Haemolytic uraemic syndrome is a serious condition that affects the blood and kidneys, most commonly triggered by bacterial infections in children. When tiny blood vessels become damaged and blocked, the kidneys can stop working properly, making this a medical emergency that requires immediate attention and hospital care.
Understanding Haemolytic Uraemic Syndrome
Haemolytic uraemic syndrome, commonly known as HUS, is a condition where small blood vessels throughout the body become damaged and inflamed. This damage triggers the formation of tiny blood clots in these vessels, which can affect organs throughout the body but most severely impacts the kidneys. The condition gets its name from three key features: the destruction of red blood cells (called haemolysis), low levels of blood platelets (cells that help blood clot), and kidney injury.[1]
When blood vessels become blocked by these small clots, red blood cells get damaged as they try to squeeze through the narrowed spaces. This mechanical damage breaks apart the red blood cells, leading to anaemia, which is when the body doesn’t have enough healthy red blood cells to carry oxygen to tissues. At the same time, platelets get used up in forming the many tiny clots, causing a shortage that can lead to bleeding problems.[2]
The kidneys are particularly vulnerable in HUS because they filter large volumes of blood through very small vessels. When these vessels become clogged with damaged cells and clots, the kidneys cannot perform their essential job of filtering waste products from the blood. This can lead to acute kidney injury, which is a sudden loss of kidney function that can be life-threatening if not treated promptly.[3]
Types of Haemolytic Uraemic Syndrome
Medical professionals recognize several different types of HUS based on what causes the condition. The most common form is called “typical” HUS, which develops after an infection with certain bacteria. This type accounts for about 90 percent of all HUS cases and usually follows an episode of diarrhoea caused by specific strains of bacteria that produce toxins.[5]
Atypical haemolytic uraemic syndrome, or aHUS, is a much rarer form that is not caused by bacterial infections. Instead, it results from genetic mutations that affect how the body’s immune system works. In aHUS, certain protective proteins in the blood don’t function properly, leading to damage of blood vessel walls. This genetic form can be inherited from biological parents and may be triggered by various factors such as illness, infection, or environmental stressors.[2]
A third category, called secondary HUS, occurs alongside other medical conditions. This form can be associated with pregnancy, certain cancers, autoimmune diseases, or reactions to specific medications including some cancer treatments and immunosuppressive drugs.[3]
How Common Is Haemolytic Uraemic Syndrome
HUS can affect anyone at any age, but it shows distinct patterns in who gets sick most often. The typical form of HUS primarily strikes young children, particularly those under five years of age. Among people who develop diarrhoea from certain dangerous bacteria, between 5 and 15 percent will go on to develop HUS.[2]
Children are much more vulnerable to HUS than adults. The condition is the leading cause of sudden kidney failure in children. When adults do develop HUS, they often experience more complicated presentations and may have worse outcomes than children. Elderly adults in particular tend to have more severe disease courses.[5]
Overall, HUS affects approximately 1.5 people per 100,000 individuals each year in the general population. Atypical HUS is exceptionally rare, occurring in fewer than 1 person per million people. The typical, infection-related form makes up the vast majority of cases, especially in children.[5]
What Causes Haemolytic Uraemic Syndrome
The most common cause of HUS is infection with certain strains of Escherichia coli bacteria, commonly abbreviated as E. coli. Not all E. coli bacteria cause HUS, only specific types that produce dangerous substances called Shiga toxins. The most notorious strain is E. coli O157:H7, but other Shiga toxin-producing strains can also trigger the syndrome.[1]
These bacteria typically enter the body through contaminated food or water. Common sources include undercooked ground beef, unpasteurized milk or juice, contaminated fresh produce, and water that hasn’t been properly treated. People can also get infected through direct contact with animals, particularly farm animals like cattle, or through person-to-person spread when hygiene practices are inadequate.[8]
When Shiga toxin-producing bacteria colonize the intestines, they release toxins that get absorbed into the bloodstream. These toxins travel throughout the body and attach to the inner lining of blood vessels, called the endothelium. This attachment triggers a cascade of damaging events: the vessel walls become inflamed and swollen, blood starts to clot inside the vessels, and blood cells get destroyed as they pass through the damaged, narrowed passages.[3]
Other bacteria can also cause HUS, though less commonly. Shigella bacteria, which cause a type of dysentery, can produce similar toxins. Streptococcus pneumoniae, the bacteria that causes pneumonia and ear infections, triggers HUS through a different mechanism by exposing hidden structures on cells that trigger immune reactions.[5]
In atypical HUS, the underlying cause is different. Genetic mutations affect proteins that normally regulate the complement system, which is part of the immune system that helps fight infections. When this system isn’t properly controlled, it attacks the body’s own blood vessel linings, causing damage similar to that seen in infection-related HUS but through a completely different biological pathway.[10]
