Cutaneous pigmented viral plaques associated with the papilloma virus are generally uncommon, but because of a breed predisposition in two of the popular breeds in the UK, Pugs and French Bulldogs, increased awareness is needed. In Pugs, an autosomal dominant mode of inheritance has been suggested (Briggs, 1985) and the condition may be associated with a gene related to innate or cell-mediated immunodeficiency (Stokking et al. 2004). Other predisposed breeds include Miniature Schnauzers and Boston Terriers. The condition has been reported in other breeds such as the Hungarian Vizsla.
It is mainly caused by canine papillomavirus (CPV) type 4, (Nagata et al., 1995; Munday et al., 2017), but CPV type 3 and 5 have also been implicated. In other breeds, factors such as immunosuppression associated with hyperadrenocorticism (spontaneous or iatrogenic), hypothyroidism and hypoalbuminaemia can allow viral replication.
It is also important to note that drugs such as glucocorticoids, oclacitinib and ciclosporin used to manage allergic skin disease are also capable of causing immune dysregulation, so dogs receiving these drugs should be monitored for viral infections (Callan et al., 2005; Favrot et al., 2005). Given that some of the breeds predisposed to cutaneous viral plaques are also predisposed to allergies, the use of these drugs in the management of allergic skin disease may pose an increased risk. However, the risk of infection should not be overlooked in other breeds receiving treatment for allergies.
The lesions appear as single or multiple circular to ovoid plaques on the ventral thorax and abdomen, and they may sometimes be seen on the proximal legs. They vary in size, from as small as 1mm spicules up to 1cm plaques. The lesions are darkly pigmented with a slightly raised scaly, papillated hyperpigmented surface (Figures 1 and 2). The lesions remain static in most cases and in others regress. Malignant transformation to squamous cell carcinoma has been reported (Callan et al., 2005).
Clinically, viral plaques can be distinguished from lentigines on appearance and on palpation. Lentigines are hyperpigmented circular lesions seen mainly in older dogs. Lentigines are flat and sharply demarcated macules which on palpation feel smooth and no different from the surrounding skin whereas the viral plaques are raised, uneven and irregular when compared to the surrounding skin.
The diagnosis of pigmented viral plaques is made on histology. The changes include focal mild to moderate acanthosis and hyperkeratosis. The epidermal surface usually has a scalloped appearance, and, in some cases, small digitations may be present on the surface. The lower epidermal layers show moderate to marked hyperpigmentation. A small number of keratinocytes in the stratum granulosum and stratum spinosum may exhibit papillomavirus-induced changes such as koilocytosis, which are characterised by the clear cytoplasm and pyknotic nuclei.
Additional tests to detect CPV DNA by PCR, immunohistochemistry and electron microscopy may be available through some laboratories in the UK.
Affected dogs should be screened for immunosuppressive disease. A full drug history should be considered in infected dogs to prevent progression and indeed allow self-resolution. Those that fail to regress should be monitored for malignant transformation.
Canine cutaneous viral plaques are considered an aesthetic problem. However, those associated with immunosuppressive agents may self-regress when the drug is stopped. Laser therapy has been used to treat viral plaques. Immunostimulants such as imiquimod and interferons have been reported in treating the condition. A report (Hansen et al., 2017) in which a Hungarian Vizsla had topical tigilanol tiglate gel applied to a selected affected area demonstrated regression of the lesions. Some of these treatments when available require informed owner consent, and the expected response versus possible adverse effects should be considered when planning to treat a condition that is generally considered aesthetic, not life-threatening. The dogs should be regularly monitored for malignant transformation.
Below are two case studies, one which had a genetic predisposition and the other associated with immunosuppressive treatment.
Case study 1
A three-year-old neutered Pug was presented with darkly pigmented slighted raised lesions on the ventral abdomen initially (Figure 1A) and then on the proximal limbs (Figure 1B). They were non-pruritic and varied in size from about 1mm to about 7.5mm in size. The dog was otherwise in good health and did not exhibit signs of immunosuppression due to disease or drugs.
Histology revealed moderate acanthosis and hyperkeratosis. The epidermal surface showed the typical scalloped appearance with large amounts of keratin. There was marked hyperpigmentation in all epidermal layers.
A diagnosis of viral plaques was confirmed and, as there were no signs of malignant transformation, treatment options of interferon and imiquimod (both unlicensed for dogs), which may or may not have altered the condition, were discussed. The owner opted for monitoring the dog.
Case study 2
An eight-year-six-month-old neutered female Staffordshire Bull Terrier was presented with numerous raised growths on the ventral abdomen and on the legs (Figure 2). She had a long standing history of allergic dermatitis that was initially managed with oclacitinib, but, as it was not effective in managing the recurrent chronic hyperplastic otitis, prednisolone was later substituted. The history at time of presentation included polydipsia, polyuria and polyphagia.
A general physical examination revealed a pendulous abdomen with hepatomegaly and muscle atrophy on the hind legs. The pinnal and aural cartilages were calcified. There were numerous papillomatous growths and hyperpigmented plaques, with scaling and irregular surfaces, of varying sizes on the ventrum and axillae and extending down the proximal legs.
A diagnosis of pigmented viral plaques was confirmed on histological findings of the typical scalloped pattern of the epidermis, increased pigmentation and hyperkeratosis.
In this case, immunosuppression associated with the glucocorticoids was implicated in rapid proliferation. The prednisolone was gradually tapered and stopped. With that, the lesions began to regress spontaneously (Figure 3) and other methods to manage the allergic skin disease have been introduced.