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Basics ⬇

Outline

BASICS

Definition!!navigator!!

Acute disease of horses characterized by immune-mediated vasculitis, edema of the head and limbs, petechial and ecchymotic hemorrhages in mucosae, musculature and viscera, and sometimes glomerulonephritis.

Pathophysiology!!navigator!!

  • PH is an immune-mediated vasculitis associated with a type III hypersensitivity reaction
  • Soluble immune complexes (mostly composed of IgA and SeM) are formed in moderate antigen excess
  • Circulating complexes become deposited in the walls of small blood vessels with subsequent fixation and activation of complement to C5a, a potent chemotactic factor for neutrophils
  • Release of proteolytic enzymes from localized neutrophils directly damages vessel walls, resulting in edema, hemorrhage, thrombosis, and ischemic changes in tissues
  • Blood vessels of the skin are primarily affected
  • Hydrostatic pressures within vessels in dependent areas of the body result in distribution of lesions in lower limbs and ventral abdomen

Systems Affected!!navigator!!

Hemic/lymphatic/immune/cardiovascular system/skin.

Genetics!!navigator!!

N/A

Incidence/Prevalence!!navigator!!

  • Sporadic
  • Reported incidence subsequent to Streptococcus equi ssp. equi varies from 0.5% to 5%
  • Highest incidence reported in outbreaks of strangles, possibly because of reinfection of horses already sensitized by previous infection(s)

Geographic Distribution!!navigator!!

N/A

Signalment!!navigator!!

There is no breed, sex, or age predilection.

Signs!!navigator!!

General Comments

Clinical signs vary from a mild, transient reaction to a severe and fatal disease.

Historical Findings

  • Exposure to, or infection with, respiratory pathogens frequently reported within previous 2–4 weeks
  • Vaccination may precede disease

Physical Examination Findings

  • Common signs include fever, depression, tachycardia, and tachypnea; reluctance to move; and reduced or absent appetite
  • Subcutaneous edema is an early sign, with swelling beginning around the nostrils and muzzle then progressing to the entire head, distal limbs, and ventral abdomen
  • Edema initially appears as well-demarcated areas of hot, sensitive swellings, which become cold and painless over time and merge into normal tissue without a line of demarcation
  • Swellings can develop suddenly or gradually, are usually asymmetric and non-pruritic, and pit with gentle pressure
  • Hyperemia and petechial and ecchymotic hemorrhages (purpura) may be observed in light-skinned areas and on mucous membranes
  • Edema and hemorrhage may progress to skin infarction, necrosis, and exudation with large, distended areas oozing red-tinged serum. Skin of the distal limbs is most commonly and severely affected. These sites may slough and leave granulating wounds
  • Mucosal surfaces may also ulcerate and slough
  • If the larynx is involved, dysphagia and dyspnea may occur owing to swelling and pain
  • Edema, hemorrhage, and necrosis can occur in other body systems resulting in lameness, colic, epistaxis, dyspnea, and ataxia
  • Subclinical renal disease is common
  • Secondary complications such as laminitis, thrombophlebitis, and localized infections are common
  • Death may ensue as a result of pneumonia, cardiac arrhythmias, renal failure, or GI disorders
  • An uncommon, more severe manifestation of PH (infarctive PH) is characterized by infarction of multiple tissues including the GI tract and muscle. Affected horses have colic and muscle swelling. The course of disease is 3–5 days, with death a common outcome due to severe colic and rapidly deteriorating metabolic status

Causes!!navigator!!

  • Antigens can be derived from infectious agents (bacteria, viruses) or drugs
  • In 1 study, approximately 32% of horses had been infected with, or exposed to, S. equi ssp. equi; 9% had been vaccinated with SeM; 17% had been infected with Corynebacterium pseudotuberculosis; and 9% had a history of apparently infectious respiratory disease of unknown cause. A further 28% of horses had no history of recent infectious diseases
  • Equine influenza virus, equine herpesviruses, equine viral arteritis, S. equi ssp. zooepidemicus, Rhodococcus equi, and prolonged drug administration have also been implicated
  • Horses vaccinated excessively with modified live, killed whole-cell, or M protein-based S. equi ssp. equi vaccines may develop PH owing to high concentrations of antibody and antigen
  • No inciting cause may be found in some cases (idiopathic vasculitis)

Risk Factors!!navigator!!

  • Role of vaccination in development of PH is contentious
  • Streptococcal vaccines have not definitively been shown to be a risk factor for development of PH, but some authors recommend that horses with high serum antibody titers to streptococcal M protein should not be vaccinated for strangles
  • Also not clear whether infected horses should be vaccinated during an outbreak of strangles owing to the potential association with development of PH

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

Immune-mediated vasculitis must be differentiated from vasculitis directly caused by infectious agents, immune-mediated thrombocytopenia, and other causes of edema or petechial and ecchymotic hemorrhages.

CBC/Biochemistry/Urinalysis!!navigator!!

  • No characteristic abnormalities are associated with PH
  • Length of illness, organ involvement, and secondary complications will influence changes observed
  • Neutrophilic leukocytosis, hyperfibrinogenemia, hyperglobulinemia, and mild anemia are commonly observed due to chronic inflammation
  • Moderate anemia occurs in some cases owing to increased red blood cell destruction
  • Creatinine may be elevated and urinalysis may show traces of hematuria and/or proteinuria due to glomerulonephritis
  • Elevations in creatine kinase and aspartate aminotransferase activities may be present owing to muscle lesions
  • Horses with infarctive PH have marked elevations in muscle enzymes and neutrophilia, and in severely affected horses there is evidence of a consumptive coagulopathy, which will result in thrombocytopenia and decreased prothrombin and activated partial thromboplastin time

Other Laboratory Tests!!navigator!!

