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Basics ⬇

Outline

BASICS

Definition!!navigator!!

An important cause of pneumonia in foals <6 months of age. Infection may also result in diarrhea, joint sepsis, intra-abdominal abscessation, and multifocal abscesses throughout the body. A proposed nomenclature change to Prescottella equi is currently under debate.

Pathophysiology!!navigator!!

  • Gram-positive pleomorphic intracellular facultative organism that normally inhabits soil
  • Inhalation of dust containing the organism thought to be the primary route of exposure for both the horse and humans
  • Then resides within the alveolar macrophages, replicates, and can produce a severe, potentially life-threatening pyogranulomatous bronchopneumonia as necrosis and destruction of lung parenchyma occur
  • Intestinal forms of the disease include ulcerative colitis and abdominal lymphadenitis. Associated GI infections likely arise from infected foals swallowing sputum containing the organism. Peyer's patches become infected and ulcerated significant mesenteric lymphadenitis can occur
  • May disseminate to other body sites and produce septic arthritis, serositis, vertebral body abscesses, and cutaneous ulcerative lymphangitis
  • Other extrathoracic manifestations—immune-mediated polysynovitis, uveitis/keratouveitis, immune-mediated hemolytic anemia, immune-mediated thrombocytopenia, hyperthermia associated with macrolide treatments, hyperlipemia, and telogen effluvium

Systems Affected!!navigator!!

  • Respiratory
  • GI
  • Musculoskeletal
  • Hemic/lymphatic/immune
  • Ophthalmic
  • Renal
  • Skin
  • Hepatobiliary
  • Nervous

Signalment!!navigator!!

  • Foals 1–6 months of age, most show clinical signs before 4 months
  • Immunocompromised adults or adults with concurrent illness

Signs!!navigator!!

  • Fever, cough, lethargy, depression, anorexia, poor weight gain, exercise intolerance, diarrhea, respiratory distress, joint distention, and sudden death
  • May have abnormal thoracic auscultation and percussion findings, although severely affected foals may not have auscultable abnormalities

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Other causes of pneumonia (Streptococcus equi ssp. equi, S. equi ssp. zooepidemicus, parasite migration, and viral respiratory infections)
  • Equine herpesvirus 2 infection may predispose foals
  • Definitive diagnosis is based on culture of Rhodococcus equi although PCR techniques may be useful

CBC/Biochemistry/Urinalysis!!navigator!!

  • CBC—leukocytosis with a mature neutrophilia
  • Increased fibrinogen and SAA
  • Increased serum protein
  • Severe disease—anemia and thrombocytopenia
  • With diarrhea—may have electrolyte abnormalities (hyponatremia and hypochloremia)
  • With dehydration—increased creatinine and blood urea nitrogen
  • Renal and/or urinary tract involvement—abnormal urinalysis

Other Laboratory Tests!!navigator!!

  • PCR testing of tracheal fluid, bronchoalveolar lavage fluid, and aspirates from other sites for the presence of the VapA antigen, which is associated with virulence
  • Testing for antibody directed against the VapA antigen is currently undergoing testing

Imaging!!navigator!!

Ultrasonography

Consolidation of lung parenchyma; pulmonary and intra-abdominal abscessation. Deep lesions will not be recognized.

Radiography

Thorax—increased interstitial density with dense patchy areas of alveolar pattern. Areas of consolidation and abscessation. Useful in monitoring response to therapy and determining the severity.

Other Diagnostic Procedures!!navigator!!

TTA

  • Cytology—Gram-positive to Gram-variable pleomorphic (“Chinese character”) intracellular rods
  • Culture—positive R. equi
  • PCR—R. equi and VapA antigen positive

Bronchoalveolar Lavage

  • Results similar to TTA
  • May recognize concurrent infection with Pneumocystis carinii, and requires specialized stain (silver stain)
  • Caution—performing either TTA or bronchoalveolar lavage may be detrimental to a foal with significant respiratory disease and those in respiratory distress

Pathologic Findings!!navigator!!

  • Related to the organ system involved
  • Bilateral bronchopneumonia with severe coalescing abscess formation. Formation may be minimal in cases with associated acute respiratory distress syndrome. Ventral lung field involvement generally more severe. Abscesses range from a few millimeters to more than 10 cm. Generalized miliary abscess formation also common
  • Pulmonary parenchyma surrounding pulmonary abscesses usually congested or consolidated
  • Bronchial and mediastinal lymphadenopathy with abscessation
  • Pleural empyema
  • Pleural inflammation unusual unless empyema secondary to abscess rupture occurred
  • GI lesions variable and may involve the entire GI tract. Mucosal villous atrophy, mucosal necrosis, diphtheritic membrane formation, ulcerative enterocolitis, and mesenteric lymphadenopathy with abscess formation
  • Pulmonary histology—pyogranulomatous abscesses have a necrotic central core with surrounding degenerate neutrophils. Adjacent areas infiltrated with macrophages, lymphocytes, and occasional giant cells. Congestion, edema, and alveolar infiltration by macrophages and neutrophils, acute suppurative bronchitis, and peribronchitis
  • GI histology—infiltration of phagocytic cells into the lamina propria. Necrosis of the villi and submucosa, mucosal ulceration, and pyogranulomatous lymphadenitis of the mesenteric or colonic lymph nodes
  • Organisms identified using hematoxylin–eosin stain and/or Gram stains. Some organisms are acid fast
  • Reports of concurrent infection with P. carinii identified via silver stains

Treatment ⬆ ⬇

Outline

TREATMENT

Appropriate Health Care!!navigator!!

