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Rocky Mountain spotted fever

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Distribution

Geographic distribution

Studies suggest that sporting and working breeds and young dogs (< 2 years) are over-represented for Rocky Mountain spotted fever. 5 to 15% of dogs in endemic areas may have positive serology for R. rickettsii. Since canine infection is supposed to mirror human infection, geographic and seasonal distribution can be considered identical for humans and dogs.

The distribution of Rocky Mountain spotted fever as the most severe and most frequently reported human rickettsial illness in the United States has been monitored extensively. Generally, dogs may be effective sentinels for the prevalence of human disease.

Over half of the human Rocky Mountain spotted fever infections are reported from the South-Atlantic region of the United States (Delaware, Maryland, Washington D.C., Virginia, West Virginia, North Carolina, South Carolina, Georgia, and Florida). Infection also occurs in other parts of the United States, namely the Pacific region (Washington, Oregon, and California) and the west South-central (Arkansas, Louisiana, Oklahoma, and Texas) region.

Figure 1: Number of reported cases of Rocky Mountain spotted fever by state and region, 1994-1998

Human infections with R. rickettsii have also been documented in Argentina, Brazil, Colombia, Costa Rica, Mexico, and Panama.

In general, the risk of exposure to a tick carrying R. rickettsii in the USA is low. Only about 1-3% of the tick population carries R. rickettsii, even in endemic areas. There may be significant variation in prevalence of R. rickettsii and RMSF within geographic areas.

Belonging into the same group of rickettsiae, Rickettsia conorii, the agent of Boutonneuse or Mediterranean spotted fever in humans – which is also rarely detected in dogs – is reported in Southern Europe, the Middle East and Southern Africa.

Seasonal distribution

Over 90% of human Rocky Mountain spotted fever infections occur between April and September. This period is the season for increased activity of adult and nymphal Dermacentor ticks.

Figure 2: Seasonal distribution of reported human cases of Rocky Mountain spotted fever, 1993-1996

Distribution among hosts

Certain small mammal hosts (chipmunks, squirrels) become infected, but clinical signs of Rocky Mountain spotted fever are transient. Despite the short infection, these small mammals serve as amplifiers of the infection in the tick population, allowing the spread of the organisms from infected to uninfected individuals, like e.g. dogs. In cats, seropositivity has been reported, but a clinical disease has not been described so far.

   

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