Ticks are small to medium arachnid parasites (0.1 to 2 cm) that are all bloodsuckers. Together with mites they form the class of acarina. Other arachnids are e.g. spiders and scorpions. Arachnids are part of the arthropodes, all those animals that have and external skeleton and jointed legs. Other arthropods are insects and crustaceans (e.g. crabs, lobsters, shrimp, etc.)
There are about 1'000 tick species worldwide. Most species have a regional distribution. Numerous species occurr only in tropical and subtropical regions (e.g. Amblyomma cajennense, Rhipicephalus appendiculatus, Boophilus microplus, etc.), whereas other species prevail in places with moderate or cold climate (e.g. certain Ixodes species).
Ticks are among the most damaging parasites of livestock in tropical and subtropical regions, in contrast with colder regions, where ticks are often only a secondary issue during the summer months. The same applies to horses, dogs and cats, whereby dogs are usually much more affected by ticks than cats.
For detailed information about particular species select an option from the corresponding menu or from the list at the end of this srticle
Anatomy of ticks
Adult ticks can be rather large, especially engorged adult females, i.e. those that have completed their blood meal and are ready to lay eggs. Engorged females of certain tropical tick species can reach the size of an olive (up to 3 cm long). Otherwise hungry (i.e. unfed) adult ticks are usually 0.3 to 1 cm long. Tick larvae may be very small (<1 mm), especially unfed ones.
The anatomy of all ticks is substantially different from that of insects and is characterized by the complete absence of segmentation, i.e. their body is not divided into head, thorax and abdomen. Ticks don't even have a proper head. Instead they have a series of mouthparts grouped into the so-called capitulum (also called false head or gnathosoma). These mouthparts are a pair of palps, a pair of chelicerae and the hypostoma. The chelicerae are used to cut and open the skin of the host. The hypostoma is like the needle that penetrates the skin to suck blood. The hypostoma has many barbs pointing backwards that help keeping it strongly attached to the host's skin.
There are two major types of ticks: hard ticks (Ixodids) and soft ticks (Argasids). Hard ticks have a clearly visible more or less protruding capitulum at the anterior end of the tick's body, whereas in soft ticks the capitulum is below the abdomen and can't be seen from upside.
Instead of thorax and abdomen ticks have just an abdomen (also called idiosoma). The abdomen contains all the major organs (digestive tract, reproductive organs, nervous system, etc.). The abdomen of hard ticks is partially covered by a shield called scutum. The scutum always has species-specific colors and patterns that sometimes help to easily recognize certain species. The skin of the abdomen is very flexible to allow expanding while feeding. In fact hard ticks, especially adult females, become virtually inflated with blood, increasing their size for up to more than 10 times their normal size. Soft ticks do not inflate as much as hard ticks.
The legs are inserted along both sides of the abdomen. Whereas tick larvae have three pairs of legs (like insects), nymphs and adults have four pairs of legs. The legs are covered with small spines and end with a claw. Spines and claws help the tick crawl and remain attached to the host or to climb in the vegetation.
As for insects and other arthropods, the body of ticks is covered with a more or less hard "skin", a kind of shell called the exoskeleton or outer skeleton. It protects the body organs against mechanical pressure, desiccation and external pathogens, and offers attachment sites for the muscles. The exoskeleton is formed of various structures, one of which is the cuticle that contains chitin. Chitin is a special molecule responsible for the hardness of the cuticle that is specific for arthropods, but is also found in some fungi and bacteria. In ticks, the exoskeleton of the abdomen is tough (i.e. resistant to pressure) but not rigid and can expand considerably, as already mentioned.
Metamorphosis and life cycle of ticks
All ticks undergo a so-called metamorphosis, i.e. development to adults does not only require increasing the size, but also changing the shape. Due to the rather rigid exoskeleton, increasing the size is only possible through molting, i.e. getting rid of the old skin and producing a new and larger one.
Ticks (and most arthropods) go through four major development stages: egg, larva, nymph and adult. The passage from one stage to the next one requires one or more molts.
The duration of the complete life cycle (i.e.) from eggs to eggs of the next generation varies a lot among the tick species and is strongly dependent on the climatic and ecological conditions. It can be as short as two to three weeks, and as long as more than two years. A typical feature of many tick species is their extreme ability to survive without feeding. Several species can wait more than a year without feeding until they find a host.