  • Affected foals may be treated at the farm
  • Severely affected foals benefit from treatment at a referral facility with climate-controlled environments and oxygen supplementation
  • Transport during cool times of the day and minimize stress
  • Should not transfer from an endemic farm to a farm with no previous history

Nursing Care!!navigator!!

  • Minimize stress
  • Climate-controlled environments, air conditioning, and good ventilation may improve the short-term prognosis with severely affected foals

Activity!!navigator!!

Exercise should be restricted. Stall confinement is not necessary as long as turnout is in a small area only. Affected foals should be completely restricted from exercise during the hot periods of the day.

Diet!!navigator!!

Parenteral nutrition for severely affected foals experiencing weight loss and anorexia.

Client Education!!navigator!!

  • R. equi probably infects all horse farms to some degree. The difference in disease appearance is related to differences in environments, management techniques, and virulence of the isolate. On enzootic farms, infections result in huge economic losses associated with costs of prevention, treatment, and the death of some foals
  • Most foals treated for R. equi infection recover. Severely affected foals are less likely to survive
  • Severely immunocompromised humans have been diagnosed with R. equi infection

Surgical Considerations!!navigator!!

Surgical drainage of easily accessible abscesses may be reasonable. Surgical removal of abdominal abscesses is unlikely to be rewarding.

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Erythromycin (10–37.5 mg/kg BID–QID PO) or rifampin (rifampicin) (5–10 mg/kg SID–BID PO) is the traditional standard therapy. Other macrolide antimicrobials azithromycin (10 mg/kg PO SID) and clarithromycin (7.5 mg/kg PO BID) are more commonly used today, have been efficacious, and may have reduced side effects
  • Hyperthermia observed with macrolide therapy may be caused by decreased ability to sweat in affected foals, secondary to the treatment
  • Treat until CBC, fibrinogen/SAA, and clinical presentation are normal and pneumonia resolved radiographically

Contraindications, Precautions, Possible Interactions, Alternative Drugs!!navigator!!

  • Some foals receiving the above-mentioned drug combinations may develop severe diarrhea. Decreasing dose may resolve the problem
  • Mares housed with foals receiving erythromycin have developed severe fatal colitis, thought to be associated with Clostridium difficile infection secondary to ingestion of small amounts of erythromycin from the foal
  • Idiosyncratic hyperthermia and tachypnea have been reported in foals receiving macrolides
  • Aminophylline should not be used in combination with erythromycin owing to potential toxicity
  • Foals diagnosed very early in the clinical course of infection may respond to trimethoprim–sulfa combinations, although this is not recommended
  • Rifampin-resistant strains of R. equi have been identified. Rifampin should never be used alone owing to the rapidity of development of resistance

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

Response to therapy can be monitored by resolution of clinical signs, normalization of CBC and fibrinogen, and radiographic improvement.

Prevention/Avoidance!!navigator!!

  • Several strategies for prevention of R. equi infection exist. Decreasing the size of infective challenge by good housing and management practices and isolation of affected foals is important
  • Early recognition of infection is important. This can be facilitated by daily temperature monitoring of foals, frequent routine physical examinations, and frequent thoracic ultrasonographic examinations
  • Passive immunization by the IV administration of R. equi hyperimmune plasma has been used as a preventative technique, but its efficacy has been questioned. Timing of this treatment, if effective, is purportedly important and depends on expected exposure; it is typically administered during the first week of life and again at 30 days of age at many facilities
  • To date, no active immunization protocol has been effective. However, experimental vaccination of mares with PNAG and the use of PNAG hyperimmune plasma appear promising for the prevention of R. equi infection in foals

Expected Course and Prognosis!!navigator!!

In well-established pulmonary cases, therapy may extend over 3–5 weeks.

Miscellaneous ⬆ ⬇

MISCELLANEOUS

Abbreviations

  • GI = gastrointestinal
  • PCR = polymerase chain reaction
  • PNAG = poly-N-acetyl glucosamine
  • SAA = serum amyloid A
  • TTA = transtracheal aspirate

Suggested Reading ⬆ ⬇

Cohen ND. Rhodococcus equi foal pneumonia. Vet Clin North Am Equine Pract 2014;30:609–622.

Cywes-Bentley C, Rocha JN, Bordin AI, et al. Antibody to poly-N-acetyl glucosamine provides protection against intracellular pathogens: mechanism of action and validation in horse foals challenged with Rhodococcus equi.PLoS Pathog 2018;14:e1007160.

Giguère S, Berghaus LJ, Lee EA. Activity of 10 antimicrobial agents against intracellular Rhodococcus equi.Vet Microbiol 201;178:275–278.

Giles C, Vanniasinkam T, Ndi S, Barton MD. Rhodococcus equi (Prescottella equi) vaccines; the future of vaccine development. Equine Vet J 2015;47:510–518.

Goodfellow M, Sangal V, Jones AL, et al. Charting stormy waters: a commentary on the nomenclature of the equine pathogen variously named Prescottella equi, Rhodococcus equi and Rhodococcus hoagii.Equine Vet J 2015;47:508–509.

Sanz MG, Oliveira AF, Loynachan A, et al. Validation and evaluation of VapA-specific IgG and IgG subclass enzyme-linked immunosorbent assays (ELISAs) to identify foals with Rhodococcus equi pneumonia. Equine Vet J 2016;48:103–108.

Author(s) ⬆

Author: Pamela A. Wilkins

Consulting Editors: Ashley G. Boyle