Serologic Diagnosis of Equine Borreliosis and Anaplasmosis: Evaluation of an In-Clinic ELISA (Snap®4Dx®)
Borrelia burgdorferi, the causative agent for Lyme disease, and Anaplasma phagocytophilum (Aph) (formerly Ehrlichia equi), the causative agent for granulocytic anaplasmosis, infects a wide range of mammalian hosts. Serological studies in horses indicate that incidence of equine Borrelia and Anaplasma infection is increasing in the northeastern United States, the Midwest, Texas and California. Clinical disease in horses has been associated with lameness, stiffness, joint swelling, lethargy, fever, weight loss, uveitis, and potentially with neurologic disease and foal mortality.
The SNAP®4Dx® assay (IDEXX Laboratories, Westbrook, ME) is a commercially available in-office test kit for the simultaneous detection of antibodies to B. burgdorferi, A. phagocytophilum, Ehrlichia canis and Dirofilaria immitis antigen in blood, plasma or serum of dogs. The test kit is an ELISA that uses synthetic peptides, C6 derived from the IR6 region within the Borrelia membrane protein VlsE, and a peptide derived from the immuno-dominant p44 protein of A. phagocytophilum. Studies with canine samples suggested that SNAP 4Dx was useful in endemic areas because it can be conveniently and reliably used in the clinic to determine the infection status of a dog (ACVIM, 2006). We evaluated the performance of SNAP 4Dx for the detection of antibodies to Lyme and Aph in equine serum samples from northeastern United States and Alaska. A total of 164 equine samples obtained from the University of Connecticut were tested by SNAP 4Dx and QualiCodeTM B. burgdorferi IgG/IgM Western Blot Kits (Immunetics, Cambridge, MA) and Aph immunofluorescence assay (IFA). Of the serum samples tested, 109 were positive for Lyme by SNAP 4Dx and 54 were positive for Aph by SNAP 4Dx. Twenty four samples were co-infected with Lyme and Aph. Of the 109 samples that tested positive for Lyme by SNAP 4Dx, only 106 were positive by Western Blot Assay. The three discordant samples were positive by IFA and had a low reciprocal antibody titer of 64. All the 54 samples that were positive for Aph by SNAP 4Dx tested positive by IFA. Thus, relative to Lyme Western Blot Assay, SNAP 4Dx had a sensitivity and specificity of 100 and 95%, respectively, for the detection of antibodies to Lyme. Relative to IFA, SNAP 4Dx had a sensitivity and specificity of 100% for the detection of antibodies to Aph.
These results indicate that SNAP 4Dx can be successfully used to detect antibodies to B. burgdorferi and A. phagocytophilum in infected horses.