Risk Factors for Developing HUS
Several factors increase the likelihood of developing HUS after exposure to Shiga toxin-producing bacteria. Young children, especially those under the age of five, face the highest risk. Their immune systems are still developing and may not respond as effectively to bacterial toxins. Additionally, young children are more likely to have practices that facilitate bacterial spread, such as less consistent handwashing and closer contact with other children in daycare or school settings.[6]
Certain dietary and lifestyle factors also elevate risk. Consuming undercooked ground beef, especially hamburgers that haven’t reached an internal temperature of at least 160 degrees Fahrenheit, represents a significant risk factor. Drinking unpasteurized beverages, including raw milk and fresh-pressed juices, can expose people to dangerous bacteria. Eating raw or unwashed fruits and vegetables, particularly leafy greens, also poses risks.[8]
People who live on farms or have close contact with livestock, particularly cattle, sheep, and goats, have increased exposure to Shiga toxin-producing bacteria. These animals can carry the bacteria in their intestines without becoming sick themselves. Children who visit petting zoos or farms need careful supervision to ensure proper handwashing after touching animals.[8]
For atypical HUS, the main risk factor is having a family history of the condition or carrying specific genetic mutations. Pregnancy can trigger aHUS in women with underlying genetic susceptibility. People taking certain medications, including some chemotherapy drugs and immunosuppressive agents, also face higher risk. Those with existing kidney disease, autoimmune conditions, or weakened immune systems are more vulnerable to developing secondary forms of HUS.[3]
During community outbreaks of E. coli infections, living in the affected area or consuming food from contaminated sources dramatically increases risk. Public health authorities typically issue warnings during such outbreaks to help people avoid exposure.[6]
Signs and Symptoms
The symptoms of HUS typically develop in stages, with early warning signs appearing first, followed by more serious complications. Understanding these progression patterns helps in recognizing when to seek emergency medical care.[1]
In typical HUS caused by bacterial infection, the first symptoms usually involve the digestive system. Most people experience diarrhoea that often becomes bloody within a day or two. The blood in the stool may be bright red or make the stool appear very dark and tarry. Severe cramping and pain in the abdomen accompany the diarrhoea. Many people also develop nausea and vomiting. Fever is common, though not everyone experiences elevated temperature. These initial symptoms typically last for several days.[1]
As the condition progresses and blood vessel damage worsens, additional symptoms emerge. People develop signs of anaemia, including extreme tiredness and fatigue that makes even simple activities exhausting. The skin becomes noticeably paler than usual, and the insides of the eyelids may look very pale instead of their normal pink color. Some people notice they bruise very easily or develop tiny red spots on their skin called petechiae, which result from bleeding under the skin due to low platelet counts.[2]
Kidney damage produces its own set of symptoms. One of the most significant is producing much less urine than normal, or in severe cases, stopping urination completely. When urine is produced, it may appear dark or cola-colored due to the presence of blood. Swelling, called oedema, develops in the legs, feet, and ankles as the failing kidneys cannot remove excess fluid from the body. Some people notice puffiness around their eyes, especially in the morning.[2]
As waste products accumulate in the blood because the kidneys aren’t filtering properly, people may experience difficulty breathing or shortness of breath. High blood pressure often develops. Headaches are common. Some people become confused or show decreased alertness and responsiveness. In severe cases, particularly when the brain is affected by the blood vessel damage, seizures can occur. Heart rhythm problems may develop if the condition affects the cardiovascular system.[2]
In atypical HUS, the symptoms may be similar but can occur without preceding diarrhoea in up to 70 percent of cases. Some people with aHUS experience recurring episodes, with symptoms appearing, improving, and then returning. Blood pressure problems are particularly common in atypical cases and may be severe.[3]
Preventing Haemolytic Uraemic Syndrome
Since most cases of HUS result from bacterial infections, prevention focuses primarily on food safety and hygiene practices. These measures can significantly reduce the risk of exposure to dangerous bacteria.[8]
Proper food handling and preparation form the cornerstone of prevention. All ground beef should be cooked thoroughly until it reaches an internal temperature of 160 degrees Fahrenheit throughout. The meat should no longer be pink in the middle, and juices should run clear. Using a food thermometer provides the most reliable way to ensure adequate cooking. People should avoid tasting meat before it’s fully cooked, as even a small amount of contaminated raw meat can cause infection.[8]
Raw fruits and vegetables require careful washing before consumption. Running water should be used to rinse all produce, including items that will be peeled. Lettuce and other leafy greens need especially thorough rinsing. Cutting boards, knives, and other utensils used for raw meat must be washed with hot, soapy water before using them for other foods. Many families find it helpful to use separate cutting boards for meat and for produce.[8]
