Equilis StrepE is a vaccine for submucosal administration containing a modified live avirulent strain of Streptococcus equi subspecies equi (Strain TW), which is a Lancefield Group C streptococci. It is the causative pathogen in the common equine infective disease known as strangles. It is administered into the horse's upper gum.
This mutant strain has part of a gene essential for the cell's metabolism deleted. Therefore, it cannot replicate in mammalian tissue. As part of its development it was tested for haemolysis, capsule synthesis and sugar fermentation, and in all these respects, it behaved like the original strain.
1Group C streptococci infections are pathogenic in horses and uncommon in humans. Infection with
S equi subspecies equi in human are rare; only five cases have been reported in the literature since 1980. Two of the cases resulted in meningitis.
2Needlestick injuries in veterinary practice have been reported in the literature, though not to the same extent as human medicine. In their review, Weese and Jack
3 note that zoonotic infections and occupational injuries have been overlooked and underappreciated in veterinary medicine. Wilkins and Bowman
4 surveyed a cohort of American female veterinary graduates with regard to needlestick injuries and found 50% of needlestick injuries involved vaccines, which caused 13% of the reported side effects. The vast majority of these vaccine-needlestick-injury side effects were mild (84%) and localised (89%). Leggat
et al5 surveyed 1094 Australian veterinarians and found that syringe devices were the commonest (63.7%) cause of needlestick and sharps injuries (NSI) and that their estimated exposure rate of 58.9 contaminated NSI per 100 person years represents a major occupational health problem.
A search of the literature revealed no prior case report of similar adverse reaction to this vaccine. There have been several case reports
6 of animal vaccines being self-inoculated into digits, resulting in amputations. However, these have mostly been a result of the oil-based nature of the vaccines in those reports that cause increased local compartment pressure and chronic granulomatous inflammation. This vaccine does not contain any other constituents, and, therefore, a significant reaction to it is unusual.
Within the veterinary literature, there have been several reports
7 8 of subsequent adverse reactions from the Equilis StrepE vaccination occurring in the horses treated. Those reactions observed were lymph-node swelling and formation of lymph-node abscesses. Although these adverse reactions were initially thought to be due to the vaccination, further investigation revealed that the horses in question had a subclinical infection with
S equi prior to vaccination.
In conclusion, it is likely that the infection and lymphangitis in this report was caused by the high dose of bacteria inoculated which were able to cause localised infection and tissue damage before they died. It is debatable whether infection occurred, as infection implies the multiplication of bacteria within tissue; this strain cannot multiply. However, the injury progressed to localised swelling and inflammation followed by lymphangitis which behaved, from the clinical perspective, like an infection and improved with antibiotics.
Learning points- The vaccine may have harmful effect on human health, if injected accidentally, but more evidence needs to be collected.
- It is likely that horses will continue to struggle when vets try to inject their gums, and it is likely that needlestick injuries will continue to occur.
- A safer injecting device, for example, self-sheathing needles, or wearing re-enforced gloves might be more appropriate.
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