We know little about the diseases of water deer and particularly their potential to transfer infections such as bovine tuberculosis, Johne's disease, bovine virus diarrhoea, liver fluke or Schmallenberg's disease to livestock. As we've seen, they can be host to a wide range of potential pathogens and, as the British Deer Society's honorary vet Peter Green once put it, it's easier to shoot the deer, thereby removing the issue, than fund research to understand the issues. Given the low numbers of water deer in England and their general avoidance of livestock they are not generally considered of concern. The situation appears different in parts of their native range, however, and writing in a 2014 paper to the Journal of Wildlife Diseases, Seol-Hee Kim and colleagues raise transmission concerns, noting that:
“... water deer are among the most common wildlife to approach farmhouses and livestock barns in Korea ...”
In captivity there have been several reports of water deer die-offs, sometimes at a significant scale. An estimated 140 water deer died during an epidemic at Whipsnade in December 1933 and January 1934, for example. In his description of the event, published in 1938, Adrian Middleton recalled that such was the scale that it necessitated sending “a lorry round the fields to collect the carcasses which were lying about”. Two animals were autopsied at London and found to have died from enteritis, while inspection by the superintendent at Whipsnade found large numbers of nematode worms. Middleton continued:
“The keepers stated that the deer moved slowly in an erratic manner, and some of them appeared to be paralyzed in the hind-quarters.”
A few years earlier, in 1930, four animals died of pneumonia soon after they were introduced to the park, while a more recent survey of 134 deaths (56 bucks and 78 does) there between 1991 and 1994 by Richard Champion reported that 50 animals were classed as thin or emaciated. In the wild at Woodwalton Fen, Arnold Cooke has observed individuals, usually bucks, with splayed front legs that impair normal movement. These deer are in poor condition.
Other conditions reported include the advanced periodontal disease on one or both sides of the mandibles that Gérard Dubost and his team found in 25% of the animals that died at between one and eight years old at Branféré Zoological Park in France. Periodontal disease was apparently more common in females (32%) than males (10%). In a paper to the Journal of Zoology in 1965, Rodney Finlayson described “mild, focal medial degeneration and calcification in the abdominal aorta of an adult male Chinese water deer” at Whipsnade with a serum cholesterol level of 0.3 mg/mL, while, also at the zoo, James Kirkwood and his colleagues lost two fawns (four- and 13-days-old) they were hand-rearing to bronchopneumonia, and a third from mycotic (fungal) pneumonia at 17 days old.
Surim Park and co-workers reported B-cell lymphoma in an adult female water deer rescued by Chonbuk Wildlife Rescue and Conservation Centre in South Korea after falling into a drainage ditch. The primary tumour was in a lymph node in the lower jaw and had metastasised to the lung and facial tissues, resulting in a swollen and lumpy appearance.
The virulent lineage II of peste des petits ruminants, roughly translated as “plague of small ruminants” or PPR, is a contagious viral disease that primarily affects small domestic and wild ruminants and has caused considerable economic loss in developing countries. In China, the disease was first identified in Tibet during 2007, and there have been at least ten reported cases in the country since then. In a 2017 paper to Transboundary and Emerging Diseases, a team of researchers led by Xue-Yuan Zhou at the Shanghai Veterinary Research Institute describe the virus in six water deer from a farm in China's Anhui Province. Prior to death, the animals were reported showing signs of fever, shortness of breath, cough, blocked noses and diarrhoea.
Finally, in recent years, a progressive neurodegenerative disorder called Chronic Wasting Disease (the “other” CWD) has become a source of increasing concern among biologists and deer managers. CWD is what we call a prion disease, which means it's caused by a contagious misfolded protein that is found predominantly in the brain. We don't have all the pieces to the puzzle yet, but we think prions spread between animals through body fluids like faeces, saliva, blood, or urine, either via direct contact or indirectly through environmental contamination of soil, food or water.
While Britain has remained clear of Chronic Wasting Disease so far, it seems to be spreading rapidly in North America and has recently been identified in Europe. Several outbreaks have also been recorded in Korea following the importation of an infected mule deer (Odocoileus hemionus) in 2001. Recently, Minji Lee and colleagues failed to find any sign of the disorder in 302 samples from 86 free-ranging water deer collected in Gangwon Province and, to the best of my knowledge, there remains no confirmed case of it in Hydropotes.