What causes STEC-HUS?
STEC-HUS happens in a few children after they get gastroenteritis, an infection in their tummy or gut. This infection is usually caused by a particular germ, a bacterium called Escherichia coli O157, which is often shortened to E. coli O157.
Most children recover from gastroenteritis caused by these bacteria. In a few children, the infection causes HUS.
About E. coli
E coli is a type of bacterium that is often found in the guts of animals, including cattle, sheep, deer and goats, and sometimes pets and birds. E. coli does not make the animal ill.
However, some strains, or types, can cause illness in humans. People may be infected with E coli O157 by eating food that has been contaminated with the bacteria (usually because it has been contaminated with the animal’s faeces, or poo).
About STEC
Some strains of E. coli make toxins, which are like poisons, that can cause serious illness. One toxin is called shiga toxin – the bacteria is called shiga-toxin-producing E. coli (STEC). (It may also be called verocytotoxin-producing E. coli (VTEC), but we will use STEC in this topic.)
The most common strain of E. coli that may cause HUS is E. coli O157.
Read more about STEC
Shiga toxin damages the lining of blood vessels in the body. This usually happens in the intestines (gut), causing a tummy upset, including diarrhoea and sometimes blood in the stool (poo). In some cases, it can affect the kidneys, causing HUS.
Each year, between 800 and 1000 cases of STEC infection are reported by doctors in England. Compared to other bacteria, such as Salmonella or Campylobacter, which also cause tummy bugs, STEC infection is rare. However, doctors take STEC infections very seriously because the disease can be fatal (cause death), particularly in infants, young children and the elderly.
The highest numbers of cases are seen in children aged under 9 years of age and the number of cases is slightly higher in females across most age groups.
Who can get STEC-HUS?
For every 100 children who get a STEC infection, about 10 will develop HUS. Often, we see STEC-HUS after an outbreak, which means that a large number of people get infected with E. coli.
Only a small number of STEC are needed to cause illness. This means that the infection can spread quickly and easily within the family and in other settings such as nurseries, primary schools, nursing homes and hospitals.
What happens in HUS
In HUS, there is damage in the small blood vessels of the kidneys.
Blood cells
When the body tries to repair the damage, this causes problems with some blood cells, the living parts of blood.
- Platelets move to the areas that have been damaged, and clump together. They partly or completely block the blood vessels. This also means that there are fewer platelets going around the bloodstream.
- Red blood cells, which carry oxygen round the body, try to pass through the blocked blood vessels. They are broken up into little pieces.
Read more about what happens to blood cells in HUS
In HUS, the body breaks down and destroys blood cells. The term ‘haemolytic’ is used because ‘haem’ means blood, and ‘lytic’ means to break down. This is what happens:
The cells that line the small blood vessels in the kidney filters (glomeruli) are damaged.
The body tries to repair this by bringing in blood cells called platelets. Platelets form clots, or clump together, at places where there is damage. These clots partly or completely block the flow of blood.
The platelets are used up trying to repair damage in the small blood vessels. This leads to a lower level of platelets in the rest of the blood, which is called thrombocytopenia. This can cause tiny pink spots on the skin and easy bruising.
Red blood cells are broken up into little pieces as they pass through the narrowed blood vessels. This causes haemolytic anaemia. Because red blood cells carry oxygen round the body (in a substance called haemoglobin), a reduction in the number of these cells can cause children to feel weak and tired and look pale.
Kidneys
The kidneys stop working as well as they should. Children may urinate (wee) less or not at all. Water, salt and waste build up in the body.
Read more about how kidneys work and what happens in HUS
How the kidney works
Inside the kidneys there are about one million nephrons. Each nephron is made up of a glomerulus (when we talk about more than one glomerulus, we say glomeruli) and a renal tubule.
- Each glomerulus acts like a sieve, helping to remove extra water and waste from the body, and holding on to blood cells and protein, which the body needs.
- Blood flows into the kidneys and to each glomerulus.
- Most of the water, and some other substances in the blood, pass through the glomeruli.
- This liquid flows into the renal tubule. Most of this liquid moves back into the bloodstream. The rest of it becomes urine.
- The urine leaves the kidney by the ureters and goes into the bladder, where it is stored until we are ready to go to the toilet.
About the urinary system and kidneys – what the kidney does
What happens in HUS
The blood clumps together and forms clots in the glomeruli. The glomeruli cannot filter blood as well as usual. The kidneys makes less or no urine, and water, salt and waste build up in the body, leading to swelling.The word uraemic means a build-up of waste in the body.
Other causes of HUS
Pneumococcal HUS
HUS may also happen after chest infections or meningitis (a disease in the area around the brain and spinal cord) caused by pneumococcal bacteria. This causes pneumococcal HUS. In serious cases, this can lead to complications with the brain.
Most children with pneumococcal HUS will need to stay in hospital for monitoring and treatment. They will need to take antibiotics, medicines that kill the bacteria. As the infection is treated, the HUS should go away.
Other infections and other causes
Other bacteria and some viruses can also sometimes lead to HUS.
HUS can occasionally be caused by some medicines, or linked to other diseases.
Atypical HUS
Atypical HUS is a rare type of HUS that is not caused by an infection, but rather is thought to be linked to genes. It may run in families.
Some children with atypical HUS have diarrhoea, but this is not usually related to a STEC infection. This type of HUS tends to keep coming back, and is more likely to lead to long-term problems with the kidneys.