What are the risk factors for rickettsial infections?

Risk Factors for Rickettsial Infections

The primary risk factors for rickettsial infections are tick exposure through outdoor activities in endemic areas during April–September, with males, older adults (aged 60-69 years), and immunosuppressed individuals at highest risk for infection and severe outcomes. 1

Geographic and Seasonal Risk Factors

Endemic Geographic Exposure:

• Residence in or travel to highly endemic regions poses the greatest risk, with over half of U.S. Rocky Mountain spotted fever cases occurring in just five states: North Carolina, South Carolina, Tennessee, Oklahoma, and Arkansas. 1
• International travel to sub-Saharan Africa accounts for approximately 90% of imported spotted fever group rickettsioses, with African tick bite fever being the second most common cause of febrile illness after malaria in returning travelers. 1
• Mediterranean spotted fever is endemic in the Mediterranean basin, Middle East, parts of Africa, and the Indian subcontinent, with case-fatality rates reaching 21% among hospitalized adults in Portugal. 1
• Travel to Central and South America (Canada, Mexico, Costa Rica, Panama, Brazil, Colombia, Argentina) poses risk for R. rickettsii infection. 1

Seasonal Timing:

• 90-93% of cases occur during April–September, coinciding with peak tick host-seeking activity and human outdoor exposure. 1
• Cases are reported year-round, but risk is substantially elevated during warmer months. 1

Demographic Risk Factors

Age-Related Risk:

• Highest incidence occurs in persons aged 60-69 years for spotted fever group rickettsioses. 1
• Highest age-specific incidences for ehrlichiosis occur in persons aged >70 years. 1
• Children aged <10 years have the highest case-fatality rate despite not having the highest incidence. 1
• Serologic studies show up to 22% of children in southeastern and south-central U.S. have evidence of previous rickettsial exposure, suggesting infection is more common than clinically recognized. 1

Sex:

• Males are at higher risk for all tickborne rickettsial diseases, possibly due to greater recreational or occupational exposures to tick habitats. 1

Behavioral and Occupational Risk Factors

Outdoor Activities:

• Recreational activities in tick-infested habitats including hiking, camping, hunting, and safari tourism increase exposure risk. 1
• Occupational exposures for military personnel on field maneuvers, humanitarian workers, and those working in endemic areas. 1
• Golfing in endemic communities has been associated with ehrlichiosis clusters. 1

Residential Exposure:

• Exposure can occur in patients' own backyards or neighborhoods, not just wilderness areas. 1
• Proximity of small rural communities to biodiverse forests with wildlife reservoirs and arthropod vectors increases spillover risk. 2

Animal Contact:

• Contact with dogs can facilitate exposure, as dogs serve as reservoir hosts for certain rickettsiae and can transport infected ticks (Rhipicephalus sanguineus) into human environments. 1
• Contact with livestock (cattle, water buffalo) in endemic regions increases risk. 3
• Concurrent infections in household dogs should raise suspicion for human family member exposure. 1

Immunologic Risk Factors

Immunosuppression:

• Persons undergoing chemotherapy, solid organ transplantation, or stem cell transplantation are at substantially greater risk for severe or fatal outcomes. 1
• Immunosuppressed patients can develop severe complications including meningoencephalitis, acute respiratory distress syndrome, and multiorgan failure. 4

Iatrogenic Risk Factors

Blood Transfusion:

• Asymptomatic or presymptomatic donors pose the greatest risk to the blood supply, with anaplasmosis being the most frequently transfusion-associated tickborne rickettsial disease. 1
• Transmission can occur despite leukoreduction of blood products. 1
• A. phagocytophilum survives in refrigerated packed erythrocytes for up to 18 days; E. chaffeensis survives for 11 days. 1
• R. rickettsii has been transmitted via whole blood stored for 9 days. 1

Solid Organ Transplantation:

• Transmission through solid organ transplantation is documented, with recipients developing acute febrile illness 20-22 days post-transplantation characterized by delirium, cytopenias, and renal failure. 1
• Donors from highly endemic regions with appropriate epidemiologic history should raise suspicion for donor-transmitted disease. 1

Clustering and Household Risk

Temporal and Geographic Clustering:

• Clustering among family members, coworkers, or persons frequenting common areas is well-recognized after exposure to natural foci of infected ticks. 1
• Family clusters of Rocky Mountain spotted fever and ehrlichiosis clusters in golfing communities have been documented. 1
• Common household exposures can place multiple family members and pets at simultaneous risk. 1

Environmental and Ecological Factors

Vector Distribution Changes:

• Climate change impacts on vector and sylvatic reservoir distributions are expanding rickettsial disease endemicity. 5
• Emerging vector competence, such as Rhipicephalus sanguineus (brown dog tick) implicated as RMSF vector in Arizona in 2005, represents evolving risk patterns. 1
• Proximity to biodiverse forests with multiple wildlife reservoirs and arthropod vectors increases peri-domestic spillover risk. 2

Common Pitfalls in Risk Assessment

• Absence of recalled tick bite does not exclude infection, as many patients do not remember tick attachment. 1, 3
• Travel history must include domestic travel within the United States, as different regions harbor different tick vectors and rickettsial species. 1
• Recent blood donation or organ donation from endemic areas warrants consideration in febrile recipients, even without obvious tick exposure. 1
• Failure to consider rickettsial disease in returning international travelers can delay diagnosis, particularly from sub-Saharan Africa where African tick bite fever is common. 1