Why Don't You Notice a Tick Bite?
― Barbs on the mouth, painkiller in the saliva. That's why you shouldn't yank it out
A mosquito bite itches right away. A tick, though, keeps its mouth in your skin and drinks blood for days, and most people never notice. Going unnoticed is the tick's survival strategy. Once you know how it works, you also know what not to do when you find one.
On an autumn holiday, you camp on a grassy riverbank. A few days after you get home, you touch the back of your knee in the bath and feel something like a small wart.
Looking closely, you see a blackish bead, a little bigger than a sesame seed, with thin legs. It is buried firmly in your skin, and pinching it doesn't dislodge it.
You have no idea when it got there. You never felt any pain or itching at all.
There are only two reasons you don't notice
Tick saliva is thought to contain substances that suppress the body's reactions of pain and swelling. The signal that you've been bitten barely reaches the brain.
A mosquito finishes in a few minutes. A tick takes several days to about 10 days, drinking only a tiny amount each hour. Your body changes too slowly to sense it.
A tick also has "barbs" and a "glue" that fix its mouth in the skin. That is why a tick is hard to remove even when you find it. Let's take it step by step.
Waiting on a grass tip for weeks
Ticks can't jump or fly. They climb to the tip of a blade of grass or a leaf, spread their front legs, and wait. At the tip of the front legs are organs that sense the carbon dioxide in breath, body heat and smell. The moment an animal or person brushes past the grass, the tick grabs onto clothes or fur.
A tick that has climbed aboard doesn't bite right away. It is thought to wander for several hours looking for soft skin. Common spots are the back of the knee, the armpit, the groin, behind the ear and the hairline.
When it bites, the tick first cuts the skin open with small blades. Then it pushes in a mouth lined with rows of tiny barbs. As Figure 2 shows, the barbs point like a fish hook: easy to go in, hard to pull out. The tick also releases a cement-like substance from its saliva and hardens the area around its mouth onto the skin.
It starts out unnoticeable, then swells all at once at the end
Once it is fixed in place, the tick drinks blood over many days. An adult female is said to reach about 100 times her weight before feeding. Use the slider in Figure 1 to change the number of days since the bite.
As Figure 1 shows, the weight hardly rises for the first few days. During this time the tick is thought to be fixing its mouth in place and preparing the site with saliva. In the last 1 to 2 days it swells all at once. A body that was the size of a sesame seed becomes the size of an adzuki bean.
This slowness is the second reason you don't notice. The amount drunk per hour is even less than what one mosquito takes in one meal (the calculation is in the fold-out at the end). Your body can barely sense the change.
Why shouldn't you pull it out?
Figure 2 is a cross-section of a tick's mouth fixed in the skin. Because it is locked in by the barbs and the cement, pinching and pulling tends to tear off the body and leave the mouth in the skin. The leftover mouth can lead to infection.
Another worry is squeezing the body hard. The contents of the tick's body may be pushed back into your skin. Ticks can carry pathogens such as those of severe fever with thrombocytopenia syndrome (SFTS) and Japanese spotted fever. The Ministry of Health, Labour and Welfare also urges people not to pull off a feeding tick by force, and to have it removed at a medical facility such as a dermatology clinic.
Ticks are most active from spring to autumn, so care is needed during autumn outings, farm work and mowing. They live not only in woods and mountains but also in the grass of riverbanks and parks and along the edges of fields. They are said to be especially common where deer and wild boar live.
So what should you do?
- Keep your skin coveredLong sleeves and long trousers. Tuck the shirt into the trousers and the trouser hems into your socks. Light-coloured clothes make it easier to spot a tick that has climbed on
- Check your body as soon as you get homeIn the bath, check behind the knees, the armpits, behind the ears and the hairline. Don't bring your jacket into the house
- If you're bitten, don't pull it out. Go to a clinicHave it removed at a dermatology clinic or similar. Watch your health for the next few weeks, and if you get a fever, tell the doctor you were bitten
If fever, tiredness, nausea, diarrhoea or similar symptoms appear within a few days to about 2 weeks of a tick bite, see a doctor right away. Severe fever with thrombocytopenia syndrome is a disease in which an estimated 10 to 30 percent of patients die. At the visit, be sure to say "I was bitten by a tick" and "I went into tall grass." If symptoms are severe, such as sudden confusion, call 119 without hesitation. Cases have also been reported of infection passed on from pet dogs and cats.
Summary
You don't notice a tick because of bad luck. It has a full toolkit for going unnoticed: saliva that stops pain, and slow feeding over many days. The same tools also make it hard to pull out.
Ticks have evolved to go unnoticed.
So "don't pull it out when you find one" and "check your body when you get home" are the best preparation.
For other dangers of autumn fields and forests, also read why bears come down to villages in autumn, folk sayings about telling poisonous mushrooms apart, and why a second bee sting is said to be dangerous.
- Prepare a pale cloth and a dark cloth, and scatter five sesame seeds at random on each.
- From about 1 metre away, count how many you can find in 10 seconds.
- Next, crumple the cloth into wrinkles and count again in the same way.
You should find it easier on light, flat clothes. This is why "wear light-coloured clothes and brush yourself off often" is recommended. Never touch a real tick with bare hands. Do your observing with cloth and sesame seeds.
