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Basics ⬇

Outline

BASICS

Definition!!navigator!!

A tick-borne, noncontagious disease caused by infection of erythrocytes by either of 2 distinct protozoan parasites, Babesia caballi and Theileria equi.

Pathophysiology!!navigator!!

  • Infection with B. caballi or T. equi results in clinical signs referable to infection and lysis of erythrocytes; dual infections occur
  • The erythrocytic stage of T. equi can lyse erythrocytes in the absence of specific immune responses; however, the precise role of immune responses to parasite antigens of T. equi and B. caballi in anemia is not known
  • Occlusion of capillaries within the pulmonary, hepatic, and CNS occurs during acute infection with B. caballi
  • Those surviving acute infection become persistently infected and represent a problem for the international movement of horses due to several countries, including the USA, restricting entry of horses based on their serologic status to T. equi and B. caballi
  • Intrauterine transmission appears to occur with T. equi but is rare with B. caballi infections; abortions due to fetal infections have been reported for both parasites

Systems Affected!!navigator!!

  • Hemic/lymphatic/immune—lysis of infected erythrocytes leads to anemia and icterus
  • Nervous, hepatobiliary, respiratory—occlusion of capillaries by B. caballi can lead to dysfunction within these organ systems

Genetics!!navigator!!

N/A

Incidence/Prevalence!!navigator!!

  • Infection and clinical disease occur when susceptible horses move into endemic areas or persistently infected horses move into a nonendemic area with tick vectors capable of transmission. Compounding concerns about movement of persistently infected horses is the lack of knowledge about the ability of tick species in nonendemic areas to transmit T. equi and B. caballi
  • According to the OIE, disease and persistent infection have been reported in southern Florida, Texas, Asia, Africa, South America, Central America, the Middle East, and southern Europe. Some countries, however, refrain from reporting identified cases. Therefore, the actual global prevalence is unclear
  • Tick vectors include species of Amblyomma, Dermacentor, Haemaphysalis, Hyalomma, and Rhipicephalus

Signalment!!navigator!!

  • Horses, donkeys, their cross-breeds, and zebras are susceptible to piroplasmosis
  • No known breed, age, or sex predilections

Signs!!navigator!!

General Comments

  • Clinical signs depend on the immune status of the horse
  • Horses that survive acute infection are immune to clinical disease on reinfection; however, an exception may be those infected with B. caballi and treated by chemotherapy
  • In endemic areas, clinical piroplasmosis seldom is seen, except when nonimmune (i.e. uninfected) horses are introduced

Historical Findings

  • Acute disease—lethargy, anorexia, fever, anemia, petechial hemorrhages of mucous membranes, and icterus
  • Hemoglobinuria can be observed in more severe cases of T. equi infection
  • Exercise intolerance (related to the degree of anemia) is common

Physical Examination Findings

  • Signs are common only during the acute phase of infection
  • During acute B. caballi infection, high fever, lethargy, hyperemia of mucous membranes with petechial hemorrhages, ventral edema, constipation, colic, dehydration, and icterus are seen
  • Acute T. equi infection is similar, with hemoglobinuria and a more pronounced icterus

Causes!!navigator!!

  • Infection of erythrocytes with the hemoprotozoan parasites B. caballi or T. equi
  • Anemia—result of hemolysis caused by replication of the erythrocyte-stage parasites
  • B. caballi sequesters in the capillaries of organ systems, including the CNS, leading to occlusion of blood flow

Risk Factors!!navigator!!

The primary risk factor is movement of uninfected (i.e. nonimmune) horses into endemic areas.

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Equine infectious anemia—infected horses are seropositive
  • Purpura haemorrhagica—petechial hemorrhages and ventral edema are common; often a history of previous exposure to Streptococcus equi or other respiratory pathogens; hemolysis is uncommon
  • Equine viral arteritis virus—hemolysis is uncommon; diagnosis can be confirmed serologically or by viral isolation
  • Equine ehrlichiosis—hemolysis is uncommon
  • Trypanosomiasis
  • Leptospirosis
  • Red maple-leaf poisoning—Heinz bodies and methemoglobinemia are common

CBC/Biochemistry/Urinalysis!!navigator!!

  • Anemia
  • Thrombocytopenia
  • Leukocytosis
  • Hyperbilirubinemia
  • Hypophosphatemia
  • Hemoglobinuria (T. equi)

Other Laboratory Tests!!navigator!!

  • Definitive diagnosis currently depends on identification of Babesia or Theileria organisms in Giemsa-stained blood smears or serologic tests (cELISA and IFAT). PCR assays have been utilized for detection of both parasites within laboratory settings
  • Direct parasitologic verification of chronic B. caballi infection is almost impossible but occasionally is successful with chronic T. equi infection. The USDA in 2005 adopted cELISA as the official serologic test for equine piroplasmosis; this test measures antibodies to an erythrocyte stage protein epitope of T. equi or B. caballi. Some countries also accept the IFAT for the international movement of horses
  • Horses that test positive on cELISA are restricted from entry into the USA. Serum submitted to state diagnostic laboratories is forwarded to the National Veterinary Services Laboratory (Ames, IA) for testing
  • T. equi and B. caballi can now be routinely cultured

Imaging!!navigator!!

