Ehrlichiosis

TL;DR. Ehrlichiosis is a serious, tick-borne bacterial infection that targets a pet's blood cells, leading to symptoms ranging from fever and swollen lymph nodes to life-threatening bleeding and bone marrow failure.

Ticks are the primary vector for transmitting Ehrlichia bacteria to pets.

What is it?

Ehrlichiosis is a common and potentially severe infectious disease caused by intracellular bacteria belonging to the genus Ehrlichia. The most notable species affecting companion animals is Ehrlichia canis, which is primarily transmitted by the brown dog tick (Rhipicephalus sanguineus). Unlike many other bacteria that multiply in the bloodstream or tissues, Ehrlichia organisms are obligate intracellular parasites. This means they must invade the host's own blood cells—specifically white blood cells like monocytes and granulocytes—to survive and replicate. Once inside, they form clusters called morulae, effectively hiding from the body's immune system while disrupting normal cellular functions.

While ehrlichiosis is most frequently diagnosed and studied in dogs, cats can also contract the infection. Feline ehrlichiosis is less commonly documented in veterinary literature, and much of our clinical understanding and treatment protocols for cats are extrapolated from canine medicine. In both species, the disease is characterized by its ability to damage the hematopoietic (blood-producing) and immune systems, leading to widespread inflammation of the blood vessels (vasculitis), abnormal clotting, and potential bone marrow failure.

The disease typically progresses through three distinct clinical phases:

1. The Acute Phase: Occurring one to three weeks after a tick bite, this phase involves the bacteria multiplying within the lymph nodes, spleen, and liver. Pets often display fever, lethargy, and swollen lymph nodes.
2. The Subclinical Phase: If the immune system does not fully eliminate the bacteria, the pet enters a subclinical stage. During this phase, which can last for months or even years, the pet shows no outward signs of illness, though the bacteria remain dormant in the spleen or other organs.
3. The Chronic Phase: In some pets, the infection progresses to a severe chronic state. This phase is marked by profound bone marrow suppression, meaning the body stops producing vital blood cells, leading to life-threatening anemia and bleeding disorders.

Causes & risk factors

The primary cause of ehrlichiosis is the transmission of Ehrlichia bacteria via the bite of an infected tick. The brown dog tick is the most common vector, but other tick species can also transmit the disease. Consequently, any dog or cat that spends time outdoors in areas with active tick populations is at risk. Warm, humid climates where ticks thrive year-round present the highest geographic risk.

According to a study cited in a leading veterinary internal medicine reference:

"In one study of 8662 dogssamplessubmitted from 14 veterinary colleges, 6 private veterinary practices, and 4 diagnostic laboratories across the south and central regions of the United States, antibody prevalence rates for E. canis and E. chaffeensis were 0. 8% and 2. 8%, respectively (Beal et al, 2012)." — Internal Medicine 5th p.1364

There are no specific breed predispositions listed for ehrlichiosis; any dog or cat exposed to vector ticks can contract the infection. However, lifestyle factors greatly influence risk. Hunting dogs, outdoor cats, and pets living in heavily wooded or tall-grass environments face significantly higher exposure rates. Additionally, pets that do not receive consistent, year-round tick preventative medications are highly susceptible.

Signs to watch for

The symptoms of ehrlichiosis can vary dramatically depending on whether the pet is in the acute or chronic phase of the disease. Because the bacteria target blood cells and blood vessels, many signs relate to abnormal bleeding, anemia, and immune system overactivity.

Common Signs

• Fever: A sudden, high body temperature is a hallmark of the acute phase.
• Lymphadenopathy: Swollen, enlarged lymph nodes, which your vet can feel during an exam.
• Thrombocytopenia: A dangerously low blood platelet count, which impairs the pet's ability to clot blood.
• Petechiae: Tiny, pinpoint purple or red spots on the gums, skin, or whites of the eyes, caused by leaking capillaries.
• Nonregenerative anemia: A low red blood cell count where the bone marrow fails to produce new cells to replace them.
• Hyperglobulinemia: An abnormally high level of globulin proteins in the blood, reflecting intense immune system stimulation.
• Hypoalbuminemia: Low levels of albumin, an essential blood protein, often due to vascular leakage or kidney loss.
• Splenomegaly: An enlarged spleen as the organ works overtime to filter out infected blood cells.
• Monocytosis: An increase in monocytes (a type of white blood cell) as the body attempts to fight the infection.

