Preventing foal pneumonia

Preventing foal pneumonia Preventive antibiotic treatment may help reduce the number of cases of foal pneumonia caused by Rhodococcus equi, according to a recent report. Rhodococcus equi (R equi) causes abscesses in the lungs. Affected animals show signs of fever, cough, rapid breathing, and nasal discharge. The disease tends to develop insidiously. Few signs may be apparent until the disease is quite advanced. The organism is often found in soil, particularly in dry dusty conditions.

Typically the disease occurs in foals up to 6 months old. However, it seems likely that foals actually pick up the infection in the first few days or weeks of life. The organism is able to live inside the cells  (alveolar macrophages and neutrophils) that are supposed to destroy it. This presents an added problem for treatment, as it is necessary to use an antibiotic that can get into these cells to reach the bacteria.

The disease may be well advanced before it is diagnosed, making treatment more of a challenge. Treatment is expensive and prolonged. A combination of two antibiotics, a member of the macrolide class (such as erythromycin, azithromycin or clarithromycin) and rifampin, is generally effective but may need to be given for up to 1 - 2 months.

Certain farms seem prone to R equi infections, and have affected foals year after year. What can they do to prevent further cases? Intravenous hyper-immune plasma can be used. But it is expensive and labour-intensive. No vaccine is currently available.

Would a course of antibiotics in the critical first few days of the foal's life prevent the infection from becoming established? Research workers at the College of Veterinary Medicine and Biosciences, Texas A & M University, conducted a study to examine the protective effects of giving foals azithromycin every other day for the first two weeks of life.

As well as being effective against R equi, azithromycin reaches high concentrations in the alveolar macrophages and neutrophils. It is absorbed well after oral administration in foals

The researchers chose farms that had suffered infection rates of more than 20% over the previous two years. The study was limited to foals that stayed on the farm from birth to 150 days of age. Foals that were born on the farm and moved off, or that moved onto the farm after birth were excluded. Many of the foals (in both groups) also received prophylactic hyper-immune plasma.

Foals were assigned randomly to a treatment or control group. The treated foals received azithromycin every 48 hours until they were two weeks old.  Participating farms and veterinarians were not blinded as to which treatment group foals were assigned to.

Overall, 13% (44) foals developed R equi pneumonia. Azithromycin-treated foals were significantly less likely to be affected than were the control group of foals.  Only 5.3% of treated foals developed R equi pneumonia compared with 20.8% of the controls.  All but one foal responded to treatment and made a full recovery.

No significant adverse effects of the treatment were reported.

One danger of using prophylactic antibiotics is the risk of encouraging the development of resistant bacteria. The researchers checked faeces and tracheal aspirates from both groups of foals and found no evidence of R equi resistant to azithromycin. However it was beyond the scope of the study to check for resistance developing in other species of bacteria.

They also found that treated foals had lower concentrations of R equi in the faeces. They suggest that this finding needs further investigation. If it is possible to reduce the excretion of the organisms that may help to reduce the environmental contamination - which may reduce the risk of infection in later years.

The authors of the report conclude that this was an effective strategy for reducing the cumulative incidence of R equi pneumonia cases. It also reduced the faecal shedding of R equi, which may have reduced the contamination of the environment with the organism.

However, although they found beneficial effects, and no evidence of resistance, they DO NOT recommend widespread use of azithromycin for prevention of R. equi, because of the risk of promoting antibiotic resistance.  Since azithromycin is currently one of the few antibiotics effective for treatment of affected foals, the authors do not encourage the use of it for prevention. This study provides evidence that preventative measures taken early in life can reduce the incidence of R. equi pneumonia.  Further studies are needed to develop other preventative measures during the early weeks of life.

For more details see:
Chemoprophylactic effects of azithromycin against Rhodococcus equi - induced pneumonia among foals at equine breeding farms with endemic infection.
MK Chaffin, ND Cohen, RJ Martens.
J Am Vet Med Assoc (2008) 232, 1035 - 1047.

Reproduced with kind permission of Mark Andrews BVM&S CertEP MRCVS
© Copyright Mark Andrews  - Equine Science Update 2008

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