The Rhodococcus equi bacterium infects several animal species, including horses, pigs and goats, and can also cause disease in humans. It grows in the soil and in damp shavings in stables. It enters the system when the horse eats grass contaminated with manure and, although Rhodococcus causes no disease in adult horses, it multiplies rapidly in the intestines of young foals, infecting the environment.
Rhodococcus reaches the lungs of foals through inhalation of contaminated dust and manure. Like its close relation, Mycobacterium tuberculosis, it does not perish when phagocytosed (devoured) by the alveolar macrophages (white blood cells in the lung tissue). Instead, it multiplies in the infected cell, which effectively shields the bacteria from antibodies that could neutralise it.
When the infected cell dies, it is phagocytosed by other macrophages, which also become infected. This results in a severe granulomatous pneumonia, that looks rather similar to tuberculosis in the lung tissue. Chronic pneumonia caused by R. equi is usually seen in foals between one and three months old and is associated with the drop in antibodies in a mare’s milk.
Often the foal’s immune system is compromised, either because it did not get colostrum, or in the case of an Arabian foal, it has severe combined immunodeficiency (SCID), a recessive genetic disease. This was first described in 1973 and about 28% of all Arabians are carriers of the gene.
Carriers show no symptoms, but foals with two copies of the gene, one from each carrier parent, usually die before three months of age from chronic pneumonia. The foals do not respond to treatment; if they test positive for SCID, it is recommended that they be euthanased.
Rhodococcus pneumonia is commonly known as the ‘rattles’, a reference to the sound a sick foal makes as it tries to breathe. The gums can be a bluish colour due to deprivation of oxygen. Respiration is shallow and laboured and there is often a slimy nasal discharge.
Although the foal may improve when treated with antibiotics, symptoms usually return a few days later and the foal’s condition slowly deteriorates as the disease progresses. The most common finding at post-mortem is lungs mottled with yellow-grey masses or granulomas.
Probably the most effective method of prevention is to breed foals from genetic lines that are free of SCID. All Arabians have to be genetically tested before they are registered, so owners are aware of carrier status for SCID.
Treatment that offers some hope
Many breeders choose to breed with carrier animals that do well in the showring, despite the risk of losing foals to Rhodococcus infection. In foals without a genetic predisposition, severe stress, lack of colostrum and outbreaks of other serious respiratory viral diseases can make them more susceptible to pneumonia.
If foals are treated with oral potentiated sulphonamides and rifampicin they can sometimes be saved.