Presentation brochure Agustín Estrada Peña
Ticks
Morphology, physiology and ecology Agustín Estrada-Peña
Ticks. Morphology, physiology and ecology
Ticks. Morphology, physiology and ecology
Ticks are a large group of arthropods, highly important in terms of public and animal health. Available information on these parasites is scattered over hundreds of difficultto-access articles, which is the reason why this book aims to bring together the most relevant information on their morphology, physiology and ecology through a practical approach and interaction with the reader. The use of high quality resources, such as descriptive illustrations and electron microscopy images, make this book the ideal tool for professionals who want to improve their understanding on this subject.
Agustín Estrada Peña
Ticks
Morphology, physiology and ecology Agustín Estrada-Peña
Ticks. Morphology, physiology and ecology
TICKS Morphology, physiology and ecology
P34310_Cubierta.indd 1
4/12/15 8:25
Author: Agustín Estrada-Peña. Format: 22 x 28 cm. Number of pages: 104. Binding: hardcover.
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Ticks make up an extensive group of arthropods of great relevance
ebook
available
in public and animal health. As there is a large amount of sources of information about this topic, this work aims to gather all the current knowledge about these parasites in a visual and updated manner, with an emphasis on the description of their morphology, physiology and ecology. A specific chapter dedicated to the pet owner is included and adapted to geographic areas. The author, a renowned specialist in this field, has prepared each of the sections with the help of detailed diagrams of their life cycle and electron microscope images, among other resources. These elements both facilitate the understanding of this issue and turn this work into a reference book for all those professionals who wish to go further into this topic.
Presentation of the book Ticks are an extensive group of arthropods of great interest in public and animal health. They are responsible for the transmission of various important pathogens, some of them with a great impact on animal production and public health. The knowledge of the morphology, ecology and transmission potential of these arthropods is one of the necessary elements to deal with their control and that of the pathogens they transmit. A correct understanding of the life cycle of ticks and their interactions with their environment is crucial to establish the tasks necessary to control and prevent the diseases transmitted by these vectors. Human and animal health professionals currently lack this knowledge due to the absence of appropriate monographs for the dissemination of the information available about this topic, which is scattered in hundreds of scientific articles that cannot be accessed easily. This monograph aims to explain, by means of a simple language and the use of numerous illustrations, the morphology, physiology and survival mechanisms of ticks as well as their complex life cycles, which are responsible for the transmission of diseases. It is a panoramic view of the different ecological aspects that have an impact on public and animal health. The book is divided into three closely related parts. The first part explores the main characteristics of the morphology (external and internal) and physiological mechanisms that regulate the life cycle of ticks. It is a summary of the main aspects to be considered for the identification of the life stages of these arthropods and the morphological details involved in their ecology. This information is used as a starting point to explain the ecological mechanisms of ticks through the organs used in their relationship with their hosts and environment. High-resolution electron microscope images are included in this part, accompanied by diagrams that visually define the importance and functioning of each organ. This way, the reader can identify the most important genera more easily.
Ticks. Morphology, physiology and ecology
The second chapter of the book focuses on the life cycle of ticks. It is a complex sequence of events that may last for years as these aim to optimise development and mortality. Since most professionals are unaware of the fact that ticks are arthropods, knowing their whole life cycle as well as their physiology and ecology is vital to obtain a clear view of the transmission of pathogenic agents. This section highlights the physiological mechanisms of blood intake, digestion, moulting and egg-laying patterns, as well as the hormonal mechanisms that control each stage of their life cycle. Furthermore, this chapter includes an exhaustive explanation of the direct life cycles of ticks and their importance to maintain infections under natural conditions. Finally, the last part of the book goes further into the ecology of ticks, which allows a better understanding of their epidemiology. It provides detailed information about their preferences and survival mechanisms to face changing climate conditions, as well as about their host-seeking methods and how they survive in plants. It also reviews, among other aspects, the activation of these vectors, the influence of photoperiod on their activity and their seasonal periods of activity. In addition, it is fundamental to relate the ecology of hard ticks (Ixodides) to their life habits, and to highlight their main differences with soft ticks (Argasides). Without specific knowledge on how ticks behave at different temperatures and in plants, and how this allows them to find a host, it is impossible to assess their epidemiological properties and pathogenic agents. Apart from these general sections, a specific chapter focused on what the pet owner should know about ticks is included, being adapted to each geographic area (depending on the request) where ticks have an important impact, such as Central-Northern Europe, Mediterranean region, North America, Central and South America, South Africa, Asia, Australia. All these characteristics turn this book into a reference in the field as it gathers all the knowledge available on ticks in a graphic and simple way, and provides clear information to those professionals who wish to go further into this subject.