Soft ticks feed several times for a few hours on a host, leave it to hide in the environment and come back later to continue feeding. Molting or egg laying takes place after enough blood has been consumed.
In contrast with this, hard ticks, either larvae, nymphs or adults have only one blood meal between molts, but always a very long one (usually several days). After each blood meal they molt and go for the next meal, the adult females lay eggs. But there are significant differences among species regarding the behavior between the blood meals.
One-host ticks remain all the time on the same host. After each meal they detach and molt but do not leave the host falling to the ground: they molt on the host. Only the engorged adult females drop off the host to lay the eggs in the soil. This means that the infective stages in the environment are only hungry larvae, i.e. only larvae actively infect animals, not adults or nymphs. Hungry (i.e. unfed) nymphs and adults do not search a new host but remain on the one they are already on. There are no free-living nymphs and adults of one-host ticks. Typical one-host ticks are cattle ticks (e.g. Boophilus microplus, Boophilus decoloratus).
Two-host ticks change the host once during development. The larvae that had infected a host complete the blood meal, detach and fall off the host to the ground. There they molt to nymphs and wait for a new host. This means that the infective stages of two-host ticks are the larvae and the nymphs. Typical two-host ticks are e.g. Rhipicephalus bursa.
Three-host ticks (also called multi-host ticks) change the host twice during development. Not only the larvae detach and fall to the ground for molting after the blood meal, but also the nymphs. This means that the infective stages of three-host ticks are the larvae, the nymphs, and the adults. Typical three-host ticks are those of the genus Amblyomma, Ixodes and Haemaphysalis.
Besides the number of hosts required for completing the life cycle, there are also differences regarding the type of hosts suitable for development. Some tick species are quite host-specific. e.g. cattle ticks (Boophilus spp) infect only cattle and related species (e.g. buffaloes, deer). They may infest and feed on other mammals (e.g. horses, donkeys, sheep, etc.) but will not complete their life cycle on such hosts. Other tick species (e.g. Amblyomma) are much less choosy and will infest whatever warm-blooded animals they meet, including birds. Larvae and nymphs often prefer smaller hosts (e.g. rodents, birds), whereas adult ticks prefer larger ones (cattle, deer, horses, large carnivores, etc.).
This different behavior regarding the free-living infective stages and the choice of potential hosts is extremely relevant regarding the population dynamics of ticks in cattle farms and the strategies for controlling them.
One-host ticks such as those of the Boophilus type are much more likely to develop resistance to tickicides, because they are much longer exposed to them than two-host or multi-host ticks, i.e., they are under a much higher selective pressure: They remain longer on treated animals, and they cannot find a refuge on untreated wildlife because they are quite cattle-specific.
Eradication of one-host ticks such as the cattle ticks Boophilus microplus and Boophilus annulatus has been more or less successfully achieved in certain regions (e.g. the USA, parts of Australia, Argentina and Uruguay). This is because they are quite cattle-specific. Independently of other factors (ecological, climatic, political, etc.) eradication was possible because basically it is enough to treat all cattle at certain intervals in a given region and to inspect all cattle that are transported into this region.
In contrast with this, for eradication of two-host or multi-host ticks treating and inspecting only cattle is not enough because many ticks can survive on whatever wild-life and straying animals, including birds that can fly and bring ticks virtually everywhere. To my knowledge only one large-scale eradication programm was tried on a multi-host tick, the Caribbean Amblyomma Eradication Program, which from 1994 to 2007 tried to eradicate Amblyomma variegatum (the tropical bont tick, TBT) from the Caribbean Islands where it was introduced from Africa in the 18th century. However, after 13 years and about $M50 spent, only a part of the islands were declared TBT-free when the program was discontinued.
Behavior of ticks
Ticks are obligate bloodsucking (so-called hematophagous) parasites, i.e., they cannot survive without blood. Larvae, nymphs and adults, both males and females need to suck blood. Adult hard tick females need a huge blood meal because they will need it for producing thousands of eggs: between 2'000 and 20'000, depending on the species. After their bloodmeal engorged adult female hard ticks can be 50 to 100 times larger than unfed.
