Few parasites can compete with mites in terms of the variety of mastered parasitism variants. It is in the subclass of ticks that one can find examples of almost all forms of parasitism that are known for arthropod invertebrates in general. In fact, ticks can be used to study parasitology in many of its classic manifestations.
And although it may seem that in this capacity, mites are curious primarily for a natural scientist, but in fact, for a person far from biological science, the parasitic way of life of mites can be interesting - at least in its most original manifestations.
And many facts from the biology of these animals are remarkable in themselves.
Types of parasitism in ticks
The most famous ticks for the layman are called ixodid ticks (the people often call them forest ticks) - they represent only a very small group of the entire subclass of ticks.
It is interesting
In total, more than 54,000 species of ticks are known today. The Ixodes family, some of which are carriers of tick-borne encephalitis and Lyme disease, includes only about 670 species - that is, just over 1%.
The form of parasitism of ixodid ticks can be characterized as obligate periodic ectoparasitism.
What does it mean?
Ectoparasites are living organisms that do not penetrate into the host to feed on the host. As a rule, they have to damage the outer integument of the host's body in order to be able to eat certain tissues (in the case of ixodid ticks, blood), but they do not live permanently in the host's body.
Unlike ectoparasites, endoparasites are those creatures that live inside the body of the host.
Ixodid ticks do not completely penetrate under the integument of the body of a person or domestic animals, that is, they are typical ectoparasites.
In the same time, Ticks also contain endoparasites.. For example, scabies itch - the causative agent of scabies, better known as a subcutaneous mite - constantly lives in the thickness of the skin, makes passages here and feeds on the epidermis.
The photo below shows what a subcutaneous tick (Sarcoptes scabiei) looks like under a microscope:
And here is a picture taken with a scanning electron microscope:
Similarly, blackhead mites, a very small member of the trombidiform mites that live in the hair follicles of most people on the planet and feed on sebum, are also an example of an endoparasite. His relatives in the order, by the way, are formidable parasites of cultivated plants.
Photo of acne gland:
There are also known cases of parasitism of ticks in body cavities. For example, cheese and flour mites, when a person eats contaminated foods, can colonize the digestive tract: exist and even multiply here in conditions of almost complete absence of oxygen, causing severe gastrointestinal disorders.
It is interesting
In the scientific community, there are disagreements among specialists regarding at what degree of penetration into the body to consider the parasite as internal, and at what degree as external. So, there are points of view in which the acne gland is referred to as ectoparasites, that is, to creatures living on the surface of the host's body. This point of view is justified by the fact that these mites do not penetrate very deeply into the integument of the body and live in the surface layer of the skin. Because of such disagreements, they even developed a system for classifying ticks into dermal, cutaneous, subcutaneous, feather and cavitary. Iron worms are most often referred to as skin endoparasites.
Another sign by which forms of parasitism are distinguished is the time spent on the surface or in the body cavity of the host. According to it, ticks are divided into permanent and temporary parasites.
Most ixodid ticks are typical temporary parasites, spending most of their lives in the topsoil and on plants. They climb onto the surface of the host's body only for feeding, and after saturation they leave it.
The opposite form is permanent parasites. Subcutaneous mites, iron mites, ear mites of the genus Otodectes can already be clearly attributed to them, the entire life cycle of which takes place on the surface or inside the integument of the host's body. If it happens that the tick is outside the body of the host, he immediately begins to search for a new one, without which he is not able to survive.
Finally, tick parasitism can be obligate and facultative.
Obligate parasitic mites are those that can only feed on the host animal, otherwise they either die or cannot reproduce. They have no other way to eat.
Facultative parasites are living organisms that can combine different ways of obtaining food. Among mites, such forms are usually represented by species that can combine predatory and parasitic types of nutrition.
Such, for example, are many water mites, mites from the family Trombiculidae (red beetles). In them, adults can attack small invertebrates and kill them by sucking out the contents of the body. And the same individuals, when meeting with a large animal, which they are not able to kill, can climb onto it, pierce the integument of its body and suck blood. That is, parasitism is not the only way for them to survive, and many of them do not parasitize even once during their lives.
On a note
Approximately 48% of parasitic mites are temporary parasites, 45% are permanent, and the rest are occasional (facultative).
Also, facultative parasites include the already mentioned flour and cheese mites, which normally do not attack a person and do not parasitize on him, but if they accidentally enter the digestive tract, they settle in it and become parasites.