Ticks. Morphology, physiology and ecology
The author Agustín Estrada-Peña Agustín Estrada-Peña is a professor at the Department of Animal Pathology of the Faculty of Veterinary Medicine, University of Zaragoza (Spain) where he coordinates the subject Parasitic Diseases. He is the founding member and a diplomate of the European Veterinary Parasitology College. He focuses his work on ticks and the diseases they transmit to domestic animals as well as the public health problems they cause. He is currently working on the development of risk models for ticks in Europe and is a consultant for the European Centre for the Control of Diseases (ECDC). He has also served as a consultant in the field of health and climate change for the WHO Regional Office for Europe and for the FAO regarding tick control in production animals. He is the main author of approximately 150 articles in indexed journals. He is the co-author of several books on the subject of ticks and the diseases transmitted by these arthropods. He has participated in four projects on vector-borne diseases funded by the European Union, and he is currently a member of the EuroVEGNEC network, a COST (European Cooperation in Science and Technology) action for the development of surveillance protocols for invasive arthropods with an impact on public health.
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Ticks
Morphology, physiology and ecology Agustín Estrada-Peña
4/12/
Table of contents 1. What do ticks look like? External morphology of ticks
Searching for a host Family Argasidae
Family Ixodidae
Family Ixodidae
Family Argasidae
Haller’s organ
Genera differentiation of Ixodidae
The influence of weather on searching for a host
The integument Chemical composition of the cuticle
2. How do they act? Physiology of ticks Life cycle of ticks Water balance Blood feeding: processes and related organs Feeding Family Ixodidae Family Argasidae
Salivary gland Microscopy of the salivary gland Structure of the alveoli Composition of the salivary secretion Saliva as enhancer of pathogen transmission
Digestion Malpighian tubules
3. How do they live? Ecology of ticks Pheromonal communication
Diapause
Tick control Traditional chemical methods Vaccines
Ticks as vectors of pathogens Tick-borne encephalitis (TBE) Babesiosis Theileriosis Anaplasmosis Rickettsiosis Ehrlichiosis Hepatozoonosis
4. What should the pet owner know about ticks? Main ticks on small animals Main tick-borne diseases on small animals Tick control
Congregation pheromones Aggregation and feeding pheromones Sex pheromones Primary pheromones
5. References
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What are they like? The family Nuttalliellidae has only one known species, Nuttalliella namaqua, which appears to represent a missing link, an abandoned road in the evolution of ticks.
1 mm
Family Argasidae
Ă— 22
Argasid ticks (Figs. 3 and 4) do not have a scletorized scutum, and their external surface appearance is reminiscent of leather. Mammillae
Palps 1 mm Ă— 22
FIGURE 3. Argasid
(Ornithodoros rioplatensis). Dorsal view.
Coxal pore
Gonopore
Anus FIGURE 4. Argasid
(Ornithodoros rioplatensis).Ventral view Post-anal groove
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Sexual dimorphism Family Ixodidae
a 500 Âľm
Sexual dimorphism is evident in adult ixodids, the scutum completely covering the dorsal surface in males (Fig. 5) but only the front half in females (Fig. 6). Because of its rigidity, the scutum limits the expansion of the body in males. Since females (and immature stages) must ingest a large amount of blood whilst feeding, they can expand their body volume thanks to the synthesis of new cuticle in body areas not covered by the scutum.
b
500 Âľm
FIGURE 5.
Male ixodid tick (Ixodes ricinus). (a) Dorsal and (b) ventral view.
Eyes, when present, are located on the dorsal scutum. Just behind the scutum are the adult foveal pores, through which they emit various pheromones.
1
What are they like?
a Palps have four segments in all ixodids. It should be mentioned that the distal segment is small and retractable, and is inserted into a small cavity of the anterior segment. Furthermore, females present the so-called porous areas — located on the capitulum dorsal surface, in the area where it joins the body—, which are responsible for producing a series of substances that provide moisture for the eggs, and are thought to prevent oxidation of the lipids lining the outer covering of the egg.
500 µm
500 µm
b
The mouthparts point forwards past the scutum, and are almost entirely visible when the specimen is viewed from the back. They consist of a pair of chelicerae two segmented palps and a hypostome, with teeth, located ventrally (in some species the hypostome has no teeth). The mouthparts section is called the capitulum.
Gonopore
FIGURE 6.
Female ixodid (Ixodes ricinus). (a) Dorsal and (b) ventral view.
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The immature forms (larvae and nymphs) are developmental stages that appear before the adults (Figs. 7 and 8). Larvae hatch from the eggs and can only be seen through a microscope. Superficially they resemble the adults, but they only have three pairs of legs and lack spiracular plates. This is because breathing is done exclusively through the cuticle.
a
200 Âľm
b
200 Âľm
After feeding and moulting, the larvae progress to nymphal stage. At this stage they resemble the adults even more, with four pairs of legs and a spiracular plate.
Spiracular plates FIGURE 7.
