The Ehrlichiae are a group of small, gram-negative, pleiomorphic, obligate intracellular cocci that infect different blood cells in various animal species and in humans. There has recently been a reclassification of the familyAnaplasmataceae to which the Ehrlichiae belong. According to this new classification there are two leukotrophic diseases in dogs that are caused by bacteria in the genus Ehrlichia, namely, Canine Monocytic Ehrlichiosis (caused mainly by Ehrlichia canis) and Canine Granulocytic Ehrlichiosis (caused by Ehrlichia ewingii). It should be noted that cross-reactivity and co-infection is common among the ehrlichiae. Classically, canine ehrlichiosis presents as a rather non-specific multisystemic disorder with the primary complaints being depression, lethargy, mild weight loss, vomiting, diarrhea, and anorexia, with or without hemorrhagic tendencies. Furthermore, patients may present with uveitis and/or retinal petechiae, polymyositis, polyarthritis, and central nervous system signs. Hematologic abnormalities most commonly associated with canine ehrlichiosis include nonregenerative anemia and thrombocytopenia. Serum chemistry commonly reveals hyperglobulinemia (monoclonal or polyclonal gammopathy), hypoalbuminemia, and low albumin-globulin ratio.
Canine Monocytic Ehrlichiosis (Ehrlichia canis)
Canine Monocytic Ehrlichiosis (CME), caused by E. canis, is an acute to chronic disease of monocytes, and is the ehrlichial disease most extensively studied. This organism is primarily transmitted by Rhipicephalus sanguineus, the brown dog tick. It is seen mostly in the southeastern and southwestern United States, although it is recognized in all states and worldwide. Amblyomma and Dermacentor ticks have also been implicated in transmission of this disease. Dogs may present with variable clinical signs, but thrombocytopenia with bleeding tendencies is the most consistent presenting complaint in dogs in both the acute and chronic stages of the disease. During the acute stage, splenomegaly and lymphadenomegaly are common. In the chronic stage, widespread hemorrhage and increased mononuclear cell infiltration of organs may also be evident. Hematologic changes include nonregenerative anemia, thrombocytopenia, and leukopenia. Pancytopenia may occur as a result of hypoplasia of all bone marrow precursor cells, more commonly in the severe chronic phase. Some dogs may develop a secondary immune-mediated hemolytic anemia (IMHA) and have an acute hemolytic crisis, and, thus, a positive direct antiglobulin (Coombs’) test.
Canine Granulocytic Ehrlichiosis (Ehrlichia ewingii)
Canine Granulocytic Ehrlichiosis (CGE) caused by Ehrlichia ewingii, is a disease of neutrophils and, rarely, eosinophils. CGE classically presents with mild signs including fever, lethargy, anorexia, weight loss, vomiting, diarrhea, severe but transient thrombocytopenia, and transient mild nonregenerative anemia with ineffective erythropoeisis. Commonly, the major presenting clinical signs associated with E. ewingii include lameness and joint swelling due to polyarthritis. This form of ehrlichiosis is generally seen in the southern and mideastern United States.Ticks including Ixodes pacificus, Dermacentor variabilis, Rhipicephalus sanguineus, Amblyomma americanum (especially in North Carolina), and Ixodes scapularis (damminni) have been implicated as vectors.
Transmission of Ehrlichia
Ehrlichia are transmitted by ticks including the Brown Dog Tick, Rhipicephalus sanguineus and the Lone Star Tick Amblyomma americanum. The immature form of the tick feeds on an animal infected with Ehrlichia. When these immature ticks or a mature form of the tick feeds on another animal, the Ehrlichia is passed on to that animal. The Ehrlichia can remain alive in the developing tick for up to 5 months. This means a tick could become infected in the fall, and infects a dog the following spring. Because the disease is transmitted by these ticks, it can occur wherever Brown Dog and Lone Star Ticks are found. Almost every state in the United States has reported a case of ehrlichiosis.