Elevated IgA titers to S. equi ssp. equi and IgG–SeM titers >1:6400 are suggestive of PH.

Imaging!!navigator!!

N/A

Other Diagnostic Procedures!!navigator!!

  • Diagnosis often based on history, clinical signs, exclusion of other causes of vasculitis, and response to therapy
  • Documentation of leukoclastic vasculitis and/or presence of immune complexes support the diagnosis
  • Full-thickness skin punch biopsies (≥6 mm diameter) should be obtained and preserved in 10% formalin and Michel's transport medium
  • Multiple biopsies of early lesions (<24 h) are required as distribution of lesions is patchy and lesions >24 h may be nondiagnostic

Pathologic Findings!!navigator!!

  • Subcutaneous edema and numerous, discrete, petechial and ecchymotic hemorrhages are seen in affected tissues
  • Leukoclastic vasculitis (neutrophilic infiltration of small blood vessels with nuclear debris around involved vessels and fibrinoid necrosis) may be observed in the skin, lung, muscle, and GI tract
  • Horses with infarctive PH have dark red to black, multifocal coalescing hemorrhages in skeletal muscles, lungs, and GI tract. Histologic examination reveals coagulative necrosis

Treatment ⬆ ⬇

Outline

TREATMENT

Aims!!navigator!!

Treatment involves removal of the inciting cause, reduction of inflammation, reduction of the immune response, and provision of supportive care.

Appropriate Health Care!!navigator!!

Administration of any drugs should be discontinued as PH could be caused by an adverse drug reaction.

Nursing Care!!navigator!!

  • Horses with PH usually require immediate, aggressive nursing care
  • Hydrotherapy can minimize edema, and pressure wraps are used to decrease limb swelling
  • Isotonic fluid administration (IV or enteral) may be required to maintain hydration in animals with severe depression or dysphagia
  • Sloughed areas of skin should receive topical wound therapy

Activity!!navigator!!

Limited hand-walking may increase peripheral circulation, especially in cases with significant edema of the limbs.

Diet!!navigator!!

Swelling of the head and pharynx may necessitate placement of a nasogastric feeding tube to permit enteral feeding of dysphagic horses.

Client Education!!navigator!!

N/A

Surgical Considerations!!navigator!!

  • Any accessible, mature abscesses should be drained
  • Emergency tracheotomy may be required to relieve respiratory distress and prevent asphyxiation

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Corticosteroid use remains controversial and mild cases may resolve without immunosuppressive therapy
  • Horses with life-threatening edema or organ dysfunction require early, aggressive corticosteroid treatment
  • Dexamethasone (0.05–0.2 mg/kg IV SID) is initially administered until edema and inflammation are reduced
  • Prednisolone (0.5–2.0 mg/kg PO BID–SID) can be used as a maintenance dose after initial dexamethasone treatment
  • Once edema starts to resolve, the dosage of corticosteroids can be reduced (10–15% every 1–2 days) over 7–21 days while carefully monitoring for recurrence
  • Some cases require >4–6 weeks of corticosteroid therapy, and relapse may occur if therapy is truncated or if dexamethasone is switched to prednisolone too early (<10 days)
  • For horses suffering a relapse, the corticosteroid dose may need to be increased over the previously efficacious concentrations
  • Antimicrobials should be used with corticosteroids to reduce the occurrence and severity of septic sequelae and to address suspected streptococcal infection. Penicillin should be given for 1–3 weeks or until clinical signs resolve
  • Flunixin meglumine or phenylbutazone may help reduce vascular inflammation and edema and provide analgesia

Contraindications!!navigator!!

None

Precautions!!navigator!!

High-dose corticosteroid therapy may be associated with laminitis or secondary infections.

Possible Interactions!!navigator!!

None

Alternative Drugs!!navigator!!

None

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

Glomerulonephritis can progress to chronic renal failure. Creatinine and urine protein should be monitored weekly.

Prevention/Avoidance!!navigator!!

  • Control of strangles reduces incidence
  • Caution should be given to the use of streptococcal vaccines in horses at low risk of developing strangles
  • Horses with M protein antibody titers >1:32 000 should not be vaccinated

Possible Complications!!navigator!!

  • Skin sloughing may be followed by exuberant granulation tissue
  • Laminitis and various infections such as cellulitis, pneumonia, colitis, and thrombophlebitis may occur due to long-term corticosteroid therapy

Expected Course and Prognosis!!navigator!!

  • Most horses recover with early aggressive therapy and supportive care within 2–4 weeks; sequelae may prolong convalescence
  • Prognosis depends on severity and extent of disease
  • Extensive skin sloughing, evidence of internal organ involvement, development of secondary septic processes, or laminitis are poor prognostic indicators

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

N/A

Age-Related Factors!!navigator!!

N/A

Zoonotic Potential!!navigator!!

None

Pregnancy/Fertility/Breeding!!navigator!!

N/A

Synonyms!!navigator!!

None

Abbreviations!!navigator!!

  • GI = gastrointerstinal tract
  • Ig = immunoglobulin
  • PH = purpura haemorrhagica
  • SeM = S. equi ssp. equi M protein

Suggested Reading ⬆ ⬇

Hunyadi LM, Pusterla N. Purpura hemorrhagica. In: Felippe MJB, ed. Equine Clinical Immunology. Ames, IA: Wiley Blackwell, 2016:31–38.

Author(s) ⬆

Author: Jennifer L. Hodgson

Consulting Editors: David Hodgson, Harold C. McKenzie, and Jennifer L. Hodgson