Anemia in dogs has many possible causes, but they all come down to three basic problems: the dog is losing red blood cells faster than it can replace them, something is destroying red blood cells inside the body, or the bone marrow isn’t producing enough new ones. A healthy dog’s red blood cell percentage (hematocrit) normally falls between 41% and 58%. When it drops below that range, the dog is anemic, and finding out why is the critical next step.
Blood Loss From Injuries, Tumors, or Parasites
The most straightforward cause of anemia is blood loss. Trauma, surgery, or a ruptured internal tumor can cause sudden, severe drops in red blood cells. Splenic tumors, particularly a cancer called hemangiosarcoma, are a common culprit in older dogs. These tumors grow on blood-rich organs and can rupture without warning, causing massive internal bleeding. A retrospective study of 456 anemic dogs found that cancer-related anemia was one of the most frequent diagnoses.
Chronic, slower blood loss is harder to spot. Bleeding ulcers in the stomach or intestines, growths in the gastrointestinal tract, and clotting disorders can all cause a dog to lose small amounts of blood over weeks or months. Because the body recycles iron from internal bleeding (say, blood pooling in the abdomen), only external blood loss, where blood actually leaves the body through the gut, urinary tract, or a wound, leads to iron deficiency anemia over time.
Hookworms and Fleas
Parasites deserve special mention because they’re one of the most dangerous causes of anemia in puppies. The hookworm Ancylostoma caninum is a voracious blood feeder that can literally bleed a puppy to death. Puppies infected through their mother’s milk can become critically anemic before any worm eggs even appear in their stool, making early diagnosis tricky. The pattern typically starts as straightforward blood loss anemia, then progresses to iron deficiency as the puppy’s reserves run dry. Heavy flea infestations can have a similar effect on very young or very small dogs, draining enough blood to cause dangerous drops in red blood cell counts.
Immune-Mediated Red Blood Cell Destruction
In immune-mediated hemolytic anemia (IMHA), the dog’s immune system stops recognizing its own red blood cells as normal. It produces antibodies that target circulating red blood cells, flagging them for destruction. The body’s cleanup cells then break them apart, sometimes faster than the bone marrow can produce replacements. In some cases, the antibodies attack red blood cell precursors in the bone marrow itself, shutting down production at the source.
IMHA comes in two forms. Primary (idiopathic) IMHA has no identifiable trigger. The immune system simply malfunctions. Secondary IMHA is set off by something specific: an underlying cancer, an infection, an inflammatory disease, certain medications, or occasionally a vaccination. Identifying the trigger matters because treating the underlying cause gives the dog the best chance of recovery.
Certain breeds face a much higher risk. A study published in the Journal of the American Veterinary Medical Association found that Cocker Spaniels were 12 times more likely to develop IMHA than dogs of other breeds. Bichon Frises, Miniature Pinschers, Rough-coated Collies, and Finnish Spitz also showed significantly elevated risk. Earlier research flagged English Springer Spaniels, Poodles, and Old English Sheepdogs as well.
Tick-Borne Infections and Other Pathogens
Several infectious organisms directly damage or destroy red blood cells. Tick-borne diseases are among the most common. Babesia parasites invade red blood cells and rupture them from the inside. Ehrlichia and Anaplasma bacteria, also transmitted by ticks, can suppress bone marrow function or trigger immune-mediated destruction as a secondary effect. Mycoplasma canis, a bacterial infection, destroys red blood cells and is a particular concern for dogs that have had their spleen removed, since the spleen normally helps filter damaged cells from circulation.
These infections can also act as triggers for secondary IMHA, meaning the infection alters the surface of red blood cells enough that the immune system begins attacking them even after the original pathogen is controlled.
Toxic Foods and Household Poisons
Onions and garlic are surprisingly dangerous for dogs. All plants in the Allium family contain sulfur compounds that, once absorbed, cause oxidative damage to red blood cells. This damage creates abnormal clumps called Heinz bodies on the cells, marking them for early destruction. The process begins within 24 hours of ingestion and peaks around 72 hours. In dogs, eating 15 to 30 grams of raw onion per kilogram of body weight has produced clinical signs, though smaller amounts consumed repeatedly can also cause problems.
Zinc toxicity is another overlooked cause. Puppies that swallow pennies (minted after 1982, which contain a zinc core) or chew on zinc-coated hardware can develop severe hemolytic anemia as the zinc dissolves in stomach acid. X-rays are sometimes the first clue, revealing the swallowed object before anyone suspects poisoning.
Bone Marrow Failure and Chronic Disease
When the bone marrow can’t keep up with the body’s demand for new red blood cells, the result is non-regenerative anemia. This means the body isn’t just losing red blood cells; it’s failing to replace them. Chronic kidney disease is one of the most common causes, because the kidneys produce a hormone that signals the bone marrow to make red blood cells. As kidney function declines, that signal weakens and red blood cell production drops.
Bone marrow cancers like leukemia can crowd out the cells responsible for producing red blood cells. Chronic inflammatory diseases, including long-standing infections or autoimmune conditions, also suppress production. In the large retrospective study mentioned earlier, inflammatory disease and cancer together accounted for the most frequent causes of anemia in dogs, and many of those cases involved multiple overlapping factors rather than a single clear-cut explanation.
Fragmentation Anemia From Blood Vessel Disease
A less well-known category involves red blood cells being physically sheared apart as they pass through damaged or abnormal blood vessels. Conditions like vasculitis (inflamed blood vessels), hemangiosarcoma, and disseminated intravascular coagulation, a dangerous clotting disorder, create rough surfaces or fibrin strands inside vessels that slice red blood cells into fragments. This is sometimes called microangiopathic hemolytic anemia, and it typically shows up alongside the underlying disease rather than as a standalone diagnosis.
How Anemia Is Identified
The visible signs depend on how quickly the anemia develops. Sudden blood loss causes a rapid heart rate, pale gums, and low blood pressure. When red blood cells are being destroyed internally, the breakdown products can cause jaundice, a yellowish tint to the gums, skin, or whites of the eyes. Dogs with slowly developing anemia have time to partially compensate, so their symptoms creep in gradually: reduced energy, weakness, poor appetite, and sometimes a heart murmur that develops as the heart works harder to circulate fewer red blood cells.
A complete blood count is the starting point, but it’s rarely the whole picture. A technician examines the blood smear under a microscope to check for abnormal cell shapes, parasites inside red blood cells, or fragmented cells that point toward specific causes. A reticulocyte count tells the veterinarian whether the bone marrow is actively trying to produce new red blood cells (regenerative) or has gone quiet (non-regenerative), which fundamentally changes the list of likely diagnoses. Depending on what the initial tests suggest, the workup may expand to include stool checks for hidden blood and intestinal parasites, X-rays to look for swallowed objects or internal masses, clotting tests if there’s unexplained bruising, urine and organ function panels, or in some cases a bone marrow biopsy under sedation.
Because anemia in dogs so often involves multiple overlapping causes, pinning down the primary driver can take several rounds of testing. A dog with a splenic tumor might be losing blood internally while also developing fragmentation anemia and immune-mediated destruction simultaneously. That complexity is part of why anemia is treated as a finding to investigate rather than a diagnosis on its own.