What’s a Vector?
In epidemiology, a vector is an organism that transmits the disease-causing agent between one organism and another. (If an infectious agent can’t be transmitted by being swallowed, breathed in, or through direct contact with an infected person, chances are the disease in question is vector-borne--unlike, say, cancer or heart disease, which are not infectious.)
The Troublesome Tick
Ticks are parasites that feed by latching on to an animal host and sucking its blood. Their life cycle has four stages: egg, larva, nymph, and adult. Larvae and nymphs each need a blood meal in order to proceed to the next stage, and adult females need a blood meal before laying their eggs. These meals may be months, or even years, apart. Because ticks feed on multiple individuals in the course of their life cycles, they are ideal vectors for infectious microbes. Nine diseases are known to be tick-borne in the U.S. alone, including Rocky Mountain spotted fever, relapsing fever, and tularemia.
The bacterium that causes Lyme disease isn’t a new species; Borrelia burgdorferi is thought to have been in the U.S. for at least a hundred years. It may have been brought over in ships from Europe by rats or livestock, or infected ticks may have hitched rides on migrating birds. “What’s happened during the last twenty years is that the range of tick that carries the bacteria that causes the disease has expanded,” explains Professor Tom Daniels, of the Vector Ecology Lab at the Louis Calder Center of Fordham University.
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