Below in the photo is a cheese mite (Acarus siro), capable of causing intestinal acariasis:
It is interesting that many types of ticks (there are many of them, for example, among the red beetles) are parasites at the nymph stage, and turning into an adult, they turn into predators. In such cases, however, one cannot speak of facultative parasitism. Here we are talking about different ways of feeding at different stages of development: if the nymphs of such ticks are obligate parasites, then the adults are obligate predators.
The most famous ticks - ixodid, argas, subcutaneous - are obligate parasites and are not able to feed on anything other than the biological materials of animal hosts.
On a note
It is noteworthy that there are fewer parasitic mites than predatory mites and those that feed on various organic residues. For example, there is a whole family of barn mites that feed on grain and plant debris. Dust mites are very widespread in apartments, feeding on pieces of the epidermis that crumble from the body of people, and thousands of species of microscopically small representatives of this subclass living in the soil and consuming the decaying remains of plants and animals have been described.
That is, despite the "image" of parasites that has developed in ticks, not all of them lead a parasitic lifestyle.
There is also a huge number of species of mites that are parasites of plants - feeding on the juices of leaves and stems and harmful to agriculture.
The example of the railways already mentioned above is curious. Their way of interacting with a person is not always typical parasitism, since in most cases a person does not suffer from their activity and does not at all feel the presence of these creatures on the skin or inside it. Despite the fact that glandular glands are found in almost all people over 70 years of age and in more than half of adults worldwide, cases of developing skin diseases caused by these mites are infrequent.
Consequently, most often people do not suffer from coexistence with these arthropods. In the absence of such antagonism, the interaction between the host and the "guest" is called not parasitism, but commensalism.
It is worth noting here that acarologists do not have a unanimous opinion on whether to consider glanders as parasites or commensals. This is another example of the variety of forms of interaction between ticks and their hosts.
Single-host, two-host and three-host ticks
Important in parasitology is the classification of ticks according to the number of hosts. In accordance with it, different types of ticks are divided depending on the minimum number of host animals that one individual of a particular species must change in order to fully realize its reproductive cycle.
For example, all parasitic mites can be divided into three types according to this feature:
- Single host mites. Their full development from the larva to the mature individual occurs on the same host, without changing it. The larva sucks blood, molts into a nymph, feeds again, molts into adults, mates with an individual of the opposite sex, sucks blood again, after which the female leaves the host's body to lay eggs in the soil or elsewhere. Such species include, for example, the bull tick and the species Hyalomma scupense, representatives of the family of ixodid ticks;
- Two-host ticks - those in which larvae and nymphs feed on the same host, after turning into a nymph and another bloodsucking, leave his body, turn into an imago, which then attacks the second host, sucks blood to enable fertilization, and then detaches to mate and (for females) lay eggs. Such a cycle of development is characteristic of some species of the genera Hyalomma and Rhipicephalus;
- Three-host ticks are species in which an individual changes its host at each stage of development.This group includes most representatives of the family of ixodid ticks. In particular, taiga and dog ticks are three-hosted.
In all these forms, the number of hosts is not identical to the concept of species specificity. That is, it would be a mistake to believe that all individuals of one or another species of a single-host tick can develop, for example, only on dogs, while individuals of a two-host species carry out the larval stage and the nymph stage, for example, on rats, and in adult form attack only cows.
In reality, "hostility" means only the number of host changes during the life of one tick. Individuals of the same species of single-host ticks can develop on hedgehogs, on rodents, on hares, on dogs, or on cattle. Where a particular parasite will grow depends only on which particular host animal it can attack.
Almost all types of ticks that change owners do not have strict species specificity in relation to their “hosts”. Even the names of ticks such as "dog" or "bovine" are not strict indications of the type of prey: many individuals of the dog tick successfully develop on cattle or on hedgehogs, and the bull tick can safely suck blood from people, poultry, rats and the same dogs. Very often, ixodid ticks attack even cold-blooded animals - turtles, frogs, lizards and snakes.
It is interesting
Many acarologists consider (and use) hedgehogs as a kind of "vacuum cleaner" for ticks in the wild. The fact is that it is difficult for a hedgehog to take care of the surface of its back and clean out parasites here, and therefore, at the end of spring, in many individuals, the entire back is literally studded with ticks of different ages and degrees of fatness.There are cases when, in order to collect ticks in natural habitats, specialists specially caught a hedgehog, removed parasites from it, then let it go and simply followed it so as not to lose sight of it, and once every few hours they took it and removed new attached ticks. In jargon, the expression "hourly" even appeared, meaning the number of ticks that a hedgehog can collect on itself in one hour of moving in the grass.
