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Even after many years in practice, the dog with immune-mediated hemolytic anemia (IMHA) can still leave us baffled. Should we refer to a specialist, transfer to emergency, or send the dog home on medication? When should we recheck—tomorrow, in a week, in a month? What drugs do we pick? Or should we just run screaming from the building?
IMHA cases usually start out in the emergency room, but sometimes they are caught early during routine bloodwork or the patient presents in general practice with mild early disease. A better understanding of how to manage these cases will help with early diagnosis, catching relapses and even managing them on our own. You don’t need to become an expert to get the ball rolling on a good diagnostic and treatment plan. At a recent Fetch dvm360 conference, Elisa Mazzaferro, MS, DVM, PhD, DACVECC, shared her approach to IMHA cases.
This is the first of two articles on this topic. Here, we discuss the causes and diagnosis of IMHA. The second article will shed light on best practices for short- and long-term therapy as well as monitoring IMHA patients.
IMHA is a disease in which the body attacks its own red blood cells (RBCs). It is among the most common immune-mediated diseases that we see in dogs.1 IMHA has a 30% to 70% mortality rate despite aggressive therapy, Dr. Mazzaferro noted, and relapse rates range from 11% to 15%.2,3 Upon initial diagnosis, Dr. Mazzaferro said she tells her clients that their hemolyzing pet has roughly a 40% to 50% chance for long-term survival, then she customizes this prognosis based on follow-up diagnostics and response to treatment. Many patients do not make it through the initial hemolytic crisis, but those that do can die later as a result of a thromboembolic disease, renal failure, liver failure, relapse or complications of medical therapy. Dogs with concurrent immune-mediated thrombocytopenia, hypoalbuminemia or very rapid hemolysis can have a significantly worse prognosis.
The median age at presentation is 6 years. Although most cases of IMHA occur in female dogs, males statistically have a worse prognosis. Cocker spaniels, sheepdogs and poodles, among others, are particularly susceptible. IMHA in susceptible breeds is usually far more severe and requires more rapid, aggressive treatment than that in non-predisposed breeds. Pit bull terriers are predisposed to Babesia and have secondary IMHA from this.
Intravascular hemolysis is less common than extravascular hemolysis. The difference between the two—besides the site of RBC destruction—is that with intravascular hemolysis you must be sure to rule out zinc toxicity (and some of the infectious diseases), and you should expect the clinical progression to be more severe, with a higher risk for disseminated intravascular coagulation (DIC). You still treat these patients the same way, but be prepared to be more aggressive, and don’t forget to address the thromboembolic and coagulopathy risks using medication and coagulation testing.
Seventy percent to 80% of canine IMHA cases are idiopathic,4 known as primary IMHA. Secondary IMHA is caused by an immunoglobulin (Ig) M or IgG antibody attack against RBCs in response to infection, neoplasia, an inflammatory condition, drug or toxin, and maybe vaccination. Some toxicities cause direct RBC damage, which can be mistaken for IMHA but is not immune mediated.
Following are potential underlying causes of secondary IMHA:
- Infection: Babesia, Anaplasma, Mycoplasma spp, Ehrlichia
- Drugs: Nonsteroidal anti-inflammatory drugs, sulfonamides, cephalosporins
- Vaccinations: There is still not substantial evidence that vaccines can cause IMHA, but we suspect that they might, Dr. Mazzaferro said. There is a 2% to 26% incidence of vaccine-implicated IMHA depending on which study you look at,5 she pointed out. It is also not at all clear what the duration from vaccination to onset of IMHA is supposed to be.
- Neoplasia: Any malignancy is potentially causal, either directly or indirectly. In patients that are significantly hypoproteinemic, be sure to rule out erythrophagocytic histiocytic sarcoma, Dr. Mazzaferro said.
- Toxins: Zinc, onions and large amounts of garlic (although these toxicities are not truly immune mediated, they cause Heinz body anemia)
- Other infections or inflammatory conditions in the body that can overstimulate the immune system (i.e. cholecystitis, pyelonephritis, pancreatitis)
Clinical presentation and history
Clinically, patients with IMHA present with lethargy, weakness, vomiting, diarrhea, pale or yellow gums, collapse, discolored urine and petechiae if they have immune destruction of both RBCs and platelets. Alternatively, patients can appear clinically normal with just spherocytes and a mild anemia on routine bloodwork.
They can have a regenerative or non-regenerative anemia. We are encouraged to see regeneration, but it takes the body four to five days to mount a significant regenerative response, so about 30% of cases are nonregenerative on presentation. Spherocytes are present in only 67% to 94% of dogs with IMHA.6 They are not pathognomonic to IMHA, but spherocytes are great markers for diagnosing and monitoring this disease. Very few other things cause this.
Slide agglutination testing
This type of test can be tricky. Here’s how to do it right.
Place one drop of blood from a purple-top tube on a slide, visualize it for clumping (agglutination), then add four tiny incremental drops of 0.9% saline with a needle-on syringe to the edges of the blood drop to see if the blood cells disperse or stay clumped. If they stay clumped, the result is positive.
Always look for macroagglutination first (you can see it with your own eyes). If macroagglutination is negative (RBCs either do not clump or disperse quickly with saline), then examine for microagglutination. Microagglutination is the same test but under a microscope, wet and unstained. Use a coverslip, and look for RBCs clinging or clumping together.
