The bacterial genus Anaplasma contains several tick-transmitted species associated with livestock disease worldwide. Anaplasma can be considered haemoparasites as they persist within either the white or red blood cells of their hosts. In the United Kingdom, the species of primary veterinary concern is A. phagocytophilum, a parasite of neutrophils, which is associated with tick-borne fever in sheep and pasture fever in cattle. Tick-borne fever typically causes high fever (as high as 42.0 °C) lasting for up to 10 days accompanied by increased respiratory and pulse rates and often a cough. Sheep may also appear dull and may lose weight. Pasture fever presents in a similar manner, with cows developing respiratory distress and cough, often associated with loss of appetite and reduced milk yield. Both tick-borne fever and pasture fever occur primarily in young animals or new stock as they are turned out onto tick-infested pastures in spring and early summer. Although both tick-borne fever and pasture fever are rarely life-threatening and can be effectively treated with oxytetracyclines, abortion is a possible complication in pregnant sheep and cattle (Woldehiwet 2020). Anaplasma phagocytophilum also indirectly contributes to the development of tick pyaemia, a disease that primarily affects lambs (<12 weeks old) and is characterised by lameness and paralysis, resulting from the formation of disseminated abscesses in joints and other parts of the body.

Staphylococcus aureus is the primary agent isolated from these abscesses, where it is an opportunistic, skin-derived pathogen that exploits the generalised immunosuppression provoked by concurrent A. phagocytophilum infection (Woldehiwet 2006). Tick pyaemia can be life-threatening, but treatment with penicillin or tetracycline can be effective, provided the abscesses are not too advanced. No outbreaks of tick-borne fever or tick pyaemia in farmed deer have been reported and the frequency of disease associated with A. phagocytophilum in wild ruminants is unknown beyond a handful of case studies from Norway that implicated (rather than demonstrated) its involvement in disease in roe deer and moose (Jenkins et al. 2001; Stuen et al. 2001, 2006).

Infection with A. phagocytophilum is frequently asymptomatic. Surveys of apparently healthy animals have repeatedly revealed infections at high prevalence (e.g. 38% of sheep in northeast Wales, Ogden et al. 2002). Furthermore, infections are not restricted to livestock; A. phagocytophilum has been detected in the blood of a wide range of wildlife, including deer. A handful of surveys of UK deer have been reported, with infection detected in roe deer (prevalence of between 7 and 47%; Alberdi et al. 2000; Bown et al. 2009; Duscher et al. 2020), muntjac (1%; Duscher et al. 2020) and red deer (5%; Johnson et al. 2021). Surveys of deer elsewhere in Europe and the countries where exotic deer species originated have also demonstrated fallow deer (Fabri et al. 2021) and Japanese sika deer (Silaghi et al. 2020) are frequently infected; thus, all these species should be considered as potential reservoir hosts. Other wildlife species implicated as reservoir hosts for A. phagocytophilum infections include rodents and small insectivores (Bown et al. 2011), carnivores (Lesiczka et al. 2023), bats (Afonso and Goydadin 2018) and birds (Jahfari et al. 2014). As A. phagocytophilum is encountered in such a vast range of hosts and is widely distributed across the northern hemisphere and beyond, numerous tick species must be involved in the transmission of infection. In the United Kingdom and the rest of Europe, I. ricinus is the key vector, although other tick species are also competent (Bown et al. 2009).

Anaplasma phagocytophilum is associated with zoonosis, causing human granulocytic anaplasmosis (HGA). HGA most commonly presents as fever, headache, chills (feeling cold and shivering) and muscle aches. Treatment with doxycycline is usually effective. Left untreated, far more profound and potentially life-threatening manifestations can develop. Interestingly, whereas thousands of cases of HGA are reported each year in the United States, the annual incidence across Europe is estimated at less than 300 (Matei et al. 2019). This discrepancy may reflect differences in the diversity of A. phagocytophilum strains in the Old and New Worlds. Molecular fingerprinting has delineated European A. phagocytophilum strains into four ‘ecotypes’, only one of which is associated with zoonosis (Jahfari et al. 2014), whereas the zoonosis-associated ecotype appears to be dominant in the United States (Aardema 2023).

