A Rickettsia amblyommatis vaccine to prevent transmission of Rickettsia rickettsii by brown dog ticks

Rocky Mountain spotted fever (RMSF) is a severe acute tick-borne illness caused by Rickettsia rickettsii. Signs and symptoms of disease are undifferentiated and may mimic a variety of other infectious diseases early in its course. RMSF is quite severe though, and if not treated within the first 5 days of illness, severe manifestations (e.g., respiratory failure, acute kidney injury, gangrenous digits, long-term neurological deficits) and death may occur. Indeed, the untreated case fatality rate in the United States during the pre-antibiotic era was roughly 25%. In the U.S. cases of RMSF are usually sporadic and acquired through the bite of infrequently infected Dermacentor ticks in a sylvatic cycle. More recently, in Arizona, RMSF has emerged as a public health threat with high infection rates in children, frequent hospitalizations, and high case fatality rate. Similarly, RMSF epidemics are occuring in regions across northern Mexico where the case fatality rate has been recorded to be as high as 44%. In Arizo-na, Mexico, and other areas of Latin America, R. rickettsii is transmitted by Rhipicepha-lus sanguineus – the ubiquitous brown dog tick. Unlike the sporadic cases transmitted by Dermacentor species in the U.S., RMSF associated with Rh. sanguineus can occur at much higher incidence within a human population and can even cause familial clustering. This is due to the perido-mestic and opportunistic nature of Rh. sanguineus, which ubiquitously infests dogs and can cause infestations in and around homes. When dogs are infected with R. rickettsii, they serve as amplifying hosts to infect ticks and put human populations at risk for RMSF. As a reservoir, dogs are one of the pivotal points in the ecology of R. rickettsii. Methods to disrupt the cycle of R. rickettsii transmission to humans are des-perately needed. An effective canine vaccine that reduces or prevents the acquisition of R. rickettsii from Rh. sanguineus would be one of the most efficient approaches to break the cycle of RMSF in hyperendemic regions. Such a vaccine would be a crucial step towards significantly decreasing the burden of RMSF. In this proposal we outline the use of Rickettsia amblyommatis as a live vaccine to help break the cycle of RMSF. Previous studies in guinea pigs have confirmed that experimental immunization with R. amblyommatis causes no illness, yet it protects guinea pigs from illness caused by usually lethal doses of R. rickettsii. The first objec-tive of this application is to test R. amblyommatis as a vaccine in dogs to demonstrate safety and immune response protecting against infection from R. rickettsii. To accom-plish this objective, dogs will be inoculated with R. amblyommatis. The dogs will then be observed clinically. Controls will consist of dogs not vaccinated with R. amblyom-matis. Blood specimens will be collected longitudinally to assess key clinical and im-munologic laboratory testing. Six weeks after R. amblyommatis inoculation, dogs will be challenged with R. rickettsii. The dogs will then be followed clinically and with laboratory assays. Clinical status, laboratory values, humoral immune response, and cellular im-mune response will be compared between the two groups (vaccinated versus unvac-cinated). The second objective of this application is to demonstrate the effectiveness of canine vaccination on reducing or preventing acquisition of R. rickettsii by brown dog ticks. To accomplish this objective, dogs will be immunized with R. amblyommatis be-fore challenge with R. rickettsii as previously described. Controls will consist of R. rickettsii-infected dogs not previously immunized with R. amblyommatis. After R. rick-ettsii challenge, Rh. sanguineus nymphs will be allowed to feed to repletion on the dogs. After feeding, the ticks will be tested for the presence of bacteria before and after molting. The rates and magnitude of bacterial acquisition by feeding ticks will be com-pared between the two groups (vaccinated versus unvaccinated). We will also assess whether immunization impacts the maintenance of R. rickettsii across tick life stages. We hypothesize that these experiments will demonstrate that R. amblyommatis is safe and will elicit an immune response that protects dogs from clinical illness and death from infection with R. rickettsii. We also hypothesize that vaccination with R. amblyommatis will prevent or reduce the acquisition of R. rickettsii by feeding brown dog ticks. The use of R. amblyommatis as a live vaccine is a pragmatic approach to a criti-cal problem. This approach includes several advantages over other methods. It is supported by prior work in guinea pigs (the classic animal model for lethal RMSF), and it has potential for rapid onset immunity with durable protection after a single dose. The ability to protect dogs from illness and significant bacteremia should significantly reduce the ability for Rh. sanguineus to acquire, and thus transmit, R. rickettsii. In are-as that face substantial populations of free roaming Rh. sanguineus-infected dogs, vaccine-induced protection of canines would disrupt the cycle of R. rickettsii transmis-sion. Ultimately, this would mitigate the risk of transmitting this severe disease to hu-mans.