Ehrlichiosis is caused by bacteria classified within the group of the alpha-proteobacte- ria, order Rickettsiales, family Anaplasmataceae, genus Ehrlichia [1] . This genus consists of obligate intracellular Gram-negative bacteria that mainly infect leukocytes such as monocytes, macrophages, and granulocytes [2] .

Monocytotropic canine ehrlichiosis is caused by Ehrlichia canis, which is found in the form of morulae in the cytoplasm of lymphocytes, monocytes, and macrophages [3] . The incubation period ranges from 8 to 20 days [4] followed by the acute, subclinical, and chronic stages of the disease.

The most important clinical signs of infection are fever, depression, lethargy, anorexia, weight loss, cardiac arrhythmias, hemorrhagic diathesis, central nervous system signs, lymphadenomegaly, splenomegaly, polyarthritis, uveitis anterior, ocular signs and blindness from retinal edema or bleeding and detachment. The most significant laboratory findings are thrombocytopenia, anemia, leukopenia, hyperglobulinemia, proteinuria, lymphocytic pleocytosis, and bone marrow plasmacytosis [4] .

The acute phase lasts between two to four weeks [5] and is characterized by fever, weight loss, anorexia, depression, lymphadenomegaly, splenomegaly, vasculitis, and ocular and musculoskeletal signs [6] [7] [8] . Thrombocytopenia is the most common abnormality in naturally or experimentally infected dogs in this phase of the disease [8] .

The subclinical phase has a duration that varies from months to years [5] [9] . Dogs in this stage are known to be carriers of E. canis and remain clinically healthy for months or years [10] until they spontaneously recover from infection or develop severe illness. During this phase, the animal may show high anti-E. canis antibody titers [11] , persistent thrombocytopenia, and leucopenia with no other clinical signs [9] [10] [11] . Generally, bacterial DNA is not found in blood samples in this phase; spleen and bone marrow are the most appropriate tissues to analyze [3] [12] . Severe pancytopenia, hemorrhagic diathesis, and organism debilitation may characterize the chronic phase [5] . Although the conditions that can lead the animal to this stage are not totally clear, it has been suggested that factors such as breed, immune system deficiency, stress conditions, co-infections, virulence of the strain, and the geographical region could have some influence on the outcome of infection [10] .

E. canis was reported for the first time in dogs of Costa Rica by Meneses [13] . Romero et al. [14] analyzed blood samples from dogs with clinical signs or suspected of suffering ehrlichiosis, and found highly significant differences in hematocrit and hemoglobin values between PCR positive and negative dogs. In this population, 26.6% (8/30) were determined to have acute (seronegative and PCR positive) E. canis infections, while 20.0% (6/30) had subclinical (seropositive and PCR negative) and 6.6% (2/30) had chronic (seropositive and PCR positive) E. canis infections [14] . Another study carried out in four regions of Costa Rica found 58.0% (29/50) of dogs with subclinical E. canis infections [15] . The present study describes and analyzes, in a more representative sampling frame, the clinical and hematological presentation of E. canis infection in dogs of Costa Rica.

Of the total of dogs (n = 441) that participated in the study, 399 had owners, and it was possible to conduct an owner interview, while 42 (9.5%) dogs were residents of the parks and had no known owner. A total of 55.9% of the owners had observed ticks on their pets, only 2.0% (9/441) of the dogs were diagnosed previously by a veterinarian with ehrlichiosis; 77.8% of them had been treated (28.6% with doxycycline and 71.4% did not remember the name of the medication) to: “ehrlichiosis; seven dogs had been treated (2 with doxycycline, and 5 did not remember the name of the medication), and...” in 3.6% of cases (16 dogs) the veterinarians suspected that the pets had ehrlichiosis. Clinical signs suggestive of ehrlichiosis observed by the owners at some point in the lives of their pets were: bleeding (17.0%), petechiae (9.5%), weight loss (4.5%), and hematuria (1.8%) (Table 1).

Clinical examination encountered abnormalities in only a minority of the dogs, such as attitude (weak 0.2%, depressed 0.7%, docile 16.1%), capillary refill time > 2 seconds (5.2%), mucous membranes (very pale 1.1%, pale 11.1%), and rectal temperatures ≥ 39.5˚C (19.5%). Clinical abnormalities included weight loss (4.5%), lymphadenomegaly (2.0%), petechiae (1.6%), cough (0.9%), scrotal edema (0.9%), ataxia (0.5%) and hematuria (0.2%) (Table 1).

Hematocrit and complete blood count (CBC) results from the total dog sample are shown in Table 2.

Analysis of hematological values of dogs that were seropositive and seronegative to E. canis determined a highly significant difference (p < 0.001) in mean hematocrit, hemoglobin and platelet count values, and a weak difference (p = 0.05 - 0.10) in mean corpuscular hemoglobin concentration (MCHC), white blood count (WBC), specifi- cally, lymphocytes and monocytes values. In all cases, mean values for all the above variables were lower in seropositive than in seronegative dogs.

Also, analysis of dogs that were PCR positive and PCR negative to E. canis determined a highly significant difference in the mean values of lymphocytes and basophils, and a weak difference in the hematocrit and mean values between PCR positive and PCR negative dogs. In all cases, mean values for all the above variables were lower in positive than in negative dogs (Table 2).