Intramuscular vaccination of young calves with a Salmonella Dublin metabolic-drift mutant provides superior protection to oral delivery

In homologous and heterologous challenge trials using calves ≤ 6 weeks old, this comparative study investigated safety, in vivo behaviour and protective properties of oral and intramuscular vaccination with recently developed live attenuated Salmonella Dublin mutant N-RM25. Neither oral nor intramuscular vaccination produced unacceptable side effects. However, the vaccine strain was isolated for up to eight days from the faeces of orally vaccinated calves, but not intramuscularly vaccinated calves. Irrespective of the vaccination route, N-RM25 was isolated in low numbers (≤ 1 x 10² cfu/g) from the liver and spleen of calves euthanized at different time points post-vaccination. Vaccination by either route significantly reduced clinical signs and faecal shedding, prevented the development of systemic infection and protected calves from homologous lethal challenge conducted within 14 days post-immunisation. No challenge strain was isolated from major organs and the gut at 18 days post-challenge (except for a single mesenteric lymph node (MLN) specimen from the intramuscular group, but only following enrichment). Following heterologous challenge with a virulent Salmonella Typhimurium strain at 14 and 20 days post-immunisation, all vaccinated animals exhibited significantly fewer clinical signs and colonisation of the intestinal tract than non-vaccinated controls. When compared to oral vaccination, intramuscular vaccination significantly reduced the frequency of faecal shedding of S. Typhimurium (p = 0.0023) and markedly reduced colonisation ofMLN. The findings indicate that intramuscular administration of N-RM25 was safer in terms of environmental contamination by the vaccine and provided better early onset protection in young calves following both homologous and heterologous challenge.Tetsuo Mizuno, Malcolm McLennan and Darren Trot