a very broad range of warm- and cold-blooded animal species. The latter are named paratyphoid salmonella.
Among host-specific salmonellas, Salmonella Pullorum and Salmonella Gallinarum are causative organisms of pullorum disease (PD) and fowl typhoid (FT) respectively. These two diseases are reportable diseases, that is if suspected or diagnosed, they are to be immediately reported to federal authorities.
Forty years ago, Salmonella Gallinarum and Salmonella Pullorum infection were rampant in poultry and plagued producers. Mortality in birds infected with these two salmonellas was very high – up to 80 percent in chicks. Hens, chickens and even turkeys would die from septicemia and multiple abscesses spread to various organs.
Because of the major economic impact these two diseases represented, American authorities created the National Poultry Improvement Plan (NPIP) in 1935. This plan was designed in part to control these two diseases and was successful in significantly reducing incidences of PD and FT. Over the years, the poultry industry has managed to test, control and eradicate these two diseases, first in breeders, then in all other sectors.
Today, the NPIP still sets standards for the evaluation of poultry breeding stock and hatchery products with respect to freedom from hatchery-disseminated diseases such as Salmonella Pullorum, Salmonella Gallinarum, Salmonella Enteritidis, Mycoplasma gallisepticum, Mycoplasma synoviae, and Mycoplasma meleagridis. The Canadian poultry industry complies with these standards, but in spite of surveillance systems, no one is protected from an outbreak.
For example, in November 1997, Salmonella Pullorum was isolated from backyard hens on Vancouver Island, in British Columbia. The flock was destroyed and the Canadian Federal Inspection Agency (CFIA) conducted an investigation to identify other possible positive poultry on the island. All flocks were quarantined and blood tested. A total of 2,900 positive chickens in 27 backyard flocks were destroyed.
But even though everybody believed that PD had been controlled, two other layers from a small backyard flock were found positive in September 2001, proving that nobody is ever fully protected from the resurgence of poultry diseases once believed to have been eradicated from our country.
We often refer to non host-specific salmonellas as paratyphoid salmonellas. These salmonellas are very numerous, with more than 2,600 different serovars, and are responsible for a large quantity of food poisoning cases in human beings. The majority of cases are linked to the consumption of contaminated animal products, and very often poultry and its by-products are associated with these events. However, all animal species can be affected with this bacterium, and unfortunately there appears to be an increase in the number of salmonella occurrences not only in humans, but also animals.
Only 10 percent of the 2,600 different salmonella serovars have been isolated in poultry. There are various means to classify salmonellas, but the most widely used consists of classifying salmonellas into serogroups and then by serovar. When submitting a sample to the laboratory, the first step consists in identifying the bacterium as a salmonella, which takes about 48 hours (this is to allow growth of the bacterium).
Then, once isolation and identification of the salmonellas are made, they are classified in serogroup by means of a very quick test that takes a matter of minutes. If the isolated salmonella is classified in the serogroup B or D, this raises alarm because Salmonella Typhimurium or Salmonella Enteritidis is suspected. These two salmonellas are recognized as presenting a health risk to consumers.
Because there are several salmonellas in the B and D serogroups, it is important to obtain a definitive serotyping. Since several days can pass between classification and definitive serotyping, many laboratories will warn the submittee of this potential health risk. Usually a single laboratory per province does the serotyping for salmonellas, and consequently this is why it takes a minimum of two weeks to get a final answer.
If the serovars are found to be Salmonella Typhimurium and Salmonella Enteritidis, the isolates are then sent to the reference laboratory in Guelph, Ontario for final identification. This is important in regards to human health as some lysotypes (a further type of classification) of serovars B and D are more pathogenic to humans than others. In the case of Salmonella Typhimurium, the identification of a lysotype DT104 will cause concern to human health authorities, while a lysotype 4 for Salmonella Enteritidis will create a lot of worry.
In 2003, the Canadian Integrated Program for Antimicrobial Resistance Surveillance (CIPARS), randomly sampled chicken caecums in Canadian slaughterhouses. Interestingly enough, only 16 percent of caecums turned out positive for salmonellas. Among these positive samples, the most frequently isolated serovars were Salmonella Heidelberg (50 % of samples), Salmonella Kentucky (14.3 % of samples), and Salmonella Hadar (11.9 % of samples). (It is important to note that the frequency of isolated serovars for a given region changes over time.)
