What Is Immune-Mediated Haemolytic Anaemia?
Immune-mediated haemolytic anaemia, commonly abbreviated to IMHA, is a condition in which a dog's immune system attacks and destroys its own red blood cells. Rather than protecting the body from foreign threats, the immune system mistakenly identifies red blood cells as targets, triggering their premature destruction faster than the bone marrow can replace them. The result is a rapid and potentially catastrophic drop in red blood cell numbers.
IMHA is one of the most common immune-mediated diseases in dogs and one of the most serious. Without prompt and appropriate treatment, it can be fatal within days. Even with treatment, mortality rates in hospitalised dogs range from 20 to 80 percent depending on the study and the severity of presentation. It demands early recognition and aggressive veterinary management.
Primary Versus Secondary IMHA
IMHA is classified as either primary or secondary, a distinction that matters for both treatment and prognosis. Primary IMHA, also called idiopathic IMHA, occurs when no underlying trigger can be identified. The immune system simply begins attacking red blood cells without an identifiable cause. This is the most common form, accounting for the majority of cases.
Secondary IMHA occurs when an underlying condition triggers the immune attack. Known triggers include:
- Infectious diseases such as babesiosis, ehrlichiosis, or leptospirosis
- Certain cancers, particularly lymphoma
- Drugs or vaccine reactions in a small number of cases
- Bee stings or other toxin exposures
- Other immune-mediated diseases
Identifying an underlying cause, when one exists, is important because treating only the anaemia without addressing the trigger is unlikely to result in sustained improvement.
Which Dogs Are at Risk
IMHA can affect dogs of any age, sex, or breed, but certain patterns emerge in the data. Middle-aged to older dogs are overrepresented, and female dogs are affected more commonly than males, a pattern seen in many immune-mediated diseases across species. Certain breeds appear at higher risk, including Cocker Spaniels, Springer Spaniels, Irish Setters, Poodles, Old English Sheepdogs, and Dobermanns, suggesting a genetic predisposition in these lines.
It is worth noting that IMHA can occur in young dogs with no apparent risk factors. It is not a disease restricted to older or predisposed animals.
Recognising the Symptoms
The symptoms of IMHA arise from the consequences of anaemia — a shortage of red blood cells means less oxygen is carried to the tissues. The severity of symptoms often reflects the speed of the red cell loss as much as the absolute level of anaemia. Rapid destruction causes acute collapse; gradual destruction allows some compensation.
Common symptoms include:
- Profound lethargy and weakness
- Exercise intolerance — dogs may collapse after minimal exertion
- Pale, white, or yellowish gums and inner eyelids
- Rapid heart rate and breathing as the body compensates for reduced oxygen delivery
- Jaundice — yellowing of skin, eyes, and mucous membranes caused by bilirubin from destroyed red cells
- Dark, orange-brown, or reddish urine, reflecting the excretion of haemoglobin and bilirubin
- Loss of appetite
- Collapse in severe cases
The gum colour is one of the most informative things an owner can check quickly. Healthy dog gums should be a bright salmon-pink and should return to colour within two seconds of being pressed. Pale or white gums in a lethargic dog are a veterinary emergency.
Diagnosis
IMHA is diagnosed through a combination of clinical examination and blood tests. The packed cell volume — a measure of the proportion of blood that consists of red cells — will be significantly reduced. A normal dog has a PCV of around 35 to 55 percent; IMHA dogs often present at 15 percent or lower.
Crucially, examination of a blood smear under a microscope often reveals spherocytes — abnormally shaped red blood cells that have been partially attacked by the immune system — and agglutination, where red blood cells clump together. A positive saline agglutination test is strongly supportive of IMHA. The Coombs test can confirm the presence of antibodies on the surface of red blood cells.
Additional tests assess liver and kidney function, look for underlying infectious causes, and evaluate clotting ability. IMHA carries a significant risk of thromboembolism — blood clot formation — which is a major cause of death in these patients.
Treatment and Management
Treatment of IMHA is intensive and typically requires hospitalisation. The two main pillars of management are immune suppression and supportive care.
Immunosuppressive therapy usually begins with high-dose corticosteroids such as prednisolone. These suppress the misdirected immune attack on red blood cells, buying the bone marrow time to replenish the red cell population. If corticosteroids alone are insufficient or if the response is too slow, additional immunosuppressive agents such as azathioprine, ciclosporin, or mycophenolate mofetil may be added.
Supportive care includes:
- Blood transfusions when anaemia is severe enough to be life-threatening
- Anticoagulant therapy with heparin or clopidogrel to reduce clot risk
- Intravenous fluids
- Oxygen supplementation for dogs with severe respiratory distress
- Cage rest to minimise oxygen demand
Blood transfusions are not a cure but a bridge — they temporarily increase red cell numbers while immunosuppression takes effect. Response to treatment is monitored through serial PCV measurements, usually daily in the acute phase.
Long-Term Outlook
Dogs that survive the acute phase of IMHA face a long treatment road. Immunosuppressive drugs must be tapered slowly over months — sometimes up to a year — to avoid relapse. Tapering too quickly is a common cause of recurrence. Regular blood monitoring during the taper phase allows treatment to be adjusted based on response.
Relapse rates are significant. Studies suggest that between 10 and 15 percent of dogs that recover from IMHA will relapse at some point, often during or after drug tapering. Owners should be counselled about relapse signs and advised to seek veterinary attention without delay if symptoms recur.
Some dogs go into long-term remission and live normal lives after treatment is withdrawn. Others require ongoing low-dose immunosuppression to maintain remission. The unpredictability of the condition is part of what makes IMHA one of the most challenging diagnoses in small animal medicine.