For both tests, if the blood cells disperse evenly with the saline, the result is negative. For the microagglutination test it is even more important to use the saline to differentiate agglutination versus rouleaux. Assess the slides immediately; letting the blood sit on the slide for even a few minutes can give the appearance of false-positives as the edges dry out.
If test results are unclear, the ACVIM consensus guidelines recommend washing the blood sample and retesting. Agglutination is caused by cross-linking of IgM immune complexes across RBCs, so it should still be happening after washing.
History is very important. Ask whether the dog is or has:
- Been traveling?
- On adequate flea and tick prevention?
- Taking any drugs or supplements?
- At higher risk for tick exposure?
- Had recent vaccinations?
- Been exposed to onions or garlic?
- Eaten something like a penny or other zinc-containing toxin?
Getting a platelet estimate
To do a platelet estimate, count the number of platelets in each of 10 random views on high-power field (100x oil immersion) near the feathered edge. Add them up, divide by 10 (basically the average number of platelets per high-power field), and then multiply by 10 to 15. That is your platelet estimate range in thousands.
Now look at the feathered edge. If there are huge clumps of platelets, factor these in as best you can as they will falsely lower your total estimate.
The baseline diagnostic workup is more extensive than for most diseases and includes the following:
- Routine testing. Complete blood counts (CBCs) and chemistries, a urinalysis with culture and minimum inhibitory concentration; a pathologist’s review of your CBC is a must.
- Packed cell volume/total solids (PCV/TS; in-house). Note that the PCV is more accurate than hematocrit because agglutination can interfere with calculated hematocrit.
- Slide agglutination test (in-house). (See Slide agglutination testing)
- Coombs test. If both macro- and microagglutination tests are negative, do a Coombs test. If your patient is already auto-agglutinating, a Coombs test will not give you any extra information. (Nobody is going to fault you, though, if you send out a Coombs test because you are unsure of your in-house agglutination test findings. However, there is no good substitute for a slide agglutination test, or for the immediate information that it gives you if it is positive.)
- Blood smear (in-house). It should be a smear with a beautiful feathered edge, dried and stained with Diff-Quik. You are looking for reticulocytes, spherocytes, shistocytes and ghost cells in a monolayer near (but not at) the feathered edge—all of which are consistent with IMHA—and using this to do a platelet estimate while you are waiting for the CBC from the lab. (See Getting a platelet estimate)
- Radiographs for metastatic neoplasia or metallic foreign body ingestion.
- Abdominal ultrasound to complete your screening for neoplasia.
- Infectious disease panel. Always send out an infectious disease panel. North Carolina State University’s Vector Borne Disease Lab offers a panel that historically has been the best test, but Idexx offers a similar comprehensive version. The Idexx test does have a polymerase chain reaction (PCR) test for Babesia conradae (seen in the Western U.S.) that the NCSU test does not offer. If using Idexx, always test for both antibodies and genetic material (i.e. both PCR and serology).
- Lactate. This may be of limited availability except at emergency or specialty hospitals. If the lactate level is elevated, your patient may need a transfusion, as elevated lactate can indicate that too little oxygen is getting to the cells (oxygen debt).
For more in-depth information and detailed resources about IMHA diagnosis, the most recent ACVIM Consensus Statement on the Diagnosis of Immune-Mediated Hemolytic Anemia in Dogs is now available online.
Dr. Johnson practices emergency medicine at Berkeley Dog and Cat Hospital in Berkeley, California, and general practice at Cameron Veterinary Hospital in Sunnyvale, California. Her nonveterinary loves are writing, dressage with her Iberian warmblood mare Synergy; watercolor painting on yupo; vinyasa yoga; and running with her dog Tyson. Try as she might, her curly-coated Scottish Fold, Hootie, refuses to go jogging with her.
- McCullough S. Immune-mediated hemolytic anemia: understanding the nemesis. Vet Clin North Am Small Anim Pract. 2003;33(6):1295-1315.
- Piek CJ, van Spil WE, Junius G et al. Lack of evidence of a beneficial effect of azathioprine in dogs treated with prednisolone for idiopathic immune-mediated hemolytic anemia: a retrospective cohort study. BMC Vet Res 2011;7:15.
- Weinkle TK, Center SA, Randolph JF, et al. Evaluation of prognostic factors, survival rates, and treatment protocols for immune-mediated hemolytic anemia in dogs: 151 cases (1993-2002). J Am Vet Med Assoc 2005;226(11):1869-1880.
- Piek C. Immune-mediated hemolytic anemias and other regenerative anemias. In: Ettinger SJ, Feldman EC, Côté E, Eds. Textbook of veterinary internal medicine. 8th ed. St. Louis: Elsevier; 2017:829-837.
- Garden OA, Kidd L, Mexas AM, et al: ACVIM consensus statement on the diagnosis of immune‐mediated hemolytic anemia in dogs and cats. J Vet Intern Med 2019;33(2):313-334.
- Day MJ. Immune-mediated anemias in the dog. In: Weiss DJ, Wardrop KJ, Eds. Schalm's veterinary hematology. 6th ed. Hoboken: Wiley-Blackwell; 2010;216-225.