Rhodococcus Equi Pneumonia Of Foals
Table of Contents
Rhodococcus equi, (previously known as Corynebacterium equi) causes a persistent bacterial pneumonia in foals, and may become established as an endemic disease on some breeding farms. It may result in considerable losses through costs of diagnosis and treatment, and in some cases, through death. A better understanding of the disease may reduce its significance. There appears to be an increase in the importance of this bacterial disease, with the increased concentration of foals on a smaller number of breeding farms.
A recent survey at the Ontario Veterinary College showed that Rhodococcus equi infection accounted for 10% of all foals sent for post mortem examination and 45% of all foals with pneumonia. These numbers, however, may not reflect the true incidence of the disease in Ontario. While the infection does not seem to be a problem on many breeding farms, on farms with endemic disease, all the foals may become sufficiently ill to require treatment.
Rhodococcus equi is a robust soil organism widespread in the environment. It has simple nutritional requirements, which are perfectly met by materials in the manure of animals, whether from horses, cattle, pigs, or even chickens. The organism will potentially multiply wherever there is horse manure. Temperature also plays a major role in the growth of Rhodococcus equi, with optimum growth at 30ºC. In Ontarios hot summer of 1988, death of foals was not restricted to those born to mares on breeding farms, under ideal temperatures, one organism may multiply 10,000 times in a few days in an area contaminated by horse manure.
There seems to be a direct relationship between the number of Rhodococcus equi in the environment of young foals and the number of cases of pneumonia. Because the organism reaches the lung by inhalation, dusty manure-contaminated environments (such as are commonly found in loafing paddocks on horse breeding farms in the summer) are potentially lethal sources of infection. The equivalent of one tablespoon of dusty soil kicked up by a mare, perhaps into the face of her sleeping foal, can contain a million of these bacteria. To prevent Rhodococcus equi infections, foals should thus ideally be born in January (when the organism remains frozen in the ground).
Rhodococcus equi can also multiply to dangerous levels in the intestine of a young foal. Foals establish a normal intestinal flora of bacteria from birth to 12 weeks. In this period, they seem to be susceptible to being infected by abnormally high levels of Rhodococcus equi. Levels of up to 10,000 organisms per gram of manure are sometimes shed from infected foals. After 12 weeks, the bacteria rapidly decline in number and no longer multiply in the intestine. The ability of the organism to multiply in foals, but not in adult horses is likely another reason why horse breeding farms may become progressively infected with Rhodococcus equi. Any of the bacteria then found in the intestine result from acquisition of grass.
These facts help to explain why some farms become heavily infected with the bacteria and develop problems each year with foal pneumonia. Little is known about the susceptibility of this robust organism to disinfectants. However, because many horse breeding farms do not use concrete, disinfection procedures are often difficult to carry out in the stalls and impossible in the loafing paddocks. In the areas where the foals are kept, regular disposal of manure and dust control have proven effective in reducing the levels of bacteria.
It is also important to ensure that loafing paddocks are well grassed, and not totally grazed, reducing them to dusty sandpits.
The incidence of the disease peaks in foals at 6-12 weeks; this coincides with the period when maternal antibody, derived from colostral milk, has largely declined and before antibody produced by the foal has developed. It is thus important to ensure that foals get their full share of colostral antibody immediately after birth.
In future, scientists may be able to find ways to promote antibody production by mares or foals to protect them during what is apparently a time of critical susceptibility to the disease. In the first 4 months of life, foals must be protected from potentially serious damage to the lung caused by inhalation of the Rhodococcus equi bacteria. Why young foals are so prone to this organism, compared to other offspring (such as piglets or calves) is not known, but they grow out of this susceptibility by 5-6 months of age. Probably, most foals develop antibodies to the organism, without the disease being apparent. The development of natural immunity appears to be slow and is inadequate in the young foals in the face of heavy challenge by inhalation. No vaccines are available to control this infection and it seems unlikely that an effective vaccine will be produced in the near future. Although the basis of immunity to this pneumonia in foals is not adequately understood, it is difficult to produce vaccines to promote the cellular type of immunity which is thought to be particularly important in immunity to Rhodococcus equi pneumonia.
The disease usually occurs in foals aged 4-12 weeks. Clinical detection of the disease in its early stages requires considerable experience. The disease begins with increased diffuse bronchial sounds, often accompanied by a cough, which later develops into a wheeze. This may be localized to a very small part of the lung. Fever follows within 1-2 days, and is associated with an increased respiratory rate (over 40 respirations per minute). Untreated foals develop progressive crackling sounds which can be heard all over the lung, and harsh inspiratory sounds which can he heard without a stethoscope. The lungs of a seriously affected foal are shown in Figure 1.
Foals will often be critically ill before anyone detects that something is seriously wrong. The disease is unique in that foals with fever will appear vigorous. Sick foals will remain bright and alert, sucking the mare well. The disease may also develop insidiously on a farm over many years, and before anyone is aware, the farm is heavily infected with the organism.
The clinical signs of the disease are not particularly distinctive from other types of pneumonia. These signs may, however, be readily recognized on endemically affected farms by people experienced with the disease. The best way to detect an infection is to isolate the organism by bacterial culture techniques. This can be done by a veterinarian.
The disease should be suspected on any breeding farm where several foals, between 4-12 weeks of age appear healthy but show low grade fever and coughing. The more heavily a breeding farm is contaminated with the organism, the younger will be the age of the foals affected. Mares with foals at foot from unaffected farms may be sent for breeding to a heavily infected farm and, upon their return, bring back an infected foal. Lack of experience with the disease on the unaffected farm and the insidiously progressive nature of the pneumonia in foals may mean that the disease will go undetected until it is too late. It is useful for brood mare owners to know the status of the breeding farm as it pertains to Rhodococcus equi.
Affected foals need prolonged treatment because of the persistence of the bacteria within abscesses in the lung and because immunity to lung infection is poor, the disease tends to recur. Recent research reveals the combination of the antibiotics, erythromycin and rifampin is effective in the treatment of the disease. These expensive drugs can be given orally, a definite advantage over alternative treatments. These drugs penetrate the phagocytic cells (the cells which ingest foreign material) where Rhodococcus equi are found, and are not toxic when used over prolonged periods. The combination should be used for at least one week past cure, determined clinically by the use of x-rays or by blood tests for normal fibrinogen levels. Other drugs, such as gentamicin or trimethoprim-sulfamethoxazole, are effective but need to be injected. Because of its toxicity, gentamicin cannot be used for prolonged periods in foals suffering from this disease. However, controlling the infection on endemic farms solely by treating diseased foals is both an ineffective and expensive approach.
The responsibility of diagnosis and treatment of Rhodococcus equi pneumonia in foals remains a challenge for veterinarians. Horse owners, particularly breeders, can contribute to the control of the disease through understanding the ecology of the organism:
With proper design and operation of the breeding farm, this insidious disease can be controlled.
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