Salmonella Typhimurium was found in a few carcasses, and two of these isolates were resistant to more than five antibiotics (PICRA, Report 2003, chart 12, p. 32, http://www.phac-aspc.gc.ca/cipars-picra 2003_e.html).
Salmonella Enteritidis (SE)
Salmonella Enteritidis has traditionally been associated with eggs. To date, there has been only one reported case of Salmonella Enteritidis foodborne infection related to chicken meat consumption. However, SE is a constant source of concern for the egg industry, and the potential danger it represents for human health justifies the various surveillance programs set up by provincial table egg marketing boards.
These programs vary in terms of sampling size, and have been very effective for detecting SE. A good example is the Quebec Egg Board SE surveillance and eradication program, created in 1997. This was in response to the Salmonella Enteritidis food poisoning outbreak related to the consumption of SE contaminated eggs, which affected more than 150 people in northwest Quebec in 1996. Since the implementation of the surveillance and eradication program, no other case of epidemic of salmonella poisoning connected with egg consumption has been reported in the province.
Why do we fear Salmonella Typhimurium DT 104?
Salmonella Typhimurium is associated with meat consumption. In humans, it causes diarrhoea (sometimes bloody), fever, abdominal pains and vomiting accompanied with migraines. Certain foodborne infections can turn into blood-poisoning, become widespread and can even lead to death, with young, old and\or immunosuppressed patients being most at risk. For those developing blood-poisoning, antimicrobials usually cure the disease.
Unfortunately, in the case of Salmonella Typhimurium DT 104 (STDT104), the bacterium is resistant to several antibiotics, hence complicating greatly the treatment. Health Canada considers this bacterium dangerous enough to have recommended the destruction of breeder flocks found positive for Salmonella Typhimurium in recent years.
Salmonella transmission across the poultry industry
Salmonellas can very easily be passed on and this transmission can take one of two paths, or both.
The transmission is deemed vertical when the mother infects her progeny. For example, an infected hen can harbour salmonella in her ovaries and lay infected eggs; this is called transovarian transmission. The majority of studies on vertical transmission have used Salmonella Enteritidis as the model, but it is now known that this mode of transmission is relatively infrequent and variable (between 0.1 and 1 % of eggs laid according to different studies).
But freshly laid eggs can also become infected if they come into contact with hens’ droppings. When freshly laid, the egg lacks its last barrier of defence, the cuticle. While cooling down, a ‘vacuum’ will be created for the formation of the air chamber. This vacuum can then suck in bacteria present on the egg surface. The egg has certain defence mechanisms present in the white, but both ageing and high temperatures (as those encountered during egg incubation) will allow the bacterium to pass from the white to the yolk and multiply. When put in incubation, some of these infected eggs will be transformed into “firecrackers,” a phenomenon known well but unappreciated by hatchery managers.
So, the breeders can contaminate their offspring, and infected eggs can contaminate their neighbouring eggs or chicks in the incubator or pens. The latter is what is known as horizontal transmission.
In the case of horizontal transmission, animals are contaminated directly by other animals (this includes other birds and rodents) or indirectly through food, dust, etc. For example, several cases of Salmonella Typhimurium DT104 in small wild birds (Passeriformes) in Quebec in the spring of 2000 were linked to gatherings at bird feeders. These birds may be an important potential source of infection for poultry.
Contamination is mostly oral (direct contact with contaminated birds, ingestion of contaminated food) and even respiratory. Younger and stressed animals, having a less efficient immune system, are the most susceptible. Infected animals (those that become ill or are carriers) excrete the bacteria in their droppings and contaminate the environment.
This excretion is variable, having anywhere from 10 bacteria or less up to 10,000,000 bacteria per gram of droppings. It may be continuous or sporadic. It is probably for these reasons that during previous episodes of Salmonella Enteritidis infections, it was impossible to find the bacterium two weeks after the episode, as laying hens most likely had stopped excreting the bacterium. Laying hens however remain carriers and when stressed, such as at the beginning or peak of lay, excretion might be reactivated.
Food can represent a source of contamination and that is why feedmills have to control many critical points (animal or vegetable raw materials, equipments, finished feed, etc.) during the various steps in feed handling, manufacturing, storing and transporting.
As you can see, potential causes of contamination are numerous, and one can seem helpless in front of salmonellas. There are however various measures to control this bacteria. This will be covered in Part II, where I’ll review how salmonella survives in the environment, surveillance protocols, prevention and control methods.