Babesia are apicomplexan protozoa that have a complex life cycle involving sexual reproduction in ticks, then proliferation in the red blood cells of their vertebrate hosts. Many different species have been encountered in a wide range of host species, some of which are associated with disease in livestock and humans. In the United Kingdom, B. divergens is the species with the highest economic impact, being the causative agent of redwater fever in, primarily, cattle. Redwater fever is characterised by, as the name suggests, infected animals producing red/red-brown urine as a result of ‘haemoglobinuria’ or Babesia-induced lysis of red blood cells. If untreated, the disease can progress and be life-threatening. Imidocarb is the drug of choice and is usually effective in resolving signs and clearing babesia infections (Meunier 2020). Babesia divergens can also provoke similar disease in humans, but almost all cases to date have occurred in splenectomised people (Zintl et al. 2003). A second, far more recently characterised Babesia species, Babesia venatorum, has also been associated with human babesiosis in continental Europe (Herwaldt et al. 2003).

Deer are susceptible to babesia infections and there have been reports of babesiosis in deer in the United Kingdom and across Europe. A large outbreak of B. divergens-associated babesiosis occurred in captive reindeer in Scotland in 2001 (Langton et al. 2003) and case reports of B. venatorum-associated babesiosis have been reported in captive reindeer in the Netherlands and Switzerland (Kik et al. 2011; Novacco et al. 2019). However, surveys of apparently healthy deer in the United Kingdom and elsewhere in Europe have revealed that asymptomatic infections are common and widespread. Furthermore, in addition to B. divergens and B. venatorum, other species not yet associated with disease are also encountered. These include Babesia capreoli and a partially characterised species often referred to as ‘Babesia odocoilei-like’ (e.g. Hrazdilová et al. 2020). Babesia divergens was first reported in healthy red deer over 50 years ago (Latif and Adam 1973) and a survey of 105 red deer from across the United Kingdom revealed five asymptomatic infections caused by the B. odocoilei-like organism (Johnson et al. 2021). A separate study of 84 red deer in the Scottish Highlands revealed babesia infections in 22 (26%); nine of these infections were caused by B. divergens and 13 by the B. odocoilei-like organism (Gray et al. 2021). The B. odocoilei-like organism has been detected in a roe deer in Cumbria and, in the same study, this Babesia and B. venatorum were detected in questing I. ricinus ticks collected on Cumbrian farmland (Perrin 2017). Furthermore, a large-scale study of questing I. ricinus ticks collected from sites across England and Wales detected B. venatorum as well as B. divergens, B. capreoli and B. odocoilei-like organisms, albeit at low prevalence (overall 0.4% of nearly 4000 ticks tested; Gandy et al. 2024). Babesia venatorum infections have also been reported in apparently healthy sheep in the United Kingdom, thereby implicating sheep as a reservoir for this species (Gray et al. 2019).

Surveys of other deer species in the United Kingdom have yet to be reported, but in continental Europe, surveys of roe and fallow deer have encountered babesia infections. For example, B. capreoli and B. venatorum were encountered in roe deer and the B. odocoilei-like organism was encountered in fallow deer surveyed across Germany (Kauffmann et al. 2017). Similarly, a survey in Czechia reported a 65% infection rate in deer, with frequent B. capreoli and B. venatorum infections in roe deer, infections due to B. divergens, B. capreoli and the B. odocoilei-like organisms in red deer, and B. divergens and B. capreoli infections in sika deer (Hrazdilová et al. 2020). Thus, given the likelihood that these four species are transmitted by I. ricinus, it is likely all deer species in the United Kingdom harbour a similar diversity of Babesia.

Borrelia burgdorferi is a spirochaete bacterium that causes Lyme disease in humans. Lyme disease is the most commonly reported vector-borne zoonosis across temperate regions of the northern hemisphere. In the United Kingdom, about 1600 laboratory-confirmed cases were reported in 2021, but it is estimated that the actual number of cases each year is at least three times higher than this figure, as many cases are diagnosed solely on clinical grounds. The most recognisable symptom of Lyme disease is an intense ‘bull’s eye’ rash (erythema migrans) spreading in concentric circles around the site of a tick bite, but this pathognomic sign is not always present. Other common symptoms include fever and sweating, chills, fatigue, neck pain or stiffness, headaches, joint or muscle and/or nerve pains [neuralgia]. More serious symptoms may develop if Lyme disease is left untreated or treatment is delayed (treatment is, typically, a three-week regimen of doxycycline). These include inflammatory arthritis, neurological problems such as numbness, paralysis of the facial muscles, memory problems and difficulty concentrating, also meningitis, which can manifest as severe headache, a stiff neck and/or increased sensitivity to light. In many patients, these presentations resolve with treatment, albeit slowly in some cases, but a few go on to develop a range of chronic, debilitating symptoms despite treatment.

