Simple Summary There are limited data on Lyme borreliosis, a tick-borne disease caused by the bacteria of the Borrelia burgdorferi sensu lato complex, in horses. Seropositivity is not necessarily associated with clinical disease. Data on seropositivity against Borrelia burgdorferi sensu lato and Anaplasma phagocytophilum in German horses are sparse. Therefore, serum samples from horses (n = 123) suspected of having Lyme borreliosis and clinically healthy horses (n = 113) residing at the same stables were tested for specific antibodies against Borrelia burgdorferi sensu lato and Anaplasma phagocytophilum. First, the horses were screened for antibodies against Borrelia burgdorferi sensu lato. Afterward, the horses were screened for antibodies against Borrelia burgdorferi sensu lato and Anaplasma phagocytophilum (SNAP(& REG;) 4Dx Plus(& REG;) ELISA). The clinical signs of suspect horses included lameness (n = 36), poor performance (n = 19), and apathy (n = 12). Twenty-three percent (n = 26) of suspect horses and 17% (n = 18) of clinically healthy horses were seropositive for having a Borrelia burgdorferi sensu lato infection (p = 0.371), showing that the detection of specific antibodies against Borrelia burgdorferi alone is not sufficient for a diagnosis of equine LB. Anaplasma phagocytophilum-seropositivity and seropositivity against both pathogens was 20%/6% in suspect horses and 16%/2% in the clinically healthy population, showing only minor differences (p = 0.108). Unspecific testing for antibodies against Borrelia burgdorferi sensu lato without clinical suspicion of Lyme borreliosis is not recommended since the clinical relevance of seropositivity against Borrelia burgdorferi sensu lato remains unknown. There are limited data on Lyme borreliosis (LB), a tick-borne disease caused by the Borrelia burgdorferi sensu lato complex, in horses. Seropositivity is not necessarily associated with clinical disease. Data on seropositivity against Borrelia burgdorferi and Anaplasma phagocytophilum in German horses are sparse. Therefore, serum samples from horses (n = 123) suspected of having Lyme borreliosis and clinically healthy horses (n = 113) from the same stables were tested for specific antibodies against Borrelia burgdorferi sensu lato and Anaplasma phagocytophilum. The samples were screened for antibodies against Borrelia burgdorferi (ELISA and an IgG line immunoblot assay). Furthermore, the samples were examined for antibodies against B. burgdorferi and Anaplasma phagocytophilum with a validated rapid in-house test (SNAP(& REG;) 4Dx Plus(& REG;) ELISA). The clinical signs of suspect horses included lameness (n = 36), poor performance (n = 19), and apathy (n = 12). Twenty-three percent (n = 26) of suspect horses and 17% (n = 18) of clinically healthy horses were seropositive for having a Borrelia burgdorferi sensu lato infection (p = 0.371), showing that the detection of specific antibodies against B. burgdorferi alone is not sufficient for a diagnosis of equine LB. Anaplasma phagocytophilum seropositivity and seropositivity against both pathogens was 20%/6% in suspect horses and 16%/2% in the clinically healthy population, showing only minor differences (p = 0.108). Unspecific testing for antibodies against B. burgdorferi without clinical suspicion of Lyme borreliosis is not recommended since the clinical relevance of seropositivity against Borrelia burgdorferi sensu lato remains to be elucidated.
