Ticks

This page offers information about ticks and the role ticks play in Lyme disease. Ticks are the so-called vector (an organism that carries and transmits an infectious pathogen) of Lyme disease. Specifically the information and images are about the hard-bodied Ixodes ricinus tick, but most of it also applies to all other ticks of the genus Ixodes.

In Europe, the vector of Lyme disease is Ixodes ricinus (the sheep tick or castor bean tick). In Eastern Europe and Asia, including China and Japan, Ixodes persulcatus (the taiga tick) is probably the most important vector. In North America, Ixodes scapularis (the black-legged tick or deer tick) is the main vector on the east coast, while on the west coast the main vector is Ixodes pacificus (the western black-legged tick). Note: ticks can also carry and transmit other infections, like babesiosis, and tick-borne encephalitis (TBE).

What are ticks?

Ticks are classified as arthropods (Arthropoda), a phylum (biological category) of invertebrates with legs with joints, a segmented body, and an exoskeleton. Within this phylum, ticks belong to the class of arachnids (Arachnida), a class that also includes spiders and mites, but not insects (Insecta).

Kingdom: Animalia (Animals)
Phylum: Arthropoda (Arthropods)
Class: Arachnida (Arachnids)
Order: Acarina

Ixodes ricinus tick
Ixodes ricinus tick

How to recognize a tick?

At first glance, a tick looks like an insect, but ticks are not insects. There are some characteristic differences. A tick can be identified by the number of legs (note: adult ticks have eight, but a tick larva have only six), the number of body segments (parts) and the mouthparts that the tick inserts into the skin at the head of the tick. The three main differences between insects and ticks are:

• Insects have three body segments, but ticks have two.
• Adult insects have six legs, but adult ticks have eight.
• Insects have wings and antennae, but ticks do not have them.

Adult Ixodes ricinus tick
Adult Ixodes ricinus tick: female tick

How many species of ticks are there?

Approximately 820 species of ticks have been identified worldwide. Of these, 100 are capable of transmitting pathogens such as bacteria, viruses and protozoa, and some tick even poisons, to humans. In Europe it is almost always Ixodes ricinus..

Ticks can be divided into two categories: hard ticks, which have a wholly or partially shield on their backs, and soft ticks, which have no shield. Among the different types are differences in terms of life span, life environment, and ability to transfer pathogen unto humans. There are about 650 different hard ticks, and 170 different soft ticks.

Life cycle of the tick

Ticks have four life stages: egg, larva, nymph, and adult. The larva, nymph, and adult (male and female) are the active stages when a tick is able to spread infections.

Tick Ixodes ricinus: larva – nymph – male – female
Tick Ixodes ricinus: larva – nymph – male – female (photo: Fedor Gassner)

On the photo above you can see how tiny ticks are in larva and nymph stages! Even in the adult stage, ticks are only a few millimeters in size. Further you can see that the tick in the larva stadium has 6 legs, while the tick in nymph and adult stadiums has 8 legs.

A tick changes from one active stage to another active active stage by shedding their skin, which means that the tick casts its old outer skeleton and a new one develops that fits the new body structure of the tick. The molting takes place right after the tick has fed and usually takes a few days. The entire life cycle of a tick takes about two years, but may take up just one year or up to three years, depending on the availability of food.

  1. Egg: The life cycle of the tick begins with a female tick laying eggs, which she lays on one big mass or seperated in several smaller masses.
  2. Larva: The larva that emerges from the egg, has six legs and no no sex characteristics yet; future males can not be distinguished from future females. A tick larva is smaller than one millimeter, but becomes three times as large when it feeds. After a blood meal, the tick larva morphs into nymph tick.
  3. Nymph: The nymph has eight legs and still has no sex characteristics. A nymph is about 1 to 1.5 millimeters in size. After a blood meal the nymph molts into an adult tick.
  4. Adult: The adult stage (also called imago) is the mature and last tick stage. Adult male ticks and female ticks have eight legs and are distinct from each other. They are approximately 3 to 5 millimeters in size. Only the female tick has become as large as 1 cm after she has fed and is fully engorged. The tick feeds for about 7 days, and thereafter the tick mates. The female tick then lays eggs and dies shortly thereafter.

Ticks: larva en fully engorged female tick
Tick Ixodes ricinus: larva - fully engorged female tick (photo: scarabaeus)

On the photo above you see on the left a tick in larva stage and on the right a fully engorged female tick. An enormous difference in size! Only female ticks grow that much in size when they take their last blood meal. After that they lay thousands of eggs.

How ticks feed

As parasites ticks are dependent for their nutrition entirely on the blood and tissue fluid from a host, what can be a person, a pet or a wild animal, including mammals, birds and reptiles. Ticks are located in forests, dunes but also often in gardens. Ticks thrive in a moist environment and normally live in the vicinity of potential hosts.

Ticks climb in grasses, shrubs and undergrowth and wait until a host comes right along and then the tick grabs onto the host. They detect their potential host by the radiated body heat, carbon dioxide and possibly also by odors. A tick usually remains less than one meter above the ground and can not jump or fly. That a tick drops out of trees when a a host comes by, is probably a misconception.

Most ticks feed preferably not on people, but on deer, mice, squirrels, and birds. The larvae and nymphs often feed on small mammals, and adult ticks often on larger mammals. But, if a person happens to come along at the right time, it can also be seen as a suitable host.

The tick can climb up via the legs of a person and settles preferably in skin folds, but also just on a leg or arm. Besides the wild mammals, especially dogs, cats and humans are bitten.

The tick bite

The tick sucks up blood with a special mouthpart called a hypostome. This is a harpoon-like mouthpart with barbs that the tick inserts into the skin and with which the tick fixes itself. On both sides of the snout are the palps which protect the snout when it is not being used.

Mouthparts of the tick with the hypostome and on both sides the palps
Mouthparts of the tick with the hypostome in the middle and on both sides the palps

The saliva of the tick contains both a narcotic substance and a substance that counteracts blood clotting. Because of this a tick bite is often not noticed and the tick can settle itself imperceptibly. A tick can stay on the same host for a few days to as long as a week.

The problem with ticks is that they can be infected with the Lyme-bacterium and other pathogens. The percentage of infected ticks can vary a lot in different locations. For example in the Netherlands about a quarter of the ticks is infected with Borrelia burgdorferi, but the infection rate varies significantly throughout the country.

In some species the adult female tick can pass some of the pathogens to the eggs, so that the eggs and larva can already be infected. But most ticks become infected by feeding on blood of an infected host. Especially the nymph, that have already sucked blood one time and so are more likely to be infected, pose a significant risk for people. Since they are smaller than adult ticks, they are more easily overlooked.

The longer an infected tick is attached, the greater the chance of an infection. Therefore a tick has to be remove as soon as possible, but in a proper way. See tick removal for information and tips on the removal of ticks.