Pathogenesis of Ehrlichiosis
The pathogenesis of infection with E. canis is the most extensively studied; therefore this discussion will focus on this particular species. Infection occurs through salivary secretions of the tick at the attachment site during ingestion of a blood meal or through blood transfusions. If the adult Rhipicephalus sanguineus engorges on the dog during the acute stage, it can transmit the disease to other dogs for at least 155 days following detachment. Transmission by Rhipicephalus sanguineus is transstadial: the tick 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. The life cycle of Ehrlichia is not yet completely understood but it is thought that it occurs in three intracellular forms. The initial bodies are small spherical structures (1-2 micrometers in diameter) which are believed to develop into larger multiple membrane-bound units known as morulae. The morulae are inclusions within the cytoplasm of the leukocyte as seen in Figure 1. This morula is thought to then dissociate into small granules called elementary bodies.
After an incubation period of 8-20 days, the acute phase of infection occurs this lasts 2-4 weeks. At this time, the organism 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 and induce a vasculitis and subendothelial tissue infection. This subsequently leads to platelet consumption, sequestration, and destruction that results in the thrombocytopenia seen during this acute phase. Variable leukocyte counts and anemia may also develop progressively during this stage. After 6-9 weeks, dogs will either eliminate the parasite (if immunocompetent) or develop a parasitemia in which clinical signs absent to mild to severe. This stage is also characterized by variable persistence of thrombocytopenia, leukopenia, and anemia. Dogs that cannot mount an effective immune response will become chronically infected.
Symptoms of Ehrlichiosis
Ehrlichiosis can have three phases. Signs of the acute phase of the disease usually develop 1-3 weeks after the bite of the infected tick. The acute phase of the disease generally lasts 2-4 weeks. The Ehrlichia enter white blood cells and reproduce inside of them. In addition to the blood, these cells are found in the lymph nodes, spleen, liver, and bone marrow. Platelets, the small cell fragments that help blood to clot, are often destroyed, as well. As a result of the infection, the lymph nodes, liver, and spleen are often enlarged. Anemia, fever, depression, lethargy, loss of appetite, shortness of breath, joint pain and stiffness, and bruises are often seen. Many dogs will be able to fight off the infection. If not, they enter the subclinical phase.
In the subclinical phase the animal may appear normal or show only slight anemia. During this phase the Ehrlichia live inside the spleen. This phase can last for months or years. Ultimately, the dog either eliminates the Ehrlichia from the body or the infection may progress to the chronic phase.
The chronic phase can be either mild or severe. Weight loss, anemia, neurological signs, bleeding, inflammation of the eye, edema (fluid accumulation) in the hind legs, and fever may be seen. Blood tests show that one or all of the different blood cell types are decreased. One cell type, the lymphocyte may increase and be abnormal in appearance. This can sometimes be confused with certain types of leukemia. If a dog becomes chronically infected, the disease can keep coming back, especially during periods of stress. In some cases, arthritis or a kidney disease called ‘glomerluonephritis’ may develop.
A decrease in the number of platelets (platelets help the blood clot) in the blood is the most common laboratory finding in all phases of the disease. Changes in the protein levels in the blood are common. The most common protein, albumin, is decreased and other types of protein called ‘globulins’ are increased.
Since one tick could be infected with and transmit more than one disease (e.g.;haemobartonellosis or babesiosis), it is not all that uncommon to see a dog infected with more than one of these diseases at a time, which generally causes more severe symptoms.
Definitive diagnosis of CME requires visualization of morula within monocytes on cytology, detection of E. canis serum antibodies with the indirect immunofluorescence antibody test (IFA), polymerase chain reaction (PCR) amplification, and/or gel blotting (Western immunoblotting).
On cytology, ehrlichiae stain dark blue to purple with Romanowsky stain. The morulae are well-defined, round to oval, eosinophilic to basophilic bodies found in host membrane-lined vacuoles within the cytoplasm of the mononuclear cells.
