A voluminous 2011 Institute of Medicine report, Critical Needs and Gaps in Understanding Prevention, Amelioration, and Resolution of Lyme and Other Tick-Borne Diseases, calls the lowly tick “the Swiss Army knife of disease vectors...able to host greater variety of disease agents than any other arthropod.”
As the report’s title indicates, there’s a lot scientists still don’t know about ticks and the complex interactions between them, their hosts, the habitats they occupy, and the pathogens they carry.
But we do know that both tick populations and the diseases spread by ticks have risen dramatically in the past couple of decades, and continue to rise.
The diseases ticks can transmit
Many tick-borne diseases are serious or life-altering, even life-threatening.
New diseases, or at least newly-discovered diseases ticks can spread, seem to crop up with increasing frequency, such as the recently named Heartland virus, the deer-tick Powassan virus, and a Lyme-like disease spread by the bacterium Borellia miyamotoi.
Ticks and tick-borne diseases occur in every state in the U.S., though some have become endemic, even epidemic, in certain regions. Ticks may even carry and simultaneously transmit two or more infections, complicating diagnosis and treatment of these illnesses.
Some diseases may be more common than official reports suggest. For example, about 30,000 cases of Lyme disease, the most common tick-transmitted disease, are reported each year, but studies conducted by the Centers for Disease Control and Prevention (CDC) suggest the number is closer to 300,000.
A few tick facts
The recent discovery of a common tick ancestor suggests that ticks and their unique blood-feeding capabilities emerged around 250 million years ago, with the two tick families (hard and soft) diverging and the species within them evolving from about 240 to 230 million years ago.
Long before humans appeared on the scene, ticks had already evolved the astonishingly complex assortment of anatomical, physiological, and biochemical tools that enable them to disable or bypass the protective defenses of their hosts and allow them to feed efficiently and exclusively on blood.
Today, almost 900 species of ticks live in terrestrial regions all over the world, according to Jerome Goddard, a medical entomologist with Mississippi State University, an expert on the ecology and epidemiology of tick-borne diseases.
Of the many tick species, Goddard says five are most significant in human and/or veterinary medicine in the U.S.:
The Lone Star tick, may transmit ehrlichiosis, Southern tick-associated rash illness (STARI), Heartland virus, a mysterious illness called “red meat disease,” bobcat fever (which can be fatal to house cats), and tularemia.
The Gulf Coast tick, which can spread a form of bacterial illness similar to Rocky Mountain spotted fever.
The brown dog tick, a serious pest of dogs, can complete its entire life cycle indoors. It thrives in kennels and homes, and may cause canine ehrlichiosis and canine babesiosis. It also transmits Rocky Mountain spotted fever.
Dogs and cats, as well as livestock and some wildlife species, are susceptible to tick bites and a number of tick-borne illnesses. Check with your vet to see which ones are prevalent in your area.
Life cycle of most hard ticks
The Ixodid (“hard”) ticks that may carry and transmit most of the pathogens that cause human disease have long and complex life cycles.
For example, adult female black-legged ticks (Ixodes scapularis) lay their eggs, often several thousand at a time, in the spring. In mid-August the eggs hatch into minute, six-legged larvae, which immediately begin seeking their first blood meal, generally finding it from a small rodent, sometimes from a bird that brushes by.
“The current thinking on Lyme disease is that the larval tick picks up the bacterium causing Lyme disease, Borrelia burgdorferi, when they take their blood meal from small rodents, such as white-footed mice," says Jim Dill, extension entomologist and pest management specialist with the University of Maine. "The scientific community is uncertain as to whether or not the bacterium is transmitted ‘transovarially' to the eggs and, hence, resulting larvae would hatch infected."
Infected mice, chipmunks, and birds don’t become sick themselves with the diseases they carry, says Dill, so they serve as reservoirs of these pathogens, passing them along to the next tick that bites them.
The six-legged larvae feed for a few days, then drop to the ground. They lie dormant over winter, digesting their meal and using their energy to transform into eight-legged nymphs about the size of a poppy seed. These nymphs emerge in late spring-early summer and begin seeking their second meal, usually from a larger animal such as a deer, a dog, or a human.
“Nymphs, which feed actively in late spring and early summer, are the stage most dangerous for humans, mainly because they’re so hard to see,” says Dill, who has contracted Lyme disease twice himself.
