A horse affected by sarcoids. A new treatment is on the horizon for the tumours, which are common in horses.
Equine sarcoids are fibrosarcoma-like tumours which occur in between 1% to 2% of horses.
There is strong evidence suggesting bovine papillomavirus (BPV) type 1 or type 2 play a role in the development of equine sarcoids.
No effective treatment is available. Relapses are common, even after surgical removal.
Researchers at the German Cancer Research Centre, headed by Dr Martin Müller, trialed a vaccine-based treatment, with promising results. "We are certain that our vaccine is able to prevent development of equine sarcoids because of the antibodies the vaccine induces," Dr Müller told Horsetalk. "However, this is very difficult to prove.
"Either one would challenge vaccinated and control horses with BPV (nothing you would like to do!) or vaccinate healthy animals and follow them up to see whether they are protected against natural infection.
"Now you can imagine that this is very difficult: With the rate of equine sarcoids you probably need a couple of hundreds horses which you observe for four to five years. Very expensive!"
Dr Müller's research team developed "chimeric virus-like particles" (CVLPs) based on the bovine papillomavirus in the hope it would create an immune response that would attack the sarcoids. The CVLPs contained a small part of a protein from bovine papillomavirus found in sarcoids.
In the first phase of the clinical trial, 12 horses suffering from equine sarcoids with an average number of more than 22 tumours per animal were vaccinated with the material, with the dose increasing as the study progressed.
In all, each horse received two or three vaccinations.
The 12 animals were followed-up for 63 days, with a further eight of the 12 horses monitored for more than a year. The researchers watched for side-effects, monitored immune-system responses and assessed the appearance of tumours.
The researchers found that the horses tolerated the injected material well and that it created a "robust" antibody response in all but one of the horses.
Two of the horses showed a clear improvement after treatment, with a reduction in the number of tumours.
In another horse, regression of five sarcoids was observed, but three of them relapsed during the study.
Two animals showed tumour regression, but also growth of new sarcoids. In two horses the clinical status remained unchanged, while in another two horses the growth of existing tumours or growth of additional tumours was observed.
The remaining three animals showed simultaneously regression and growth of existing tumours.
Reinforcing the link between sarcoids and bovine papillomavirus, the researchers found evidence of BPV DNA in tumours of 11 of the 12 horses before the study.
In all, 10 of the 12 horses were assessed as showing either an improvement or their condition remained stable.
Dr Müller said the next step is to establish tests to measure the actual immune responses in the immunized horses.
"This is quite difficult as horses are not the typically model animal for immunologists. But we need to know why some animals do not respond," he said.
He also intends exploring adjuvants that could assist the vaccine, and whether vaccination would decrease recurring tumors after surgical removal.
The study has been published in the January issue of the Journal of General Virology. It is entitled "Immunotherapy of equine sarcoid: dose-escalation trial for the use of chimeric papillomavirus-like particles".