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This article was published in 1944
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.
FOOD-BORNE DISEASE OF BACTERIAL ORIGIN
D. F. Stewart. B.V.Sc., Dip. Bact, (Lond.), Division of Food Preservation, C.S.I.R.
The term "food poisoning" is not really applicable to all cases of illness arising from the ingestion of foods. The term survices (sic) from the misconception that illness of this type was due to the presence of so-called ptomains or toxic amines derived from the breakdown of proteins. It is now recognised that before a foodstuff can reach a state wnereat it might contain such substances it would be inedible, or at least, decidedly unattractive.
It would be better to term this class of illness arising from the ingestion of foodstuffs or liquids infected with pathogentic bacteria, or contaminated with toxic products elaborated by bacteria, as bacterial food-borne diseases. Accepting this wider interpretation, the ingestion of contaminated foodstuffs can be the source of many diseases, but it is proposed to limit this talk to dealing in some detail with the three main types of bacterial food-borne disease and to refer only briefly to those diseases which can be, but are not usually acquired in this manner.
The Salmonella group of bacteria is one upon which considerable attention has been directed in recent years. It comprises a group of organisms which have many characteristics in common, and includes the typhoid and paratyphoid bacilli. They are essentially and typically intestinal parasites of man and animals. As they do not form spores they are not particularly resistant heat, being destroyed by 60ºC. maintained for a period of 30 minutes. Therefore, they should not be a source of disease in products processed in such manner as to give effective heating of that order.
The disease in humans is characterised by an incubation period of 12 to 24 hours and symptoms of gastro-enteritis accompanied by fever. It is not commonly fatal, but deaths are by no means rare, as they occur in 1-2% of cases. At least, this is the classical conception of the disease caused by these organisms. However, they have been found recently to give rise to a variety of pathological conditions and have been isolated from abscesses and occasional cases of meningitis. Furthermore, although the overall mortality is of the order of 1-2% closer investigation is demonstrating that some species of this group give rise to a large number of cases of mild disease, whereas others are very frequently fatal and consequently few people become infected by contact.
In domestic animals there is a greater variety of conditions caused by infections with this group of organisms. They may vary from abscess formation to pneumonia.
The ways in which infection can be acquired by humans are several. It should be understood that all classes of domestic animals consumed as meat are susceptible to infection with these organisms. In Australia comparatively little work has been done, but we have found in recent years mortalities in sheep, cattle, pigs and ducks.
While the risk of direct infection from well-cooked meat is not likely, it might be remembered that in large joints cooking might merely mean incubation of the interior, particularly it not refrigerated soon afterwards. This frequently has been the story when the meat has been consumed safely when first cooked, but when served cold or rehashed in some fashion it has given rise to infection. Products such as pies and patés are particularly liable in this regard. Salmonella organisms grow well in the range 20ºC to 40ºC.
However, there is another aspect of the necessity of good storage conditions after cooking. There is an endemic disease of rats and mice due usually to S. typhimurium or S. enteritidis. Contamination of foodstuffs with the excreta of rats and mice can be the source of infection.
A third important method of transmission is from human carriers who contaminate foodstuffs, milk or water. Human carriers might arise from recovered cases or from sub-clinical cases, who have never had reason to suspect that they have suffered from infection. The organisms are excreted through the intestines. Finally, the possibility of transmission by flies from contaminated sewage cannot be disregarded in places where sanitation is poor.
Another type of food-borne disease which has come in for much attention during recent years is that caused by a bacterial product known as "enterotoxin." This substance is most commonly formed by staphylococci, either of the aureus or albus types. However, other organisms such as Bact. coli and species of the Proteus group have been known to elaborate a similar substance. In contrast to the disease just described, which was eseenttally an infection, this illness is an intoxication. The organisms grow in the foodstuffs and the toxin is formed prior to consumption.
The symptoms of the disease are characteristic. One of the chief features is that there is a very short incubation period varying from two to six hours and usually about four hours. The onset is very sudden, and the symptoms are violent vomiting, occasionally diarrhoea, extreme prostration and shock, followed by complete recovery in 24 hours.
The foodstuffs involved are usually of a semi-liquid nature derived from milk or incorporating milk. Custards, cake-fillings, ice-cream, cream fillings, eclairs, or milk itself are most commonly involved. However, cured meats, which exercise a selective effect favouring growth of staphylococci, are frequently the cause. Ham and tongue sandwiches and meat paste also have been recorded, and we have encountered an outbreak involving some 60 individuals due to the ingestion of ham. It will be noted that there is a marked similarity between the products in that they are usually stored for some hours at room temperature prior to consumption. While staphylococci grow best at about body temperature they grow quite readily in the 15-20ºC. range.
While staphylococci are ubiquitous the contamination of the foodstuffs probably occurs in one of two ways. Mastitis caused by staphylococci is quite common in dairy cattle and these organisms are responsibler for about 10% of all cases of mastitis in dairy cattle of this State. While in acute cases of staphylococci mastitis the change in the nature of the secretion from the affected quarter is such that it would not be included in the supply, sub-acute and chronic cases also occur and shed millions of cocci into the milk, and no doubt much of this finds its way into households. Apart from the dairy origin, staphylococci are a very common cause of skin lesions in humans, and food might easily become contaminated from this source. There is little doubt that this condition is more common in this country than is generally recognised.
