Ehrlichia is a type of bacteria transmitted through tick bites that infects a dog’s white blood cells, triggering a disease called ehrlichiosis. It causes fever, lethargy, loss of appetite, and a dangerous drop in platelet counts that can lead to abnormal bleeding. Dogs typically develop symptoms one to three weeks after being bitten by an infected tick, and the disease progresses through distinct stages that range from easily treatable to life-threatening.
How Dogs Get Infected
The most common culprit is Ehrlichia canis, a species of bacteria spread primarily by the brown dog tick. Once the tick feeds on an infected dog, it picks up the bacteria and passes it along to the next dog it bites. A second species, Ehrlichia ewingii, is transmitted by the lone star tick and causes a related but distinct form of the disease that targets a different type of white blood cell.
Unlike many infections, ehrlichiosis doesn’t spread directly from dog to dog or from dogs to people. Humans can contract ehrlichiosis, but only through their own tick bite. A 2023 case in Italy confirmed that Ehrlichia canis can infect people when a tick carrying the bacteria attaches to them, though this remains uncommon. The shared risk factor for your household is the tick itself, not your dog.
What Ehrlichia Does Inside the Body
Once in the bloodstream, the bacteria invade white blood cells called monocytes and begin replicating inside them. This hijacking of the immune system sets off a chain reaction: the lymph nodes swell, the spleen enlarges, and the number of platelets in the blood drops. Platelets are responsible for clotting, so as their count falls, dogs become vulnerable to spontaneous bleeding from the nose, gums, or under the skin.
If the infection persists, it can spread beyond the blood and affect multiple organs. The kidneys, lungs, eyes, and even the brain and spinal cord can become inflamed, leading to a wide range of symptoms depending on where the damage occurs.
The Three Stages of Ehrlichiosis
Acute Stage
This is the first wave, appearing one to three weeks after the tick bite. Dogs develop a fever, become lethargic, lose interest in food, and may show lameness or joint pain. Swollen lymph nodes and an enlarged spleen are common. Bloodwork at this point often reveals low platelets, anemia, or both. Some owners also notice unusual bleeding, like nosebleeds or tiny red spots on the gums and belly. Eye inflammation (uveitis) can occur as well, sometimes showing up as redness, squinting, or cloudy eyes.
Subclinical Stage
If the acute phase goes undiagnosed, many dogs appear to improve on their own. They seem outwardly healthy, but the bacteria are still present, quietly persisting inside white blood cells. Bloodwork may show lingering low platelet counts or elevated antibody levels, but the dog acts normal. This stage can last months or even years, and some dogs eventually clear the infection on their own. Others do not, and the disease silently advances.
Chronic Stage
Dogs that fail to clear the subclinical infection can progress to chronic ehrlichiosis, the most dangerous form. At this point, the bone marrow itself may be damaged, causing a severe drop in all blood cell types: red cells, white cells, and platelets. Severe weight loss is common. Dogs may develop kidney failure, uncontrolled bleeding, or secondary infections because their weakened immune system can no longer fight off other threats. The prognosis for chronic ehrlichiosis is poor, with severe drops in all blood cell lines being a strong predictor of death.
How Ehrlichiosis Is Diagnosed
Diagnosis involves a combination of clinical signs, bloodwork, and specific testing. A routine blood panel showing low platelets, anemia, or elevated liver and kidney values raises suspicion, but it’s not enough on its own.
Many veterinary clinics use a point-of-care screening test (an ELISA-based test, often run alongside heartworm screening) that detects antibodies to Ehrlichia in a few minutes. A positive result means the dog has been exposed, but antibody levels can remain elevated for months to years after treatment, so a positive test alone doesn’t confirm active infection. A dog with antibodies but no symptoms doesn’t necessarily need treatment.
PCR testing looks for the bacteria’s DNA in a blood sample and is strongest evidence of active infection. It works best early in the disease, before antibiotics are started. Once treatment begins, PCR sensitivity drops significantly within 48 hours. A negative PCR result doesn’t completely rule out infection, though, since the level of bacteria circulating in the blood can fluctuate.
The most definitive serological test is an indirect fluorescent antibody (IFA) test, which measures antibody levels in two blood samples taken weeks apart. A fourfold rise in antibody levels between the first and second sample confirms a current or recent infection. Antibody testing is frequently negative in the first week of illness, so early samples may need to be repeated later.
Treatment and Recovery
Doxycycline, an antibiotic, is the standard treatment for ehrlichiosis in dogs. A typical course lasts 28 days. Most dogs in the acute stage respond quickly, with fever breaking and appetite returning within the first few days. The prognosis for acute ehrlichiosis is good when treatment starts early.
Dogs in the subclinical stage also respond well to antibiotics, though the lack of obvious symptoms sometimes means the infection isn’t caught until routine screening picks it up. Chronic ehrlichiosis is a different situation entirely. By the time bone marrow damage has set in, even aggressive treatment may not reverse the decline. Some dogs in the chronic stage require blood transfusions or supportive care alongside antibiotics, and not all recover.
One frustrating reality of ehrlichiosis is that antibody levels often persist for months or years after successful treatment. This means follow-up screening tests can remain positive long after the bacteria have been eliminated. Your vet will typically rely on symptom resolution and normalization of bloodwork rather than a negative antibody test to confirm that treatment worked.
Preventing Tick-Borne Infection
Since the only route of transmission is through a tick bite, prevention comes down to keeping ticks off your dog. Year-round tick preventatives, whether oral, topical, or collar-based, are the most reliable defense. No preventative is 100% effective, so checking your dog for ticks after time outdoors adds another layer of protection, especially around the ears, neck, and between the toes.
Dogs that live in or travel to areas with heavy brown dog tick or lone star tick populations face higher risk. The brown dog tick is unusual among tick species because it can complete its entire life cycle indoors, meaning kennels, homes, and sheltered areas can become infested even without nearby woods or tall grass. If your dog has been diagnosed with ehrlichiosis, inspecting your home and yard for ticks is worth the effort to prevent reinfection.
Annual screening through a combination heartworm and tick-borne disease test catches many cases during the subclinical stage, before organ damage begins. Early detection and a four-week course of antibiotics can resolve what would otherwise become a serious, potentially fatal infection.