Introduction

• Cause: serious tick-borne rickettsial infections of dogs acquired in parts of the tropics, sub-tropics, and warm temperate regions. In Europe the distribution of Ehrlichia canis correlates to the distribution of the vector Rhicephalus sanguineus with infections mainly seen in the area around the Mediterranean basin; occasional cases have been seen further north in the UK and Finland.
• Signs: non-specific in the acute phase, sudden onset of fever, depression, anorexia and in the chronic phase, hemorrhages including epistaxis. A subclinical phase can exist in some cases after the subsidence of clinical signs and persist for several years.
• Diagnosis: serology or documentation of the presence of Ehrlichia morulae in lymphocytes or monocytes by cytology or PCR.
• Concurrent infection with Babesia spp and Leishmania infantum is common.
• Treatment: doxycycline 10 mg/kg daily for 4 weeks.
• Prognosis: good when acute phase is treated properly and the subclinical phase avoided; guarded if severe chronic form of canine pancytopenia develops.
  Print off the owner factsheet on Ehrlichiosis Ehrlichiosis to give to your client.

Pathogenesis

Predisposing factors

General

• Exotic dogs imported as adults into endemic areas from non-endemic areas.

• Immunological status of the host.
• Stress including malnutrition and pregnancy.
• Virulence of the strain of the organism.
• Size of challenge dose.

• Dogs can remain infected in a sub-clinical carrier state for several years and therefore iatrogenic transmission can occur.

Specific

• The vector for E. canis is the three-host brown tick, Rhipicephalus sanguineus Rhipicephalus sanguineus.

• Failure to treat adequately may lead to the development of the sub-clinical or progression to the chronic form, tropical canine pancytopenia.

Pathophysiology

• A tick-transmitted rickettsial infection in which, depending on the Ehrlichia species involved, mononuclear cells or granulocytes are parasitized.
• Clinical signs of acute monocytic ehrlichiosis tend to be non-specific with the insidious development of the chronic form in which bone marrow suppression results in pancytopenia and a hemorrhagic crisis.

• E. canis is an obligatory intracellular parasite multiplying inside cytoplasmic vacuoles in mononuclear phagocytic cells.
• Hematogenous → splenomegaly, hepatomegaly and lymphadenopathy.
• Vasculitis affecting various organs including lungs, kidneys and possibly the meninges.
• Bone marrow depression → thrombocytopenia, anemia and leukopenia during the acute phase and may persist during the subsequent subclinical phase.
• Platelet dysfunction also contributes to the occurrence of hemorrhages.
• Infection appears to induce an immune-mediated thrombocytopenia and immune-mediated hemolytic anemia.
• Severely affected animals are susceptible to secondary infections. Persistent latent infection → lymphoproliferative syndrome, with accumulation of large numbers of plasma cells in various organs including lymphopoietic tissues, kidneys and meninges, develops in some dogs several weeks after the primary clinical episode (chronic form known as Tropical Canine Pancytopenia).
• Hemorrhages, especially epistaxis Epistaxis , and various other symptoms, depending on the organs/tissues involved, occur in affected animals.

Timecourse

• Incubation period of monocytic canine ehrlichiosis ranges from 8-20 days.
• The course of the illness is variable with the acute phase lasting 2-4 weeks.
• Clinical signs of the acute phase can resolve spontaneously in 1-4 weeks.
• Death may occur within a week of the onset of clinical signs.
• The subclinical phase lasts for months or years before the development of the chronic phase in some infected dogs.
• Death may occur within a few hours of the onset of clinical signs in tropical canine pancytopenia due to severe hemorrhage associated with thrombocytopenia.

Epidemiology

• E. canis, the etiologic agent of canine monocytic ehrlichiosis is transmitted by Rhipicephalus sanguineus Rhipicephalus sanguineus.
• Transtadial but not transovarial transmission occurs in infected ticks.
• Rhipicephalus sanguineus can survive for up to 500 days as unfed adults remaining infected for at least 155 days.
• Ticks are more abundant and acute infections more common during the warm season.
• E. canis can be transmitted by blood transfusions.

Diagnosis

Treatment

Prevention

Outcomes