The prognosis for most animals with endocarditis is grave. Frequently the sysdisease is advanced and the damage to the affected valve is extensive; however, on some occasions treatment can be successful. If the horse has sufbreeding potential or sentimental value, aggressive antibiotic treatment may be justified. Some animals have returned to athletic performance after successful treatment of endocarditis; however, this is uncommon and depends on the degree of valvular compromise.
The aim of treatment is to achieve a bacteriological cure, to reduce systemic embolisation of infected thrombi, and to reduce valve damage and subsequent incompetence. Echocardiographic examination is important not only to help to confirm the diagnosis, but also because it allows the degree of valvular damage and resultant volume overload to be assessed. If there is substantial volume overload associated with the damage to the valve structure, the long-term prognosis is poor even if a bacteriological cure is achieved.
Successful treatment depends on early diagnosis and treatment with approantibiotics. High doses of bactericidal drugs, to which the organism is sensitive, are required for prolonged periods. Ideally the drug should also have good properties for penetration of the mass of fibrin which accumulates around an infected valve. Additional considerations are the potential side-effects of long-term antibiosis and the cost of treatment.
Initially, the agent involved and its drug sensitivity are seldom known. A wide variety of bacteria have been reported to cause endocarditis; no one species appears to be particularly prevalent. Treatment with broad-spectrum antibiotics is therefore indicated, while the results of culture and sensitivity tests on multiple blood culture samples are awaited. Two antibiotic regimes are widely used. Potentiated sulphonamide preparations should be used intravenously initially (15 mg/kg bid). Oral treatment at a dose of 15 mg/kg bid can be instituted once the initial systemic illness has resolved. Long-term use of potentiated sulphonamides has been associated with development of diarrhoea. Another option is to use a combination of penicillin and gentamycin. This must be administered parideally four times daily initially, and is much more expensive. In addithe potential nephrotoxicity of gentamycin should be considered if treatment is long-term. It is helpful to monitor plasma fibrinogen levels, which are a good guide to the presence of an inflammatory process. It is usually advisable to maintain antibiosis until fibrinogen levels have been normal for approximately two weeks. It is wise to repeat the fibrinogen assay periodically thereafter, to ensure that the infection has completely resolved.
Non-steroidal anti-inflammatory drugs (NSAIDs) have also been recomas part of treatment. They may reduce the pyrexia and improve the horse's demeanour, and are particularly valuable in animals with gram-negative bacteraemias and endotoxaemia. In addition, NSAIDs may decrease further platelet aggregation on the vegetative lesion. Phenylbutazone or flunixine meglumine can be used, although low doses are advisable in the light of possible toxicity associated with long-term use, particularly if renal function is compromised