Salmonella in humans is from an international perspective often linked to the consumption of eggs, meat or meat products from poultry. The other type is caused by the serotypes Salmonella Pullorum and Salmonella Gallinarum. These cause severe disease in poultry but are rare in humans.
Poultry can become infected with many different types of salmonella; about 10 percent of all Salmonella spp. have been detected in poultry. The most important are Salmonella Typhimurium and Salmonella Enteritidis. Other serotypes frequently occurring in poultry worldwide are, for example, Salmonella Hadar, Salmonella Livingstone, and Salmonella Senftenberg.
In most cases, the birds are not sick and the production is not affected. The degree of illness depends on factors of both the bacteria and the host. The bacterium’s serotype and phage type is of significance but also the type of animal, age and general health status. S.Typhimurium and S. Enteritidis may induce clinical symptoms in poultry.
It is mainly in very young chickens aged up to two weeks that salmonella can cause disease and death. The symptoms may vary and include weakness, loss of appetite and poor growth. The animals are crowded close to heat sources and sit with drooping wings and their eyes closed. Watery diarrhoea may also occur. In adult poultry, disease is rarely seen even if they have bacteria in the blood. Infected hatching eggs may lead to low hatchability with increased embryo mortality and increased mortality among newly hatched chickens.
Spread of infection and preventive measures
Basically, salmonella is a faecal-oral infection. Infected birds can clear themselves of infection after some time, but some excrete bacteria in droppings for several months. It is practically impossible to rid a salmonella infected flock from the infection when kept on permanent bedding.
Several serotypes have the ability to invade, i.e. they have the ability to infect inner organs and not just the gastrointestinal tract. This leads to increased risk of eggs becoming infected in the fallopian tube of the hen. S. Enteritidis has a high ability to invade, but even S. Typhimurium, S. Hadar, and S. Heidelberg have some ability to do so. The infection can thus be passed on to humans via infected consumption eggs or to the chicken if the egg is to be hatched.
Poultry usually become infected through contaminated feed, infected animals or eggs, but also through contact with a contaminated environment.
The voluntary salmonella control program for poultry has taken possible routes of infection into account and is based on biosecurity. Examples of measures taken within the program are requirements to heat-treat all feed thus eliminating any salmonella bacteria. All-in-all-out production, with no introduction of new animals during the production period and a thorough cleaning and disinfecting between batches is also important, as well as strict hygiene measures like changing shoes and washing hands before entering the animal premises and rodent control to prevent infection from the environment. It is also important that wild birds are kept out of contact with poultry, in for example ventilation systems.
A voluntary control program for poultry has existed in Sweden since 1970. Since 2007, the program is laid down in a regulation but is administered by the Swedish Egg Association and the Swedish Poultry Meat Association. Besides the voluntary program, there is also a mandatory salmonella control of poultry regulated by one Swedish and several EU legal acts. This makes provisions for poultry to be sampled before slaughter and laying hens during the production period. The surveillance of breeding poultry and hatcheries is particularly important to prevent the spread of infection downstream in the production chain. The legal acts also make provisions for how sampling should be performed to guarantee safe results. If salmonella is detected in poultry the disease is generally controlled by stamping out and cleaning and disinfection of the premises.
Salmonella Pullorum & Salmonella Gallinarum
Cause and host
Salmonella Pullorum and Salmonella Gallinarum are especially adapted to poultry. They cause diseases known as pullorum disease (S. Pullorum) and fowl typhoid (S. Gallinarum). Both mainly affect hens and turkeys, but birds of other species may also become infected and fall ill.
Several other animals can also carry these two types of salmonella. Occasional findings of S. Pullorum and S. Gallinarum are reported in humans. If mammals, including humans, become infected the disease is relatively mild and transient.
Symptoms in poultry and turkey flocks infected with S. Pullorum or S. Gallinarum are very similar. It is mainly young birds that develop disease symptoms, including weakness, loss of appetite, poor growth and white, loose droppings. If signs of high mortality, up to 100 per cent, and uneven growth in young chickens (0 to three weeks of age) is noted it is important to investigate if S. Pullorum or S. Gallinarum could be the cause.
A reduced hatchability can be seen since chicken foetuses are dying in the egg before hatching, due to the transmission of the bacteria from the hen to the egg before it is laid. The animals can also become infected by direct contact with infected animals and through the droppings, sometimes as early as in the incubator.
S. Gallinarum can, unlike S. Pullorum, cause disease in adult birds. Symptoms that can be seen are increased mortality in the flock, poor general condition (such as ruffled feathers, pale and small crest), diarrhoea, decreased feed consumption, reduced egg production and decreased hatchability.
The chickens that survive infection with S. Pullorum or S. Gallinarum, as well as birds that are infected in adulthood, often become chronic carriers of the bacteria without seeming ill. These chronic carriers will excrete the bacterium through droppings and eggs for a long time.
History and disease control
Controlling pullorum disease and fowl typhoid has a long historical tradition in Sweden. Already in the 1920´s the poultry industry took, at that time Sweden’s General Poultry Breeding Association, the initiative for mandatory blood testing for the control of poultry farms in the country. In the early 1930´s, the diseases were included in Swedish legislation and they came to be fought very actively and at a considerable cost to the poultry industry and individual animal owners during the following decades. The last case was reported in 1962 and thereafter the disease was considered to be completely eradicated. Nevertheless, in 2001 pullorum disease was detected in two chicken flocks in Stockholm County. The disease was eradicated and has not been detected since. It is unclear how the disease entered the flocks.
To ensure that Swedish commercial breeding poultry (breeding stock of laying hens as well as chickens and turkeys for fattening) are free of S. Pullorum and S. Gallinarum the infections are routinely monitored by mandatory blood testing in the so-called Poultry Health Program. There are no records of the status of pullorum disease and fowl typhoid in backyard poultry. The possibility to detect S. Pullorum and S. Gallinarum in these birds is dependent on animal owners sending in dead animals, blood-, or faecal samples for examination.
In case of suspected infection
There is a strong determination to prevent pullorum disease and fowl typhoid from becoming endemic in Sweden again. Mainly because of major disease problems in infected flocks but also because of the hazard it presents to human health (although the symptoms are mild). Therefore, all poultry owners in Sweden are requested to contact a veterinarian when experiencing hatching problems and high mortality in young chickens.