Ixodid nymph (Ixodes ricinus). (a) Dorsal and (b) ventral view.
6
On the ventral side of the body, nymphs and adults of Ixodidae present spiracular plates, located just behind the insertion point of the fourth pair of legs. The spiraculum opens approximately in the centre of each plate, and the gas exchange takes place through it.
1
What are they like? The ventral side of the body is often sclerotized in the males of various genera, although some of them only have some quitinized plates.
a
In the centre of the ventral side is the gonopore or genital pore. Immature stages (larvae and nymphs) superficially resemble adults, but do not have a genital pore, porous areas or foveal glands (organs that are observed only in the adult stages). In the Ixodidae family there is only one nymphal stage.
b
FIGURE 8. Ixodid
larva. (a) Dorsal and (b) ventral view.
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Feeding Haemoglobin (blood) Faeces (guanine)
Water (from the host)
2
Cement
Pathogens and antibodies
Enzymes, anticoagulants, toxins and pharmacoactive substances
Entry of blood and cellular debris
FIGURE 3.
Tick attaching to the host's skin in order to feed.
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2
2
How do they act?
Family Ixodidae After finding and perching on a host, ixodids tend to walk for several hours along the host surface before finding the place that might seem appropriate to complete the feeding process. Once they have found it, ticks place themselves at an angle of about 40 degrees on the surface, contract their dorsoventral muscles and project the chelicerae forward, applying pressure on the host's skin. The digits of chelicerae move, making small cuts in the skin, and move forward to drill the skin down to the stratum corneum. This process can take between 3 and 5 minutes, after which the tick tipically moves to a nearby place and repeats the whole process. Once the chelicerae have settled in the lesion, the tick movements gradually increase and the hypostome settles in what will be the feeding area. It is here where the discharge of cement occurs, as mentioned, contributing to the tick's attachment.The whole process takes about 1 hour, and ends when the palps open sideways allowing deep penetration of the hypostome in the skin. In ticks with a long capitulum (Ixodes or Hyalomma), the hypostome penetrates the skin deeply, and can even reach the hypodermis. In those species with a short capitulum, the hypostome reaches approximately up to the dermis. In any case, its insertion involves the breaking of blood vessels on its path.
However, mechanical factors alone can not provide the blood supply the tick needs for feeding. The inflammation and necrosis foci progress as the tick feeds from the original point of insertion of the hypostome to the areas of muscles and blood vessels surrounding it. During the first two or three days of feeding ixodids do not ingest blood, but a mixture of lysed cells and tissues. Because ixodids can feed for days, or even weeks, the participation of substances to evade the host immune response is necessary. Given the length of the feeding process and the infusion of such combination of pharmacoactive substances in the host, an immune response will undoubtedly appear sooner or later, depending even on the number of ticks attached to the same host. However, the array of active substances in the salivary mixture ensures full evasion from the immune response. This has been proved in the attempts to produce a vaccine against ixodids: none of the tested compounds (isolated from salivary glands) has been effective.
Family Argasidae The attachement of argasids to the host is similar to what has been said for ixodids, but lasts only a few minutes. In this case a small haemorrhage, produced by the rupture of a few small capillaries, forms at the point where the hypostome is inserted.
Argasids do not usually feed from large blood vessels, since they alternate rapid feeding periods with times when feeding is practically interrupted, which facilitates salivation within the feeding cavity.
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Digestion a Empty gastric ceca
Metabolically inactive intestinal cells
b
Gastric ceca fill with the host's blood FIGURE 4. Ixodid intestine structure, (a) before and (b) after blood ingestion.
22
Intestinal cells initiate their metabolic activity by producing secretory granules, that are poured into the intestinal lumen
2
How do they act?
The ticks intestine consist of a central cavity, the stomach, and a number of large diverticula (ceca), together with a small and narrow tube which directs the digestion residues outwards through the anus. Most of the intestine is comprised of diverticula that spring in all directions and on the three planes from the central cavity. In fasting ticks, these diverticula appear as thin and narrow tubes, whereas in gorged females, they appear as large dilated sacks. During ingestion the ticks intestine can increase its size several times in order to accommodate the more than 8 cubic centimeters of blood that some ixodids may ingest.
The tick's intestine is a long tube with lateral diverticula that occupies a large portion of its body.
Histologically, the intestine consists of a thin layer of cells and a membrane that binds it to the muscle layer. The peristaltic movements of these muscles make the contents move. The concept of digestion as such in ticks does not only apply to the processes involving the conversion of blood into energy. When the host is infected with any type of pathogens, the structure of the ticks intestine, together with the biochemical processes that take place in it, determine the destiny these pathogens are going to face during the digestion of the host's blood.
Ticks can feed on the blood of phylogenetically widely separated hosts due to the stability of blood composition in very separate (from a taxonomic point of view) hosts.
