During their post mortem of 25 water deer from Whipsnade during the spring of 1937, John Leiper and Phyllis Clapham found a variety of nematode (roundworm) species: Bunostomum trigonocephalum (a small intestine worm), Capillaria bovis, Chabertia ovina (the large-mouthed bowel worm), Haemonchus contortus (barber's pole worm), Nematodirus fillicollis, Ostertagia lyrata (brown stomach worm), Ostertagia griihneri, Strongyloides sp., Trichostrongylus axei (stomach hair worm), Trichostrongylus retortaeformis, Trichostrongylus colubriformis, Trichostrongylus vitrinus and Trichostrongylus cervarius. Six deer had no helminth parasites. The researchers noted that worm burdens ranged from 28 parasites to nearly 5,000, but none of the animals could be considered to have a pathological infection. Capillaria was the most widespread nematode parasite, particularly abundant among young deer; older deer had up to 35 worms, while several young deer carried more than 200.
Other studies on Whipsnade deer have yielded the long bowel worm Oesophagostomum venulosum from the colon of one animal and nematodes of the family Ostertagiinae from the small intestine of another. Additionally, the bacterium Corynebacterium pygoenes was isolated from a water deer with a hepatic abscess and pleurisy in 1981, and haemolytic streptococci were found in widespread subcutaneous pustules containing Demodex (mange) mites on two water deer in 1977. Indeed, microbes are ubiquitous in the environment, although it tends to be only those known to cause problems for humans or livestock that are actively searched for by researchers. Several species of protozoa, bacteria, fungi and virus that are of potential concern have been isolated from water deer, although at the time of writing I know of no confirmed zoonotic (animal to human) infection involving Hydropotes.
Virologist Michael Lawman and his colleagues conducted a preliminary survey of livestock viruses in British deer collected between 1961 and 1973. The team looked for the presence of antibodies to these viruses, an indicator that the animal had been exposed to the pathogen at some point in the recent past. Two of the 18 water deer tested were positive for antibodies for adenovirus group A that can cause respiratory infections in cattle, and three for bovine virus diarrhoea. In their 1978 paper to the British Veterinary Journal, the researchers point out that, despite their findings, water deer are unlikely to have contact with cattle in Britain.
Seol-Hee Kim and colleagues surveyed 305 water deer from three provinces of South Korea (Gangwon, South Chungcheong, and Gyeongsang) between 2010 and 2012 and found evidence for low prevalence infection by the bovine viral diarrhoea virus (5/60 = 8%) and rotavirus (1/60 = 2%), but reasonably high prevalence of Brucella abortus (33/56 = 59%), which can cause calf loss in cattle. Compared with a Brucella prevalence of 10-15% in most other species studied to date (e.g. roe deer, goral, and raccoon dogs), this is high, and there is some concern they may act as a transfer host, moving the bacteria between farms. The researchers found no evidence of either coronaviruses or Mycobacterium bovis, although Ji Hyung Kim and his team identified bovine coronavirus (BCoV) in the nasal cavities of three (4%) out of the 77 water deer samples supplied by Chungnam Wild Animal Rescue Center in South Korea between November 2016 and September 2017. Writing in Transboundary Emerging Diseases during 2018, they suggest:
“As the water deer coronavirus was detected from nasal swabs during the winter season, it could be assumed that the water deer coronaviruses might also cause respiratory disease and circulate during the winter season like other BCoVs.”
The presence of Coxiella burnetii, a bacteria pathogen that causes Q fever in livestock (particularly goats, sheep, and cattle) and very occasionally humans, was reported in just over 9% of the 196 serum samples of wild water deer in South Korea analysed by a team at Chonbuk National University, led by Gee-Wook Shin, between 2010 and 2012. The data, published in the Journal of Veterinary Medical Science in 2014, showed prevalence to be variable with location, ranging from just under 6% in Gyeonggi province to almost 13% in Chungnam.
Sung-Suck Oh and team found severe fever thrombocytopenia syndrome (SFTS) viral RNA in one of the 39 water deer samples they analysed from South Korea in 2013, although 24% were seropositive for SFTS viral antibodies implying some previous exposure. More recently, Hyun Seo Lee and colleagues at the National Institution of Environmental Research in South Korea found a low prevalence of this tick-borne viral disease in tissue samples collected between March and November 2017 from provinces around South Korea. Three (2.3%) of the 129 samples yielded the virus, two from Gyeongsangnam-do and one from Gangwon-do. The virus causes fever, thrombocytopenia (low blood platelet), leukocytopenia (low white blood cell count), multi-organ dysfunction, lymph node swelling and diarrhoea. It is an emerging infectious disease in Asia that commonly affects livestock and, in China at least, is often carried by the ticks Haemaphysalis longicornis and Haemaphysalis flava.
