Effect of living Rev. 1 vaccine in producing long-term immunity against Brucella melitensis infection in sheep in Iran

Evaluation of shedding, tissue burdens, and humoral immune response in goats after experimental challenge with the virulent Brucella melitensis strain 16M and the reduced virulence vaccine strain Rev. 1

Brucella melitensis is the causative agent of brucellosis in small ruminants and is of considerable economic and public health importance in many countries worldwide. The control of disease in humans depends on the control of disease in livestock; however, few counties with endemic B. melitensis infection have been able to successfully eradicate this pathogen. This underscores the need for further research on the pathogenesis of both virulent and vaccine strains of B. melitensis in the small ruminant host. The aim of the present study was to characterize clinical effects, tissue colonization, shedding, and humoral immune response following B. melitensis infection in goats. Both virulent (16M) and reduced virulence (Rev. 1) strains of B. melitensis were studied. Pregnant goats were infected at 11–14 weeks of gestation with 8 x 10⁶ or 8 x 10⁷ CFU of B. melitensis. Infection of goats with B. melitensis 16M resulted in an 86% abortion rate. This strain disseminated widely in pregnant does post-infection with none of the 15 sampled tissues spared from colonization. Importantly, we report the first isolation of B. melitensis from muscle tissue in ruminants. Pathogenesis of Rev. 1 infection was variable with two does showing minimal colonization and one doe exhibiting disease similar to that of animals infected with fully virulent 16M. Shedding of B. melitensis in milk occurred in all 16M- and Rev. 1- infected goats. In pregnant animals challenged with virulent B. melitensis, median time to seroconversion was 21 days; however, 2 animals did not seroconvert until after abortion.

Essential role of vaccines in brucellosis control and eradication programs for livestock

Brucellosis, in particular infections with Brucella abortus, Brucella melitensis or Brucella suis, remains a significant human health threat in many areas of the world. The persistence of pathogenic Brucella spp. in domestic livestock or free-ranging wildlife remains unresolved, despite decades of regulatory efforts worldwide. Although vaccination is probably the most economic control measure, administration of currently available vaccines alone is not sufficient for elimination of brucellosis in any host species. Complacency in brucellosis control programs usually results in failure, or at best, limited reductions in disease prevalence or incidence of human infections. New brucellosis vaccines with high efficacy and safety are needed that address the diversity in host species and can be more widely applied under field conditions. Development of safer and more efficacious vaccines alone, or combined with enhancements or increased emphasis on other regulatory program components, could have tremendous impact on reducing the worldwide prevalence of brucellosis and the associated zoonotic infections.

Progress in Brucella vaccine development

Brucella spp. are zoonotic, facultative intracellular pathogens, which cause animal and human disease. Animal disease results in abortion of fetuses; in humans, it manifests flu-like symptoms with an undulant fever, with osteoarthritis as a common complication of infection. Antibiotic regimens for human brucellosis patients may last several months and are not always completely effective. While there are no vaccines for humans, several licensed live Brucella vaccines are available for use in livestock. The performance of these animal vaccines is dependent upon the host species, dose, and route of immunization. Newly engineered live vaccines, lacking well-defined virulence factors, retain low residual virulence, are highly protective, and may someday replace currently used animal vaccines. These also have possible human applications. Moreover, due to their enhanced safety and efficacy in animal models, subunit vaccines for brucellosis show great promise for their application in livestock and humans. This review summarizes the progress of brucellosis vaccine development and presents an overview of candidate vaccines.

DeltaznuADeltapurE Brucella abortus 2308 mutant as a live vaccine candidate

To create a new, safe brucellosis live vaccine, a double mutant strain was constructed from Brucella abortus 2308. Using the DeltaznuA B. abortus 2308 mutant, a second mutation was introduced by deleting purE gene. The DeltaznuA DeltapurE B. abortus 2308 strain was less capable of surviving in macrophages. When evaluated in vivo, it was cleared within 8 weeks (wks) from mice, causing significantly less inflammation than spleens obtained from wild-type B. abortus 2308-infected mice. Furthermore, two doses of DeltaznuA DeltapurE B. abortus 2308 conferred 0.79 log protection, similar to S19 as did a single dose of DeltaznuA B. abortus 2308. Thus, this study shows the DeltaznuA DeltapurE B. abortus 2308 strain to be a potential livestock vaccine candidate.