Canine Ehrlichiosis or Tropical Canine Pancytopenia

Filed Under: Dogs, Diseases

Canine Ehrlichiosis is also known as tropical canine pancytopenia, canine typhus, canine hemorrhagic fever, idiopathic hemorrhagic syndrome, Nairobi bleeding disorder, canine rickettsiosis, and tracker dog disease. Ehrlichiosis is caused by obligate intracellular rickettsial organisms that parasitize thrombocytes (the precursor of platelets) of the animal affected. Platelets are important in the function of blood clotting.

During the Vietnam War as many as 1,600 military working dogs were deployed in Vietnam at one time. Most dog handlers expected to take their assigned dog’s home with them when their tour of duty was over. Because Ehrlichia was endemic in Vietnam, the U.S. Department of the Army enforced strict criteria for determining which dogs were eligible to be returned to the States in order to prevent the introduction of Ehrlichiosis to the U.S. Under strict guidelines only 150 to 200 dogs were eligible for return to the U.S. The dogs remaining in Vietnam were transferred to the Vietnamese military or euthanized. Despite those precautionary measures, Ehrlichia has become a common tick transmitted disease in the U.S. and is now worldwide in distribution.

Three species of Ehrlichia may cause disease in the dog: Ehrlichia canis, E. ewingi, and E. chaffeensis. Of these Ehrlichia species, E. canis is the most commonly seen rickettsia in dogs and will cause the most severe clinical infections. Researchers in South Africa have obtained evidence that E. ruminantium, a type of Ehrlichia infecting cattle, may also be capable of causing disease in dogs and people. E. ruminantium is the causative agent of heartwater in cattle and is currently endemic in Africa. Restrictions preventing cattle importation from Africa has limited the spread of E. ruminantium to other parts of the globe. If E. ruminantium is now able to infect these additional species, the transport of dogs or people could lead to the introduction of heartwater into other parts of the world previously unaffected. The worldwide spread of heartwater would have serious consequences to global cattle production.

Ehrlichia canis infects both wild and domestic members of the canine family. The disease produced by E. canis is called canine monocytic ehrlichiosis because it infects certain white blood cells called monocytes or lymphocytes. E. ewingi and E. chaffeensis primarily affects people, although dogs may become infected, and it will specifically infect granulocytes (neutrophils and eosinophils) within the blood and is termed granulocytic ehrlichiae.

Ehrlichia canis is transmitted by Rhipicephalus sanguineus also known as the brown dog tick. The brown dog tick becomes infected after ingesting an infected blood meal containing the appropriate blood cell. The brown dog tick is a three-host tick, and once the tick is infected transstadial transmission occurs (any stage of the tick life-cycle from that day forward may transmit disease). E. chaffeensis and E. ewingi are transmitted by Amblyomma americanum, which is also known as the lone star tick.

The number of Ehrlichia infections appears to be more common in the southern U.S., where infestations of the brown dog tick are more commonly seen.

The incubation period for Ehrlichiosis is 8 to 20 days. The incubation period is followed by the acute phase of infection which lasts 2 to 4 weeks during which the organisms multiply and spread throughout the body, although spontaneous recovery may occur. During the acute phase non-specific clinical signs are seen which include fever, anorexia, weight loss, hemorrhagic discharge from the eyes or nose, splenomegaly (enlarged spleen), edema of the limbs and scrotum, and enlarged lymph nodes. Aspiration of the lymph nodes usually indicates hyperplasia (an activated lymph node).

Laboratory abnormalities during the acute period include anemia, thrombocytopenia (lack of thrombocytes and therefore a lack of platelets), and a variable leukocyte count. The acute phase resolves spontaneously, leading to the subclinical phase and will usually last 40 to 120 days. In the subclinical phase, the dog will appear clinically normal although laboratory abnormalities of thrombocytopenia and hyperglobulinemia (elevated amounts of globulin in the blood) may persist. Serum antibody levels rise 7 to 28 days after infection and continue to rise during the subclinical phase. The subclinical phase may last for years unless the dog is stressed or becomes immunosuppressed.

The chronic phase may be mild, chronic, or severe. In the mild chronic phase vague signs of illness reappear in which the dog will appear unthrifty, lethargic or have some loss of appetite.

