Ehrlichiosis in dogs: symptoms, tests and treatment

Overview

In Europe, a major form of canine ehrlichiosis affects monocytes, a type of white blood cell. The disease may be acute, cause few symptoms or become chronic. Acute symptoms can develop within 1–3 weeks of infection. An incubation period of 8–20 days is also reported.

The acute symptomatic phase usually lasts about 1–3 weeks. A phase in which the dog seems symptom-free may continue for weeks or months. Some dogs develop chronic disease even after the initial symptoms have eased. Severe chronic disease has a poor outlook, particularly when all blood cell lines are reduced.

Causes

The bacterium passes from a tick to a dog

The brown dog tick, Rhipicephalus sanguineus, is a particularly important transmitter. The bacterium is intracellular: it needs living host cells to multiply. In dogs, it multiplies in white blood cells, forming small bacterial clusters called morulae. It then spreads with immune system cells to the liver, spleen and lymph nodes.

Brown dog tick larvae, nymphs and adults can acquire the bacterium while feeding on an infected member of the dog family. The bacterium can remain in the tick as it develops from one stage to the next. Infection can also overwinter in infected ticks. Transmission through a blood transfusion is possible if the donated blood contains the bacterium.

Dogs are the main host, but other members of the dog family can also maintain the infection in the wild.

Several factors contribute to bleeding

The bacterium can impair platelet function and increase platelet destruction and removal from the circulation. Platelets are cell fragments that help blood clot. Bleeding can therefore occur even when a dog's platelet count is only mildly reduced. In chronic disease, bone marrow function may deteriorate and blood cell production decline.

In chronic disease, immune complexes, structures formed by antibodies and their targets, may accumulate in different organs.

Risk factors

Exposure to brown dog ticks is a key risk factor for infection. In Europe, infections in dogs have been reported in countries including France, Italy, Portugal, Greece and Spain. Cases have also been identified in Germany, Switzerland and Britain. Travel abroad and previous residence in risk areas therefore form part of the assessment of a dog's infection risk.

Dogs returning from travel or being imported can carry brown dog ticks in their coats. Brown dog ticks can survive indoors even in cold climates. Any dog can develop chronic ehrlichiosis. Long-standing disease is not tied to a particular season in the same way as recent tick exposure.

Disease severity is influenced by the bacterial strain and the dog's own immune response.

Typical symptoms

In acute disease, changes in general wellbeing and appetite are particularly noticeable at home.

• A feverish dog is quiet, tired and weaker than usual.

• Your dog loses its appetite or starts losing weight.

• Your dog vomits or becomes generally unwell without an obvious local problem.

• Nosebleeds or tiny pinpoint spots of bleeding appear on the skin and mucous membranes.

• The gums look pale or bruises develop more easily than usual.

• Your dog is stiff, lame or reluctant to go for a walk.

As the disease persists, pus-like discharge may appear from the eyes or nostrils. The hindlimbs and a male dog's scrotum may swell. When the nervous system is affected, a dog may be painful to touch and have poorer control of its movements. During the phase with few symptoms, however, it may seem completely healthy despite blood abnormalities.

Inflammation of the nervous system may also cause neck pain. The eyes may make involuntary jerking movements, known as nystagmus.

Complications and consequences

Anaemia, a shortage of red blood cells, can occur with ehrlichiosis. Prolonged infection may reduce red blood cells, white blood cells and platelets together, causing pancytopenia. Severe blood abnormalities associated with bone marrow damage worsen the outlook. Heavy bleeding can cause anaemia requiring a blood transfusion.

Chronic ehrlichiosis can cause uveitis, inflammation within the front of the eye. Retinal bleeding or detachment may lead to loss of vision. The kidney's filtering structures may become inflamed, allowing protein to leak into the urine. Inflammation of muscles, joints and blood vessels can also occur.

Diagnosis at the vet

Blood cell counts and organ function

The vet first establishes the history of tick exposure and considers the symptoms alongside blood cell counts and biochemistry results. Blood counts often show low platelets and mild anaemia in which the bone marrow does not adequately replace the missing cells. Blood tests may also reveal raised liver enzymes and globulins. Protein in the urine and low blood albumin may form part of the wider pattern of laboratory findings.

Bacterial clusters, or morulae, can sometimes be seen inside cells on a blood smear. However, a routine blood smear has low sensitivity, so a negative result cannot rule out infection. Examining the white blood cell layer or a lymph node sample may increase detection sensitivity to about 60–66%. Blood cell changes also occur in other vector-borne diseases, so separate tests identify the cause.

Some affected dogs have an increased total white blood cell count, even though low blood cell counts are common in this disease. Morulae on a blood smear do not distinguish Ehrlichia canis from Ehrlichia chaffeensis infection. Leptospirosis, inflammation of the heart's inner lining and immune-mediated platelet loss can also cause findings resembling acute ehrlichiosis. Other explanations for chronic blood abnormalities include diseases affecting the bone marrow and certain types of drug poisoning.

