Ticks: Biology, Ecology, and Diseases

By Nicholas Johnson

Ticks: Biology, Ecology and Diseases provides a detailed overview of the fascinating world of tick biology and ecology. This book discusses disease transmission to humans and livestock, assesses the impact of human behavior and climate change on tick biology, and details how this will affect future disease transmission. Written by an expert on ticks and their transmitted diseases, this book explores the unique biology of ticks and how it influences the transmission of some of the most devastating diseases. In a series of detailed chapters, the book provides up-to-date information on the interrelationship between ticks and the vertebrates they feed on.

In addition, the book covers information on recent scientific discoveries surrounding ticks, along with reviews on control methods and disease transmission. Other sections cover the recent emergence of tick-borne pathogens, making this book an ideal source for interested scientists, clinicians, veterinarians and experts in the field of tick biology.

• Offers an overview of tick anatomy to assist tick identification
• Provides a thorough and complete update on emerging tick-borne diseases
• Considers current and future options for controlling tick populations

• Language: English
• Publisher: Academic Press
• Release date: Jan 7, 2023
• ISBN: 9780323998475

Nicholas Johnson

Nick Johnson has worked in biological research for over 20 years working on projects as diverse as HIV pathogenic mechanisms to transmission of bat borne diseases. The main focus of his research over the past fourteen years has been the investigation of the epidemiology and disease mechanisms of zoonotic viruses including rabies virus, West Nile virus and tick-borne encephalitis virus. He has published extensively through original research in peer-review journals, commissioned reviews and book chapters.

Book preview

Preface

For most humans, ticks are not a major concern. However, a chance encounter, something as trivial as brushing past tall grass while walking in woodland or along the side of a field, can lead to a tick bite. If not removed promptly, it can result in transmission of a virus, bacteria, or protozoan pathogen that in the worst cases cause a life-threatening illness. Even the tick bite itself can lead to an autoimmune disease and allergy. Each year, thousands of people are affected by diseases such as tick-borne encephalitis, Lyme's disease, and babesiosis, a cause of anemia. This is mainly based on the reporting of disease in North America, Europe, and parts of Asia. The full impact of tick-borne disease in large parts of Africa and South America is not recorded. Despite the threat to human health, the greatest impact is to domestic animals, including companion animals, but especially livestock. The concentration of increasingly large numbers of cattle and sheep has provided a ready target for a range of tick species that have exploited this ever since animals were first domesticated. Beyond the welfare impact to the animals of heavy tick infestations, the worldwide cost of tick predation through the transmission of disease runs to billions of dollars annually. And the use of chemicals to suppress tick infestations are not inexpensive and may rapidly lose their efficacy due to the development of acaracide resistance. The combination of agricultural losses and the impact on human health provide the main drivers for research on the biology of ticks, their ecology, and the diseases they transmit.

In this book I have tried to cover all three aspects, but a book could be written on each of the main chapters that consider the historical investigations that have led to our current understanding, recent developments and disagreements, particularly around tick taxonomy, and future innovations. I have tried to condense this breadth of information into a series of chapters that review the key elements of tick biology, tick-borne pathogens of humans and animals, and the control of ticks. No book should be without a chapter on the impact of climate change, and this one is no exception.

I must acknowledge a legion of scientists who have contributed to our knowledge and understanding of the subject of ticks and their pathogens. While not naming them, I hope that the citation of their work throughout this book acknowledges their impact. I also need to thank the many scientists, mainly in Europe, who I have worked with since developing an interest in tick-borne viruses. Of these, the one I will name is my former colleague, Paul Phipps. After fifty years as a scientist within the British Civil Service, Paul retired in March 2022. Much of his career was in the field of livestock parasitology, and he maintained an interest and expertise in ticks and tick-borne diseases long after it was deemed uninteresting or fundable. Despite my interest in viruses, he encouraged me to consider the vector alongside the pathogen and to step outside of the laboratory to literally look in the field where transmission occurs. This emphasized the importance of investigating the biology of the vector to understand the transmission process, appreciate the ecology of the vector to understand when and where transmission occurs, and then to marry this with what we know of the pathogen to begin to prevent or control disease.

Nicholas Johnson 2022

Chapter 1: A brief introduction to ticks

Abstract

Ticks are fairly unpleasant creatures, and for most people, the first encounter with one will be the discovery of a tick in the process of biting them or their pets. Unlike most other biting arthropods, ticks hang around at the bite site, so are often caught in the act. Unless removed promptly, there is also the possibility that the resulting encounter may lead to the transmission of one of a vast range of pathogens that in some cases can cause debilitating disease and in extreme cases death. This raises the question of what role, either positive or negative, they play within the ecosystems they exist in. For the owners of livestock around the world, ticks represent a threat to the health and well-being of virtually all domesticated animals and one to which huge resources are directed at suppressing tick infestation. The combination of a source of disease and the economic threat they pose means that ticks cannot be ignored and that understanding their biology, ecology, and disease transmission is an important first step in combatting tick-borne infections.