Ticks are very sophisticated bloodsuckers. Tick bites are usually not painful compared with the bites of deer, horse or stable flies. This reduces possible defensive reactions of the irritated host (rubbing, scratching, biting, etc.) that would kill many blood-sucking ticks. This is important for ticks, because their blood meals last for hours and even days, and they are very solidly attached to the host, i.e., they cannot quickly detach and fly away as biting flies typically do when the host gets irritated and strikes back.
The saliva of some ticks has a lot of properties to ensure a successful biting. It contains substances that work as a glue that helps the hypostoma to remain firmly attached to the host's skin, and that dissolves after feeding. As in many biting parasites, the tick's saliva contains anticoagulants that prevent the host's blood from clogging during the extremely long blood meal. But in contrast with other biting insects (e.g. flies and mosquitoes) the tick's saliva contains little or no inflammatory substances that cause itching and swelling, another adaptation to reduce the host's reaction to bites. Finally it is also known that the tick's saliva contains substances that reduce the immune response of the host.
After their blood meal, adult females detach from the host, fall to the ground and begin to lay eggs. Egg-laying can last for several days, and the female's body virtually empties itself almost completely. The number of eggs ranges from 2'000 to 20'000 and depends on the species but also on physiological and environmental conditions. Once egg-laying is completed the female dies.
Larvae need a few days or weeks to hatch out of the eggs, depending on weather conditions. Hatching is faster by hot and humid weather. Soon after hatching larvae instinctively climb to the tip of grasses or on leaves of shrubs and bushes in the vegetation. There they wait for a suitable host to pass by. This behavior is called questing.
When a suitable host passes by and touches the grass or the leaves, the questing ticks just hang on to it. They recognize a suitable host by its body heat and exhaled CO2 (carbon dioxide), which they can perceive with sensory organs, e.g. the so-called Haller's organ in the legs. In fact, an easy method to collect ticks consists in putting some dry ice (= frozen carbon dioxide) in a jar burried in the ground or over a piece of white cloth on the grass, and wait. After a few hours the ticks will have gathered in the jar or on the cloth.
Unfed nymphs and adults of two or three-host ticks show exactly the same questing behavior.
This behavior means that most hard ticks neither jump onto their preys, nor drop down from the trees. Australian Ixodes holocyclus seems to be an exception: it does climb trees and falls down onto people sitting underneath. As a general rule hard ticks are not transmitted from one animal to another one, neither within a herd, nor from pets to humans, nor among pets. A hard tick (larva, nymph or adult) that has found a suitable host will soon attach for feeding and have its bloodmeal for several days. It is extremely unlikely that it is passed to another animal before or after the blood meal.
This is diferent for soft ticks. Their blood meals are shorter and they leave the host repetedly. Between the meals they remain in their nests, which they leave for a new meal on whatever host they can attach to. The good news is that soft ticks are significantly less abundant than hard ticks in most regions of the world.
Harm and damage to livestock, horses, dogs and cats caused by ticks
All tick species and all their development stages (larvae, nymphs, adults) are obligate bloodsucking parasites, i.e. they have no alternative food than blood of their hosts.
Ticks cause a lot of harm to livestock and pets. For livestock, especially cattle, harm can be enormous because infected animals may carry thousands of ticks. For pets and horses the major risk is disease transmission.
The bite of a hard tick causes little or no immediate pain to the host. But since the bite lasts for days, the injected saliva causes immure reactions that lead to inflammation of the skin, which usually means itching (=pruritus) and more or less pain. The affected host reacts through more or less intense scratching, rubbing or biting of the affected skin areas. All this means stress and low appetite, which on livestock results in lower productivity: less milk, lower weight gains, reduced fertility, etc. On livestock, the scars built at the biting sites can significantly reduce the hide quality. Bites of soft ticks, especially by Otobius species are notoriously more painful.
