Serum samples from hunters (n ؍ 440), their hunting dogs (n ؍ 448), and hunters without dog ownership (n ؍ 53) were collected in The Netherlands at hunting dog trials and were tested for antibodies against Borrelia burgdorferi by a whole-cell enzyme-linked immunosorbent assay. Additionally, 75 healthy pet dogs were tested. The results of this study indicate that the seroprevalence among hunting dogs (18%) was of the same order as the seroprevalence among pet dogs (17%) and hunters (15%). The seropositivity of a hunting dog was not a significant indicator of increased risk of Lyme borreliosis for its owner. No significant rise in seroprevalence was found in dogs older than 24 months. This indicated that seropositivity after an infection with B. burgdorferi in dogs is rather short, approximately 1 year. In humans this is considerably longer but is also not lifelong. Therefore, the incidence of B. burgdorferi infections among dogs was greater than that among hunters, despite a similar prevalence of seropositivity among hunters and their hunting dogs. Because no positive correlation was observed between the seropositivity of a hunter and the seropositivity of the hunter's dog, direct transfer of ticks between dog and hunter does not seem important and owning a dog should not be considered a risk factor for Lyme borreliosis.Lyme borreliosis (LB) is a zoonotic disease caused by the spirochete Borrelia burgdorferi (5, 41). An animal reservoir of approximately 40 mammals and birds has been established (15) in Europe. The disease is transmitted primarily by ticks feeding on mammals and birds, with the most common vector in Europe being the tick Ixodes ricinus (1). In humans, LB in its early stages is characterized by influenza-like symptoms, followed in 60 to 80% of the cases by erythema migrans (40), a skin lesion that spreads outward from around the site of a tick bite. If untreated, the disease may proceed to a second or a third stage in which neurological disorders and arthritis are common symptoms (42). Much less is known about LB in animals than is known about the disease in humans. The most common symptom of LB in dogs is migratory arthritis (30) without divergent radiographic findings. Other but less common symptoms reported in dogs are carditis (25), glomerulonephritis (17), and neuritis (2; B. M. Feder, R. J. Joseph, S. D. Moroff, et al., Abstr. Proc. 9th ACVIM, p. 892, 1991). B. burgdorferi infections or serologic evidence of B. burgdorferi infections have been reported in dogs in the United States (3,7,26,29,30). In Europe, relatively few reports exist on LB in animals. In Sweden (13), Denmark (18), Germany (20,21,35,45,47), The Netherlands (19), the United Kingdom (32), Belgium (33), France (9,11,12,14), Switzerland (37), Slovakia (43), Slovenia (34), and Spain (10), antibodies to B. burgdorferi and/or clinical symptoms of LB have been found in dogs. However, in Europe, the use of dogs as sentinel animals for the estimation of the risk of Lyme borreliosis for humans in that region has not been examine...