1160 Scientific Reports: Original Study JAVMA, Vol 221, No. 8, October 15, 2002 RUMINANTS N eospora caninum is a recently recognized proto- zoan parasite that has been associated with abor- tion and, rarely, neurologic dysfunction in several species worldwide but is of primary importance in cat- tle. 1-3 Percentages of abortions attributed to N caninum infection have been estimated to be 12.5% for English and Welsh dairy cattle 4 and 38.7% for dairy cattle in northwestern Spain, 5 and seropositive cattle in endem- ically affected herds have been shown to have 3 times the risk of abortion, compared with seronegative cat- tle. 4,6,7 Seropositive cattle in herds experiencing abor- tion epidemics have been shown to have 40 times the risk of abortion, compared with seronegative cattle. 8 Infection with N caninum is usually acquired via vertical (congenital) transmission. 1,4,5,9,10 However, hor- izontal transmission (eg, oral ingestion) has been demonstrated to be necessary to maintain prevalence of N caninum infection in herds through multiple gen- erations 11 and has been linked to abortion epi- demics. 12,13 The annual economic impact of N caninum for dairy industries, albeit difficult to estimate, has been suggested to be in the range of $35 million in California, 14 $44 million in Australia, 15 $6.78 to 7.2 million in New Zealand, 16,17 and $11.6 million in Japan. 18 Assumptions regarding the effect of infection on milk production are integral to accurate estimates of economic impact. However, the few studies that have analyzed this effect have reported conflicting results. A study 19 of 305-day mature equivalent milk production of 372 first-lactation Holstein cows in a single 2,000-cow dairy in California, for instance, con- cluded that seropositive cows produced 384 kg (845 lb) less milk than did seronegative cows, a decrease of 1.3 kg/cow/d (2.8 lb/cow/d). Similarly, a study 20 of 565 multiparous Holstein cows in a single 700-cow dairy in Florida found that seropositive cows produced 385 kg (847 lb) less milk, a decrease of 1.3 kg/cow/d (2.8 lb/cow/d). In contrast, a study 21 of a single crossbred, spring-calving, 600-cow dairy herd in New Zealand demonstrated an increase in milk volume of 0.4 L (0.11 gallons) in seropositive cows. Finally, a large population-based study 22 in eastern Canada of 2,425 cows from 90 randomly selected farms (mean N can- inum seroprevalence of 19.2%) failed to demonstrate a significant difference in 305-day projected milk pro- duction between seropositive and seronegative cattle. Differences in the observed association between N caninum serostatus and milk production in these studies may, in part, be a result of differences in herd management. For instance, the pasture-based herd management systems of New Zealand are vastly differ- ent from the dry-lot systems of the southwestern United States. Alternatively, differences could be a result of unique characteristics of the individual herds Neospora caninum serostatus and milk production of Holstein cattle Jamie C. Hobson, BSc, DVM; Todd F. Duffield, DVM, DVSc; David Kelton, DVM, PhD; Kerry Lissemore, DVM, DVSc; Sharon K. Hietala, PhD; Ken E. Leslie, DVM, MSc; Beverly McEwen, DVM, PhD, DACVP; Gerard Cramer, DVM; Andrew S. Peregrine, BVMS, PhD, DVM Objective—To determine whether Neospora can- inum serostatus was associated with milk production among Holstein cattle in Ontario. Design—Case-control study and cross-sectional observational study. Animals—3,702 Holstein cows in 83 herds (case- control study) and 3,162 Holstein cows in 57 herds. Procedure—Herds in the case-control study were grouped on the basis of N caninum abortion status. Herds in the observational study were considered representative of Ontario dairy herds. The N caninum serostatus of individual cows was determined with a kinetic ELISA. Milk production was modeled to com- pare seropositive with seronegative animals while controlling for parity, days since parturition, and herd clustering. Results—In the case-control study, 305-day milk pro- duction of seropositive cows was significantly less than milk production of seronegative cows in herds with abortions attributable to N caninum infection and in herds with abortions attributable to pathogens other than N caninum, but not in herds without abor- tion problems. In the observational study, 305-day milk production for seropositive cows was not signif- icantly different from milk production of seronegative cows. Conclusions and Clinical Relevance—Results sug- gest that the association between N caninum serostatus and milk production in Ontario Holstein dairy cattle may depend on abortion status of the herd. In herds with abortion problems, regardless of cause, N caninum-seropositive cattle produced less milk, whereas in herds without abortion problems, N caninum-seropositive cattle produced the same amount of milk as seronegative cattle. (J Am Vet Med Assoc 2002;221:1160–1164) From the Departments of Population Medicine (Hobson, Duffield, Kelton, Lissemore, Leslie, Cramer) and Pathobiology (Peregrine), Ontario Veterinary College, and the Animal Health Laboratory (McEwen), University of Guelph, Guelph, ON N1G 2W1, Canada; and the California Animal Health and Food Safety Laboratory System, University of California, Davis, CA 95616 (Hietala). Supported by the Ontario Ministry of Agriculture, Food and Rural Affairs, the Ontario Cattlemen’s Association, and Dairy Farmers of Ontario. Presented in part at the 34th Annual Conference of the American Association of Bovine Practitioners, Vancouver, BC, Canada, September 2001. Address correspondence to Dr. Hobson.