Novel papillomavirus identified in a New Zealand warmblood horse

The horse presented with two masses, about 3cm across, over the left front fetlock (identified in A with the arrows). They had a hard, roughened surface. The masses were debrided using a saline swab and imiquimod cream was applied daily for 10 days. While the area, shown in B, is markedly reddened, only mild thickening of the skin occurred. Image C shows the masses after treatment for two additional weeks, with fewer applications of the cream. The area is mildly reddened and covered with thick serocellular crusts. However, no thickening of the underlying skin was detectible at this stage. Images: Munday et al.

Researchers believe a novel papillomavirus was responsible for two lesions treated on the fetlock of a horse in New Zealand.

Papillomaviruses are small circular double-stranded DNA viruses that are typically highly host specific.

Nine equine papillomaviruses have currently been fully sequenced and deposited in GenBank — the National Institutes of Health genetic sequence database.

Equus caballus type 1 (EcPV1) is the most common. It was the first papillomavirus type fully sequenced from horses. It causes papillomas — benign growths — in horses.

John Munday, Michael Hardcastle and Melissa Sim reported in the open-access journal Pathogens on the unusual papillomavirus case in a 10-year-old horse.

The warmblood gelding had two roughened masses, about 3cm across, on the left front fetlock.

The masses were first noticed by the owner three weeks earlier and had grown rapidly since then.

One was partially removed and sent for routine microscopic evaluation. The evidence was consistent with papillomavirus infection.

DNA work was undertaken, revealing sequences that were most similar to EcPV1.

“However, as the sequences were only around 73% similar to EcPV1, they appear to be from a novel papillomavirus type that is likely to be within the Zetapapillomavirus genus,” the researchers reported.

Munday and his fellow researchers noted that the clinical presentation of the papillomas in the present case was unusual and markedly different from papillomas caused by EcPV-1, which are typically small, numerous and around the face.

The papillomas were successfully treated with the application of imiquimod and resolved within 14 weeks, they said.

The study team concluded that, based on the DNA sequences obtained, the lesions were most likely caused by a novel PV type. “This suggests additional papillomavirus types are able to infect horses.”

Discussing their findings, the researchers noted that papillomas only develop when a host is infected by a papillomavirus type for the first time.

“As most horses are infected by EcPV1 early in life, warts caused by this papillomavirus type typically develop in young horses.

“In contrast, the presently described horse was 10 years old when the papillomas developed.

“This suggests that the horse was not exposed to the putative novel papillomavirus type until later in life and therefore this papillomavirus type may be less prevalent than EcPV1.”

If it does indeed prove to be rare in horses, this would explain why this papillomavirus type appears to have remained undetected until now, they said.

As virally induced squamous papillomas are expected to self-resolve, the degree to which the treatment with imiquimod cream influenced lesion resolution is unknown.

The use of the cream in the present case was associated with significant ulceration and reddening of the skin. “This is thought to be due to the stimulation of local inflammation and is a well-recognized side effect in people using this topical treatment.”

Munday is with the School of Veterinary Science at Massey University; Hardcastle is with Gribbles Veterinary Pathology Ltd in Auckland; and Sim is with Franklin Vets in Pukekohe.

Munday, J.S.; Hardcastle, M.R.; Sim, M. Detection of a Putative Novel Papillomavirus Type within a Large Exophytic Papilloma on the Fetlock of a Horse. Pathogens 2020, 9, 816.

The study, published under a Creative Commons License, can be read here

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