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Rhodococcal Pneumonia in Foals

By: Dr. Philip Johnson

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Diagnosis

Diagnosis of Rhodococcus equi pneumonia is initially based on recognition of the typical signs, especially in particular geographical locations in which rhodococcal disease has been previously confirmed. Particularly, common initial findings include:

  • Fever
  • Increased respiratory rate and effort, often very pronounced
  • Tendency to cough
  • Nasal discharge (although it is rarely profuse)
  • Abnormal breath

    Abnormal breath sounds are variable. Although the affected foal may be breathing hard, breath sounds may be difficult to detect in areas of the lung field which have been affected. In other areas of the lung, breath sounds may be harsh or even musical. Your veterinarian may detect the presence of abscesses at the lung surface by tapping the chest and listening for the degree of resonance; tapping the chest over normal lung elicits a resonant (booming) sound and tapping the chest over an abscess elicits a dull thud.

    The presence of inflammatory fluids in the airway often causes affected foals to ventilate their lungs with noisy breaths. For this reason, the disease is commonly known as the "rattles," but it should be noted that other infectious lung diseases may also cause similar sounds in the absence of Rhodococcus equi infection.

    Further diagnostic tests that support the diagnosis of pneumonia include routine blood tests (white blood cell count and plasma fibrinogen are elevated). Ultrasound scanning of the chest can easily be used to identify abscesses at the surface of the lung. Chest radiographs are valuable in determining the extent and severity of pneumonia.

    The specific diagnosis of Rhodococcus equi pneumonia depends on positive isolation of the bacterial pathogen in fluids retrieved from the airway. Airway fluid analysis is routinely undertaken by veterinarians and may be accomplished with minimal difficulty using a transtracheal wash technique. In addition to checking for the presence of Rhodococcus equi in airway fluid, the bacteriological analysis includes investigation for other bacterial organisms that might be operating at the same time as Rhodococcus equi and an evaluation of the antimicrobial susceptibility of all cultured microorganisms. Antimicrobial sensitivity testing enables your veterinarian to select the most effective antibiotic for a given infection.

    Although blood testing for antibodies against Rhodococcus equi has become popular recently, a positive blood test result merely implies that the foal has been exposed to Rhodococcus equi and should not necessarily be interpreted as proof of the infective disease process itself.

    Treatment

    Treatment of affected foals is most effective if treatment can be started as soon as possible in the course of the infection, which is often advanced at the time a problem is recognized by the foal's owner. Foals affected with pneumonia should be isolated from other foals and the communal environment during treatment because their manure contains very high levels of virulent Rhodococcus equi. Affected foals represent a very important source of Rhodococcus equi contamination for the environment.

    It is also important that affected foals should be brought into a well-ventilated, shaded, cool barn environment with minimal excitement. The most appropriate treatment for Rhodococcus equi pneumonia is usually a combination of two antimicrobials, erythromycin and rifampin administered orally. It is important to ensure that erythromycin estolate is the type of erythromycin selected.

    The specific dose and duration of this treatment depends to some extent on the severity of the problem in a given foal. In rare cases (less than 10 percent), other antibiotics may be needed because either the isolated variant of Rhodococcus equi is resistant to erythromycin or rifampin or other bacterial pathogens are involved at the same time. Foals must be protected from direct sunlight and high temperatures during treatment because there is risk of death from hyperthermia in affected foals.

    In those foals presented with severe respiratory distress, it is sometimes necessary to administer oxygen through a tube passed into the nasal passages. Other treatments that may be considered by your veterinarian include the use of nonsteroidal anti-inflammatory drugs to reduce airway swelling and bronchodilators to increase the airway diameter.

    Newer treatments are being evaluated that affect the foal's immune system in such a manner as to increase the defense mechanism's ability to recognize and kill Rhodococcus equi.

    Veterinarians usually treat foals affected with Rhodococcus equi pneumonia for two weeks following cessation of clinical signs, which typically takes 6 to 8 weeks. Monitoring the foal's clinical signs, which include respiratory rate and effort at rest, rectal temperature, presence of cough and nasal discharge, presence of abnormal breath sounds detected by the stethoscope, is very important during the treatment period. Clinical improvement can also be evaluated by follow-up radiographic and ultrasonographic examinations of the chest. Repeated blood tests for the white blood cell count and plasma fibrinogen concentration are also used routinely by veterinarians to monitor progress.

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