The veterinary importance of B. burgdorferi is less well-established. Numerous case reports of Lyme disease in companion animals have been published (e.g. Silvestrini et al. 2023); substantial rates of seropositivity (reflecting exposure) to B. burgdorferi are reported in animals but with no relevant medical history (Miró et al. 2022). Neither cattle nor sheep are thought to be susceptible to systemic B. burgdorferi infection (Ogden et al. 1997; Sprong et al. 2020).

Borrelia burgdorferi exploits a wide range of wild-living vertebrate species as reservoir hosts, between which it is transmitted by ixodid ticks. Reservoir hosts include a wide range of small mammals such as wood mice (Apodemus sylvaticus), bank voles (Myodes glareolus), hedgehogs (Erinaceus europaeus) and squirrels (Sciurus carolinensis), also foraging birds such as blackbirds (Turdus merula) and thrushes (Turdus philomelos; Mannelli et al. 2012). Remarkably, deer are not reservoir hosts for B. burgdorferi; thus, ticks that have fed on deer do not carry the pathogen. However, this does not mean deer are irrelevant in terms of Lyme disease risk, as they are integral ‘blood-hosts’ for ticks and thus play an important role in maintaining tick populations across most of the United Kingdom. Studies have shown that even though the proportion of infected ticks is lower when deer are present, usually the abundance of ticks is increased by so much that the abundance of infected ticks is also augmented (Gandy et al. 2021). Thus, the likelihood of a human encountering an infected tick in a woodland inhabited by deer is greater than in a woodland where deer are absent. Such is the importance of deer as hosts for ticks that they have been targeted in efforts to reduce tick numbers and therefore reduce Lyme disease risk. Reducing deer numbers is widely considered a key part of effective community-level intervention to reduce the risk of Lyme disease (Telford 2017) but the scale of reduction needed is unclear (Hofmeester et al. 2017). Alternative interventions have also been proposed, including acaricide treatment of deer using baits or topical application in the field (Stafford and Williams 2017) and vaccination of deer with anti-tick vaccines (Contreras et al. 2020). However, these remain in development and are still a long way from being available for routine use, including in the United Kingdom.

Bartonella species are fastidious haemotrophic Gram-negative bacteria that persist within the erythrocytes of their natural hosts. These hosts include many terrestrial vertebrates including deer. Two of the 40 or so valid Bartonella species, Bartonella capreoli (Bermond et al. 2002) and Bartonella schoenbuchensis (Dehio et al. 2001) were first isolated from apparently healthy roe deer in France and Germany, respectively. Subsequent studies have demonstrated infections by these Bartonella species and a third, Bartonella bovis (originally isolated from cattle) occur at high prevalence in cervid populations across Europe; B. schoenbuchensis has been detected in roe deer, red deer and moose (Skotarczak and Adamska 2005; Adamska 2008; Welc-Falȩciak et al. 2013; Razanske et al. 2018; Wijburg et al. 2022), B. bovis has been detected in roe deer, red deer, elk and moose (Adamska 2008; Doudo et al. 2013; Razanske et al. 2018; Sacristán et al. 2021) and B. capreoli has been detected in roe deer, elk and Japanese sika deer (Sato et al. 2012; Welc-Falȩciak et al. 2013; Wijburg et al. 2022). Most of these studies employed molecular (polymerase chain reaction-based) methods to detect bartonellae, although isolation of bacteria on blood-rich agar is relatively straightforward; examination of Giemsa-stained blood smears can also reveal their presence.

Surveys of other cervids present in the United Kingdom (and Europe), such as fallow deer, muntjac deer and (Chinese) water deer, have yet to be reported; there are currently no published data on the epidemiology of bartonella infections in UK deer populations. How deer-associated bartonellae are transmitted between hosts is still debated, but there is mounting evidence that deer keds (L. cervi

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