The disease is caused by the Gram-negative bacterium Anaplasma phagocytophilum and is transmitted by I ricinus, which is also responsible for spreading Lyme disease.

There have been numerous reports of tick-borne fever affecting dairy herds in the UK (Anonymous, 2016). The organism A phagocytophilum is currently considered endemic in dogs in the UK (Silvestrini et al, 2023).

The intra-erythrocytic parasites of the genus Babesia, which cause babesiosis in humans and animals, are present in questing nymphs of I ricinus (deer tick) in England and Wales at a low prevalence.

Although the risk of encountering Babesia-infected ticks is lower compared to other pathogens such as Borrelia burgdorferi sensu lato, localised areas with higher risk may still exist (Gandy et al, 2024).

Various Babesia species, including B caballi, have been identified in D reticulatus ticks in regions of west Wales and southern England, suggesting their presence in areas where horses graze.

This underscores the importance of raising awareness about the risk of equine piroplasmosis in areas where D reticulatus ticks are established, due to the presence of B caballi (Sands et al, 2022).

Nymphs of I ricinus collected from cattle farms in south-west England harbour Babesia, primarily B divergens and B venatorum, with indications of B divergens clustering within these ticks.

This highlights the extensive presence of zoonotic B venatorum in environments frequented by livestock, emphasising implications for both animal and human health (Sands et al, 2022).

Canine babesiosis is attributed to B canis, B vogeli, and B gibsoni, which are regarded as non-native to the UK. Reports indicate instances where dogs have contracted B canis and B vogeli after traveling in Europe (Tappin, 2009; Johnson et al, 2013).

However, cases of locally transmitted B canis by D reticulatus ticks were documented in Harlow, southern England, between 2015 and 2016 (de Marco et al, 2022). Recently, B vulpes infection has been identified in a dog that had never travelled outside the UK, indicating that certain canine Babesia species might be establishing themselves within the country (Silvestrini et al, 2023).

Canine babesiosis has the potential to spread widely, as its vectors, D reticulatus and an unidentified Ixodes species tick, are endemic in the UK.

Ehrlichia canis, a Gram-negative, obligate intracellular rickettsial bacterium, has been first detected in the south-east of England in dogs with no travel history outside the UK (Wilson et al, 2013).

This organism is transmitted by Rh sanguineus and infects monocytes and macrophages, leading to a condition known as monocytotropic ehrlichiosis in dogs.

E canis was recently detected in 2 out of 131 (1.5%) imported dogs seized during an animal welfare investigation led by the RSPCA (Wright et al., 2023).

Louping ill virus (LIV) is the only native virus transmitted by ticks in the UK, causing viral encephalomyelitis primarily in sheep, which exhibit neurological symptoms, such as incoordination and ataxia.

Other mammalian infections are rare, but red grouse are economically significant in terms of susceptibility (Reid, 1975). LIV is closely related to tick-borne encephalitis virus detected in Eurasia and primarily affects humans instead of livestock. It is prevalent in upland areas of the British Isles, with occasional cases reported in sheep from regions including Cumbria, west of Scotland, Devon and Wales (Gilbert, 2016).

Lyme disease stands as one of the most widely recognised TBDs in the UK. It is caused by the spirochete bacterium Borrelia burgdorferi sensu lato complex and transmitted to humans through bites from infected ticks, predominantly the I ricinus species.

Common symptoms include fatigue, fever, headache and a distinctive bullseye rash. If untreated, Lyme disease can progress to more serious complications affecting the joints, heart, and nervous system. Within the B burgdorferi sensu lato complex, several genospecies are notable in the UK, with B burgdorferi sensu stricto, B garinii, and B afzelii being the most significant.

Additionally, Borrelia miyamotoi, another spirochete bacterium associated with Lyme borreliosis, has also been identified in the UK, showing a broad, but relatively low-level distribution across geographic regions (Layzell et al, 2018).

Areas with high tick populations are often referred to as Lyme borreliosis “hotspots,” although they may not necessarily harbour the highest proportion of infected ticks (Layzell et al, 2018).

In 2019, the first evidence of tick-borne encephalitis virus (TBEV) was discovered in the UK, notably in two locations: Thetford Forest in East Anglia and along the border of Hampshire and Dorset in southern England.

The discovery challenges previous predictions that TBEV could not establish in the UK due to climate conditions. The Thetford strain is similar to a strain from Norway (Holding et al, 2020), while the Hampshire-Dorset strain resembles one from the Netherlands (Holding et al, 2019), indicating that there were likely multiple introductions into the UK, potentially through migratory birds transporting infected ticks.

While no confirmed cases of locally acquired TBE in humans have been reported in the UK, a suspected case was noted in close proximity to where TBEV was detected in Hampshire (Kreusch et al, 2019), highlighting the potential risk to public health, especially since TBEV can cause encephalitis, a severe inflammation of the brain.

Diagnosis in the UK is complicated by cross-reactivity with other endemic viruses, making confirmatory testing challenging. Additional research is necessary to comprehend the distribution and prevalence of TBEV in the UK. Health care providers should remain alert for TBEV infections in patients who have not travelled recently, particularly if they have experienced recent tick bites.

Theileriosis is a TBD affecting livestock such as cattle, sheep, goats and equids, caused by Theileria species, apicomplexan protozoa closely related to Babesia. Unlike Babesia, Theileria species are transmitted transstadially within the tick vector.

This means that infection occurs when larval or nymphal ticks feed on infected animals and persists through subsequent nymphal and adult stages. Unlike Babesia, no evidence exists of Theileria species being transmitted from adult female ticks to their offspring (transovarial transmission).

Both Babesia and Theileria species are transmitted through bites from infected ticks. Babesia species directly invade red blood cells, whereas Theileria species initially undergo a lymphocytic phase (called schizogony) before invading host red blood cells for further development (Folly et al, 2020).

The presence of Theileria has been documented in cattle in southern England, where it is transmitted by Haemaphysalis punctata ticks (Brocklesby and Barnett, 1972). Additionally, Theileria species have been identified in mosquitoes that fed on cattle in areas where H punctata is prevalent (Fernández de Marco et al, 2016).

Through genomic analysis, Theileria orientalis has been detected, a pathogen known for causing severe disease in cattle across Asia and Australasia. Despite these findings, clinical cases of bovine theileriosis have not been reported in the UK.

In north Kent, cases of ovine theileriosis caused by Th luwenshuni have been linked to a dense tick population (Phipps et al, 2016).