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Rhodococcal Pneumonia in Foals

By: Dr. Philip Johnson

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Young foals are at risk for developing severe pneumonia due to infection by bacteria Rhodococcus equi, a soil-based bacterial organism that used to be known as Corynebacterium equi in the past. Rhodococcus equi pneumonia, sometimes known as rattles, represents a potentially devastating disease that necessitates expensive and time-consuming treatment. Now that the treatment is known, however, Rhodococcus equi rarely causes death in foals unless the infection is recognized too late.

This organism is found almost everywhere in the environment, but it is especially concentrated in grounds where horses and cattle have been intensively accommodated. Rhodococcus equi is passed onto the ground in feces by infected foals and adults where it thrives and multiplies. Infection by this organism can cause severe disease in foals aged between 6 to 24 weeks. Rhodococcal pneumonia tends to occur on a seasonal basis in foals; most foals are infected during the warmer months, April through September. As a rule, disease associated with Rhodococcus equi does not effect adult horses.

Risk Factors

Of particular concern for horse owners is the fact that symptoms of disease in infected foals are too often not recognized until the disease is advanced. The success of treatment for this condition is highly dependent on early recognition of the clinical signs.

Although a majority of young foals are exposed to Rhodococcus equi in their environment, healthy foals are able to develop good immunity and fight off the infection. In human beings, infection by Rhodococcus equi is almost always recognized as a complication of the immunocompromise associated with HIV infection. Similarly, in foals, Rhodococcus equi is an opportunist that is able to cause infection if the foal's immune system is below par.

Foals are also more likely to become infected if they are exposed to relatively large numbers of Rhodococcus equi in the environment, such as might occur on farms on which large numbers of foals have been raised over many years, which leads to build-up of Rhodococcus equi.

Furthermore, it is now being recognized that certain virulent strains of Rhodococcus equi are more likely to cause infection than nonvirulent strains, and that foals exposed to these virulent strains of Rhodococcus equi are at substantially greater risk of developing disease.

Rhodococcus equi causes infection of the foal's lungs when it gains access through the airways on contaminated dust particles in inspired air. Inspired air is most likely to be contaminated with Rhodococcus equi in hot, dry and dusty conditions. It has been well documented that foals accommodated on dusty dry lots in hot weather are at particular risk for developing Rhodococcus equi pneumonia. Infection is much less likely in foals raised on grassy paddocks in temperate climates.

Reduced Immunity in Foals

Newborn foals acquire their immunity (antibodies) in the mare's first milk or colostrum. Colostral antibodies are able to fight off infection until the foal begins to produce antibodies himself. A natural phase of waning immunity occurs between the ages of 4 and 8 weeks, during which time the foal's total antibody levels are relatively low.

This temporary phase of low immunity is the period in which the antibodies obtained from the mare are being used up and yet the foal's own antibody production has not caught up. During this phase, the foal is at particular risk for infection by opportunistic bacterial pathogens, such as Rhodococcus equi. Foals that receive marginal quantities of antibodies from the mare are at greater risk than foals receiving a plentiful antibody supply.

Other factors that may adversely effect the foal's immunity include the numerous stresses to which young foals are exposed early in their life. Typical and common examples of stressful factors that adversely effect immunity and increase risk of Rhodococcus equi pneumonia include:

  • Excessive and rough handling
  • Transport
  • Inclement weather (hot or cold)
  • Other diseases
  • Various drugs and injections
  • Parasite (roundworm) challenge
  • Overcrowding
  • Mixing with other equids of different origin
  • Hospitalization

    Ensuring that the foal obtains a plentiful supply of antibodies against Rhodococcus equi can prevent pneumonia associated with this infection. These antibodies can be injected by the veterinarian if necessary.

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