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Rhodococcus equi Diagnosis, Prevention and Treatment
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Rhodococcus equi continues to present a major problem for foals world wide. Earlier work indicated the pathogenic strains of R. equi have a unique 15-17 kd antigen that is plasmid mediated [1,2]. This is thought to be why some farms have a lot of R. equi and others do not. All studies using vaccines directed against these 15-17 kd virulence factors as of 2012 are yet to be proven effective.
I. Clinical Presentations of R. Equi Infection (3 weeks to 6 months of age)
- Subclinical
- Foals are not observed to be ill but have lung abscesses that resolve on their own. Radiographs reveal abscesses, serology positive and transtracheal wash are positive.
- Pneumonia
- May be bronchial, broncho-interstial or lung abscesses.
- Osteomyelitis
- Can be in any bone but 3 reports of vertebral osteomyelitis in male Quarter horses [3].
- Septicemia-Uveitis
- Present with severe acute uveitis.
- Gastrointestinal
- Abscesses in mesentery and gut wall.
II. Diagnosis
- Transtracheal wash
- R. equi usually cultured if it is there [4].
- A high percentage of foals on an endemic farm may culture positive at some time [4].
- Use gram stain and culture - Often have mixed infections with Strep. spp.
- Ultrasonography [2] (See Thoracic and Abdominal Radiography)
- Radiography
- Polymerase Chain Reaction
- Blood culture
- May be positive in some disseminated infections and with osteomyelitis.
- Aspirate of bone or abscess
- Serology
- A positive ELISA indicates exposure and not necessarily clinical disease.
- AGID test touted to indicate active infection when positive.
Editor's Comment - In my opinion this test is positive with exposure and clinical infection. It lagged behind tracheal wash in picking up early infection in our studies.
III. Treatment
Editor's Comment - Because of problems with diarrhea associated antimicrobial therapy, new approaches have been suggested.
- Azithromycin 10 mg/kg once daily orally [1]
- Alone or used with rifampin in combination
- Clarithromycin - 7.5 mg/kg q 12 hrs orally
- It can reach greater concentrations in lung
- When used in combination with rifampin was superior to erythromycin-rifampin and azithromycin-rifampin [1]
- Erythromycin (25 mg/kg TID) orally has been used for several years, although it is mostly replaced by azithromycin and clarithromycin due to less side effects and better efficacy.
- Antibiotic resistance is being detected.
Duration of treatment is difficult to determine - Minimum of 3 weeks (up to 2 months) and use ultrasound ≥ radiographs if possible to determine resolution of infection.
IV. Early Antibiotic Treatment Prophylaxis
- Azithromycin orally daily for the first 2 weeks of life [5]
- Study showed a decreased incidence of clinical R. equi using 10 mg/kg azithromycin orally every 48 hrs during the first week of life.
Editor's Comment - Don’t give anything orally in the first 12 hrs because of risk of enhanced absorption of the open gut. This approach may increase antibiotic resistance.
V. Immunoprophylaxis
- Hyperimmune Plasma Sources
- Vaccination of adult horses
- Donor selection-horses must be tested negative for EIA and anti-erythrocyte antibodies.
- Administer R. equi bacterin intramuscularly 3 times at 3 week intervals
- Test serum before and at 3 weeks following last bacterin injection for R. equi titer via ELISA or AGID specific for R. equi antibody. To be an adequate donor R. equi ELISA titer should be > 90 units. Serum total IgG levels do not correlate with specific R. equi antibody levels.
- Collect plasma via plasmapheresis, store frozen in 1 liter bags.
- Commercial plasma sources
- Several companies that manufacture and sell plasma have hyperimmunized donors with R. equi bacterin. (See Plasma Therapy)
VI. Administration of Hyperimmune Plasma [6,7]
- Timing of administration
- Proper timing of administration is paramount to success of the prevention program.
- Plasma must be given prior to significant R. equi exposure. It is not effective as a treatment. If a foal is incubating a subclinical infection, plasma will not stop the progression.
- Determine from previous farm history and weather conditions when foals first develop R. equi in your area.
- Foals born early in the year (January and February) in our area are not transfused until spring because R. equi is not a problem until that time. Do not start plasma too late.
- After the onset of spring or in areas with immediate post birth exposure to R. equi, administer plasma within 48 hours of birth.
- Plasma may need to be repeated in 4-6 weeks. Measure R. equi ELISA titer at 2 week intervals in some foals to determine trend for decline of antibody. Repeat if levels go below 20 ELISA units.
- Levels of R. equi ELISA antibodies rise from natural exposure at 6-12 weeks.
- A high level of ELISA antibody indicates either immunity or active infection and late ineffective production of antibody.
- Method of administration
- Restrain foal and place jugular 14 ga. catheter
- Administer 1 liter of plasma IV over approx. 15 minutes (See Plasma Therapy)
- Mare vaccination with R. equi
- Our experiments indicate colostrum derived antibody from R. equi vaccinated mares is not protective against R. equipneumonia
- If mares are vaccinated we still recommend administration of hyperimmune R. equi plasma to the foal.
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School of Veterinary Medicine, University of California-Davis, CA, USA.
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