Blood loss due to a few ticks is irrelevant for a host's health. Most dogs and cats will usually catch only a few ticks; let's say 1 to 10. But livestock, especially cattle, can be affected by thousands of ticks, all sucking blood. By these numbers blood loss is highly significant and can cause serious anemia. A single female hard tick of the larger species (e.g. Amblyomma or Rhipicephalus) can take something between 1 and 5 ml blood during a meal that lasts for one to two weeks. 100 such ticks would suck up to 0.5 liters, 1000 ticks up to 5 liters of blood! Thousand ticks per cattle are not the average, but not unusual in numerous tropical countries in Africa and Latin America.
Biting ticks often build clusters, typically in the ears, udders, below the tail, often according to species-specific preferences. When many ticks are biting in a small area of skin open wounds can develop that can be easily infected or attract screwworm flies that lay eggs on the wounds, whose larvae will eat themselves into the wound.
Poisoning is another possible harm caused by tick bites. Ticks do no inject a strong toxin as snakes, spiders or scorpions. But the tick's saliva contains a complex mix of several substances that strongly affect the host's immune system. For several tick species this can lead to a general toxemia (= poisoning) of the host. Such toxemia can lead to paralysis of the host that can be fatal. This is the case for Ixodes rubicundus in Southern Africa that can caused tick paralysis on sheep; Ixodes holocyclus in Australia that can cause tick paralysis of humans, horses, dogs, and cattle; and Dermacentor andersoni that can cause tick paralysis of cattle in North America. Hyalomma truncatum causes the sweating sickness in Northern Africa, which is characterized by massive hair loss (alopecia).
Ticks can transmit numerous viral, bacterial and protozoan diseases to livestock, pets and humans, i.e. they are vectors of such diseases. Typical tick-borne diseases are African Swine fever, Crimean-Congo hemorrhagic fever, tick-borne encephalitis, borreliosis (e.g. Lyme disease), anaplasmosis, ehrlichiosis, babesiosis, theileriosis, etc. Most of these diseases have a regional occurrence, often associated with the seasonal abundance of the various tick species.
Most of these diseases are highly harmful for livestock in tropical and subtropical countries, a few are often fatal, especially for cattle. But the importance of tick-borne diseases for pets, horses and humans is increasing in regions with moderate climate, perhaps due to global climatic and ecological changes that favor survival and spreading of some tick species in these regions.
Livestock as well as horses and pets naturally exposed to such diseases usually develop a significant degree of immunity, which results in a so-called enzootic stability. This means that although the animals are continuously exposed to ticks and such ticks actually re-infest the animals with microorganisms, most of them are not harmed because they have progressively built up their own antibodies. Such antibodies are often passed from mothers to their offspring with the first milk (calostrum). This immunity of the hosts is maintained as long as they remain re-infested by the ticks.
If the animals start to be treated with tickicides, and treatment is effective, the treated animals will progressively lose their disease immunity. As long as they remain adequately protected against the ticks, this is OK. However, if for whatever reason tickicide treatments are interrupted (e.g. due to financial, supply, or whatever problems), a sudden outbreak of any of such diseases can be a disaster, with dozens of fatalities.
If animals (livestock, horses, pets) living in a tick-free environment and thus without acquired immunity to tick-borne diseases are transported to tick-infested regions and thus suddenly exposed to tick-borne diseases, the risk of severe complications when exposed to ticks is particularly high.
A problem with tick-borne diseases is that it is very difficult to predict the potential impact they can have on livestock, horses or pets. Within a tick population, not all ticks are infested with pathogens. Maybe only 10% of the ticks are actually infected. Additionally, the level of infestation of the infected ticks is also variable, i.e. some ticks may carry thousands of pathogens, others only a few ones. And finally, the virulence of the pathogens can also be different, whereby very few pathogens of a very virulent strain can be as harmful as many pathogens of a rather benign strain.
From the economic point of view, hard ticks are by large the most damaging ticks for the cattle industry in tropical and subtropical regions. Economic loss due to ticks was estimated to be about 2 billion USD only for Brazil in 2002. Ticks are usually not the primary parasitological problem on sheep and goats, but they can have regional and seasonal importance. Ticks are not an issue for modern industrial pig & poultry production facilities, but can be a problem under traditional or backyard farming conditions. Ticks are becoming increasingly important for horses and dogs.
