Bacterial and Rickettsial Diseases

Abstract

Diseases caused by bacterial and rickettsial pathogens transmitted by arthropods have caused incredible suffering throughout human history. The twin scourges, plague and typhus, may have killed more people than all wartime battlefield injuries combined (Zinsser 1934). Although improvements in sanitation and treatment of diseases have decreased these vetorborne agents around the world, recent events have shown how quickly the threat of these diseases can cause chaos in human society. Unlike diseases that prevail where poverty is high and living standards are low, bacterial and rickettsial agents transmitted by arthropods recently have emerged among the world’s wealthiest residents. Lyme disease clearly is tied in the northeastern USA to the reversion of agricultural land to wealthy suburban communities, with accompanying regrowth of secondary forests and deer populations free from hunting. These changes have led to a reemergence of the blacklegged tick, Ixodes scapularis, and the 3 human diseases transmitted by these ticks: babesiosis, Lyme disease and human granulocytic ehrlichiosis. Dr. Harry Hoogstraal, the late eminent tick biologist, predicted that changing land- scape conditions caused by human society would dramatically impact tickborne agents in the future. The emergence of 3 new agents associated with a single tick species late in the 20th Century emphasizes Dr. Hoogstraal’s prediction. The plasticity of the vectorborne bacterial and rickettsial agents allows them to change and survive in parallel with human landscape changes.