For those who want more ― terms, formulas and links to textbooksEach part is labelled by level, from junior high science to university specialist courses
- Junior highCovered in junior high school science
- High schoolCovered in high school biology
- HS+Advanced high school material, or textbook column content
- UniversityUniversity specialist courses (medical entomology, infectious disease) not taught in high school
- ResearchTopics researchers are still investigating, not taught as settled fact even at university
Junior highTerms: this phenomenon has names
- Tick: A relative of spiders (eight legs), a large mite that drinks animal blood. It is a different group from the tiny mites found in houses.
- Hypostome: The part of the mouth pushed into the skin. It carries rows of fine backward-pointing teeth (the barbs).
- Cement substance: A substance released from the saliva after biting, which fixes the mouth to the skin.
- Haller's organ: A sense organ at the tip of the front legs. It is thought to detect carbon dioxide, heat and smell.
Junior highHigh schoolCheck with a formula: how much does a tick drink per hour?
"Swells to 100 times its weight" sounds like a huge amount. But spread over several days, the amount per hour is very small. We also estimate how much the body size changes.
| In symbols | q ＝ ( W1 − W0 ) ÷ T 、 L1 ＝ L0 × ∛( W1 ÷ W0 ) |
| In words | Amount drunk per hour ＝ weight gained ÷ hours of feeding. Body length grows by the cube root of the weight ratio |
| Where the formula comes from | Weight increases by the mass of blood retained (conservation of mass). If the body swells while keeping the same shape, volume (weight) is proportional to the cube of length |
| Weight of an adult female before feeding W0 (rough guide) | 3 mg |
| Weight ratio after feeding (rough guide) | About 100 times |
| Feeding period T (rough guide) | 7 days |
| Body length before feeding L0 (rough guide) | 3 mm |
| Blood one mosquito takes in one meal (rough guide) | Said to be 2–3 mg |
Meaning of the symbols: W0 is the weight before feeding, W1 the weight after feeding (in mg), T the feeding time, and L0 and L1 the body length before and after (in mm).
| Weight after feeding W1 | 3 × 100 ＝ 300 mg |
| Weight gained | 300 − 3 ＝ 297 mg |
| Feeding time (converted to hours) | 7 × 24 ＝ 168 hours |
| Amount drunk per hour q | 297 ÷ 168 ≒ 1.8 mg |
| Cube root of the weight ratio | The cube root of 100 is about 4.6 |
| Body length after feeding L1 | 3 × 4.6 ≒ 14 mm |
At about 1.8 mg per hour, that is even less than one mosquito takes in one meal. In reality the feeding is concentrated in the last 1 to 2 days, so the first few days are slower still. The calculated body length is about 14 mm, but because the flat body swells round, a real tick is said to be around 1 cm. Also, a tick returns part of the water in the blood to the host as saliva, so the total blood taken is said to be more than the weight gain suggests.
High schoolHS+How saliva stops pain and itching
High schoolWhen skin is injured, blood clotting, which seals the wound, and inflammation, which drives out foreign matter, begin. In inflammation, histamine and similar substances are released and cause itching, swelling and pain. These mechanisms are taught in the "immunity" and "blood clotting" units of high school biology.
HS+Tick saliva is thought to contain many proteins: substances that stop blood from clotting, substances that weaken the action of histamine and the like, and substances that suppress immune responses. To keep feeding at the same spot for days, the tick must hold down the body's defences for a long time.
UniversityTick-borne diseases and feeding time
Diseases carried by ticks are grouped as "tick-borne diseases." In Japan, severe fever with thrombocytopenia syndrome (SFTS), Japanese spotted fever and Lyme disease are known. For some pathogens, it is thought to take time after the bite before transmission, so finding and removing the tick early is considered worthwhile. In medical entomology, the behaviour of lying in wait for a host is called "questing."
📖 Going further: Ticks ― Wikipedia (Japanese edition) ／ Severe fever with thrombocytopenia syndrome ― Wikipedia (Japanese edition)
ResearchWhat is still not clear
- Do we know every component of the saliva? Tick saliva is said to hold several hundred kinds of proteins, and the mix changes with the stage of feeding. The role of each is still being studied.
- Can we make a vaccine using ticks? Research is under way on building immunity to saliva components so that ticks can't feed well.
- How will their range change? It has been pointed out that as deer and wild boar increase and temperatures change, the areas with ticks and tick-borne disease are spreading, and monitoring continues.
So the content of this article, too, is "an explanation of what is known now." The numbers vary greatly by species and individual.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: "Classification of animals," "Stimulus and response" | Ticks as relatives of spiders; how we feel pain |
| High school | Biology: "Immunity," "Blood clotting" | Inflammation, histamine, how blood clots |
| HS+ | Advanced biology | Saliva suppressing the body's defences |
| University | Medical entomology, infectious disease | Questing, tick-borne diseases |
| Research | Parasitology, ecology | Saliva components, vaccines, range changes |
| ― | Links to daily life | Clothing in tall grass, checking after returning home, not pulling the tick out |
- Ministry of Health, Labour and Welfare (厚生労働省), "Severe fever with thrombocytopenia syndrome (SFTS)"
- Wikipedia (Japanese edition), "Ticks"
- Wikipedia (Japanese edition), "Severe fever with thrombocytopenia syndrome"
- Sonenshine, D. E., & Roe, R. M. (Eds.). (2014). Biology of Ticks (2nd ed.). Oxford University Press.
- Francischetti, I. M. B., et al. (2009). The role of saliva in tick feeding. Frontiers in Bioscience, 14, 2051–2088.
※This article is a general-audience science explainer. The figures given are rough guides to help you understand how things work, and they vary by tick species and individual. If you are bitten, or if your health changes, see a medical facility and follow the instructions of doctors, public health centres and local authorities.