Respiratory Microbiology

A healthy respiratory system is critical for the optimal performance of the equine athlete. Bacterial and fungal agents are frequently identified as either the primary cause of respiratory disease or secondary invaders following viral infection.

Strangles (Streptococcus equi subsp. equi)

Strangles is a highly contagious diseases of horses caused by the bacterium Streptococcus equi subsp equi and is characterised by high temperature, nasal discharge and swelling of the lymph nodes of the head and neck. While not usually fatal, complications are not uncommon and even routine infections will keep horses out of work for an extended period. Discharges from horses with active infection are a significant source of infection for other horses and may be spread by direct contact between horses or indirectly through buckets, tack, equipment, etc. Infection may also be spread by carrier animals which appear outwardly healthy but carry the bacteria in their guttural pouch and shed it intermittently. The bacterium does not survive for more than a few days in the environment.

Testing for the presence of Streptococcus equi subsp. equi is done by bacterial culture or PCR analysis of nasopharyngeal swabs, nasal washes, pus or guttural pouch washes. Antibody tests can be used to demonstrate previous exposure to Streptococcus equi subsp. equi.

Rhodococcus equi

Rhodococcus equi is a bacterium which can cause pneumonia, with abscessation of the lungs, in foals. Infection may also be seen in other body sites such as the intestines, bones and joints. Overt infection in adult horses is uncommon. Rhodococcus equi is a soil-based organism and is commonly found on many horse farms. The main route of infection is through inhalation of dust particles containing the bacterium. Some farms have problems with Rhodococcus in foals every year but the reasons for this are not fully understood.

Diagnosis of Rhodococcus equi infection is done with bacterial culture or PCR analysis of tracheal wash samples or other appropriate sample.

Treatment of Rhodococcus equi infection requires a combination of 2 drugs for a period of some weeks. Recently, strains of Rhodococcus equi resistant to these antibiotics has emerged and is a cause for concern as treatment options are limited besides. The IEC is actively involved in research on these drug-resistant strains.

Aspergillus fumigatus

Aspergillus fumigatus is a fungal organism which is commonly found in soil, dead vegetation and organic debris. This fungus rarely causes disease in healthy animals but is very quick to exploit any weakness in the immune defences of
the horse. For example, Aspergillus fumigatus can invade superficial lesions on the surface of the eye leading to more extensive ulceration of the cornea. Similarly, horses with weakened pulmonary defences, either from previous infection or from corticosteroid administration, may develop fungal pneumonia or infection of the nasal cavity or sinuses. Diagnosis of Aspergillus fumigatus infection is done via fungal culture. Susceptibility testing of fungal isolates is available on request.