Has your dog suddenly developed an anemia or lameness? Have you had a problem controlling ticks this summer? Then your pet may be suffering from an infection called anaplasmosis.
Anaplasmosis in dogs is caused by a rickettsial infection with Anaplasma phagocytophilum or Anaplasma platys, which are both obligate intracellular bacteria. Both of these bacteria cause a separate disease syndrome. A. phagocytophilum was previously known as Ehrlichia equi, and A. platys was previously classified as Ehrlichia platys. Both of these bacterial species have been found to cause disease worldwide, and are endemic in the Midwest, East, and Northeastern and western coastal regions of the United States, as well as in Europe and South America. In the United States the disease is seasonal in occurrence with the majority of clinical cases coinciding with seasonal increases in tick populations.
A. phagocytophilum may cause disease in dogs, horses, ruminants, white-tailed deer, and humans, as well as several species of small rodents. The primary reservoir for the organism in nature is the white-footed mouse, or Peromyscus leucopus.
Ixodes scapularis, also known as the deer tick, serves as the primary vector for A. phagocytophilum in the Midwest and eastern portions of the United States, while I. pacificus ticks, also known as the western blacklegged tick, are the primary ticks involved in canine anaplasmosis transmission in the western portions of the U.S. The lone star tick, or Amblyomma americanum, can also transmit anaplasmosis. In Europe, I. ricinus is the primary tick involved in transmission.
Disease transmission to a mammalian host is believed to require prolonged contact with the tick, around 24 hours or more. The organism infects granulocytes (a type of white blood cell). A. phagocytophilum binds to a cell surface protein of the granulocyte and is taken into the cytoplasm where it resides within cell vacuoles.
Disease in the dog may be seen as an acute (disease develops quickly) or chronic (disease develops slowly), subclinical (no clinical signs of infection) and persistent infection. Clinical disease is most commonly seen with the acute infection, which typically lasts from one to several days.
Common clinical signs of canine anaplasmosis mimic those seen with Lyme disease, another tick-transmitted disease, and include high fever, depression, anorexia, lethargy, and inflammation of multiple joints (polyarthritis). Neurologic signs including ataxic seizures and neck pain may also be observed. Observed less commonly are GI symptoms such as vomiting and diarrhea, or respiratory clinical signs such as coughing and labored breathing. The most consistent finding is the presence of an immune-mediated thrombocytopenia (lack of thrombocytes involved in blood clotting), often in association with an immune-mediated hemolytic anemia.
Clinical anaplasmosis can be difficult to distinguish from Lyme disease.
Most laboratory test abnormalities will be observed only in the acute phase of the infection where it is possible to observe platelet dysfunction or thrombocytopenia (low platelet counts), which usually occurs due to immune-mediated destruction of platelets as well as a degree of myelosuppression (decrease in platelet production). Despite the platelet dysfunction dogs rarely exhibit any bleeding problems with A. phagocytophilum infections. A mild to moderate nonregenerative anemia (low red blood cell levels are not being replaced) is occasionally exhibited. Lymphopenia (lack of lymphocytes) also occurs characteristically on hematology.
Hypoalbuminemia (low levels of albumin in the blood) is the most common serum abnormality. An increase in serum alkaline phosphatase (a liver enzyme) and amylase (a pancreatic enzyme) activity may be demonstrated on serum chemistry analysis.
Chronic or subclinical disease typically results in a mild, flu-like disease that is self-limiting in people or animals.
The SNAP 4Dx® test from Idexx Laboratories can be used to test for antibodies to A. phagocytophilum and is easily conducted at most veterinary offices. The 4Dx® test is professed to be 99.5% effective as early as eight days after infection. Alternative methods of confirmation include demonstrating the presence of Rickettsial inclusion bodies within neutrophils (a type of white blood cell) on blood smears. An IFA test, available at regional laboratories that will demonstrate seroconversion as soon as two to five days following infection.
Treatment with doxycycline twice daily or tetracycline three times daily typically results in a favorable prognosis. Marked improvement usually occurs within 24 to 48 hours of therapy. Chronically infected dogs typically appear clinically healthy, and since there is no effective therapeutic regimen for clearance of the organism from an infected individual, treating an apparently healthy appearing pet is of questionable benefit.
Duel infections with other tick born diseases are not uncommon. Dogs infected concurrently with B. burgdorferi, or Lyme disease are nearly two times more likely to develop clinical disease than those infected with anaplasmosis alone. The administration of corticosteroids may cause the reappearance of a bacteremia with anaplasmosis, although most of these animals remain clinically normal.
A. phagocytophilum is not a zoonotic infection. People typically obtain infections from the same tick population infecting their dogs by frequenting the same geographical areas. To date there has never been a documented case of a direct anaplasmosis transmission between a dog and a human.
Anaplasma platys is also worldwide in distribution and is the only intracellular infectious agent capable of infecting the platelets of people and animals.
The ticks involved in the transmission of A. platys include the Rhipicephalus and Dermacentor genus. Dogs are the most commonly infected species, although A. platys infections have been reported in cats, impalas, and sheep.
Clinical disease with A. platys may be mild and inapparent to severe. The most commonly seen clinical signs include a fever, pale mucous membranes, petechial hemorrhages (small skin and gum hemorrhages), epistaxis (nose bleeds), and lymphadenopathy (enlarged lymph nodes). These animals appear lethargic, and when seen with dual infections will have more severe clinical manifestations.
The appearance of clinical signs typically occurs eight to fourteen days following infection. The organism causes a thrombocytopenia which may result in bleeding. Typically, platelet counts will rebound one to two days following the onset of clinical disease, reaching near-normal levels within three to four days. This cycle will often be repeated on a one-to two-week intervals resulting in a cyclic thrombocytopenia. The severity of the thrombocytopenia tends to decrease with each subsequent episode.
Although there appears to be a great deal of cross-reactivity between A. platys and A. phagocytophilum on the SNAP 4Dx® test from Idexx, microscopic identification of the organism within circulating platelets or an IFA test may be necessary to confirm the diagnosis.
Doxycycline is also effective in the treatment of an infection with A. platys. In severe cases of bleeding diathesis a blood transfusion may become necessary. Cases not responding rapidly to treatment should be tested for additional tick-borne infections.
Prevention requires strict tick control and careful screening of blood used for transfusions.
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