NEW TYPE OF STRANGLES VACCINE I N THE WORKS
significant reduction in the average surface temperatures where injections had been given compared to the temperature of the noninjected areas. The biggest difference was seen two hours after injection, but significant temperature changes were detectable for as long as 12 hours. The researchers say the decrease in temperature could be caused by reduced local circulation related to swelling or tissue degeneration in the wake of the injection.
The researchers conclude that this technology could potentially be used as a screening tool to identify horses who had received jugular injections prior to competition. With only a few exceptions, the United States Equestrian Federation prohibits the administration of injections in the 12 hours prior to the start of competition.
The researchers note, however, that the lesscontrolled environment of showgrounds or competition housing would complicate the use of infrared thermography because of outside influences on skin temperature such as sunlight, exercise, bathing and wind.
A new type of vaccine to protect horses against strangles may be available by the end of the decade.
Caused by Streptococcus equi bacteria, strangles is characterized by high fever and enlargement of the lymph nodes of the horse’s head and neck. Over the course of the disease, which typically lasts several weeks, the abscessed lymph nodes may burst open to drain copious amounts of thick yellow pus from the nostrils. Horses who are severely affected may have trouble breathing, leading to a “strangling” noise that gives the condition its common name.
Although vaccines against strangles are currently available, researchers have been eager to develop alternatives. “Colleagues of mine have published articles on [strangles] vaccine adverse reactions and vaccines causing clinical disease,” says Carl Robinson, PhD, of the Animal Health Trust in Suffolk, England. Current strangles vaccines also cause a horse to test positive for the organism after inoculation, which can confound diagnostic efforts during an outbreak.
The new vaccine, called Strangvac 4, is a protein vaccine, which triggers the horse’s immune response using proteins associated with S. equi rather than the whole organism itself. “We have sequenced the whole genome of S. equi and identified a number of what we call surface or secreted proteins that are likely to interact directly with the host, performing various functions including attachment, immune evasion and nutrient acquisition,” says Robinson. “We cloned the genes encoding eight of these for the Strangvac 4 vaccine, produced recombinant protein and mixed them in the final vaccine formulation.” He adds that as far as he knows this is first protein vaccine developed for horses.
A major benefit of this type of vaccine, says Robinson, is that it confers immunity without interfering with diagnostic tests that may need to be conducted in an outbreak: “The really big advantage of Strangvac 4, apart from the efficacy, is that it does not trip the currently available tests for strangles. The PCR tests for the disease are based on DNA targets, which Strangvac 4 does not contain, and the ELISA tests look for antibodies to S. equi proteins, which the Strangvac 4 does not contain, and so would also not trip those tests. This allows you to vaccinate where disease is circulating with the knowledge you can tell whether the animal is diseased with active infection or vaccinated.”
In three preliminary trials of various vaccine formulations, researchers immunized ponies and then exposed them to S. equi. In all of the experiments, only three of 16 ponies vaccinated with Strangvac 4 developed a fever, compared to all 16 ponies treated with a placebo.
“There are virtually no vaccines available that will give 100 percent protection,” says Robinson. “It’s a common misconception that this will be the case. [Strangvac 4] prevented all clinical signs in some ponies, reduced them
in others, and was not effective in others. What we are trying to achieve is a point where good vaccine efficacy, good diagnostic tests, good outbreak management, good biosecurity and good husbandry all come together to reduce the impact of the disease and reduce the number and severity of outbreaks.”
Robinson says that the experimental side of Strangvac 4 development is complete and once the regulatory processes are finished, the vaccine may be available commercially by 2019 or 2020.
INFECTION: Strangles causes enlargement and abscessing of the lymph nodes.