Flea-Borne Transmission Model To Evaluate Vaccine Efficacy against Naturally Acquired Bubonic Plague

Yersinia pestis, the bacterial agent of plague, remains an international public health concern. Recent plague outbreaks in India and parts of East Africa, where the disease had been dormant for decades, have raised fears of a new resurgence (6, 33). More troubling was the isolation of multidrug-resistant strains of Y. pestis during the ongoing plague epidemic in Madagascar (10, 12). Added to these concerns is the recognized potential of Y. pestis as a bioterrorism agent (19). The potential threat of plague outbreaks caused by naturally occurring or deliberately released antibiotic-resistant strains increases the urgency of possessing an effective plague vaccine, but none is available; the previously used killed whole-cell vaccine is no longer being produced. Y. pestis is transmitted primarily by fleabite among its many rodent reservoir hosts. Transmission by direct contact or ingestion of infected tissues can occur in some cases, but the maintenance of plague in its natural environment is thought to depend on rodent-flea-rodent transmission cycles, and most human cases of plague also result from fleabites (21). During the first week after transmission, the bacteria disseminate from the peripheral fleabite site to the regional lymph nodes and produce a severe acute lymphadenitis, characterized by a swollen, painful lymph node called a bubo. Bubonic plague can be treated successfully with an appropriate antibiotic. If not treated, however, a fulminant, high-density, and usually fatal septicemia rapidly develops. Hematogenous spread to the lungs causes secondary pneumonic plague in ∼5% of human cases (5). New second-generation bivalent recombinant plague vaccines consisting of the Y. pestis F1 capsular and V antigens have shown promise in trials using rodent and primate animal models (13, 20). To evaluate protection against bubonic plague, vaccinated animals were challenged by subcutaneous (s.c.) or other parenteral injection of bacterial suspensions prepared from in vitro-grown cultures. There are important differences between this challenge model and natural challenge by fleas, however. To produce a transmissible infection in the flea, Y. pestis blocks the proventriculus, a valve that connects the esophagus to the midgut, by forming a large aggregate that is embedded within an extracellular matrix. When these blocked or partially blocked fleas attempt to feed, blood containing Y. pestis from the blocking mass is refluxed into the bite site (3, 15). It is in this flea-specific context, which is not duplicated by artificial challenge models, that Y. pestis exits the flea and enters the mammal. For other arthropod-borne pathogens, it is known that the route of transmission can affect the initial interaction with the host immune system and vaccine efficacy. For example, the saliva of blood-feeding arthropods contains immunomodulatory components that inhibit complement activation, phagocytosis, T-cell proliferation, and cytokine secretion (24, 26). Notably, a vaccine that fully protected mice against needle-injected Plasmodium sporozoites was much less protective against mosquito challenge (31). Because of the unique phenotype and microenvironment of Y. pestis present at transmission and its potential to influence the immune response, we developed a model to explicitly evaluate the abilities of candidate plague vaccines to protect against fleabite challenge.