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Antibodies to equine influenza in real life -a study of Swedish trotters in training

Published 2019-03-06

The general aim of this study was to increase knowledge on the outcome of influenza vaccination in competition horses. Specifically, we monitored the influenza antibody levels in Standardbred trotters in training vaccinated twice yearly during a 13-month period.

The study comprised 56 horses from four trainer’s yards, 21 mares, 27 stallions and 8 geldings aged between 2 and 8 years at the start of sampling. The horses were trained and raced as usual during the study and were high to average performers according to the trainers. Horses were vaccinated twice yearly, in December and in June, with a commercial equine influenza vaccine according to the trainers’ standard procedures. Serum samples were collected monthly, from August year 1 to August year 2 and in total 549 serum samples were available for the present study. Antibody levels to the influenza virus strain A/equi 2/Borlänge/91 (an antigen included in the vaccine) were determined with an in-house ELISA.

Results showed that all horses were seropositive to influenza virus throughout the study period although antibody levels showed a large variation between individuals. Significant increases in antibody levels were observed at sampling occasions one and two months after vaccinations where after antibody levels declined to near pre-vaccination levels. During the present study period the December vaccination induced significantly higher increases in antibody levels compared to the June vaccination. The December vaccination was also preceded by the lowest antibody levels during the study. Moreover, a correlation between low pre-vaccination and high post-vaccination antibody levels was observed. A selection of 105 samples with the highest and lowest antibody levels according to the ELISA were also tested using SRH-methodology that is considered to give correlates of protection against virus shedding and clinical disease. From these results it was calculated that the antibody levels detected in our material at infection with virus strains in the vaccine conferred protection against clinical disease in 100 – 94 % of the horses and protection against virus shedding in 97 – 45 % of the horses during the 13-month period.

Taken together, the kinetics of vaccination induced antibodies in this study differ from what has earlier been reported from experimental studies with this vaccine type as we observed more rapid declines in antibody levels in the present material. This is the first report of long-term kinetics of influenza vaccine induced antibody responses in competition horses and the results will contribute to the evaluation and design of vaccination regimes.