Veterinary Medicine - April 2008 - (Page 218)

Canine anaplasmosis PEER-REVIEWED The cyclic appearance of clinical cases that coincide with tick season indicates that canine anaplasmosis is an acute disease that occurs in dogs a week or two after organism inoculation by ticks.5,12 Because chronic infection has not been directly related to clinical disease and because a therapeutic regimen effective in clearing the organism from an infected animal has not been established, treating clinically healthy, seropositive animals is of questionable bene t. However, a seropositive reaction to A. phagocytophilum in a clinically healthy dog should not be disregarded. At a minimum, implement an aggressive tick-control program designed to minimize exposure to ticks, and, hence, to other tick-borne diseases. It is clear that coinfection with two or more tickborne agents is common and that dogs coinfected with B. burgdorferi and A. phagocytophilum are nearly two times more likely to develop clinical disease than are dogs infected with either agent alone.9 There is also some concern that chronically infected carriers could be adversely affected by therapeutic agents that compromise the immune system or by a concurrent illness that might alter an animal’s immune status. The administration of immunosuppressive doses of corticosteroids to infected, asymptomatic dogs resulted in the reappearance of bacteremia, although the animals remained clinically normal.6,7,15 4. Two large, dark-blue-staining Anaplasma platys morulae in a circulating platelet from an infected dog. The normal, smaller, pink-staining platelet granules are also observed in the cytoplasm of the infected platelet (Wright’sGiemsa; 100X). Tick vector and mammalian hosts The natural mode of disease transmission has not been conclusively determined, but A. platys DNA has been ampli ed from Rhipicephalus and Dermacentor species ticks.19,20 Therefore, tick transmission is highly suspected. Dogs are by far the most common mammalian host, although rare reports of infections in cats, impalas, and sheep have been documented outside the United States.21,22 organisms will reappear at one- to twoweek intervals, and the process repeats, resulting in a cyclic thrombocytopenia. The percentage of parasitized platelets and the severity of the thrombocytopenic episodes decrease with subsequent cycles of parasitemia. Diagnosis Anaplasma platys infection can be diagnosed by light microscopic identi cation of morulae in circulating platelets. An IFA test is also commercially available, and serologic testing has indicated that seropositivity in dogs can be high in the Southeast and Midwest.23 There appears to be some cross-reactivity between A. platys and the A. phagocytophilum with the SNAP 4Dx assay. With this assay, many animals infected with A. platys will have positive test results for A. phagocytophilum. The organisms are closely related and evidently share epitopes used in this ELISA. PCR testing for A. platys, which can distinguish between the two organisms, is typically done in experimental laboratories for research purposes, but a PCR test is also commercially available (e.g. Zoologix). Clinical and laboratory ﬁndings Most dogs with naturally occurring infection have mild clinical disease, although more severe clinical signs of fever, lethargy, pale mucous membranes, petechial hemorrhages, epistaxis, and lymphadenopathy can occur. As with other tick-transmitted diseases, coinfection of A. platys with other infectious agents such as Ehrlichia, Bartonella, or Borrelia species can result in more severe clinical manifestations. Experimentally, clinical signs appeared between eight and 14 days after inoculation.18 The organism appears in circulating platelets, resulting in a severe thrombocytopenia, typically < 20,000/µl. Platelet counts usually remain below 20,000/µl for only one or two days. The organisms disappear rapidly from the peripheral blood, and platelet counts rise quickly, reaching a normal value in three or four days. Subsequently, ANAPLASMA PLATYS INFECTION Anaplasma platys (formerly Ehrlichia platys) is the causative agent of infectious cyclic thrombocytopenia in dogs. Epidemiology Anaplasma platys was rst reported in the United States in 1978 and has since been recognized to have a worldwide distribution, being reported in many European, Asian, and South American countries.18 This agent is unique—it is the only intracellular infectious agent described in people or animals to speci cally infect platelets (Figure 4). Treatment and prognosis Doxycycline at the dosage described above for A. phagocytophilum is apparently effective in treating A. platys infection.12 However, animals with circulating A. platys organisms that have moderate to severe clinical dis- 218 April 2008 VETERINARY MEDICINE

Table of Contents Feed for the Digital Edition of Veterinary Medicine - April 2008

Veterinary Medicine - April 2008 Contents Letters Leading Off The Veterinary Medicine Interview: Dr. Barrak Pressler Idea Exchange Help! My Dog Licks Everything An Update on Anaplasmosis in Dogs CE Form Advertiser Index Marketplace/Classifieds Mind Over Miller

Veterinary Medicine - April 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.