When to see a vet urgently
Go to an emergency vet immediately if your cat has any of the following signs:
- difficulty breathing or open-mouth breathing;
- collapse, fainting or an inability to stay on their feet;
- almost white gums and marked weakness;
- rapidly increasing weakness and yellowing of the mucous membranes.
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Overview
Red blood cells carry oxygen from the lungs to the tissues. In IMHA, antibodies and complement cause them to be destroyed prematurely. The bone marrow cannot replace them quickly enough, leading to oxygen deprivation, weakness and life-threatening complications.
Two forms are distinguished:
- Primary (idiopathic) IMHA: the cause is unknown, and the immune system attacks red blood cells without an apparent trigger.
- Secondary IMHA: develops as a complication of infections, tumours, medication or other systemic diseases.
An antibody is a protein made by the immune system that recognises its target. Complement is a system of proteins involved in the immune response. In IMHA, these mechanisms are mistakenly directed against red blood cells. The problem is not simply that a nutrient is missing from the blood: the cells' lifespan is shortened.
Red blood cells may break down within blood vessels or be removed in greater numbers than usual, particularly in the spleen and liver. Their breakdown produces bilirubin, which can cause jaundice if it accumulates. The same visible yellowing can also occur with other diseases, however. Establishing haemolysis and demonstrating its immune-mediated cause are therefore separate parts of the investigation.
Causes
The main causes of secondary IMHA in cats include:
- viral infections, including FeLV, FIV and coronavirus;
- bacterial and parasitic diseases, including haemoplasmosis;
- malignant tumours;
- chronic inflammatory diseases;
- medicines, rarely;
- autoimmune diseases.
Finding an underlying disease is essential for treatment. Haemoplasmas, for example, are bacteria associated with red blood cells that can cause their destruction. Anaemia caused by these bacteria and anaemia primarily caused by an immune system disorder do not automatically need the same treatment. The severity of anaemia alone does not distinguish these situations.
Risk factors
Susceptibility is greater in:
- cats with chronic infections;
- older cats;
- breeds with a tendency towards immune disorders;
- cats with cancer or severe inflammatory diseases.
Information on predisposing factors for feline IMHA is limited. Inherited IMHA has not been demonstrated in cats, so the general mention of breed does not establish an individual cat's risk. Investigating infections, tumours and other concurrent diseases is more relevant in practice than inferring the diagnosis from breed.
Symptoms
Typical signs of IMHA in cats include:
- pale or yellowish mucous membranes (icterus);
- tiredness, weakness and listlessness;
- faster breathing and heart rate;
- loss of appetite;
- fever;
- dark urine with severe haemolysis;
- fainting and collapse in severe cases.
The signs of anaemia arise largely from reduced oxygen transport. Your cat may move little, stop playing or tire more quickly than usual. Faster breathing and heart rate are the body's attempt to compensate for the reduced oxygen supply. In a severe situation, rest does not correct the shortage of red blood cells.
A change in urine colour, yellowish whites of the eyes or loss of appetite are useful details to report at the appointment. Also explain how quickly the change occurred and mention any vomiting. Symptoms cannot reliably indicate the haematocrit, the proportion of blood volume made up of red blood cells. A blood sample is needed for this.
Complications and consequences
Immune-mediated haemolytic anaemia can cause:
- severe oxygen deprivation in the tissues;
- thromboembolism, involving blood clot formation associated with red blood cell destruction;
- liver and kidney failure;
- death in severe cases.
Diagnosis at the vet
Investigations used to confirm the diagnosis include:
- a blood count showing low haematocrit and regenerative anaemia;
- a blood smear showing damaged red blood cells and spherocytes;
- blood biochemistry showing raised bilirubin;
- a Coombs test to detect antibodies on the surface of red blood cells;
- ultrasound and X-rays to rule out tumours and other diseases;
- PCR tests for haemoplasmosis, FeLV and other infections.
IMHA does not always appear immediately as regenerative anaemia, in which red blood cell production increases. The bone marrow response takes time, and some immune-mediated diseases also disrupt the formation or survival of young red blood cells. A limited regenerative response early on therefore does not rule out the diagnosis alone.
Reticulocytes are young red blood cells. Their number indicates how the bone marrow is responding to cell loss. The blood count also assesses white blood cells and platelets to see whether the changes affect more than red blood cells. A blood smear, a thin layer of blood examined under a microscope, provides information about cell shape and possible abnormalities.
Interpreting the Coombs test
The direct Coombs test looks for antibodies attached to red blood cell surfaces. A positive result can support an IMHA diagnosis, but does not prove on its own that the antibodies are causing the anaemia found. They may also occur with some infections and other diseases. The test result is therefore considered alongside evidence of anaemia and red blood cell breakdown.
A negative result does not reliably rule out immune-mediated disease either. Sample age, antibody levels and previous treatment, for example, can affect the test. Tell your vet about medicines used before investigations begin. The vet and laboratory assess what the result means in the particular situation and whether more investigation is needed.