Some specificity may be associated with the structural features of the sense organs and the ecology of a particular type of tick. For example, an adult dog tick most often lies in wait for its prey, sitting on grass stalks, and here it is more likely to “catch” a large animal than a hedgehog or lizard. And taiga tick nymphs, on the contrary, in search of prey more often climb into burrows and cavities under stones, where they are most likely to encounter mice, voles or lizards.
On a note
Argas ticks even have homovampirism - a behavior in which a hungry individual attacks a well-fed one, pierces the integument of her body and sucks blood from it, which was previously fed by a fellow victim. Simply put, ticks don't care who they attack and whose blood they suck, but evolutionary adaptations help each species develop a certain specialization.
At the same time, the concept of "housekeeping" is not relevant for endoparasitic ticks. It is impossible, for example, to say that the scabies mite is single-host, although from a terminological point of view this is true - the entire development of one individual takes place on the same host animal. The number of hosts is spoken about only for temporary parasites, which necessarily spend some part of their lives freely, without contact with the host's body.
Interesting facts about parasitic mites
The parasitic way of life to a large extent influenced the characteristics of the biology of ticks. And in many cases, these features have become so unique that they have become real phenomena.
Like most other free-living ectoparasites, mites can starve for long periods. This is a necessary guarantee of their survival, given that the lurking type of hunting for the owner requires a long wait. So, ordinary ixodid ticks of the genus Hyalomma can starve up to 10-12 months, and adults of some other species - up to 2-3 years.
Hyalomma marginatum:
Some mites that parasitize birds live in nesting litter in bird colonies and feed when the bird sits on the nest, and reproduce most actively when the chicks appear. It is parasites that often cause chicks to die, literally biting them to death.
On a note
During the entire period during which birds fly south or (for Antarctic species) north, these ticks are starving and waiting for the return of their hosts, and such a hunger strike for 8-9 months a year is a normal part of their life cycle. It is due to such adaptations to the life cycle of the hosts that ticks were able to settle, including on the rocky Arctic and Antarctic islands, where there are practically no other arthropods.
For 9-10 months a year, under a layer of snow and ice, the nymphs and adults of these species are in a state close to suspended animation - to wait for the arrival of spring, move to the nest and get enough blood again.
As with any other parasites, ticks have a high mortality rate. Less than 1% of individuals hatched from eggs survive to adulthood, and a huge number of eggs are destroyed by predators and superparasites (for example, some riders).However, the mites have managed to adapt to this by multiplying in huge numbers.
Ticks are also distinguished by the highest prevalence and breadth of the spectrum of animal hosts. They can parasitize (and parasitize) almost all mammals and birds, reptiles and amphibians, and water mites can attack fish. Even terrestrial species normally tolerate prolonged submersion under water and do not die for several hours under water, while at this time they suck the blood of the victim. This allows them to parasitize on animals leading a semi-aquatic lifestyle.
Finally, poisonous mites are known. Most of them are among the argas ticks, whose saliva is so toxic that it can cause acute pain at the bite site, anaphylaxis, and even muscle paralysis. In particular, bird mites of the species Ornithodorus coriaceus in the southern United States and Mexico are considered more dangerous than rattlesnakes, precisely because of the pain of their bites.
How They Became Parasites: Hypotheses for the Evolution of Parasitism
Most of the theories about the development of parasitism in ticks are hypotheses with varying degrees of certainty, however, some of these hypotheses for different species have the most evidence, and therefore are considered the main ones.
In particular, the parasitism of ixodid ticks is most likely a consequence of the predation of their ancestors. It is known that ticks are representatives of the arachnid class, and there is reason to believe that it was the ancient spiders that were the ancestors of modern ticks, and not vice versa.
Most spiders are predators, feeding by catching prey, injecting saliva with digestive enzymes into its body cavity, and then sucking out the resulting "broth", leaving the covers intact.
Perhaps some ancient spiders and ticks attacked their victims and began to devour them before the victim died. Examples of such hunting are also known among modern species. Some of these ticks could go on to attack larger victims who did not need to be killed. All that was needed for this was the ability to suck blood or lymph without causing acute pain in the host, and gradually it developed in an evolutionary way - those individuals survived whose saliva caused the least irritation to the host, until parasites appeared that bit generally painlessly. They became the first obligate parasitic mites.
On a note
Fossil mites have been known since the Devonian, when vertebrates had not even begun to conquer land. There is an assumption that already quite separate morphological species sucked the blood of dinosaurs.
