Contamination of food with salmonellae can arise in many different ways and can come from a wide range of natural sources. Salmonella infections are common among:-
The organisms may be found in their eggs.
Occasional cases of food-poisoning result from eating lightly-cooked individual eggs, but far more serious trouble arises from the mixing and spray-drying or freezing of large batches of eggs for use in the catering trade. Eggs often travel from one country to another in these forms, and can carry large numbers of salmonellae with them.
The danger of such egg preparations is increased by the fact that they are often used in preparing food items that undergo little cooking. The carcasses of the birds themselves, especially of those reared in the crowded conditions of modern intensive production, are often heavily contaminated with salmonellae by the time that they reach the kitchen.
Pigs and cattle may have salmonella infections, and some human infections are due to eating inadequately cooked meat. In connection with this and all forms of food-poisoning it is important to appreciate that heat penetrates very slowly to the center of large joints of meat or other large volumes of solid or semisolid food during cooking, so that even vegetative organisms may still be alive at the center long after the periphery is well cooked.
Oysters grow particularly well near sewage outlets, and readily become contaminated with fecal bacteria. Since they are eaten raw, it is important that they should be kept for long periods in fresh water before they are marketed. Rats and mice are commonly carriers of salmonellae and must therefore be kept away from food stores. Finally, human carriers may contaminate food with salmonellae, of the food-poisoning or the enteric varieties.
Transmission can easily occur via a substance such as ice-cream, which is readily contaminated during manufacture, is a good culture medium and is not cooked before it is eaten. Sometimes the route from man to man is long and complex.
Although typhoid is not strictly 'salmonella food-poisoning', international transmission of salmonellae is well illustrated by the 1964 Aberdeen typhoid outbreak, which was the largest of several that occurred in Britain within a few years in association with the distribution of corned beef imported in large cans from South America. These cans were in effect cultures of typhoid bacilli, which they had presumably acquired from river water, contaminated with human excreta, that had been used to cool the cans after sterilization and had been sucked into them through faulty joints, as described below in connection with an outbreak of staphylococcal food-poisoning.
The large scale of the Aberdeen outbreak-507 cases—was a result of contamination of a slicing machine and other utensils in the shop in which the corned beef was sold, and consequent transfer of typhoid bacilli to various other foods sold in the same shop. A fortunate feature of this series of outbreaks was the very low frequency of secondary cases—i.e. of patients who had not themselves eaten the contaminated food but had acquired their infection from those who had. Salmonellae usually produce gastro-enteritis only when ingested in large doses. Their multiplication, which is encouraged if infected foods of appropriate composition are allowed to stand for some hours in a warm room, is even further stimulated by 'warming up' the food for too short a time to kill the bacteria and then allowing it to cool slowly through the temperature range that suits their growth.
The presenting symptoms are:-
They usually come on 12 to 36 hours after eating the offending food. Vomiting may occur but is not commonly a prominent feature.
Recovery may take a week or two, and convalescents may remain carriers for long periods. A small proportion of cases are fatal.