Looking for infections and other causes
PCR is a key method for investigating haemoplasma: it identifies the bacterium's genetic material in blood. A routine blood smear is less sensitive for detecting the infection. Clinically healthy carriers also exist, so the significance of a finding is assessed in the context of all the symptoms. Not every positive infection result automatically explains the full severity of the anaemia.
Blood biochemistry and urine examination provide further information about organ function. X-rays or ultrasound may look for concurrent diseases, such as tumours. The aim is to identify an underlying factor that affects treatment. A bone marrow examination is considered particularly when the poor red blood cell production response remains unexplained.
Treatment and monitoring recovery
Severe anaemia may require hospital care and a blood transfusion. The transfusion supports oxygen transport while the cause is investigated and other treatment begins to work. Before transfusion, blood type and compatibility are checked to reduce the risk of reactions. A blood transfusion does not itself stop immune-mediated red blood cell destruction.
If there is a treatable infection or another identified cause, it is treated at the same time. Haemoplasmosis, for example, is treated with an antimicrobial chosen by the vet. Medication that suppresses the immune system is not an automatic solution for every such anaemia. The roles of infection and immune-mediated destruction are distinguished as far as possible.
Primary IMHA may require immunosuppressive treatment, which reduces immune system activity. This can include a corticosteroid and other medicines prescribed by the vet if needed. Treatment choices are based on the diagnosis, the severity of anaemia and your cat's other diseases. In a complex situation, assessment by a vet with expertise in internal medicine may help.
Do not stop or reduce medication on your own initiative even if your cat seems brighter. When the anaemia improves or stabilises, the vet plans gradual changes to medication. Relapse is possible during reduction. Continuing treatment is therefore not decided solely on the basis of one good day or normal-looking gums.
What should you record at home?
Monitor appetite, activity, breathing effort and any new colour changes in the mucous membranes or urine. Also record whether you manage to give the medication and any changes in your cat's condition. Contact the treating vet if your cat becomes weaker again. Difficulty breathing or collapse requires emergency care even after treatment has begun.
Follow-up checks monitor the blood count and selected other tests according to treatment response and underlying diseases. Red blood cell numbers and reticulocytes describe different things: the former show cell numbers, while the latter indicate the production response. Kidney and liver values and urine tests can help assess overall condition. The examination at the appointment complements the laboratory results.
Prognosis
The prognosis depends on the cause identified, your cat's condition at the start of treatment and the response. Anaemia associated with haemoplasma may respond well to appropriate infection treatment. A tumour or serious underlying viral disease can complicate the situation. No single promise about the prognosis of primary or secondary disease therefore applies to every cat.
Prevention
There is no specific prevention, but the risk of secondary IMHA may be reduced through:
- vaccination against viral infections, including rhinotracheitis, panleukopenia, calicivirus and FeLV;
- monitoring and treatment of chronic diseases;
- regular blood tests for cats with a predisposition to autoimmune disease;
- careful use of medicines.
Vaccinations are chosen according to your cat's infection risk, but they do not prevent every cause of IMHA. A routine vaccination programme cannot reliably prevent the disease. New anaemia needs investigation in a vaccinated cat too. Tell your vet about medicines and supplements so that possible exposures are taken into account.
What it means for owners
Immune-mediated haemolytic anaemia is rare in cats but extremely serious. Owners should remember that sudden weakness, pale mucous membranes and yellowish skin are reasons to seek immediate veterinary assessment. The earlier treatment begins, the better the chances of stabilising your pet's condition.
Note: this material provides general information and does not replace veterinary advice. If symptoms develop, contact a vet immediately.
Frequently asked questions
Is IMHA the same as iron deficiency?
No. In IMHA, the immune system is involved in premature red blood cell destruction. Iron deficiency works through a different mechanism, and an iron supplement does not replace investigating and treating the cause of IMHA. Blood tests identify the type of anaemia.
Does a positive Coombs test confirm the diagnosis?
Not alone. The result shows antibodies associated with red blood cell surfaces, but its significance depends on the anaemia, evidence of haemolysis and other diseases. The vet interprets it alongside the other investigations.
Can IMHA return after treatment?
Yes. Relapse is possible, particularly when medication is reduced or stopped. The treating vet plans follow-up checks and medication changes. Report new weakness, pallor or difficulty breathing without delay.
This article is for general information only and is not a guide to treatment. If you notice the first signs of illness, contact your vet.
Paw4care editorial team
Sources
- Anaemia, Cornell Feline Health Centre, accessed 2026-11-02.
- Anaemia in Cats, Merck Veterinary Manual, accessed 2026-11-02.
- Overview of Anaemia in Animals, Merck Veterinary Manual, accessed 2026-11-02.
- Autoimmune Haemolytic Anaemia in Cats, VCA Animal Hospitals, accessed 2026-11-02.
- Guideline for Haemoplasmosis in Cats, European Advisory Board on Cat Diseases, accessed 2026-11-02.
- Direct Coombs Testing, Cornell University Animal Health Diagnostic Centre, accessed 2026-11-02.