Research carried out in the Netherlands and Ireland in the search for a safe and effective vaccine against strangles has produced encouraging results.
Strangles is a contagious disease of horses caused by Streptococcus equi subspecies equi. Typical signs include fever and abscess formation in the lymph nodes of the head and neck. The disease occurs throughout the world and causes heavy economic loss in terms of the cost of treatment, quarantine measures and occasionally the death of affected animals.
Associated with the cell surface of Streptococci is a substance called M- protein. It is regarded as an important virulence factor. Most of the commercially available strangles vaccines available worldwide contain the protein - either in an extracted form or as part of the inactivated whole cell. These vaccines, however, frequently produce adverse effects and induce poor immunity against experimental infection.
Previous studies have suggested that a systemic response which relies on circulating antibodies in the blood does not protect as well as a local immune response in the mucosal lining of the nose and throat. It should be possible to trigger such a response by using attenuated live vaccines, or purified antigens in a mucosal adjuvant.
Dr Ton Jacobs and his colleagues tested three different vaccines and three different vaccination routes in Shetland ponies. Two weeks after the last vaccination the ponies were challenged by intra-nasal application of a virulent strain of S. equi.
One vaccine was based on the M-protein extract and was given into the muscle. All animals vaccinated with that vaccine developed clinical signs of strangles within 6 days of challenge. The response to an inactivated whole cell vaccine was not much better. Two out of three vaccinated animals developed strangles when challenged. The results indicated that inactivated whole cell or sub-unit vaccines given into the muscle did not protect against strangles.
A live attenuated vaccine was produced from genetically modified Streptococci. It was administered by three different routes.
When given into the muscle it protected against infection , but produced an unacceptable swelling.
Intra-nasal administration produced no adverse reactions but it did not confer adequate immunity. Two of three ponies developed strangles when challenged.
When administered by sub-mucosal injection into the upper lip, small swellings (2-3cm diameter) developed, which resolved completely within 2 weeks. These swellings had no effect on appetite and caused no apparent discomfort. All five foals vaccinated submucosally remained free of strangles when challenged.
Of the vaccines tested, the live vaccine, produced from genetically modified S equi administered by injection on the inside of the upper lip appeared to be the only safe and effective method of vaccination.
For further details see: A.A.C. Jacobs, D. Goovaerts, P.J.M. Nuijten, R.P.H. Theelen, O.M.Hartford, T.J. Foster. Investigations towards an efficacious and safe strangles vaccine: submucosal vaccination with a live attenuated Streptococci equi. Veterinary Record (2000) 147,