Across Europe and North America, sheep and goats can become infected with obligate intracellular bacteria of the genus Anaplasma. Whereas Anaplasma phagocytophilum is widespread in many European countries, an infection with Anaplasma ovis mainly occurs in the Mediterranean Basin [1]. However, reports about the focal occurrence of A. ovis in Central European countries like Hungary, Slovakia and Germany are increasing [2,3,4]. Both pathogens are also present in the US sheep population, but detailed information about the dissemination is lacking [5,6].

Wild ruminants may act as a reservoir for both pathogens in Europe and North America [3,7,8,9,10,11]. The transmission of Anaplasma spp. usually happens through tick bites [1]. The main vectors of A. phagocytophilum are Ixodes ricinus in Europe, as well as Ixodes scapularis, Ixodes pacificus and Ixodes spinipalpis in North America [1,11,12,13]. Different tick species belonging to the genera Dermacentor, Rhipicephalus and Hyalomma are considered to transmit A. ovis [13,14]. In recent years, A. ovis was also found in sheep keds (Melophagus ovinus) but their vector competence remains doubtful [15,16].

The replication of A. phagocytophilum takes place within the vacuoles of neutrophil granulocytes and sometimes also lymphocytes [17]. This causes granulocytic anaplasmosis in many domestic animals, such as horses [18,19], cattle [20,21], dogs [22,23] and cats [24,25], and also in humans [12]. In small ruminants, A. phagocytophilum results in tick-borne fever (TBF) and affected animals suffer from high fever, anorexia and dullness [26,27,28]. Neutropenia and thrombocytopenia are the haematological key findings in affected sheep and goats [26,29]. Immunosuppression causes a high susceptibility to secondary infections like Mannheima haemolytica and Bibersteina trehalosi and leads to respiratory distress in lambs [30,31]. Furthermore, A. phagocytophilum favours co-infections with staphylococcal bacteria which cause tick-pyaemia with polyarthritis [29]. TBF and co-infections can be fatal for lambs [29,30,31]. However, mild courses of A. phagocytophilum were reported but affected lambs had reduced growth rates [28]. Goats show similar clinical signs to sheep after an infection with A. phagocytophilum, but to a lesser extent [26,32,33].

Anaplasma ovis mainly affects the ovine and caprine erythrocytes [34] but can also be found in wild ungulates like roe deer (Capreolus capreolus) and red deer (Cervus elaphus) [7,15,35]. Humans rarely become infected [36]. The pathogen causes ovine anaplasmosis especially in sheep in poor health [37]. Main clinical signs in sheep are fever, severe anaemia, extreme weakness, anorexia, and weight loss [34,37,38,39]. Moreover, haemoglobinuria and icteric carcasses were also described in sheep infected with A. ovis [2,39]. An acute infection results in decreased values of red blood cells, haemoglobin and packed cell volume [40]. Although the same signs are described for goats as for sheep, A. ovis appears to be more pathogenic for goats [41].

In Germany, A. phagocytophilum was identified in I. ricinius across the country with detection rates between 1.9% and 5.4% [42,43,44]. Although A. phagocytophilum has been well described in domestic animals [19,20,25,45] and wild ungulates [9,46], knowledge of the occurrence of the pathogen in German sheep and goat flocks is still limited. A molecular investigation revealed an infection rate of 4% (n = 255) in sheep from Northern Germany [8] and a clinical case of TBF was described in a goat from western Germany [47]. Recently, A. phagocytophilum was detected by PCR in five sheep flocks located in the southern part of the country [4]. In the same study, A. ovis was identified for the first time in a German sheep flock.

2. Animals