“I know runners who have suffered a tick bite and ended up with Lyme disease. I'll take an angry moose any day.” - Don Kardong, American Runner and Author
“The confirmed man of trout should resolve to get along with wood ticks. Any other procedure would fail because the wood tick is determined to get along with the fisherman.” - Gordon MacQuarrie, American Author and Journalist
The winter doldrums are behinds us, and nature reminds us every day that spring has finally arrived in Minnesota. There is nothing that I enjoy more at this time of year than walking through the woods as they are coming back to life with budding trees and birdsong. There is one entity, however, whose reemergence I do not welcome—ticks. These blood-sucking arachnids seize the opportunity to latch onto us as we enjoy outdoor activities. And while their bites extract only a tiny amount of blood, it is the number of pathogens that ticks carry that can wreak havoc on our species. In this Germ Gems post, I provide a brief summary of things everyone should know about ticks.
Tick…tick…tick. There are 899 known species of ticks in the world. Fortunately, only a small percentage transmit microbes that cause infectious diseases in humans. The four tick species that are the most common sources of human disease are: the black-legged tick (aka the deer tick), lone star tick, American dog tick, and brown dog tick. All are found in overlapping regions of the United States. Geographic location becomes especially important, however, when considering whether tick bites require treatment. Here is an overview of these tick species and some of the diseases they carry.
Blacklegged tick (Ixodes scapularis). This tick species is by far our biggest enemy. Immature ticks, called nymphs, that feed during the spring and summer months are the main culprits. These nymphs are very tiny, <2mm, or about the size of the period at the end of this sentence. Nonetheless, they can transmit six different microbes: three bacterial pathogens, two viral ones, and one protozoan, each of which is responsible for a great deal of human misery. Of the bacterial pathogens, our biggest threat is from the species that cause Lyme disease, Borrelia burgdorferi (found throughout the Northeastern and Upper Midwestern United States), Borrelia mayonni (in the upper Midwest), and a related tick species, Ixodes pacificus, found on the West Coast.
The Center for Disease Control and Prevention (CDC) recently announced that the number of people diagnosed each year with Lyme disease has climbed to nearly half a million which is a jump of 59% over the 300,000 estimate previously listed on the CDC’s website a year ago. Lyme disease is, however, rarely fatal, and early treatment with an antibiotic is highly effective. In the 2021 Clinical Practice Guidelines of the Infectious Diseases of America, it is recommended that a prophylactic antibiotic (i.e., a single dose of oral doxycycline) should be given to adults and children within 72 hours of removal of an identified high-risk tick bite.
Lone star tick. The native range of this tick is from Texas through Nebraska, but it has shown up recently on the Eastern seaboard. The bacteria that cause Rocky Mountain spotted fever (RMSF) and ehrlichiosis are transmitted by bites of this tick.
American dog tick. California and the Eastern half of the continental United States is the native range of this tick species. It can transmit the bacterial species that cause RMSF and tularemia, a rare infectious disease that typically involves the skin, eyes, lymph nodes, and lungs.
Brown dog tick. This tick is found worldwide, including the entire continental United States, although it is most commonly found in our southern states. All stages of this tick (larval, nymphal, and adult) are active year-round and can transmit the bacterium that causes RMSF.
Minimize exposure. If you are engaging in outdoor activities this spring or summer, here are some things that can help minimize your risk of exposure to ticks:
Wear long pants and a long-sleeved shirt;
Stay in the middle of the trail and out of the brush (or, if golfing, out of the rough); and
Implement personal protective measures including using any one of a number of tick repellants, such as, DEET, picaridin, IR3535, oil of lemon eucalyptus, PMD, 2-undecanone, or permethrin. (The bonus of repellants is that they also deter mosquitoes from feeding on us.)
Tick check and removal. After any activity that might have exposed you to ticks, a thorough body search for these critters is in order. It is a good idea to do so as soon as possible after the potential exposure. It takes some time for the transmission of pathogens to occur. For example, the tick vector of Lyme disease, Ixodes scapularis, has to feed for more than 36 hours before it can transmit the offending bacterium, Borrelia burgdorferi. Therefore, if you can remove the tick quickly, there’s a chance you will have caught it in time to avoid infection.
It is easy to remove a tick that has not yet attached to the skin. Simply pluck it off, kill it and dispose of it. If is not engorged (i.e., it is still flat and not full of your blood), it’s unlikely to have transmitted a pathogen. But when removal isn’t easy, you’ll need a tweezers or special device for tick removal. It is important that the tick is removed completely from your body. If you need directions on how to remove ticks correctly, you can find an excellent demonstration on the Internet at: Tick Removal-YouTube.
Identifying the tick. The probability of catching a disease from a tick depends on three factors: the tick species, where you picked it up (geography), and how long the tick was feeding. Once you’ve removed a tick, it is important to identify it in order to determine what pathogens it might carry. There are two sites on the Internet that you may find useful: Tick Identification-YouTube; or Tick Encounters—which is provided free by the University of Rhode Island.
A boom time for ticks. Ticks are having a field day. Climactic changes such as warmer temperatures globally and more moisture regionally have enabled ticks to expand their geographic range and extend their reproductive season. Researchers recently looked at past temperature and rainfall in the United States to estimate their impact on Lyme disease. Using the results to model the future they found that even if the temperature increases are held to 1.8C, Lyme disease cases will jump by 34,000 cases annually by 2100.
And as the temperature warms, more frequent and larger disease outbreaks of RMSF, a disease with a mortality of 5% to 10%, are also predicted. Recently, scientists reported that brown dog ticks, which are vectors of Rickettsia rickettsia (i.e., the bacterium that causes RMSF), shift their preference to humans over dogs at warmer temperatures. The reason for this intriguing change in taste is, however, currently unknown.
For citizens of Minnesota, where warmer temperatures (especially after the our long winters) are greeted enthusiastically, we are chastened by headlines such as these appearing on April 9 in Yarmouth, Maine, “Deer ticks making a big comeback as temperatures rise” and in Hancock, Michigan, “With increasing temperatures, be on the alert for ticks.” It is important that all of us heed these warnings. Ticks can be dangerous; the diseases they spread can be debilitating and some are lethal. Nonetheless, ticks and the threat of a tick bite shouldn’t stop us from engaging in outdoor activities this spring. We just have to take the necessary precautionary measures to ensure that ticks don’t hitch a ride.