HUS is a disorder characterised by thrombocytopenia, microangiopathic haemololytic anaemia (anaemia secondary to red blood cell fragmentation) and renal (kidney) failure. Like TTP, both acquired and inherited forms exist.

VTEC-associated HUS

About 90 per cent of HUS cases are caused by a certain group of bacteria known as verocytotoxin-producing E.coli (VTEC). The most commonly associated strain is E.coli 0157:H7.

E.coli infection is a common cause of food poisoning and can be acquired from foods such as unpasteurised milk or uncooked meat. It can also spread from person to person. In the US, it has been estimated that each year eight E.coli 0157:H7 infections occur per 100,000 people. However, only about 5 per cent of these infections lead to HUS.

HUS results as a consequence of damage to the lining of the kidneys caused by a secreted toxin (verotoxin). Treatment of the acute diarrhoeal illness with either antibiotics or medication to reduce bowel activity is associated with an increased risk of developing HUS. Young children (under fives) and the elderly are also more susceptible to the disease.

Familial (congenital) HUS

Congenital HUS may occur during infancy or in adult life. It is associated with deficiency of factor H.

Factor H is a plasma protein synthesised by the liver, which regulates the complement pathway. Complement is a series of plasma proteins forming part of the immune system. Activation of the complement pathway ultimately leads to destruction or removal of foreign material from the body. Deficiency of factor H has been associated with a particular type of kidney disease (type II mesangiocapillary glomerulonephritis) in addition to HUS. It is possible that some of the sporadic HUS cases occurring without a diarrhoeal phase are also associated with low factor H levels. Exactly how low factor H levels result in HUS remains unknown.

Patients usually seek advice after an acute illness characterised by abdominal cramps, bloody diarrhoea, nausea and vomiting. Fever may be minor or absent. Characteristically about one week later further symptoms similar to those described for TTP develop. A patient may therefore become jaundiced, pale or experience symptoms of anaemia (eg lethargy and breathlessness) with excessive bruising or even bleeding. Patients may pass less urine or find their urine becomes red or brown in colour. Sometimes there are no symptoms of kidney failure and this is only found on blood testing. Similarly high blood pressure is not apparent but is discovered by medical staff on clinical examination. Although less common than in TTP, about 25 per cent of people with HUS develop nervous system symptoms such as headaches or confusion and fever can also be present.

In familial HUSthere is no preceding diarrhoeal illness but the symptoms are otherwise the same. Since it is inherited, there may be a family history of the condition.