Tick species of livestock, horses and pets
Hard ticks (Ixodidae)
The following hard ticks are particularly important for livestock and pets (for additional information click the link on each genus):
- Genus Amblyomma
- Amblyomma americanum, the lone star tick; important for dogs; parts of North America
- Amblyomma cajennense, the Cajenne tick; important for livestock and dogs; Americas, tropical and subtropical
- Amblyomma hebraeum, the bont tick, the Southern Africa bont tick; important for livestock; Africa, South of the equator
- Amblyomma maculatum, the Gulf Coast tick; North and South America
- Amblyomma variegatum, the tropical bont tick; important for livestock; parts of Africa, Caribbean
- Genus Boophilus (nowadays Rhipicephalus): the most important ticks for the cattle industry
- Boophilus annulatus (= Rhipicephalus annulatus), the American cattle tick; parts of America, Near and Middle East, Mediterranean, parts of North and South America
- Boophilus decoloratus (= Rhipicephalus decoloratus), the tropical cattle tick, the blue tick; important for livestock; Africa, South of the equator
- Boophilus microplus (= Rhipicephalus microplus), the cattle tick; the southern cattle tick; important for cattle; parts of Australia, Americas, parts of Africa and Asia
- Genus Dermacentor
- Dermacentor andersoni, the Rocky Mountain wood tick; North America
- Dermacentor marginatus, the ornate sheep tick; Europe, temperate and Mediterranean; Northern Africa
- Dermacentor nitens (= Anocentor nitens), the tropical horse tick; important for horses; tropical America
- Dermacentor reticulatus, the marsh tick, the ornate cow tick, the European meadow tick; important for dogs; Europe
- Dermacentor variabilis, the American dog tick, the wood tick; important for dogs; parts of North America
- Genus Haemaphysalis
- Haemaphysalis concinna is found in Central Europe, East and Southeast Asia, China, Japan
- Haemaphysalis inermis is found around the Mediterranean, in East Europe, Near and Middle East.
- Haemaphysalis leachi leachi, the yellow dog tick, the African dog tick. Occurs in Tropical and Southern Africa.
- Haemaphysalis longicornis, the bush tick, Australia and New Zealand
- Haemaphysalis punctata, the red sheep tick; Europa, Middle East
- Genus Hyalomma
- Hyalomma anatolicum, important for livestock; Southern Europe, Near and Middle East, Asia, Northern Africa
- Hyalomma dromedarii, Asia, Northern Africa
- Hyalomma marginatum, Southern Europe, Near and Middle East, Asia
- Hyalomma truncatum, important for livestock, Northern Africa
- Hyalomma lusitanicum, Southern Europe
- Genus Ixodes
- Ixodes holocyclus the paralysis tick of Australia; the scrub tick, etc., important for dogs and horses; Australia
- Ixodes pacificus, the Western black-legged tick; North America (West Coast)
- Ixodes persulcatus, the taiga tick, Central, North and East Europe, Siberia, North China, Japan
- Ixodes ricinus, the castor bean tick, the sheep tick, the European wood tick; important for dogs; Europe, Near and Middle East, Northern Africa
- Ixodes rubicundus, the karoo paralysis tick; Southern Africa
- Ixodes scapularis = dammini, the deer tick, the black-legged tick; important for dogs; North America, East and Gulf Coast
- Genus Rhipicephalus
- Rhipicephalus appendiculatus, brown ear tick; important for livestock; Africa South of Equator
- Rhipicephalus bursa, Southern Europe, Near and Middle East,
- Rhipicephalus evertsii, the red-legged tick; important for livestock; Africa South of Equator
- Rhipicephalus sanguineus, the brown dog tick; important for dogs; worldwide
Soft ticks (Argasidae)
The following soft ticks are particularly important for livestock and pets:
- Argas miniatus, the chicken tick, the South American bird argas; important for poultry; Central America and Caribbean
- Argas persicus, the fowl tick, the poultry tick; important for poultry; worldwide in warm climates
- Argas radiatus, the North American bird argas; North America and Caribbean
- Ornithodorus moubata, the eyeless tampan; the tampan tick; important for poultry; Africa
- Ornithodorus savignyi, the sand tampan; important for livestock, Africa, Asia
- Otobius megnini, the spinous ear tick; important for horses; Americas, Africa, India