After feeding, the nymph drops off and molts into an adult, which must find another blood meal before mating to begin the cycle again.
‘Questing’ for a host: How ticks find you
Lacking wings, ticks can’t fly. They don’t leap or hop, drop from trees, or even move very fast. Instead, tick larvae, nymphs, and adults all seek their hosts with a behavior entomologists call “questing.”
Responding to carbon dioxide, body heat and odors, vibration, and moisture, they climb stems of grasses and foliage plants, and extend their forelegs (which contain sensing organs), grabbing on when a suitable host brushes by.
What happens as a tick feeds
When it finds a suitable spot on the host body, a hard tick inserts its barbed, harpoon-like mouthpart (see photo), called its hypostome, through the skin, injuring small blood vessels and feeding from the blood pool that collects. Most hard ticks secrete and anchor themselves with a special cement that holds them in place until they are fully engorged, after ingesting up to 100 times their original weight in blood.The rasp-like teeth on the hypostome, with some help from the cement, is what makes ticks so hard to remove.
As feeding progresses, the tick secretes numerous biologically active salivary compounds that prevent the host’s blood from clotting, disable sensations of pain and itching that would stimulate the host to pull off the tick, prevent blood vessels from constricting, inhibit inflammatory responses, and suppress the immune responses that would disrupt tick feeding.
Co-evolution of ticks and pathogens
Over tens of millions of years, many microorganisms have co-evolved with ticks, exploiting the sophisticated ecological niche of the tick salivary system. Ticks have developed an ingenious two-way system that sends concentrated nutrients from the blood meal into the tick’s gut in one direction, while returning the excess fluid--along with any pathogens living in the tick’s saliva--back into the body of the host. The host’s disabled immune response may help the injected disease-causing organisms off to a good start.
Why ticks have been so successful
Ticks inhabit many terrestrial regions around the world, but flourish best in warm, humid climates because of their need for moisture and their lengthy life cycle, which is inhibited by extreme cold.
What's the secret to their success in colonizing the globe?
Goddard answers quickly: “They have few predators. Some birds, including guinea fowl and backyard chickens, will eat them, but they don’t do much to control tick populations.”
Adds Dill, “Ticks are so adaptable. For example, the black-legged tick has adapted to a wide variety of small rodents, birds, and large mammal hosts.”
Entomologists also say that other factors such as shifts in populations of hosts and reservoir animals, changes to local landscapes, human settlements encroaching on wildlife habitats and into areas of abundant tick populations, are playing a role.
Although drying out is the primary cause of tick mortality, the tick’s salivary system has another extraordinary property that allows a fasting tick to survive months without drinking. It uses its forelegs to detect a source of higher humidity, then moves to that spot and secretes a substance from its mouth that’s able to absorb water vapor from the air.
When the substance is saturated, the tick pulls it back, where it evaporates and helps rehydrate the tick’s body.
Finally, Goddard says, “You can’t escape the conclusion that climate change is playing a role in the explosion of tick-borne illnesses, when you see tropical ticks and diseases expanding their range northward, even up mountainsides.”
Why the dramatic spike in ticks and tick-borne illnesses?
Given the co-evolution of ticks and the pathogens they carry, humans and other animals have probably always suffered from tick-transmitted diseases.
Scientists at Oregon State University have found a tick preserved in amber believed to be about 15 million years old, containing a species of Borrelia bacteria similar to the one that causes Lyme disease today. The same team has also found amber-encased tick fossils dating back around 100 million years in Myanmar, containing bacterial cells similar to those that cause Rocky Mountain spotted fever.
But why the big spike in recent decades?
“There are simply more opportunities for human/tick encounters,” says Goddard. “A primary reason is the increase in the population of the white-tailed deer [preferred host of adult black-legged ticks]. He notes that many states encourage an abundant deer population to attract hunters and wildlife enthusiasts.
“In 1910, the U.S. had a deer population of around 300,000. A century later, there were four million of them.”
Vigorous ongoing research around the world aims at developing vaccines that could protect humans and other animals against the pathogens ticks transmit or against the ticks themselves.
But self-protection currently remains the best line of defense against tick bites and any tick-borne illness.
