Rhodococcus equi Diagnosis, Prevention and Treatment

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)

1. Subclinical

1. 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.

2. Pneumonia

1. May be bronchial, broncho-interstial or lung abscesses.

3. Osteomyelitis

1. Can be in any bone but 3 reports of vertebral osteomyelitis in male Quarter horses [3].

4. Septicemia-Uveitis

1. Present with severe acute uveitis.

5. Gastrointestinal

1. Abscesses in mesentery and gut wall.

II. Diagnosis

1. Transtracheal wash

1. R. equi usually cultured if it is there [4].
2. A high percentage of foals on an endemic farm may culture positive at some time [4].
3. Use gram stain and culture - Often have mixed infections with Strep. spp.

2. Ultrasonography [2] (Chapter 53)
3. Radiography
4. Polymerase Chain Reaction
5. Blood culture

1. May be positive in some disseminated infections and with osteomyelitis.

6. Aspirate of bone or abscess
7. Serology

1. A positive ELISA indicates exposure and not necessarily clinical disease.
2. 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.

1. Azithromycin 10 mg/kg once daily orally [1]

1. Alone or used with rifampin in combination

2. Clarithromycin - 7.5 mg/kg q 12 hrs orally

1. It can reach greater concentrations in lung
2. When used in combination with rifampin was superior to erythromycin-rifampin and azithromycin-rifampin [1]

3. 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.
4. 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

1. Azithromycin orally daily for the first 2 weeks of life [5]

1. 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

1. Hyperimmune Plasma Sources

1. Vaccination of adult horses

1. Donor selection-horses must be tested negative for EIA and anti-erythrocyte antibodies.
2. Administer R. equi bacterin intramuscularly 3 times at 3 week intervals
3. Test serum before and at 3 weeks following last bacterin injection for R. equi titer via ELISA or AGID specific for R. equiantibody. 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.
4. Collect plasma via plasmapheresis, store frozen in 1 liter bags.

2. Commercial plasma sources

1. Several companies that manufacture and sell plasma have hyperimmunized donors with R. equi bacterin. (See Chapter 10)

VI. Administration of Hyperimmune Plasma [6,7]

1. Timing of administration

1. Proper timing of administration is paramount to success of the prevention program.
2. Plasma must be given prior to significant R. equi exposure. It is not effective as a treatment.

1. If a foal is incubating a subclinical infection, plasma will not stop the progression.

3. Determine from previous farm history and weather conditions when foals first develop R. equi in your area.
4. 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.
5. After the onset of spring or in areas with immediate post birth exposure to R. equi, administer plasma within 48 hours of birth.
6. 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.
7. Levels of R. equi ELISA antibodies rise from natural exposure at 6-12 weeks.
8. A high level of ELISA antibody indicates either immunity or active infection and late ineffective production of antibody.

2. Method of administration

1. Restrain foal and place jugular 14 ga. catheter
2. Administer 1 liter of plasma IV over approx. 15 minutes (Chapter 10)

3. Mare vaccination with R. equi

1. Our experiments indicate colostrum derived antibody from R. equi vaccinated mares is not protective against R. equipneumonia
2. If mares are vaccinated we still recommend administration of hyperimmune R. equi plasma to the foal.