During 2008 and 2009, Jun-gu Kang and colleagues assessed the spleens of 66 South Korean water deer collected in Chungcheongbuk-do, Gangwon-do, Gyeonggi-do, and Jeollanam-do provinces and Ulsan City. Their PCR analysis identified Anaplasma phagocytophilum in 64% of samples and Anaplasma bovis in 35%. Anaplasma are bacteria that can cause anaplasmosis, an emerging infectious disease in both humans and livestock, and the authors expressed concern that water deer may act as a reservoir for this pathogen. In their paper to Vector-Borne and Zoonotic Diseases in 2011, the researchers note:
“Our results indicate that the [water deer] in the [Republic of Korea] were naturally infected with A. phagocytophilum and A. bovis. These findings suggest that [water deer] may play an important role in the enzootic maintenance of Anaplasma spp. in the [Republic].”
Similarly, also in South Korea, Giyong Seong and co-workers used a genomic DNA analysis to diagnose parasitism of ten water deer during 2014. Eight samples tested positive for Theileria sp., protozoans that can cause important cattle and equine diseases as well as babesiosis in humans, four were infected with Ehrlichia canis, a bacterium responsible for canine monocytic ehrlichiosis, and two yielded Anaplasma phagocytophilum. In their 2015 article in the Korean Journal of Parasitology, the authors raised the same concern as Kang and his team:
“Our results suggest that the Korean water deer may serve as a major reservoir for these tick-borne pathogens, leading to spread of tick-borne diseases to domestic animals, livestock, and humans.”
In their literature review of wildlife-associated diseases from the Republic of Korea, a team led by Jusun Hwang at Seoul National University listed seven for which the water deer is a confirmed or potential host: Anaplasma spp.; Bartonella grahamii; bovine viral diarrhoea virus; Brucella abortus (occasionally zoonotic, but mostly affects cattle and farmed deer); Coxiella burnetti; severe fever thrombocytopenia syndrome (a zoonotic virus); and Theileria spp. Sungjin Ko and his team found Bartonella grahamii and Bartonella schoenbuchensis bacteria in 16% and 13% of spleens, respectively, of water deer sampled in South Korea between March 2008 and August 2009. Bartonella can cause various rare diseases in humans, including cat scratch disease, lymphadenitis, and peliosis hepatitis, but more commonly affect cattle.
More recently, Seung-Uk Shin and colleagues recorded a number of tick-borne pathogens from water deer killed on roads in South Korea during the autumn of 2018. Their list, published in Vector-Borne and Zoonotic Disease in 2020 included: Anaplasma phagocytophilum (21% of carcasses); Anaplasma capra (14%); Babesia capreoli (4%); and Coxiella burnetii (4%). The authors' concerns are in line with those of their predecessors:
“[the water deer] was considered a reservoir host for A. phagocytophilum. Our results suggest that Korean water deer harbor A. phagocytophilum that may be infectious for humans, dogs, horses, and livestock; thus, they could be relevant reservoirs for granulocytic anaplasmosis in humans and domestic animals in the [Republic].”
Anaplasma and Theileria are pathogens of particular concern, and in South Korea theileriosis is one of the more important diseases of grazing cattle. Jae-Ik Han and colleagues at Chungbuk National University in South Korea identified two cases of infection with Theileria in wild water deer found dead on the roadside in Chungbuk province: one in June and the other in December 2008. Another study conducted around the same time found that 13 (72%) of 18 blood samples taken from wild water deer in Chungbuk province between June 2008 and January 2009 were positive for markers of infection by Theileria sp., with three strains indicated: an unidentified strain that was phylogenetically extremely close to the pathogenic Theileria sp. reported from China and Japan, and Theileria ovis and Theileria capreoli detected individually in two deer. No clinical signs of theileriosis were apparent in the deer, suggesting that they may be asymptomatic carriers.
Finally, a team at Kyungpook National University led by Dorene VanBik detected Borrelia afzelii, a bacterium known to cause Lyme disease in humans, in one of 48 Haemaphysalis longicornis ticks collected from water deer killed on the roads in Gyeonbuk Province, Korea between 2013 and 2015.