The most serious food-borne disease, not in its incidence but in its effects, is botulism. Although uncommon, when botulism does occur it is characterised both by a high morbidity and a high mortality. (U.S.A. 60-70%, Europe 20-40%).
Again the disease is an intoxication. There is no satisfactory evidence that the organism can invade the body, multiply and elaborate its toxin. The organism, Clostridium botulinum, is a natural inhabitant of the soil and if it possessed any inherent properties of invasiveness, one could well imagine that there would be many individual cases of the disease for there is no doubt that the majority of individuals have taken in the spores of Cl. botulinum at one time or another.
The action of the toxin is essentially a bulbar paralysis, being absorbed through the intestinal tract and exerting its effects on the central nervous system. Visionary disturbances are usually the first symptoms, followed by constriction of the throat, inability to swallow, with a final terminal pneumonia or terminating with respiratory paralysis. There is extreme muscular flaccidity but usually no gastrointestinal disturbances, except constipation. Cl. botulinum is an anaerobe and will not grow in the presence of air. Its requirements for growth in foodstuffs are not fully understood but it will not grow in acid foodstuffs (below pH 4.5) unless spoilage by some other organism raises the pH to a suitable level, However, we have evidence ourselves that other factors than pH will also influence the growth of the organism. Cl. botulinum frequently contaminates canned foodstuffs. It forms spores which are very resistant to heat and if the canned products are underprocessed, or if contamination is excessive, and conditions are suitable within the container, the organism can grow and elaborate its toxin. Manifestations of its growth are somewhat variable in different products or in different specimens of the same product. Although the organism is proteolytic the product might not be of such a nature as to permit off-odours to develop. Although sufficient gas is frequently produced to swell cans, this is not always so and the can may remain flat and yet be toxic.
The toxin is the most potent known in its lethal effects but not in the rapidity of its action. It can be absorbed through abraded surfaces or mucous membranes as well as through the intestinal wall.
Most outbreaks of this disease are due to home canning of vegetables where usually processes are grossly inadequate. Very frequently the unfortunate housewife is the victim as she tastes the product if at all doubtful of its appearance, It is most unwise to test spoilage in non-acid foods by tasting.
The time taken to destroy toxin by heat up to temperatures of boiling point is variable. It would be very foolish to depend upon such a process to detoxify spoiled material.
Another type of food-borne infection is attributed by some authorities to a non-haemolytic streptococcus which in one instance was identified as Str. faecalis, a member of Lancefield's group D. The incubation period is reported as varying from 5 to 18 hours and the symptoms, nausea, colicky pains, diarrhoea and sometimes vomiting. While the evidence is not yet clear it is thought to be an infection rather than an intoxication.
Apart frcm these three diseases there are many of bacterial aetiology which can be, but are not usually food-borne.
Water-borne diseases can flourish only under poor conditions of sanitation and hygiene, or under conditions where safe sanitary provisions break down.
In this country, however, milk-borne diseases are of much more importance than water-borne. Milk-borne diseases can arise in one of two Ways. Firstly, by direct transmission of milk derived from an infected cow and secondly by contamination of the milk during distribution by a human carrier of pathogenic organisms.
Foremost amongst these is tuberculosis of the bovine variety, acquired particularly by children. It is estimated in Great Britain that 2,000 children under the age of five years contract tuberculosis of bovine origin annually due to the consumption of infected milk. The bovine variety of Mycobacterium tuberculosis is responsible for 7% of total deaths due to tuberculosis.
The sore-throat, scarlet fever syndrome caused by infected milk is common. The group A. streptococcus which causes puerperal fever, scarlet fever and erysipelas in humans occasionally infects udders of cows and milk from the infected cow gives rise to such an epidemic. Also, dairymen and milkmen can infect milk if they are mechanical carriers of the organism. It is known that 10 to 20% of normal people are carriers of group A. streptococci in their throats. Under the system of milk distribution prevalent in this country there is ample opportunity to infect the milk by sneezing and coughing. Of samples of milk taken from 48 dairies in the U.S.A., where hygienic standards were considered to be particularly high, group A. streptococci were isolated from 7.
Infection of dairy cattle with Brucella abortus reaches 10-15%. This organism causes Contagious Abortion among cattle and is excreted in the milk of infected cows. Brucella abortus is also responsible for Undulant Fever of human beings. This disease is not usually fatal but is a very distressing and disabling one and runs a long course.
Infection of the external surfaces of the cow's udder with Corynebacterium diphtherae is very rare but has been recorded as the cause of human cases of diphtheria. Again human carriers could infect milk during its distribution.
Typhoid and Salmonella infections also can be spread by milk either from infected cows, which is uncommon and refers only to Salmonella, or by human carriers which is not so uncommon and is exemplified in the recent outbreak in Melbourne.
While the dilution factor usually prevents disease from this source reaching epidemic proportions, there is no doubt that it could be responsible for many individual cases of disease.
The story would not be complete without some reference to fly transmission in respect of water and milk. The recent finding of the virus of poliomyelitis in sewage and in flies infesting epidemic areas throws new light on the spread of this disease. Spread by flies under poor sanitary conditions must be considered a strong possibility.