The severe chronic phase is characterized by bleeding problems, vomiting, ataxia and shifting leg lameness. Joints may be swollen and painful. Bleeding problems include epistaxis (nose bleeds), melena (blood in the stool), petechia and ecchymoses (bleeding under the skin from pinpoint to larger). Additional bleeding disorders that can occur include hematuria (blood in the urine), hyphema (blood in the eye), hematemesis (bloody vomit), hemarthrosis (blood in the joints), retinal and cerebral hemorrhage (bleeding in the brain). Central Nervous System Signs may include ataxia, seizures, muscle twitching, hyperesthesia (increased sensitivity in the skin), and even mental stupor.

The most common laboratory signs are thrombocytopenia and nonregenerative anemia. The anemia is nonregenerative due to continued destruction of red blood cells (RBCs), chronic blood loss or decreased production of blood cells secondary to bone marrow hypoplasia. Pancytopenia (lack of all blood cells) is reported to be seen more often in German shepherd dogs. Hyperglobulinemia (increase in globulins) and an increase in serum proteins are common.

Infection with more than one species of Ehrlichia may occur. Dual infections with other tick-transmitted diseases such as Babesia and Anaplasmosis are not uncommon.

Diagnosis is made on the basis of clinical signs, characteristic laboratory findings, and increasing antibody levels. A positive titer is generally considered indicative of current infection. Most experimental infections have been shown to become seronegative 6 to 9 months following effective treatment. Reinfection may occur but is typically not as severe.

Idexx Laboratories has a Snap 3Dx® or Snap 4Dx® antibody test kit for the detection of antibody to Ehrlichia canis in whole blood, serum or plasma for use in veterinary clinics. The test does not detect antibody to various other types of Ehrlichia but will screen for Borrelia burgdorferi antibody which causes Lyme disease, heartworms, and with the Snap 4Dx Anaplasmosis as well.

Ehrlichia may be treated with tetracycline, oxytetracycline, doxycycline, minocycline, imidocarb dipropionate and chloramphenicol. Doxycycline or tetracycline is usually considered to be the treatment of choice. Enrofloxacin has been used and will suppress clinical signs of the disease but will not effectively clear the organism from the blood of infected animals. Treatment with antibiotics should be conducted for a minimum of 3 to 4 weeks and up to 8 weeks in chronic cases. Due to staining of the teeth from tetracyclines they should be avoided in young animals in which the teeth are still developing when possible. Blood transfusions should be given as needed to correct anemia and bleeding problems. Imidocarb dipropionate is given as two doses, two weeks apart and has variable effectiveness against babesia and Ehrlichia.

The key to prevention is tick control. To prevent infection effective measures should be used to prevent exposure to ticks: Acaricides such as fipronil or Frontline®, produced by Merial; amitraz used in a tick collar known as Preventic®, distributed by Virbac or available as a spot on as ProMeis®, available from Fort Dodge Animal Health; and permethrin, contained within the spot-on treatment K9 Advantix®, available from Bayer, are all generally effective against ticks when used according to label directions.

Blood donors should also be screened for Ehrlichia before using their blood for a blood transfusion.

Ehrlichia is a zoonotic infection (capable of infecting people). Dogs do not play a major role in the development of human infections. Tick infected areas do. E. chaffeensis and E. ewingi may cause meningoencephalitis, acute renal failure, and acute respiratory failure in man.

References:

Ettinger, Stephen and Edward Feldman. Textbook of Veterinary Internal Medicine. 6th Edition. Elsevier 2000. Vol.1. Pp. 632-636.

Evans, Rebecca, DVM. John R. Herbold DVM. Et al. “Causes for Discharge of Military Working Dogs from Service: 268 cases (2000-2004).” JAVMA, Vol 231, No.8, October 15, 2007. Pp. 1215-1220.

Kahn, Cynthia Editor. The Merck Veterinary Manual. 9th Edition. Pp. 638-641.

Ryan, William and Doug Carithers. “Reducing the Risk of Tick-Borne Diseases.” Clinician’s Brief, Supplement to NAVC Clinician’s Brief. November 2007. P. 2.

“Ticks are on the Move, Study Finds.” Veterinary Practice News. March 2008. P.13.

Woody, Benny DVM and Johnny Hoskins DVM. “Ehrlichial Diseases of Dogs”. The Veterinary Clinics of North America. Small Animal Practice. Vol. 21, No.1. January 1991. Pp. 75-97.

Topics: ticks

Symptoms: discharge, fever, loss of appetite, weight loss

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