Occasional Signs

• Epistaxis: Sudden, unexplained nosebleeds due to low platelet levels.
• Polyarthritis: Joint pain, stiffness, or swelling, causing the pet to walk with a stiff, painful gait.
• Proteinuria: The presence of excess protein in the urine, indicating potential kidney damage from circulating immune complexes.
• Vomiting and Diarrhea: General gastrointestinal distress.
• Leukopenia: A decrease in the overall number of white blood cells, leaving the pet vulnerable to secondary infections.

Petechiae on the gums are a common sign of low platelet counts associated with ehrlichiosis.

How vets diagnose it

Diagnosing ehrlichiosis requires a combination of clinical history, physical examination findings, and specific laboratory tests. Because the symptoms can mimic other diseases, such as autoimmune disorders or other tick-borne infections, your vet will perform a comprehensive diagnostic workup.

Your vet will start with baseline blood work, including a Complete Blood Count (CBC) and a chemistry profile. A low platelet count (thrombocytopenia) combined with elevated blood proteins (hyperglobulinemia) strongly points toward ehrlichiosis. To confirm the diagnosis, your vet will utilize one or more of the following specific tests:

• ELISA (SNAP Test): A rapid, in-clinic screening test that detects antibodies against Ehrlichia in a small blood sample. This is highly effective but may yield a false negative in the very early days of infection before antibodies have developed.
• IFA (Immunofluorescence Assay): A laboratory test that measures the level (titer) of antibodies in the blood, helping to determine the strength of the immune response.
• PCR (Polymerase Chain Reaction): A highly sensitive molecular test that detects the actual DNA of the Ehrlichia bacteria. This is the gold standard for detecting early infections before antibodies are present, and for confirming that the bacteria have been successfully cleared after treatment.
• Blood Smear or Buffy Coat Smear: A microscopic examination of a blood sample to look directly for the characteristic bacterial clusters (morulae) inside the white blood cells.
• Bone Marrow Aspirate: In chronic cases, your vet may need to extract a small sample of bone marrow to evaluate cell production. This helps differentiate chronic ehrlichiosis from other bone marrow disorders.

As noted in a leading veterinary internal medicine reference:

"Changes in bone marrow cell lines associated with ehrlichiosis vary from hypercellular (acute phase) to hypocellular (chronic phase). Bone marrow plasmacytosis is common in dogs with subclinical and chronic ehrlichiosis, and the disease can be confused with multiple myeloma, particularly in dogs with monoclonal gammopathies." — Internal Medicine 5th p.1365

Additionally, evaluating specific proteins in the blood can help categorize the severity of the chronic phase:

"The positive acute phase proteins (APP) C-reactive protein (CRP), serum amyloid A (SAA), and haptoglobin (Hp) and the negative APP albumin concentrations were measured in 27 dogs with nonmyelosuppressive chronic monocytotropic ehrlichiosis, 29 dogs with myelosuppressive chronic monocytotropic ehrlichiosis, and 7 healthy dogs. The acute phase protein levels correlated to type of clinical syndrome..." — Internal Medicine 5th p.1366

Blood smears allow veterinarians to look for Ehrlichia organisms directly inside white blood cells.

Treatment options

Treatment for ehrlichiosis is highly effective when initiated early, particularly during the acute phase. The primary goal is to eliminate the intracellular bacteria and support the pet's body as it recovers.

First-Line Antibiotic Therapy

The primary treatment for ehrlichiosis is a prolonged course of tetracycline-class antibiotics. The consensus among veterinary specialists is to use doxycycline. According to veterinary guidelines:

"Several different tetracycline, doxycycline, chloramphenicol, and imidocarb diproprionate protocols have been used. The ACVIM Infectious Disease Study Group currently recommends doxycycline (10 mg/kg PO q24h for at least 28 days)." — Internal Medicine 5th p.1367

It is vital to complete the entire 28-day course of antibiotics, even if your pet appears completely recovered within the first few days. Stopping treatment early can allow the bacteria to survive in a dormant state, leading to a relapse or progression to the dangerous chronic phase.