Antibody testing and PCR complement each other

Antibodies usually become detectable 1–4 weeks after infection. A very recent infection may therefore give a negative antibody result. Conversely, antibodies can persist for months or years, so a positive result may no longer indicate current disease. Antibodies against different Ehrlichia and Anaplasma species may cross-react in tests.

In an unclear acute case, a quantitative antibody test can be repeated after 2–4 weeks. An increase in antibody level of at least 4-fold supports the diagnosis. A qualitative rapid test alone cannot measure this change in concentration. PCR looks for bacterial DNA and can detect infection before antibodies form.

PCR may be negative if there are few bacteria in the sample or the dog has already received an antimicrobial. Samples may include blood or, depending on the case, tissue such as a lymph node or bone marrow sample. PCR can also distinguish bacterial species. A negative PCR result therefore does not rule out infection in every situation.

Treatment

Antibiotics and supportive care when needed

Doxycycline is the first-choice antibiotic for ehrlichiosis.

One reason for its use is its ability to enter the cells where the bacterium lives. The vet may use minocycline if the first-choice medicine is unavailable. If infection is strongly suspected, treatment can start before test results are available. An initial negative antibody test is not a reason to stop treatment, because antibodies may still be absent early in the disease.

In appropriately treated acute ehrlichiosis, fever is expected to ease within 24–48 hours of starting treatment. However, blood abnormalities in chronic disease may persist for 3–6 months. A dog with fever, bleeding or marked weight loss may need supportive care as well as antibiotics. Severe bleeding may require a whole blood or platelet transfusion.

Not every antibiotic that improves laboratory findings necessarily eliminates the bacterium from the body. For example, rifampicin improved blood test findings in experimental infections but did not eliminate Ehrlichia canis infection.

Confirming the response to treatment

In a dog with chronic disease, antibody levels and whole-blood PCR can be checked at 6 months to assess treatment success. If the antibody level is still weakly positive then, it can be checked again after another 6 months. A positive PCR immediately after treatment can sometimes reflect residual DNA from dead bacteria. The timing of follow-up testing therefore affects interpretation.

Prevention

Preventing infection primarily relies on protecting your dog against ticks. A dog travelling abroad needs protection suited to the destination's insect and tick risks. Tick protection is started according to the product instructions before arrival in a tick-risk area. Ticks can remain unnoticed in the coat even after careful inspection.

Check your dog's coat after walks and carefully remove any ticks you find. In tick-risk areas, avoid grassy vegetation and leaf litter away from paths. Screening blood donor dogs for infection reduces transfusion-related risk. Even so, a very recent infection may be missed at the donor's first screening.

An attached tick can be removed by grasping it with fine-tipped tweezers close to the skin and pulling straight out. Checking a dog after travel is also important because brown dog ticks brought indoors can be difficult to eradicate.

Important for dog owners

Record your dog's previous countries of residence, travel destinations and travel dates. Bring details of tick protection and previous blood tests. If your dog received antibiotics before sampling, tell the vet, as this matters when interpreting the results.

Previous antimicrobial treatment can reduce the sensitivity of PCR.

Monitor and record appetite, weight changes, energy levels and any bleeding. Photograph bleeding spots on the skin or gums if you can do so without stressing your dog. Keep agreed blood tests and follow-up appointments in your calendar even if your dog seems better.

A dog with chronic disease may improve clinically before its blood counts recover.

Frequently asked questions

Can a dog be infected without symptoms?

Yes: the phase with few symptoms can last weeks or months, and a dog may look healthy throughout it. Blood tests may nevertheless show anaemia, low platelets or increased globulins. Some dogs later develop chronic disease.

Does a positive antibody test always mean current disease?

No: antibodies may remain in the blood for months or years after previous exposure. Cross-reactions also occur between different Ehrlichia and Anaplasma species. The vet interprets the result alongside symptoms, blood counts and PCR when needed.

Can the first test be negative despite infection?

Yes: antibodies take 1–4 weeks to develop, so a recent infection may still be seronegative. PCR sensitivity also falls if bacterial numbers are low or the dog has already received antibiotics. A strong suspicion may therefore require repeat testing.

How quickly should my dog feel better?

Fever in acute ehrlichiosis usually eases within 24–48 hours after appropriate treatment starts. Blood count abnormalities in chronic disease may take 3–6 months to resolve. General wellbeing often improves sooner, so laboratory tests also help monitor treatment success.

Can ehrlichiosis pass directly from a dog to a person?

Ehrlichia canis is not considered an established cause of zoonotic disease. Some other Ehrlichia species can infect people, but usually need a tick to transmit them. Infection in a dog may indicate a risk of human tick exposure in the same environment.