Excerpt from Tick-Borne Diseases, Introduction

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Ticks are excellent vectors for disease transmission. More than 800 species of these obligate blood-sucking creatures inhabit the planet. They are second only to mosquitoes as vectors of human disease, both infectious and toxic.

From the perspective of disease transmission to humans, the essential characteristic of ticks is their need to ingest a blood meal to transform to their next stage of development. Not picky in their eating habits, they take their requisite blood meal from all classes of vertebrates (eg, mammals, reptiles, birds), with the exception of fish.

Ticks feed by perching in low vegetation and waiting (questing) for a susceptible host on which they can attach and feed. Once on a host, the tick attaches its hypostome, a central piercing element with hooks, into the host's skin. Some ticks secrete a cementing material to fasten themselves to the host. In addition, Ixodes ticks secrete anticoagulant, immunosuppressive, and anti-inflammatory substances into the area of the tick bite. These substances presumably help the tick to obtain a blood meal without the host's noticing. These same substances also help any freeloading pathogens to establish a foothold in the host.

Ticks can carry and transmit a remarkable array of pathogens, such as bacteria, spirochetes, rickettsiae, protozoa, viruses, nematodes, and toxins. A single tick bite can transmit multiple pathogens, a phenomenon that has led to atypical presentations of some classic tick-borne diseases. In the United States, ticks are the most common vectors of vector-borne diseases.

In North America, the following diseases are caused by tick bites: Lyme disease, human granulocytic and monocytic ehrlichiosis, babesiosis, relapsing fever, Rocky Mountain spotted fever, Colorado tick fever, tularemia, Q fever, and tick paralysis. In Europe, the list is similar, but other diseases should be considered as well; these include boutonneuse fever (caused by a less virulent spotted fever rickettsial organism Rickettsia connori) and tick-borne encephalitis.

Most tick bites do not result in transmission of infection; in the case of Lyme disease for example, only about 2-3% of all persons bitten by Ixodes scapularis ticks in endemic areas develop Lyme disease.

Secondary infections and allergic reactions to proteins in tick saliva are also possible. In fact, one study suggests that repeated tick bites may actually protect against Lyme disease, possibly due to developed hypersensitivity from the prior bites of uninfected ticks.

For excellent patient education resources, visit eMedicine's Bites and Stings Center. Also, see eMedicine's patient education article Ticks.

Please click here to view the full topic text: Tick-Borne Diseases, Introduction