Effects of homologous and heterologous immunization on the reservoir competence of domestic dogs for Rickettsia conorii (israelensis)

Abstract

A number of spotted fever group (SFG) rickettsiae cause serious infections in humans. Several antigenically related rickettsial agents may coexist within the same geographical area, and humans or vertebrate hosts may be sequentially exposed to multiple SFG agents. We assessed whether exposure of a vertebrate reservoir to one SFG Rickettsia will affect the host's immune response to a related pathogen and the efficiency of transmission to uninfected ticks. Two pairs of dogs were each infected with either Rickettsia massiliae or Rickettsia conorii israelensis, and their immune response was monitored twice weekly by IFA. The four immunized dogs and a pair of naïve dogs were each challenged with R. conorii israelensis-infected Rhipicephalus sanguineus nymphs. Uninfected Rh. sanguineus larvae were acquisition-fed on the dogs on days 1, 7, and 14 post-challenge. These ticks were tested for the presence of rickettsial DNA after molting to the nymphal stage. The naive dogs became infected with R. conorii israelensis and were infectious to ticks for at least 3 weeks, whereas reservoir competence of dogs previously infected with either R. massiliae or R. conorii was significantly diminished. This opens an opportunity for decreasing the efficiency of transmission and propagation of pathogenic Rickettsia in natural foci by immunizing the primary hosts with closely related nonpathogenic SFG bacteria. However, neither homologous immunization nor cross-immunization significantly affected the efficiency of R. conorii transmission between cofeeding infected nymphs and uninfected larvae. At high densities of ticks, the efficiency of cofeeding transmission may be sufficient for yearly amplification and persistent circulation of a rickettsial pathogen in the vector population.

Introduction

Spotted fever group (SFG) rickettsiae are closely related antigenically and exhibit strong cross-reaction in serological tests. While this group contains some highly virulent pathogens, it also includes agents which may cause only mild or unapparent infections both in humans and animals. These agents exist in natural cycles involving arthropod vectors (mainly ticks) and their vertebrate hosts. The list of known rickettsial pathogens has tripled in the last 15–20 years and continues to grow. This extension is due to both discovery of new species and recognition of pathogenic potential in agents previously thought to be exclusive endosymbionts of ticks maintained in vector populations solely via vertical transmission (Walker et al., 2007).

Humans and animals recovering from spotted fever rickettsioses are often reported to develop a “solid immunity” that protects them from illness and death in case of reinfection (Lackman et al., 1965, Walker et al., 2007). In laboratory studies, immunizations with various pathogenic rickettsiae have been reported to protect experimental animals not only against challenges with the same (homologous) agent, but against other (heterologous) Rickettsia spp. as well. For example, guinea pigs immunized with either R. rickettsii, R. conorii, R. rhipicephali, or R. montanensis remained afebrile following heterologous challenges (Badger, 1933, Feng and Waner, 1980, Walker et al., 1984, Gage and Jerrells, 1992); similarly, sublethal infection with R. conorii and Rickettsia australis resulted in the absence of clinical manifestations of disease after challenge with a lethal dose of the heterologous agent (Feng and Walker, 2003).

Although antibody titers in dogs infected with R. conorii israelensis decline below the detectable threshold level within 8–10 months after infection, the reservoir competence of seropositive animals reinfected with the same agent appears to be lower than that in primary infection (Zemtsova et al., 2010, Levin et al., 2012). Based on the rationale that antibodies have limited access to obligately intracellular Rickettsia, it had been suggested that cell-mediated immunity is primarily responsible for curtailing a rickettsial infection, whereas humoral immunity to rickettsiae plays only a secondary role in vertebrate hosts (Jerrells, 1997). On the other hand, neutralization of bacteria inside ticks by antibodies from immune hosts has been suggested as one of the factors responsible for decreasing reservoir competence and preventing highly pathogenic rickettsiae like R. sibirica and R. conorii from infecting large proportions of vector populations (Grokhovskaya and Sidorov, 1966, Zemtsova et al., 2010).

As more than one species of tick-borne rickettsiae often exist within the same geographical area and may even be transmitted by the same vector species (Bitam et al., 2006, Carmichael and Fuerst, 2006, Carmichael and Fuerst, 2010, Eremeeva et al., 2006, Mediannikov et al., 2010, Moncayo et al., 2010, Medeiros et al., 2011, Spitalska et al., 2012), humans and reservoir animals can be exposed to multiple SFG agents circulating in the same or adjacent foci. If acquired immunity to one of these agents provides protection against others in immune animals, it may potentially reduce their reservoir competence for related rickettsial agents as well.

R. conorii and R. massiliae provide an example of closely related SFG pathogens with overlapping geographical distribution, which are transmitted by the same vector species – Rh. sanguineus in particular (Brouqui et al., 2007). The R. conorii group includes causative agents of Mediterranean spotted fever, Astrakhan fever, Israeli spotted fever, and Indian tick typhus in the Mediterranean basin and Africa, southern Russia, Middle East, and India, respectively (Zhu et al., 2005). R. conorii subspecies israelensis, the causative agent of Israeli tick typhus (ISTT), has been described in Cyprus, France, Israel, Italy, and Portugal with Rh. sanguineus being its main vector [reviewed by (Zemtsova et al., 2010)]. R. massiliae, originally isolated from Rh. sanguineus ticks collected in southern France (Beati and Raoult, 1993), has since been identified in several species of the genus Rhipicephalus in France, Greece, Italy, Portugal, Spain, Switzerland, Israel, north and central Africa, Argentina, and the United States (Bernasconi et al., 2002, Cicuttin et al., 2004, Bitam et al., 2006, Eremeeva et al., 2006, Fernandez-Soto et al., 2006, Brouqui et al., 2007, Marquez et al., 2008, Mura et al., 2008, Sarih et al., 2008, Labruna, 2009, Mediannikov et al., 2010, Milhano et al., 2010, Beeler et al., 2011, Harrus et al., 2011, Chochlakis et al., 2012, Khaldi et al., 2012).

Here, we assess effects of homologous and heterologous immune responses in a vertebrate host on the host's reservoir competence for R. conorii israelensis by comparing the efficiency of rickettsial transmission to Rh. sanguineus ticks feeding on naïve versus R. conorii- and R. massiliae-immunized dogs.