In dogs experimentally infected with E. canis, the IFA test has detected serum antibodies as early as 7 days after initial infection, although some dogs do not become seropositive until 28 days post-infection. If ehrlichiosis is highly suspected clinically in a seronegative dog, serology should be repeated in 2-3 weeks. In the past, titers of IgG antibodies of >1:80 have been considered diagnostic,but the most recent research has indicated that titers <1:80 should be deemed suspect and serology should be repeated in 2-3 weeks or a PCR or Western immunoblotting should be considered. A diagnosis should be made and treatment instituted when clinical signs and clinicopathological abnormalities consistent with canine ehrlichiosis are found.
There are a few potential downfalls of using the IFA test for the diagnosis of E. canis infection. One major concern exists in endemic areas with dogs that are chronically infected and have a positive titer, but are otherwise healthy or show non-specific clinical signs. In these dogs, a positive antibody titer does indicate past exposure to E. canis, does not prove that ehrlichiosis is necessarily an active infection or the cause of the presenting clinical signs. In dogs with non-specific clinical signs, a repeat IFA test after 1 or 2 weeks may be beneficial to differentiate between primary E. canis infection and another secondary disease. Antibody titers to E. canis should increase with active infection. Furthermore, one must consider co-infection with multiple tick-borne diseases caused by agents such as otherEhrlichiae, Rickettsia species, Bartonella species, and Babesia canis. Disease caused by any of these agents may be clinically, hematologically, and serologically indistinguishable from each other. In addition, the immunodominant proteins of E. canis have been shown to serologically cross-react with those of E. chaffeensis (the agent that causes Human Monocytic Ehrlichiosis). Studies have shown that serologic testing by IFA could not consistently distinguish between infections of these two species. Interpretation of E. canis serology should include the consideration of the disease process, cross-reactivities with other ehrlichial species, the possibility of multiple tick-borne infections, and persistent IFA antibody titers post-treatment. Antibody titers are used to gauge the success or failure of treatment of CME. Treatment success should be based on remission of clinical signs, a decline in E. canis antibody titers and a concurrent decrease in gammaglobulin concentrations.
PCR amplification is also a sensitive method for the detection of acute E. canis although there are currently several potential limitations. It is recommended that this method be used in addition to serology for the initial diagnosis of ehrlichiosis, not instead of it.
The diagnosis of CGE differs from that of CME as E. ewingii has not yet been cultivated in an in vitro system; therefore antigens have not been available for comparative serological testing. Diagnosis of CGE requires visualization of morula within neutrophils in peripheral blood (Figure 2), joint effusions, and PCR or Western immunoblot. In a study using Western immunoblots, sera from dogs that were experimentally infected with E. ewingii were tested on E. canisantigens. Although there were no reactions with the dominant E. canis antigens, the sera produced binding patterns similar to those of anti-E. canis sera with high molecular proteins. This also may help with the diagnosis of CGE.
Treatment of Ehrlichiosis
The antibiotics, tetracycline or doxycycline are used. Treatment is usually for 3-4 weeks, even though the dog’s symptoms generally improve after several days of therapy. Some dogs will need blood transfusions or intravenous fluids depending on the severity of the disease. Generally, the prognosis during the acute phase is good, if the animal is properly treated. Dogs that go on to the chronic phase have a poorer prognosis. German Shepherds and Doberman Pinschers tend to have a more severe chronic form of the disease.
The drug, imidocarb dipropionate, is sometimes used in conjunction with the antibiotics. It is given as an injection, but may not be available in all areas.
Some of the damage caused by Ehrlichia may be due to the dog’s own immune response to the organism. For this reason, if immune-mediated arthritis or decrease in platelets occurs, corticosteroids (e.g., prednisolone) may be given.
Prevention of Ehrlichiosis
Tick control is the main way to prevent ehrlichiosis. Products which repel and kill ticks such as those containing permethrins are excellent choices. Tick collars containing the active ingredient amitraz (Preventic collars) are also used, sometimes in conjunction with permethrin products in those areas with high tick infestations. If a large number of cases of ehrlichiosis are diagnosed in an area, some veterinarians recommend placing dogs on low doses of tetracycline or doxycycline during the tick season. There is no vaccine for ehrlichiosis.
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Ehrlichia Research Laboratory, College of Veterinary Medicine, The Ohio State University, Columbus, OH