Further evolution took place, most likely, in the direction of strengthening the bonds between ticks and their hosts. Three-host ticks are apparently the most ancient and least specialized; two-host ticks have already taken the first step in approaching the host. The pinnacle of this path was endoparasitic mites - itching, glanders and the like, which completely "related" to their victims and thus received constant food and "shelter". By the way, they have adapted to feeding on those tissues that are not critical for the survival of the host.
With a high probability, iron glands are younger species than pruritus. It is known that the "parasite-host" relationship is constantly evolving in the direction of reducing antagonism.. This reduces host mortality from parasite activity and increases the survival chances of host-dependent parasites themselves.Plus, in the absence of concern from the parasite, the host does not take any measures to combat it. It was the iron glands that reached this evolutionary level, from the activity of which the human body practically does not suffer in any way.
To date, it is not known how dust mites evolved - whether they switched from feeding on the epidermis directly on a person to feeding on exfoliated epidermis in room dust, or whether they initially fed on all organic debris in a person’s dwelling, and then narrowed the diet only to flaking skin residues. To clarify this issue, additional studies of the anatomy and biology of these arthropods are required.
Adaptations to a parasitic way of life
Along with the basic abilities and functions, ticks have developed numerous additional adaptations that are necessary specifically for a parasitic lifestyle.
First of all, this applies to the device of the oral apparatus. The jaws of ticks have turned into a highly effective piercing tool, which, after puncturing the skin and walls of a blood vessel, expands in such a way that it keeps the parasite on the host’s body and not only prevents it from accidentally falling, but also prevents even attempts to intentionally remove it with considerable effort. Simply put, due to the special teeth of the tick, it is difficult to tear it off the skin.
Other specific features of ticks as parasites include the following adaptations:
- Enormous extensibility of the digestive tract and cuticles. An adult female can deposit several times more blood in herself than she weighs. When bloodsucking, its size increases by more than 10 times, and the body turns from almost flat before feeding to almost round after it.This ability allows the fullest use of the possibility of feeding on one host;
- The presence of blood anticoagulants and local anesthetics in saliva. The former prevent thickening of the blood and facilitate its absorption, the latter make the bite invisible to the host;
- The already mentioned ability to a long hunger strike;
- Huge fertility. In terms of the number of eggs laid, ticks are champions among blood-sucking arthropods. Females of large ixodid ticks lay up to 20,000 eggs in their lifetime, and females of small species that live in the burrows of their hosts lay about 1,000 eggs. Such fecundity ensures that even with a low survival rate, part of the offspring will still survive to reproductive age and also take part in reproduction;
- Adaptation to the biology of the host species - reproduction phenology, lifestyle, anatomy.
In general, the influence of the parasitic way of life on the biology of mites is very great and contributes to the increasing specialization of these arthropods.
Diseases of humans and animals associated with tick parasitism
Various diseases associated with the attack of ticks on humans and animals can be considered a kind of side effect of the activity of these parasites. The fact is that the evolutionarily severe consequences of the attack of the parasite on the host reduce the likelihood of survival of both participants in such relations, and therefore are not “beneficial” for anyone.
However, such diseases are widespread and pose a danger to both humans and animals. They are called acariases, and the following are of greatest medical importance:
- Scabies that develops with constant damage to the epidermis layer by the female scabies itch.May lead to severe skin lesions and associated diseases;
- Tick-borne encephalitis is a deadly viral disease that still claims hundreds of human lives every year. It is fraught with disability even with effective treatment;
- Lyme disease (lyme borreliosis) is a deadly bacterial disease, the carrier of which develops in the body of a tick and is transmitted to humans by bloodsucking;
- Tick paralysis - occurs due to the action of toxins contained in the saliva of some ticks on the skeletal muscles of the human body. Mortality among the sick is 10-12%, mostly children get sick;
- Intestinal acariasis caused by cheese and some other mites entering the intestines and their transition to existence and even reproduction under anaerobic conditions with damage to the epithelial lining of the intestinal tract;
- Various dermatitis, also called acarodermatitis;
- Alopecia in animals and loss of plumage in birds. Abundant reproduction of some argasid mites in poultry houses sometimes causes the death of poultry;
- Allergic reactions (up to anaphylactic shock);
- Demodicosis, rosacea and rosacea, caused by the reproduction of glandular glands in very large quantities. This leads to inflammation of the hair follicles, reddening of the skin, dilation of blood vessels, and itching.
Most of these diseases are characteristic of both humans and animals. For example, a huge number of skin and trichological diseases of cattle, cats and dogs, pigeons, chickens and rabbits are caused precisely by parasitic mites.
Demodex subcutaneous mite (iron gland): video taken under a microscope
Extraction of scabies itch from under the skin (Sarcoptes scabiei)
Great article