Outbreaks of salmonellosis have been reported for decades, but in the past 25 years the disease has increased in incidence on many continents. In the Western hemisphere and in Europe, Salmonella serotype Enteritidis (SE) has become the predominant strain.
Investigations of SE outbreaks indicate that its emergence is largely related to consumption of poultry or eggs.
But because there are no symptoms the strain is hard to detect in chickens, which in turn poses a tough problem for the food industry that needs to build food safety tools to tackle the issue.
Jean Guard Bouldin, a researcher at the Agricultural Research Service (ARS), a US government science laboratory, conducted tests in which chickens were inoculated with S. enteritidis.
Eggs were then tested for hardness by compressing them until a hairline crack formed. Eggs from salmonella-infected hens cracked more easily than those from non-infected hens. Other research, say the scientists, has shown that some strains of S. enteritidis seem to target the hen's reproductive tract, which appears to result in an egg with a less resilient shell.
Bouldin and colleagues found that not only was salmonella present inside chicken eggs, but other bacteria was also detected. Since these bacteria are usually seen in eggs that have been contaminated through cracks in the shell, Bouldin theorised that poor eggshell quality allowed the bacteria to enter the egg.
At low-dose infection, Bouldin found that S. enteritidis actually stimulated egg production, particularly in older hens. This increased production, claim the researchers, may have literally spread the limited eggshell material - calcium - too thinly.
Other diseases of chickens can also decrease shell quality, but usually they result in a decrease in production and illness in hens. The researchers add that changes to eggshell quality over the lifespan of a laying hen are to be expected, and consequently a hen's age could be an additional risk factor.
Recent trends in global food production, processing, distribution and preparation are creating an increasing demand for food safety research in order to ensure a safer global food supply. In industrialised countries, the percentage of people suffering from foodborne diseases each year has been reported to be up to 30 per cent.
In the US, for example, around 76 million cases of foodborne diseases, resulting in 325,000 hospitalisations and 5,000 deaths, are estimated to occur each year, claims the United Nations body, the World Health Organisation.
Recent research on the UK egg market by analysts Mintel finds that while volume sales have risen by 10 per cent between 1999 and 2003, value sales have increased by double this amount, some 23 per cent, due to consumers trading up to premium egg varieties.
The trade up to higher-priced eggs - free range and organic - suggests overall health concerns are driving the market. In 2003 free-range eggs accounted for 30 per cent of egg sales by volume compared to just 24 per cent in 1998, representing a 38 per cent rise in sales since 1998.