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• The secondary defence system
There
are more than 2,000 different strains of salmonella bacteria - one
of the commonest causes of food poisoning in the world. The bacteria live
in the intestines of cows, poultry, pigs, pets, and many wild animals
- often doing no harm to the carrier. But they are harmful to humans.
Eating salmonella bacteria can be a nasty and sometimes even fatal experience.
•
Transmission
The most direct methods by which salmonella bacteria can be transmitted
from one animal to another are either by eating the meat of an infected
animal or by ingesting faeces containing bacteria from a host's intestine.
So, for example, if you tread contaminated faeces into the carpet at home,
you could unwittingly transmit salmonella to a toddler or pet.
There are many other indirect routes of transmission. For example, fruit and vegetables that have been contaminated by water containing infected sewage carry the bacteria. Any animal - including a human - that eats the fruit and vegetables will become infected. If an infected animal is eaten by a second animal, it too will become infected. In this way salmonella bacteria spread throughout the food chain.
•
Multiplication
Salmonella ingested with food is immediately bathed in digestive juices,
but since hundreds of thousands of salmonella bacteria can be taken in
with just one mouthful, some are likely to survive. Villi - finger-like
projections on the lining of the stomach designed to absorb water and
nutrients - provide a good safe site for the survivors. The bacteria
invade the cells on the surface of villi and start to multiply.
When a cell has been infected with salmonella for two hours, it dies. As it does so, it bursts open, spreading salmonella to surrounding cells. A process of invasion and destruction begins. Within just eight hours, the stomach is swarming with one million salmonella bacteria. This number will continue to grow exponentially: within nine hours, there are four million of the bacteria in the stomach.
•
The primary defence system
The body is not inactive while this invasion is taking place. As soon
as cells start to die, they release chemical distress signals which alert
the immune system. A complex series of attacks and counterattacks begins.
The body's immune system deploys a rapid reaction force in the form of mobile defence or 'scavenger' cells called macrophages. Macrophages capture the salmonella and release a cocktail of poisons to kill them. But salmonella bacteria have a secret weapon: they release a chemical counter-defence which disables the macrophages. The bacteria can now use the macrophages as a place to breed, untouched. The bacteria have won the first battle, turning a cell that was sent to destroy it into a convenient host. The body's primary defence system has failed.
•
Expulsion
The body is down, but not out. It has to resort to brute force. The chemical
distress signals, which are constantly being released by dying cells,
reach a critical level. They trigger a physical reaction in the intestine.
Usually, the intestine contractions work to push food down from the stomach
into the bowel. Now they begin to work in the opposite direction in an
attempt to expel the salmonella bacteria physically. Food shoots back
into the stomach from the intestine, and from there up the oesophagus.
By vomiting, the body can clear the top few inches of the intestine of
salmonella.
But most of the bacteria are lower down, and will need to be flushed out another way. The contractions lower in the intestine continue to work in the right direction - and they accelerate. Digested food passes so quickly through the gut that very little water is absorbed from it. Diarrhoea, like vomiting, is a crude defensive system. It dehydrates the body, giving the sufferer a terrible headache. Nevertheless, diarrhoea is effective. A single bout will flush around five million salmonella bacteria from the intestines.
•
The secondary defence system
Although diarrhoea can do an effective job of flushing out salmonella
bacteria, thereby preventing them from spreading, it cannot get rid of
the invaders altogether. So the immune system brings a secondary defence
system into play. Specialised scavenger cells work as traps: once infected
by salmonella bacteria, they neutralise the invader and pass it into a
lymph gland. Within the gland, the bacteria come into contact with helper
cells, part of the immune system. Once the helper cells have familiarised
themselves with the bacteria, they are better equipped to fight it -
or rather, to help the macrophages fight it. The helper cells release
a chemical stimulant which transforms the macrophages from useless bacteria-infested
cells into vicious killers. They double in size, and they become faster
and more efficient. They are able to pump out their salmonella-killing
poisons so rapidly that the bacteria are helpless. Their advance through
the body is quickly halted.
•
Treating the disease
In most cases, the body is best left to get rid of salmonella bacteria
on its own. Treatment of the bacteria is possible with antibiotics, but
is not usually recommended because the use of antibiotics tends to increase
the length of time for which the salmonella remains in the stomach and
faeces. This prolongs the symptoms of the disease and increases the risk
that the salmonella may be passed from one carrier to another. However,
if the infection spreads from the intestine, or if a patient is very old,
very young or has another disease, antibiotic treatment does become necessary.
Salmonella is becoming increasingly difficult to treat, however. Although antibiotics are rarely used to eliminate the bacteria from humans, they are used with farm animals. If bacteria are repeatedly exposed to antibiotics, they begin to become resistant to it.
In addition, the use of growth-promoting antibiotics in farm animals has led to an increase in the incidence of salmonella food poisoning. As well as strengthening the defences of salmonella against various drugs, growth-promoting antibiotics kill off normal gut bacteria in animals' stomachs which would be in competition with salmonella.
•
Treating the symptoms
Salmonella causes a wide range of unpleasant symptoms. Victims suffer
vomiting and diarrhoea as a result of the body’s attempts to expel
the bacteria. They also experience stomach cramps as a result of the increased
speed of contractions within the intestine. Meanwhile, the chemicals that
seep into the bloodstream as cells are destroyed cause muscles to swell
and ache. To round it all off, diarrhoea has the effect of dehydrating
the system, which causes severe headaches.
Sufferers of salmonella poisoning usually try to treat the symptoms with a variety of means. Painkillers will alleviate the aches and pains effectively. However, taking diarrhoea tablets is a mistake. Chemicals in the diarrhoea pills slow the contractions in the stomach, meaning that the cramps will die down and the diarrhoea will stop. But diarrhoea is one of the body’s key weapons against the infection. Stopping it will make the sufferer feel more comfortable in the short-term, but it also gives the salmonella a chance to continue breeding and spreading. As a result, the volume of chemicals released into the bloodstream will rise and the aches and pains will become worse. Sufferers are advised instead to drink lots of fluids in order to prevent dehydration.
•
The worst case
In most cases, the body is capable of ridding itself of salmonella bacteria
within five to seven days. However, in a small number of cases, the disease
takes hold and may be fatal. About 50 people in the UK die each year from
salmonella poisoning. Those most likely to succumb are those whose immune
systems are unable to counter the effects of the bacteria quickly or effectively:
the old, the very young, and those who are already suffering from a disease.
Salmonella attack | Food scares | Dangerous foods
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