February 17, 2018

Study Examines Primary Rabies Immunization in Young Dogs

Researchers found distinct differences in rabies vaccine effectiveness depending on signalment, number of vaccines administered, vaccine timing, and other factors.
By Natalie Stilwell, DVM, MS, PhD
Researchers in Greece recently performed a retrospective study examining whether variables such as age, breed, vaccine type, and number of vaccine doses affect rabies antibody titer in young dogs. The results were published in Veterinary Microbiology.

Analysis
Serum sample records were retrieved from the Greek National Reference Laboratory for Rabies, an approved rabies serology laboratory that verifies vaccine effectiveness via titer measurement using a fluorescent antibody virus neutralization test. The laboratory follows international standards for titers: 0.5 IU/mL or higher is acceptable and less than 0.5 IU/mL is considered vaccine failure.

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Serum samples were included in the study if they met the following criteria:
  • The dog was less than 12 months old at the time of the most recent rabies vaccination.
  • Serologic testing was performed prior to the first annual booster after 12 months of age.

Results
Serum antibody titer data were available for 332 male and 330 female dogs from 2008 to 2015. Purebred and mixed-breed dogs were equally represented. Included were 215 (32.5%) small dogs, 75 (11.3%) medium dogs, and 149 (22.5%) large dogs; size data were not recorded for 223 (33.7%) dogs.

Acceptable titer threshold (≥0.5 IU/mL) was achieved for 573 (86.5%) dogs, while 89 (13.5%) dogs experienced vaccine failure as indicated by titer values less than 0.5 IU/mL. Five hundred forty-six dogs (82.5%) received 1 vaccine before 12 months of age, while 116 (17.5%) dogs received 2 vaccines. Dogs that were vaccinated twice were more likely to have an acceptable titer (91.1%) compared with those vaccinated once (83.9%), and median titer was higher after 2 vaccine doses compared with 1 (13.77 vs. 4.56 IU/mL).

The age of most recent rabies vaccination was 0 to 3 months in 35 (5.3%) dogs, 3 to 6 months in 275 (42.5%) dogs, 6 to 9 months in 168 (25.4%) dogs, and 9 to 12 months in 142 (21.5%) dogs. The time interval between vaccinations was not recorded for dogs that received 2 vaccines. The median interval between most recent vaccination and blood sampling was 5.29 weeks, and seropositivity decreased with increasing time interval. Results suggest the optimal time to measure viral titers is 4 to 6 weeks after rabies vaccination.   

Most (88.7%) dogs received a monovalent rabies vaccine, which was more likely to provide acceptable titers compared with a polyvalent vaccine (91% vs. 54% acceptable rate). Mixed-breed dogs had higher antibody titers than purebred dogs did, and the vaccine failure rate was significantly higher in puppies less than 3 months old compared with older dogs. Body size did not significantly affect vaccination outcome.

Take-home Message
Administering 2 rabies vaccines resulted in significantly higher viral titers in dogs younger than 1 year. Monovalent vaccines provided a superior vaccination response compared with polyvalent vaccines, and the highest titers were achieved when vaccination was administered in puppies at least 3 months of age.

 
Dr. Stilwell is a medical writer and aquatic animal veterinarian in Athens, Georgia. After receiving her DVM from Auburn University, she completed an MS degree in Fisheries and Aquatic Sciences, followed by a PhD degree in Veterinary Medical Sciences, at the University of Florida.

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