Protecting yourself and people you care for from tick bites begins with understanding the life cycles and habitats of disease-causing ticks in your area, either staying out of those areas or taking protective measures when venturing into tick terrain.
Dill suggests, “Wear light-colored clothing and long pants tucked into your socks. Use a repellent containing either DEET or picaridin that lists ticks on the label.”
“You can also spray permethrin insecticide on your clothing, or buy clothing already treated with the pesticide. They call permethrin a repellent, but it actually kills ticks,” Dill says.
The Environmental Protection Agency has an interactive site on tick and insect repellents that allows you to find a specific product that will work for you and your outdoor activities. The agency has also just unveiled a new labeling scheme to indicate how many hours a product will repel mosquitoes, ticks or both.
Dill, who’s twice contracted Lyme disease, suggests a head-to-toe tick check after coming from a wooded or grassy area, especially if you live in an area where ticks and tick-borne illnesses are endemic. Take pains to check inside and around your ears, scalp, and around the hairline, inside the navel, behind the knees, between the toes, and in the groin and armpits. Have a loved one check your back, or use a large mirror in a well-lit space.
Don’t undress near sleeping or sitting areas of your home. Check your clothing for loose ticks before you hang it up or toss it into the laundry basket. Even hot-water washing won’t kill ticks, only drying at high temperatures.
What to do if you’re bitten
A tick larva or nymph has to get a previous blood meal from an infected animal to acquire and pass along disease microorganisms, so all ticks aren't necessarily disease carriers.
Depending on the microorganism, the nymph or adult tick has to remain attached long enough to transmit the pathogens, so all tick bites don’t necessarily result in disease.
It’s important to learn to remove an attached tick safely. Instead of disposing of the tick, you may want to drop it into a small container with an alcohol-soaked cotton ball, and take or mail the specimen to a lab in the area for identification. Check with your local Cooperative Extension office or your state’s public health authorities to find out where.
Take any tick bite seriously. Keep a record of when and where on your body (or a family member’s body) the bite occurred, and monitor your health over the next few weeks. Seek prompt medical attention if you notice any kind of unexplained rash, or if you experience fever, headache, nausea, vomiting, muscle aches, swollen lymph glands, or unusual fatigue, as these symptoms of many tick-spread illnesses.
If you suspect a tick bite has made you or someone you care for sick, it’s important to find a healthcare professional experienced in treating tick-borne infections.
Managing ticks in the home environment
Homeowners can reduce tick populations through landscape modification, perimeter sprays, and practices that reduce rodent populations and keep white-tailed deer from the property.
“Cut your grass and weeds back. Let the wind blow and the sun shine in,” says Goddard. “Ticks need shade and moisture. They’re extremely sensitive to desiccation.”
“Properly timed and located, pesticide sprays can be effective,” says Dill. “Even some botanical sprays that include essential oils have proven very effective (though pricey).”
For the most comprehensive information about tick-proofing your home grounds, check out the Connecticut Agricultural Experiment Station’s Integrated Tick Management.
The rise in tick-related diseases has raised new worries for parents eager to have their children spend more time outdoors. They’re asking for information on how to protect a child who’s going away to summer camp for two weeks, or a teenager who’s taken a job with a landscaping firm.
As one mother put it, “You can teach your children to know the risks, put on the bug spray, understand when and how check for ticks, and how to remove one safely.
“But even if they remember your advice, in that camp cabin or shower room, will they actually check in and around their ears, navels, armpits, and groins in front of other kids? I don’t think so.”
The rising incidence of ticks and tick-borne illnesses has also brought new worries to state lawmakers and tourism officials.
Says Dill, who's also a Maine state legislator, “With 1,400 reported cases of Lyme disease last year and all these ongoing programs to help raise public awareness, we have people asking, ‘What’s this going to do to tourism? Will fishermen, hunters, and hikers still want to come here?’”
Top of page: Adult female black-legged tick feeding, courtesy, Alan Eaton, University of New Hampshire Cooperative Extension.
Mid-page right: Questing male dog tick, courtesy, Alan Eaton, University of New Hampshire Cooperative Extension.
Mid-page left: Scanning electron microscope photo of mouthparts of Ixodes scapularis (black-legged tick) by Nancy Cherim. Courtesy UNH Cooperative Extension.
By Peg Boyles, eXtension, UNH Cooperative Extension alumna, firstname.lastname@example.org