Ticks are excellent vectors for disease transmission; consequently, tick-borne diseases are common.
They are second only to mosquitoes as vectors of human disease, both infectious and toxic. More than 800 species of these obligate blood-sucking creatures inhabit the planet.
The image below depicts the Ixodes scapularis and the Dermacentor variabilis.
This photo shows the relative sizes of the adult forms of Ixodes scapularis (right) and Dermacentor variabilis (left). These ticks are shown next to a common match for scale. I scapularis also is referred to as Ixodes dammini. Photo by Darlyne Murawski; reproduced with permission.
See Lyme Disease and 4 Emerging Tick-Borne Illnesses, a Critical Images slideshow, to help identify and treat several tick-borne conditions.
Also, see the Bug Bites You Need to Know This Summer slideshow for helpful images and information on various bug bites.
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, and 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 themselves and feed. Once on a host, the tick inserts 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, including 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.
North American tick-borne diseases
In North America, the following diseases are caused by tick bites:
Human granulocytic and monocytic ehrlichiosis
Red meat allergy
Powassan virus disease
Bourbon virus disease
Tick paralysis is a neurotoxidrome that occurs in individuals with an actively attached tick. It is characterized by vague symptoms of malaise followed by worsening neurologic deficits. Findings may include bulbar symptoms such as diplopia, dysphagia, and dysarthria, as well as ataxia, generalized weakness, and areflexia. Treatment involves finding and removing the tick, which typically allows for complete neurological recovery in a matter of days.
More than 40 species can cause paralysis in both animals and humans, including Dermacentor variabilis, the most common species responsible for tick paralysis in the southeastern United States. Tick paralysis is most common from March through July, when ticks are most active, and should be considered in patients where Guillain–Barré syndrome and/or botulism are being diagnosed.
Red meat allergy
Over the past decade, the lone star tick, a medium-sized, reddish-brown tick common to the southeastern United States, has been implicated as the likely cause of severe red meat allergies, which have been increasing in incidence. This allergy has been found in North America, Europe, Asia, Central America, and Africa. The lone star tick is believed to produce a sugar from its gut called galactose-alpha-1,3-galactose (“Alpha-Gal”), which is injected into the host during a tick bite. The sugar is also found in red meat (eg, beef, pork, venison, rabbit) and some dairy products.
The primary management of mammalian meat allergy is avoidance of mammalian meat. The allergy can cause symptoms ranging from urticaria to angioedema and anaphylaxis. Delayed anaphylactic shock has also been reported to occur 4-6 hours after consumption of red meat. These symptoms are treated in the usual manner.
Powassan virus disease
Powassan (POW) virus may also be transmitted to humans by infected ticks, with approximately 60 US cases reported in the last decade, mostly in the Northwest United States and Great Lakes region.
Signs and symptoms of POW infection may include headache, fever, weakness, vomiting, seizures, confusion, and memory loss. Long-term neurologic problems have been reported. POW virus infection has no specific treatment, but infected persons often require hospitalization for respiratory support, intravenous fluids, or medications to reduce brain swelling.
Bourbon virus disease
In June 2014, a newly discovered virus (Bourbon virus, named after the Kansas county in which it was found to have infected a farmer, ultimately leading to his death) is presumed to have been transmitted via tick bite, although this has not been proven. The patient’s symptoms, including fever, low red and white blood cell counts, elevated liver enzyme levels, and loss of appetite, suggested a tick-borne illness such as ehrlichiosis or the Heartland virus, but test results were negative. A laboratory with the Centers for Disease Control and Prevention (CDC) in Colorado finally determined that the virus was one that had never been encountered in the Western hemisphere.
The Bourbon virus belongs to the orthomyxovirus family and possesses a genome similar to that of viruses in Eastern Europe, Africa, and Asia.
Tests are planned to determine if prior undiagnosed but similar cases may have been caused by Bourbon virus.
European tick-borne diseases
In Europe, the list is similar to that in North America, but the following diseases should be considered as well
Boutonneuse fever (caused by a less virulent spotted fever rickettsial organism, Rickettsia conorii)
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 because of hypersensitivity developed in response to previous bites of uninfected ticks.
For patient education resources, see the Bites and Stings Center, as well as Ticks.