N/A

Other Diagnostic Procedures!!navigator!!

N/A

Pathologic Findings!!navigator!!

  • Horses that die of acute infection may demonstrate subcutaneous edema, serous exudates in the body cavities and pericardium, pronounced icterus, hepatomegaly, splenomegaly, glomerulonephropathy, and petechial hemorrhages of mucosal membranes
  • Histologically, the spleen contains macrophages with intracellular erythrocytes (erythrophagocytosis), centrilobular necrosis of the liver, microthrombi within the liver and lungs, degeneration of the renal tubular epithelium, and hemoglobin casts in the renal tubules

Treatment ⬆ ⬇

Outline

TREATMENT

Appropriate Health Care!!navigator!!

Inpatient or outpatient, depending on the severity of clinical signs.

Nursing Care!!navigator!!

Routine care; intensive care usually not needed.

Activity!!navigator!!

Restrict activity.

Diet!!navigator!!

Normal diet.

Client Education!!navigator!!

Inform clients of the reportable nature of this infection and its significance regarding the international movement of horses.

Surgical Considerations!!navigator!!

N/A

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Imidocarb dipropionate is the most effective and safest chemotherapy to date. Its ability to clear B. caballi persistent infection has been demonstrated and is effective in complete elimination of T. equi persistent infection in most cases. Recommended dosing of imidocarb dipropionate for complete chemotherapeutic clearance of either organism is 4 mg/kg every third day for a total of 4 treatments. Some horses may require more than 1 treatment regimen to achieve chemosterilization. Each dose is given IM and divided among at least 4 injection sites
  • Colic, agitation, sweating, transient salivation, and diarrhea are common after imidocarb treatment
  • Single doses of either glycopyrrolate (glycopyrronium) at 0.0025 mg/kg or atropine sulfate at 0.01 mg/kg have been administered IV to prevent the gastrointestinal symptoms associated with imidocarb treatment. Both, however, have potential negative side effects. Therefore, N-butylscopolamine is often used to mitigate the anticholinesterase activity of imidocarb dipropionate
  • When treating T. equi-infected horses with imidocarb dipropionate, do not initiate retreatment for at least 30 days after the first treatment

Contraindications!!navigator!!

  • In endemic regions, antibabesial therapy may lead to susceptibility on reinfection. This is especially true for B. caballi infections, in which chemotherapy (imidocarb dipropionate) clears persistent infections
  • Donkeys appear very susceptible to the toxic side effects of imidocarb dipropionate and should not be treated with this drug

Precautions!!navigator!!

N/A

Possible Interactions!!navigator!!

N/A

Alternative Drugs!!navigator!!

  • Several chemotherapies with antibabesial activity have been tested; these therapies include phenamidine, benenil, and diampron
  • For T. equi, parvaquone and buparvaquone have been tested
  • Imidocarb dipropionate is the most effective and safest of all therapies tested to date

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

Monitor hydration status and percentage parasitemia in the peripheral blood.

Prevention/Avoidance!!navigator!!

Control in endemic areas is most effectively directed at tick vectors.

Possible Complications!!navigator!!

N/A

Expected Course and Prognosis!!navigator!!

  • Horses that survive acute infection usually, with appropriate supportive care, can return to normal activity. Owners should be advised that such animals are persistently infected and remain a potential source of parasite transmission to susceptible horses
  • Although chemotherapy eliminates some strains of B. caballi and T. equi infection, chemotherapeutic clearance of horses dually infected with both organisms may not be achievable with currently available medications

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

N/A

Age-Related Factors!!navigator!!

N/A

Zoonotic Potential!!navigator!!

N/A

Pregnancy/Fertility/Breeding!!navigator!!

Abortion (especially with T. equi infection) is a possible outcome.

Synonyms!!navigator!!

Babesiosis

Abbreviations!!navigator!!

  • cELISA = competitive–enzyme-linked immunosorbent assay
  • CNS = central nervous system
  • IFAT = immunofluorescence antibody test
  • OIE = Office International Epizooties/World Organisation for Animal Health
  • PCR = polymerase chain reaction
  • USDA = United States Department of Agriculture

Suggested Reading ⬆ ⬇

Friedhoff KT. The piroplasms of equidae—significance for international commerce. Berl Munch Tierarztl Wochenschr 1982;95:368–374.

Holman PJ, Chieves L, Frerichs WM, et al. Culture confirmation of the carrier status of Babesia caballi—infected horses. J Clin Microbiol 1993;31:698–701.

Knowles DP. Control of Babesia equi parasitemia. Parasitol Today 1996;12:195–198.

Knowles DP. Equine babesiosis (piroplasmosis): a problem in the international movement of horses. Br Vet J 1996;152:123–126.

Wise LN, Kappmeyer LS, Mealey RH, Knowles DP. Review of equine piroplasmosis. J Vet Intern Med 2013;27:1334–1346.

Author(s) ⬆

Authors: Kelly P. Sears and Don Knowles

Consulting Editor: Ashley G. Boyle