Ehrlichia spp

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Clinical appearance of an anemic dog with Ehrlichia canis infection[1]
Ehrlichia canis seen in a membrane-bound inclusions (morulae) within the cytoplasm of a monocyte (buffy coat smear, Wright stain)[1]

Ehrlichia spp are a common Gram-negative obligate intracellular proteobacterial infection of dogs transmitted by Rhipicephalus sanguineus[2] and characterized by chronic anemia.

Species which are parasitic to dogs include:

  • Ehrlichia canis - most common - causes canine monocytic ehrlichiosis
  • Ehrlichia ewingii - less common - causes canine granulocytic ehrlichiosis[3]
  • Ehrlichia chaffeensis - rare in dogs

Co-infections with a number of Ehrlichia species is common in dogs as well as coinfections with other tick-borne parasites such as Babesia gibsoni, Anaplasma phagocytophilum, Anaplasma platys[4], Neorickettsia risticii[5], Rickettsia massiliae and Borrelia afzelii[6].

Transmission by Rhipicephalus sanguineus is transstadial, and ticks acquires the bacteria by feeding on an infected dog in either the larvae or nymph form and the tick transmits the disease to another dog as either the nymph or adult form.

After an incubation period of 1 - 3 weeks, an acute febrile phase within the dog lasts for 2 - 4 weeks, during which Ehrlichia multiplies within circulating mononuclear cells and the mononuclear phagocytes within the liver, spleen, and lymph nodes.

The infected cells are then transported in circulation to the rest of the body, with a predilection for the the lungs, kidneys and meninges. Cells infected with Ehrlichia adhere to the vascular endothelium, resulting in vasculitis.

Clinical signs are usually nonspecific and include mental depression, lethargy, mild weight loss, vomiting, diarrhea, and anorexia, with or without hemorrhagic tendencies.

Other symptoms include uveitis and/or retinal petechiae, polymyositis, polyarthritis, protozoal polyradiculoneuritis and encephalomyelitis[7].

Blood tests usually reveal varying degrees of nonregenerative anemia and thrombocytopenia. Serum chemistry commonly reveals hyperglobulinemia (monoclonal or polyclonal gammopathy), hypoalbuminemia, and low albumin-globulin ratio.

Although a presumptive diagnosis can be made on visualization of the parasite under light microscopy, a definitive test requires molecular identification of the parasite species using indirect immunofluorescence antibody test[8], PCR testing[9] or western blot[10][11].

Treatment usually involves doxycycline given daily for at least one month. The use of rifampicin has been shown to hasten hematological recovery but is ineffective at eliminating the parasite[12].

In many cases, polyarthritis associated with E. ewingii may be self-limiting.

Previous infection does not confer lifelong immunity, and dogs can become reinfected with the same or other ehrlichial species after re-exposure to infective ticks.

The mainstay of prevention of canine ehrlichiosis is tick control using drugs such as fipronil, amitraz and (S)-methoprene[13].

References

  1. 1.0 1.1 TPV Experience
  2. Ionita M et al (2013) Molecular evidence for bacterial and protozoan pathogens in hard ticks from Romania. Vet Parasitol Jan 29
  3. Ehrlichia Research Laboratory, College of Veterinary Medicine, The Ohio State University, Columbus, OH
  4. Kelly PJ et al (2013) Ehrlichiosis, babesiosis, anaplasmosis and hepatozoonosis in dogs from St. Kitts, West Indies. PLoS One 8(1):e53450
  5. Dumler JS et al (2001) Reorganization of genera in the families Rickettsiaceae and Anaplasmataceae in the order Rickettsiales: unification of some species of Ehrlichia with Anaplasma, Cowdria with Ehrlichia and Ehrlichia with Neorickettsia, descriptions of six new species combinations and designation of Ehrlichia equi and ‘HGE agent’ as subjective synonyms of Ehrlichia phagocytophila. Int J Syst Evol Microbiol 51:2145–65
  6. Hornok S et al (2013) Molecular evidence of Ehrlichia canis and Rickettsia massiliae in ixodid ticks of carnivores from South Hungary. Acta Vet Hung 61(1):42-50
  7. Neer TM, Breitschwerdt EB, Greene RT, Lappin, MR (2002) Consensus statement on ehrlichial disease of small animals from the Infectious Disease Study Group of the ACVIM. J Vet Intern Med 16:309-315
  8. Waner T, Harrus S, Jongejan F, Bark H, Keysary A, Cornelissen A (2001) Significance of serological testing for ehrlichial diseases in dogs with special emphasis on the diagnosis of canine monocytic ehrlichiosis caused by Ehrlichia canis. Vet Parasitol 95:1-15
  9. McBride J et al (1996) PCR detection of acute Ehrlichia canis infection in dogs. J Vet Diagn Invest 8:441
  10. Nazari M et al (2013) Molecular detection of Ehrlichia canis in dogs in Malaysia. PLoS Negl Trop Dis 7(1):e1982
  11. Goldman EE, et al (1998) Granulocytic ehrlichiosis in dogs from North Carolina and Virginia. J Vet Intern Med '12:61-70
  12. Theodorou K et al (2013) Efficacy of rifampicin in the treatment of experimental acute canine monocytic ehrlichiosis. J Antimicrob Chemother Mar 8
  13. Fourie JJ et al (2013) Prevention of transmission of Ehrlichia canis by Rhipicephalus sanguineus ticks to dogs treated with a combination of fipronil, amitraz and (S)-methoprene (CERTIFECT(®)). Vet Parasitol 193(1-3):223-228