The seroprevalence for MV was 18.9% and the prevalence by small lungworms was 11.6%. Protostrongylid infection was detected as a risk factor of MV with an univariate statistical analysis (Chi-squared test). SMALL LUNGWORMS- A CONFOUNDING VARIABLE IN MAEDI-VISNA SHEEP INFECTION López, C.; Viña, M.; Panadero, R.; Fernández, G.; Díaz, P.; Lago, N.; Morrondo, P.; Díez-Baños, P. Universidade Santiago de Compostela. Dpto de Patología Animal. Área de Sanidad Animal. Facultad de Veterinaria. 27002 LUGO INTRODUCTION OBJETIVE The objective of this study is to demonstrate the influence of protostrongylid infection in the prevalence of ovine MV. Serum and faecal samples were taken from 2,081 sheep from 74 semi-intensive lamb-producing flocks in Galicia (NW, Spain). All the animals were mixed breed. Faecal samples were collected directly from the rectum with plastic gloves and kept at 4 ◦ C until being analyzed by the Baermann–Wetzel technique in the same day. Briefly, 10 g of faeces were left overnight with non woven filters (Filter- Lab, Filtros Anoia, S.A., Barcelona, Spain) in glass funnels with a 12 ml centrifuge tube at the end of a silicone tube. These tubes were centrifugated 10 min at 350G and the last milliliter containing the larval population was reserved and analyzed in Favati chambers. The different protostrongylid species were counted and identified by the characteristics of the posterior section of the larvae. If the number of larvae was lower than 100, all of them were identified, and if higher than 100, at least 100 were identified. Blood samples were centrifuged and sera collected within 24 h of sampling and tested for anti-VMV antibodies by means of a commercial enzyme-linked immunosorbent assay (Pourquier® maedi-visna/CAEV, Indirect ELISA/Screening, Institut Pourquier, France), performed according to manufacturer instructions. The assay reported 98% and 97% in sensitivity and specificity, respectively. Optical densities (OD) were read at 450 nm. Sera were analysed in relation to the positive and negative controls using the formula: Samples with a S/P higher than or equal to 120% were considered positive. Statistical analyses were done with R v. 2.12.1 (R Development Core Team, 2010). Chi 2 and Mantel-Haenszel odds-ratio and signification were calculated with cc() and mhor() funtions from epicalc package (Chongsuvivatwong, 2011). Maedi-Visna (MV) is a disease provoked by a virus belonging to the lentivirus subgroup (SRLV), Family Retroviridae, causing a chronic disease in sheep. SRLV does not represent any risk to humans, but their importance is related to economic losses in animals as increases in mortality rates and decreases in lamb weaning weight and adult culling age. MV virus (VMV) is worldwide distributed and the disease has been reported in most European countries. Seroprevalence has been determined in different spanish regions, with great d ifferences (5-77%), due to variation in sampling procedures, sheep production systems, flock average size or diagnostic techniques. Small lungworm (Protostrongylidae) infection has been targeted as a risk factor for the spread of MV virus but its role in the epidemiology of MV is still controversial. MATERIAL and METHODS RESULTS CONCLUSION Protostrongylid infection is only a confounding variable with no real effect on Maedi-Visna infection in sheep. However, protostrongylid-infection effect over MV infection were neglected in a logistic multivariate regression when the age of the animals was introduced as variable. Both MV and protostrongylid infection are chronic diseases and show a cumulative pattern in time, so age and protostrongylid infection are correlated if considered as risk factors for MV infection. A Mantel-Haenszel technique (Holland and Thayer, 1988) was applied and lungworm infection was detected as a confounding variable (Mantel-Haenszel odds ratio calculation). Chi-squared = 6.13 , 1 d.f. , P value = 0.013 Stratified analysis by age OR lower lim. upper lim. P value Age 1 (<13m) 2.48 0.0525 20.71 0.371 Age 2 (13-48m) 1.21 0.6020 2.29 0.512 Age 3 (>48m) 1.25 0.8359 1.85 0.270 M-H combined 1.26 0.9115 1.73 0.163 M-H Chi2(1) = 1.95 , P value = 0.163 Homogeneity test, chi-squared 2 d.f. = 0.4 , P value = 0.819 REFERENCES: Chongsuvivatwong, V. <cvirasak@medicine.psu.ac.th> (2011). epicalc: Epidemiological calculator. R package version 2.12.2.3. http://CRAN.R-project.org/package=epicalc. Holland, P.W., Thayer, D.T. (1988). Differential Item Performance and the Mantel-Haenszel Procedure. In: H. Wainer & H.I. Braun (Eds.), Test validity (pp.129-145). Hilldale, New Jersey: Erlbaum. R Development Core Team (2010). R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. ISBN 3-900051-07-0, URL http://www.R-project.org/.