There are no verified data about the effect of different diets (from different host species) on ticks, but it appears not to be significantly important for their survival. The different hosts they parasitize are more the result of a biological co-especialization than a pure physiological specialization for a particular blood type. Ticks digestion processes include: • Removal of excess water from the blood. • Lysis of cell and tissue fragments. Haemolysis. • Degradation of haemoglobin and other proteins by hydrolysis. Storage of haemoglobin.
Removal of excess water from the blood After ingesting the blood, the main problem for both argasids and ixodids is how to concentrate a fluid that is rich in water and salts, which have no nutritional value. Immediately after ingesting the blood, argasids concentrate it by removing the excess water through some systems called coxal organs (the only feature that can be externally seen in these organs is a tiny pore which opens at the coxa I, on the ventral side of the body). A complex microtubule system inside these organs concentrates the blood by repeatedly circulating it through them. In the first few minutes after feeding, an argasid can remove up to 40 % of its weight in water and salts only. After discharging the coxal fluid, the intestine walls collapse and adjust to the remaining amount of blood. Erythrocyte concentration after removal of the extra volume of water is, in the case of argasids, approximately twice that of the host's blood.
The coxal gland is an ultrafiltration system that retains most of the proteins while eliminating the water and dissolved salts.
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Searching for a host
Family Argasidae The argasids strategy is completely different, as it involves waiting in the vicinity of the burrow or nest the host inhabits.
Searching for a host is one of the basic processes in the life cycle of ticks, as it allows them to obtain the energy required for moulting to the next stage of their life cycle. The excess energy, accumulated as remains of proteins and heme groups, allows the parasite to survive after moulting. When that energy is exhausted, the tick dies regardless of external climatic conditions.
Hosts commonly inhabit burrows for long periods of time — small and medium-sized mammals—, or return to the nest for the breeding season —the case of many birds. The waiting strategy implies that ticks are able to use their energy reserves very slowly, thus allowing them access to new hosts that reuse the nest in the breeding season.
Ticks employ two different basic strategies when searching for a host, depending on whether they belong to the Argasidae or the Ixodidae family. The physiological mechanisms triggered are different but similar due to both groups zoological proximity and co-evolution in the recent past.
Ticks usually live in the vicinity of the host, so that searching for a host simply consists of facing the closest heat source. The more or less permanent presence of sucklings or nestlings facilitates both the encounter with the hosts and the continuous feeding of successive generations of ticks.
28
3
How do they live?
Family Ixodidae The host searching strategy for ixodids is based on passively waiting for hosts in the vegetation. Although the behaviour of some stages of ixodid ticks can be the same as that of argasids, the most popular strategy is to cyclically climb up and down natural vegetation (grass, wild herbs or shrubs). This system, however, requires a number of mechanisms to prevent water loss, as well as some adaptive responses to environmental conditions.
After moulting, the resulting stage (immature or adult) becomes hydrated in the deepest layers of topsoil, in the first few millimetres above the ground. When temperature rises above 7-9 °C approximately, ticks become active and begin climbing vegetation. At the tip of this vegetation, usually at a height ranging between a few centimetres and 1 metre, they extend their first pair of legs and begin to actively detect the proximity of hosts. This search and detection is performed by the so-called Haller's organ.
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Haller's organ Haller's organ is a structure located near the end of the first pair of legs (Fig. 1). It contains a number of hairs, protected inside a tiny cavity, with autorregenerative capacity in the event of loss or amputation. The walls of these hairs have microscopic holes that allow certain molecules to escape through their wall and stimulate the nerve endings connected to the pores. These neurons can detect minuscule concentrations of carbon dioxide, lactic acid, and pelargonic acid, which are produced during respiration or form part of skin secretions of vertebrates. The concentration gradient of these molecules that reaches each of the tick's leg enables the parasite to position itself at the right angle and prepare to climb onto the host. When ticks are located in the highest areas of vegetation, they are likely to lose a large amount of passively. The variable that best defines the water loss is the vapour saturation deficit in the atmosphere, which is directly related to the temperature and relative humidity. Neither relative humidity alone nor rain are adequate descriptors of the amount of water loss.
When water losses exceed certain physiological values, ticks are forced to descend to the ground vegetation, where they passively re-hydrate. Hydration is never complete and, in each ascent/ descent cycle, ticks inevitably lose water and energy reserves. When desiccation exceeds a critical limit, or energy is exhausted before the tick finds a host, it will die.
Finding a host is therefore a passive process. No tick chooses a host actively, but rather heads for a heat source in places where orientation using the sense of smell does not usually work (as in burrows surroundings, which are saturated with urea), or uses a suitable gradient of carbon dioxide to find its way. In any case, most ticks perch on a host passively.
Haller's organ, contained in a pit to protect the sensory setae Pre-Haller setae
FIGURE 1. Electron microscopy of Haller's organ.
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