Deer have long been recognised to harbour a variety of ectoparasites (see, for example, Frank Fraser Darling’s book A Herd of Red Deer (1969), first published in 1937), which are familiar to stalkers and deer managers (Green 2020; Walsh 2022) as well as veterinary surgeons (Goddard 1994; Otranto and Wall 2024). Infestations with members of the Hypodermatinae (warbles) and Calliphoridae (myiasis) are important in deer (Scholl et al. 2019); ticks will be covered in Chapter 29.

Ectoparasite infestations may be light to moderate to severe, in terms of the number of parasites; their distribution on the body (localised to extensive). Deer that seem debilitated and have severe infestations may have intercurrent disease problems. The ectoparasite burden may reflect those problems and potentially be a secondary contributing factor to a poor body condition score.

The term warble is taken from the Scandinavian word varbulde meaning boil and refers to the furuncular lesion that the host develops in response to the presence of the L3 larval form of the fly. In general terms, the life cycle consists of eggs being laid on the hair and skin of deer during the summer months, the eggs hatch into the small L1 larvae which penetrate the skin and then migrate to take up residence, for several months, in the cutaneous tissues around the spinal column, growing in size and moulting into the L2 larvae. The palpable warble is a nodule located beneath the skin along the dorsal aspect of the trunk and consists of a growing L3 larva and the host’s inflammatory reaction, with a pore forming and the larva emerging in the summer months. The larva pupate and adults emerge to mate and lay eggs to start the cycle over.

Hypoderma bovis and H. lineatum are usually confined to cattle hosts; migration of the L1 larva is along connective tissue and fascial planes (along nerve pathways in H. bovis), L2 larva is found resting in the epidural space of the spinal column and the oesophagus respectively. There are rare reports of both species in deer (de la Fuente-López et al. 2001; Otranto et al. 2003). In the United Kingdom, H. diana is mainly found in red deer, it has also been reported in roe (Bennett et al. 2009); elsewhere it has been reported in fallow and sika deer, also reindeer (it does not infest cattle). In some European countries, red and roe deer have been described with H. actaeon infestation; while in Scandinavia reindeer have their own H. tarandi (Kutz et al. 2018). It is not clear how H. diana and H. actaeon migrate within deer, some reports suggest a subcutaneous route without involvement of body cavities, although H. diana has been reported in the epidural space of reindeer (Venjakob et al. 2019), while some authors report migration similar to H. bovis (Otranto and Wall 2024).

The life cycle of H. diana is not completed in aberrant hosts that may, occasionally, include alpaca, donkey, horse, sheep and wild boar. There are rare reports of human infestation with Hypoderma species where the larvae try to migrate after penetrating the skin.

In the United Kingdom, H. diana L3 larvae are only seen on deer carcases that originate from Scotland (Bennett et al. 2009); they are not recorded in England or Wales. Infestations can be severe and lead to condemnation, at meat inspection, of carcases going into the food chain (see Figure 26.1a–c). Some warble lesions may be observed in Scottish red deer from December onwards. It is likely that ambient conditions and climate change may influence the timing of life cycle events.

If warble lesions are suspected in cattle in Scotland, the Animal and Plant Health Agency should be contacted (https://www.gov.uk/guidance/contact-apha) because the condition is notifiable.

The most common skin tumour type in deer is benign (non-invasive) fibromatous nodules including fibromas and (fibro)papillomas; all have involvement of fibroblasts in the dermis and some have epithelial changes. Rare cases have had intraoral papilloma lesions (Zürcher-Giovannini et al. 2020). These tumours have been associated with up to nine species-specific papillomaviruses (Munday et al. 2016; Tryland et al. 2018) including European elk (Alces alces papillomavirus type 1 (AaPV1, formerly EEPV), red deer (Cervus elaphus papillomavirus type 1 and 2, CePV1 and CePV2), roe deer (Capreolus capreolus papillomavirus type 1 (CcaPV1, formerly RdPV1), reindeer (Rangifer tarandus papillomavirus types 1–4 (RtPV1–4, formerly RPV) and white-tailed deer (Odocoileus virginianus papillomavirus type 1 (OvPV1, formerly DPV). In Europe, CcaPV1 is considered to be endemic in Austria, Croatia, Hungary and Slovakia, with cases also seen in the Czech Republic, France, Italy, Portugal, Spain and the UK (Kmetec et al. 2020). Viral exposure does not always lead to skin lesions. Deer may have transient lesions which resolve over a matter of months. It is not precisely known how infection is spread and contributing factors may include damage to the skin and close contact with the virus on another deer or in a contaminated environment, where haematophagous insects may also be involved.

Lesions may be observed on the head, neck, abdomen and legs, be single to multiple, variable in size (up to 15 cm) and shape (round to oval), exophytic and some are pedunculated, they can be cream to black in colour, the skin surface may be corrugated consistent with typical wart lesions (see Figure 26.2a,b). Differentiation from what can be very large, single, locally aggressive, persistent lesions associated with fibrosarcoma (a form of soft tissue sarcoma) requires histological examination. Management may vary from excising single lesions to culling deer with extensive or substantial lesions.

Skin nodules may be associated with various other neoplasms including hamartoma, lymphoma, melanoma and squamous cell carcinoma; non-neoplastic lesions include dermoid cysts (in reindeer) and filarial nematode Onchocerca spp. infection (outside the United Kingdom and in various deer species; Kulpa et al. 2021; Manzanell et al. 2022).