A significant number of healthy Italian horses in a study tested positive for Equine Papillomavirus 2 (EcPV2), which can cause squamous cell carcinomas of the external genitals.
Katia Cappelli and her fellow researchers, writing in the journal Viruses, noted that few studies have reported EcPV2 prevalence among healthy horses. This lack of information does not allow the identification of at-risk populations and hinders the development of screening protocols, they said.
Papillomaviruses can cause both benign and malignant lesions in many species. Human Papillomavirus (HPV) is associated with cervical, ano-genital, and head and neck squamous cell carcinomas.
“There is growing evidence that papillomarvirus infections may also play a role in cancer development in horses,” the researchers said.
In particular, EcPV2 has been detected in gastric, penile, vulvar, clitoral, and oropharyngeal papillomas in horses, as well as squamous cell carcinomas.
Many studies have reported the presence of viral DNA and mRNA in these lesions, suggesting EcPV2 as the cause. Today, it is generally accepted that this virus plays an active role in the development of these tumors, they said.
However, little is known regarding the mechanisms giving rise to the cancer, the viral transmission route, whether a carrier state exists, or the impact of papillomavirus status on prognosis or therapeutic protocols.
The authors noted that, in humans, subclinical HPV infection is more frequent than clinical disease, with many infections being cleared and few becoming persistent.
“In the horse, we can speculate that asymptomatic and transient EcPV2 infection is common,” they said. However, only a limited number of studies have attempted to measure the prevalence of EcPV2 exposure or infection in apparently healthy horses.
The researchers set out to estimate the prevalence of EcPV2 on genital swabs taken from clinically healthy horses and to evaluate their innate immune response.
Sterile cytobrushes were used to swab 234 healthy horses, comprising 200 females and 34 males. The researchers applied molecular-based methods to test for both the presence of EcPV2 DNA and evidence of gene expression (RNA).
Overall, 30.3% of the horses were positive for EcPV2 DNA, with evidence of gene expression in 48% of the positive samples.
No statistically significant differences were found in terms of the virus’s prevalence in relation to sex, age, and origin. Concerning breeds, Thoroughbreds had the highest risk of infection, and Arabians had the lowest among the study horses.
Looking specifically at mares, 40.2% of them were positive for EcPV2. The chances of being positive were higher among mares that had already had foals compared to maiden mares, and were similarly higher among those whose pregnancies arose from natural service rather than artificial insemination.
The study team said their findings, which included an examination of cytokines, suggested the presence of an effective immune response in horses against the papillomarivus, which could explain the low incidence of squamous cell carcinomas in positive horses, despite a high EcPV2 genoprevalence.
The results suggest that in horses, as in humans, many infections are asymptomatic and resolve spontaneously, with an effective immune response.
The study team also found a positive association between mares who were unable to breed and a positive result for EcPV2. Although the number of samples was not sufficient to be conclusive, the finding was interesting and warranted further investigation given that, in humans, the infection is associated with low fertility.
The 19-strong study team came from a range of Italian institutions, including the University of Perugia and the National Reference Center of Veterinary and Comparative Oncology.