Alternative and Supportive Therapies

• Imidocarb Diproprionate: An antiprotozoal medication that may be used as a secondary treatment option, though it is less commonly utilized than doxycycline.
• Tetracycline: Another antibiotic in the same class as doxycycline that can be used if doxycycline is unavailable.
• Chloramphenicol: A broad-spectrum antibiotic that may be considered, particularly in young puppies where tetracyclines carry a risk of staining developing teeth, or in cats where doxycycline might cause esophageal irritation.
• Glucocorticoids (Corticosteroids): In cases where the pet's immune system is actively destroying its own platelets or red blood cells (immune-mediated thrombocytopenia or hemolytic anemia), your vet may prescribe a short course of anti-inflammatory or immunosuppressive steroids to halt this destructive process.
• Supportive Care: Severe cases may require intravenous fluid therapy, blood transfusions to treat severe anemia, and nutritional support.

The Seropositive Healthy Dog Controversy

Veterinarians sometimes face a dilemma when a healthy dog tests positive for Ehrlichia antibodies during routine screening but shows no clinical signs of illness. As noted in veterinary literature:

"Whether to treat seropositive healthy dogs is controversial. Arguments for and against testing or treating healthy dogs were reviewed by the ACVIM Infectious Disease Study Group (Neer et al, 2002). The primary reason to treat a seropositive healthy dog is to try to eliminate infection before development of chronic-phase disease." — Internal Medicine 5th p.1367

Your vet will weigh the risks and benefits of treating a healthy, antibody-positive pet based on their specific lifestyle, geographic location, and baseline blood work.

Prognosis

The prognosis for pets diagnosed with ehrlichiosis depends heavily on the stage of the disease at which treatment begins.

For pets diagnosed during the acute phase, the prognosis is excellent. Most dogs and cats show noticeable improvement within 24 to 48 hours of starting doxycycline, and complete clearance of the bacteria is highly achievable.

For pets that have progressed to the chronic phase, the prognosis is variable to guarded. If the bacteria have caused severe, irreversible bone marrow suppression (hypocellularity), the body may no longer be able to produce red blood cells, white blood cells, or platelets. Despite aggressive treatment and blood transfusions, these cases can be fatal.

Because feline ehrlichiosis is less common, long-term prognosis data in this species is limited. However, when identified early and treated with appropriate antibiotic courses, cats generally experience a favorable outcome similar to dogs.

Prevention

Because there is currently no vaccine available for ehrlichiosis in either dogs or cats, prevention relies entirely on minimizing exposure to ticks.

• Year-Round Tick Preventatives: Administer veterinary-approved oral or topical tick preventatives to your pets every month without fail. Consult your vet to choose the most effective product for your pet's lifestyle.
• Daily Tick Checks: Thoroughly check your pet for ticks after they spend time outdoors, especially in wooded, brushy, or grassy areas. Pay close attention to the ears, under the collar, between the toes, and around the groin.
• Prompt Tick Removal: If you find a tick, remove it immediately using fine-tipped tweezers or a specialized tick-removal tool. Grasp the tick close to the skin and pull straight outward with steady pressure to ensure the mouthparts are not left behind.
• Environmental Control: Keep your lawn mowed short and clear away brush piles where ticks thrive. If you live in a high-risk area, consider treating your yard with pet-safe acaricides.

Chronic ehrlichiosis can suppress the bone marrow, halting the production of vital blood cells.

When to call your vet

You should contact your veterinarian if your pet has been exposed to ticks and begins showing signs of lethargy, fever, or joint stiffness.

Seek immediate emergency veterinary care if you observe any of the following red-flag signs:

• Sudden, severe nosebleeds (epistaxis)
• Widespread pinpoint bruising on the gums, belly, or inner ears (petechiae)
• Extreme weakness, collapse, or pale to white gums (signs of severe anemia)
• Difficulty breathing or rapid, shallow respiration

Sources

• Internal Medicine, 5th Edition, pages 1364, 1365, 1366, 1367.

My highlights & notes

New note