Canine monocytic ehrlichiosis (CME) is caused by Ehrlichia canis, an intracellular parasitic bacteria and tick-borne pathogen. Recently, this pathogen has received further attention, because it leads to increasing morbidity and mortality in animals [1]. Transmission is mediated by the tick Rhipicephalus sanguineus sensu lato, and, before infection, the bacteria replicate in monocytes and macrophages [2].
Clinical presentation of CME results in acute, chronic or subclinical phases, with several clinical manifestations. The acute phase persists for 2 to 4 weeks [3] and is characterised by signs in diverse systems, yet the most common are fever, weight loss, anorexia, depression, lymphadenomegaly, splenomegaly, and vasculitis [4]. In addition, dogs in this phase show thrombocytopenia as the most common laboratory abnormality [5]. In the subclinical phase, dogs have persistent thrombocytopenia and leukopenia in laboratory analysis; however, during this phase, in some dogs, the thrombocytopenia may be mild to non-existent [6] and they usually do not show clinical signs. Duration of this phase differs from months to years [7]. Additionally, in this phase is common that the microorganism is not circulating in blood and stays housed in some target organ, such as the spleen, bone marrow or liver [8–11]. Furthermore, previous research has shown that E. canis is widely distributed in different organs of infected dogs [8, 9, 12, 13]. Otherwise, in the chronic phase dogs have severe pancytopenia, haemorrhagic diathesis, and general debilitation [3]. Immune system deficiency, stress, co-infections, virulence strain, and geographical region are factors that influence the presentation of this phase in affected dogs [8].
In recent times, diagnosis of the disease has been challenging for practicing veterinarians [14, 15]. Identification of morulae in monocytes in a blood smear is diagnostic of the disease; however, a low frequency of morulae in buffy coat smears has been reported previously. It could be to the low parasitaemia observed in the natural infection [7, 11, 16–21]. Besides, other more specific methods are used as diagnostic, including the immunofluorescence antibody test (IFA) and ELISA (enzyme-linked immunosorbent assay), which are both able of detecting specific antibodies [21–27]. Also, other molecular techniques exist such as the polymerase chain reaction (PCR) [1, 19, 36, 28–35]. Presently, the infectious disease group of the American College of Veterinary Internal Medicine (ACVIM) requires that dogs diagnosed with this disease show suggestive clinical signs and have positive tests, either by serology and/or by PCR [37]. A complication in the diagnosis comes about in dogs in the subclinical phase of the disease, because normally dogs do not have clinical signs. Furthermore, cross-reactivity and failure to differentiate between current and past infections with ELISA and IFA tests has been reported [25, 38, 39]. On the other hand, both in the subclinical and chronic phases, there is a possibility that parasitaemia is low in the dog [20, 26, 40, 41], as the bacteria are in the target organs [10]. Therefore, in these cases, the dogs will be negative in a PCR blood test [10].
Presently, the presence of DNA of E. canis in several organs, such as blood, bone marrow, spleen, liver, kidney, and lymph nodes has been demonstrated by PCR in infected dogs [7, 8, 11, 12, 40].
The goal of this study was to evaluate the occurrence of E. canis in different tissues, such as liver, spleen, lymphatic nodules and bone marrow, in dogs naturally infected with monocytic ehrlichiosis.