Beverage choices matter for prevention. Only pasteurized milk and dairy products should be consumed. The same applies to juices and ciders, which should have been heat-treated to kill any bacteria. Labels on these products will indicate whether they’ve been pasteurized. Raw milk and fresh-pressed juices that haven’t been treated pose risks, particularly for young children.[8]
Handwashing represents one of the most effective prevention strategies. Hands should be washed thoroughly with soap and water for at least 20 seconds before preparing or eating food, after using the toilet, after changing diapers, and after any contact with animals. This is especially important for children, who may need reminders and supervision to ensure proper technique.[8]
Families who visit farms or petting zoos should take extra precautions. Children should never put their hands in their mouths after touching animals or anything in animal enclosures. Hand sanitizer can be used when soap and water aren’t immediately available, but handwashing with soap and water is preferable. Eating or drinking should not occur in areas where animals are kept.[8]
People with diarrhoea should not swim in public pools, lakes, or other recreational water areas, as the bacteria can spread through water. Anyone diagnosed with E. coli or similar infections should stay home from work, school, or childcare until cleared by a healthcare provider. Food handlers and healthcare workers may need to provide negative stool test results before returning to work.[8]
For atypical HUS, prevention is more complex because the condition has genetic roots. People with a family history of aHUS may benefit from genetic counseling to understand their risks. Pregnant women with known genetic susceptibilities need close medical monitoring. Avoiding trigger medications when possible, particularly in people with known complement system abnormalities, may help prevent episodes.[17]
How the Body Changes in HUS
Understanding the physical and biochemical changes that occur in HUS helps explain why the condition affects multiple organ systems and why prompt treatment is so critical.[3]
In typical HUS, the process begins when Shiga toxin produced by bacteria in the intestines enters the bloodstream. This toxin has a particular affinity for binding to certain types of cells, especially those lining blood vessels in the kidneys, brain, and other organs. Once attached, the toxin enters these cells and interferes with their ability to produce proteins, essentially poisoning them. The affected cells swell, malfunction, and eventually die.[7]
As blood vessel lining cells become damaged, they expose underlying layers that normally remain hidden. The body’s clotting system recognizes these exposed surfaces as injury sites and responds by forming blood clots. However, because damage is occurring throughout the body’s smallest blood vessels, countless tiny clots form simultaneously. This widespread clotting uses up the blood’s supply of platelets and clotting factors, leading to both clotting problems and bleeding problems at the same time.[10]
The formation of many small clots narrows blood vessels throughout the body. As red blood cells try to squeeze through these narrowed, partially blocked vessels, they get sliced and fragmented by the fibrin strands that make up the clots. These damaged red blood cells, called schistocytes or fragmented cells, can be seen under a microscope when blood samples are examined. The destruction of red blood cells releases their contents, including hemoglobin, into the bloodstream.[10]
The kidneys suffer particularly severe damage in HUS for several reasons. First, they receive about 20 percent of the heart’s output of blood, so they’re exposed to large amounts of the circulating toxins. Second, the tiny filtering units in the kidneys, called glomeruli, contain the specific cell receptors that Shiga toxin targets. Third, as damaged red blood cells and clots accumulate in these small kidney vessels, they physically block the filtration process.[10]
When kidney filtration becomes impaired, waste products that are normally removed from the blood begin to accumulate. Creatinine and urea, which are waste products from protein breakdown, rise to dangerous levels. The kidneys also cannot properly balance electrolytes like potassium and sodium, leading to potentially life-threatening imbalances. Fluid that should be eliminated as urine stays in the body, causing swelling and potentially affecting heart and lung function.[9]
In atypical HUS, the pathophysiology involves the complement system rather than bacterial toxins. The complement system is a group of proteins in blood plasma that work together as part of the immune response. In healthy people, regulatory proteins keep this system in check, preventing it from attacking the body’s own tissues. When genetic mutations affect these regulatory proteins, the complement system becomes overactive and attacks blood vessel linings continuously. This triggers the same cascade of cell damage, clotting, and red blood cell destruction seen in typical HUS, but through a completely different initial mechanism.[10]
The brain can also be affected by the blood vessel damage in HUS. When small vessels in the brain become blocked or damaged, it can lead to decreased oxygen delivery to brain tissue. This explains symptoms like confusion, seizures, and in severe cases, stroke. The heart may be affected if the coronary arteries develop similar problems, though this is less common than kidney or brain involvement.[2]