A significant concern with the identification of parasites such as the Theileria protozoa, SFTS virus, and Borrelia bacteria in water deer, is that these are what we call tick-borne parasites. In other words, they're spread through tick bites, and deer are well-known hosts for a variety of tick species. The incidence of Lyme disease, caused by Borrelia burgdorferi, has been increasing alongside rising tick populations here in the UK in recent years.
Concern has been raised in both Europe and North America that surging deer populations are driving increasing tick numbers and, thus, cases of Lyme disease. The role of deer in the epidemiology of Lyme is complex, however, and they form part of a web of other factors. Typically, after hatching, the larvae feed on a small mammal, bird or reptile host for just under a week before detaching and moulting into the nymph instar. Nymphs can feed on medium-sized mammals such as rabbits, hedgehogs, foxes, badgers and cats. It is after the final moult, when the ticks are adult, that they tend to feed on larger mammals, such as deer and humans.
Perhaps of most significance here is the observation that larval ticks don't carry Borrelia and actually become infected while feeding on their first host. Indeed, in the few circumstances where ticks live out their entire life on deer, we do not see cases of Lyme, because deer are what we call “non-competent hosts” - in other words, they don't carry the bacteria in their bloodstream. Consequently, the role of deer in the tangled web of Lyme epidemiology appears to be in sustaining the tick populations and, potentially, spreading the ticks themselves to new areas. Several other factors, including the relentless persecution of mesopredators such as foxes and birds of prey, which help reduce and disperse small mammal populations, and the milder and wetter winters brought by climate change, which ticks are more likely to survive, are also likely to be significant contributors.
I'm not aware of any data on tick burden on wild water deer from Britain, but during his work at Whipsnade and Woburn, Raymond Chaplin found that park deer had higher ectoparasite burdens than those in the surrounding Woburn farmland. In his 1977 book Deer, he gives the relative incidence of ticks (Ixodes sp.) as 1.5 in captive versus 0.22 in the wild; values for flies/keds (Lipoptaena sp.) were 1.03 vs. 0.56, and lice 1.27 vs. 0.44. Chaplin also commented on how the animals in the low-density wild population were in better condition than those in the denser zoo population.
Chaplin went on to observe that, provided infestation was not heavy, the distribution of lice on the mother’s body was very specific. Most were found in the inguinal (groin) region, on the relatively sparsely haired surface of the mammary glands, and around the face. In other words, the parasites were concentrated on the areas in contact with the fawn. Indeed, examination of the fawns immediately after their initial grooming showed that most had three or four lice.
Between May and October 2013, Sung-Suck Oh and colleagues recovered 340 ticks from 39 Korean water deer rescued by wildlife charities in South Korea. The mean parasite burden was 12 ticks per deer and, the vast majority (83%) were Haemaphysalis longicornis; 6% were Haemaphysalis flava and 11% Ixodes nipponensis. Similarly, VanBik and her team recovered 48 ticks, 17 nymphs and 31 adults, all identified as H. longicornis, from six water deer killed in traffic between 2013 and 2015 in Korea's Gyeongbuk Province. In a short paper to the Journal of Wildlife Diseases in 2009, Jae-Ik Han and co-workers at Chungbuk National University described infection of two water deer by the tick Haemaphysalis leporispalustris, while the finding of Ehrlichia canis in water deer (see Endoparasites) implies they are also potentially a host for the brown dog tick (Rhipicephalus sanguineus) that transmits the bacterium.
The Zoological Society of London Veterinary Reports published during the mid-1970s describe four water deer suffering from folliculitis caused by the mange mite Demodex. At least three of the individuals diagnosed with demodectic mange were described as being in very poor condition. These are the only reports of mange in water deer of which I'm aware and I would be very interested to hear of any from the wild.