Keywords

Acari; Nomenclature; Parasitism

Why study ticks? (And write a book about them)

Ticks are fairly unpleasant creatures, and for most people, the first encounter with one will be the discovery of a tick in the process of biting them or their pets. Unlike most other biting arthropods, ticks hang around at the bite site, so are often caught in the act. Unless removed promptly, there is also the possibility that the resulting encounter may lead to the transmission of one of a vast range of pathogens that in some cases can cause debilitating disease and in extreme cases death. This raises the question of what role, either positive or negative, they play within the ecosystems they exist in. For the owners of livestock around the world, ticks represent a threat to the health and well-being of virtually all domesticated animals and one to which huge resources are directed at suppressing tick infestation. The combination of a source of disease and the economic threat they pose means that ticks cannot be ignored and that understanding their biology, ecology, and disease transmission is an important first step in combatting tick-borne infections.

Aristotle (384–322 BCE) described them as present on a range of domestic animals including sheep, cattle, and dogs. He may also have set the tone by describing them as disgusting parasites, a view shared by many who encounter them after venturing outdoors. The disgust comes from their only interaction with us (humans) and most other vertebrates of biting us and then remaining, often for over a week as it takes a blood meal. They do this as it is their only source of nutrition, and, like any animal that bites, this makes them excellent vectors for transmitting diseases. As a result, ticks are one of the main sources of disease for humans, livestock, and companion animals throughout the world. Ancient Egyptian art may well have captured the first images of a tick on a jackal-headed beast in 1500 BC (Arthur, 1965), but it is only in the past 150 years that research has focused on the diversity of ticks and the diseases they transmit.

Ticks are related to spiders and mites, placing them in the class Arachnida and sharing the common feature of eight legs in the adult form. Ticks can be divided into two basic categories of hard and soft ticks based on their anatomy, and this will be discussed further in the next chapter. All tick species have four life stages, egg, larva, nymph, and adult (Fig. 1.1). The adult stage is sexually dimorphic with male and female forms. All mobile life stages must locate a host, attach to it, and take a blood meal. This can take minutes to days depending on the life stage and species and can lead to a huge distension of the tick abdomen. Once the blood meal has been imbibed, the tick will detach. For larvae and nymphs, this supplies the nutrition required to enable them to metamorphose, more commonly referred to as moulting, to the next stage. In the case of adults, the female takes a blood meal to support egg development, while the male usually does not take a blood meal, focusing instead on finding a female and mating with it.

Figure 1.1  A simplified schematic of the tick life cycle from egg, through two immature forms, larva and nymph, to adult.

An engorged female, one that has swollen to its maximum size will detach from the host, seek a safe location to digest the blood meal, and allow eggs to mature. It will then lay the eggs in a process termed oviposition. This can lead to thousands of eggs that will produce the next generation of ticks.

Ticks are found wherever their vertebrate hosts are present. The greatest diversity is found in tropical regions where ticks have adapted to predation on virtually all vertebrate groups of species. Some, such as the Asian blue tick, Rhipicephalus microplus, and the brown dog tick, Rhipicephalus sanguineus, have adapted to feeding, specifically on cattle and dogs, respectively. This represents an adaption to human modification of the environment such as the domestication of animals and wholesale translocation of species around the world. This leads directly to conflict with human activities, particularly livestock management, and has prompted attempts to suppress tick feeding as a means of controlling disease transmission. However, ticks can be found on all continents. This includes extreme environments such as the Antarctic, where the seabird tick Ixodes uriae can be found infesting sea birds and their nests (Muñoz-Leal and González-Acuña, 2015). This tick is tough enough to withstand temperatures as low as −30°C experienced in polar regions (Lee and Baust, 1987; Benoit et al., 2007) and the extreme windswept conditions found in remote bird nesting colonies. Its association with birds has almost certainly led to its global distribution. In contrast, isolated regions such as New Zealand have evolved their own tick fauna in response to their separation from the major continents. Even the giant tortoises of the Galapagos Islands are plagued by ticks that have evolved with them (Hoogstraal and Kohls, 1966).

A note on naming of ticks

Throughout this book ticks will be named by their latin name for clarity. See the International Code of Zoological Nomenclature website for further information (https://code.iczn.org/). Another common feature of naming species is to include the name of the scientist that first described the species and the year in which the description was published. Indeed, it was Carl Linneaus who produced some of the first detailed descriptions of some of the species of tick discussed in the following chapters. The complete naming of a tick species is illustrated in Fig. 1.2 for the naming of the common sheep tick that Linneaus first described in 1758. This also illustrates the ephemeral nature of species naming as this can change as our understanding of the relationship between species changes and with developments in technology. Throughout this book, the first mention of a tick species within in each chapter will include all aspects of the naming but will then restrict to the latin name for clarity. Some authors, although not all, also include details within the title of a publication of aspects of the classification of a species such as its subclass and family. In the case of Ixodes ricinus, this would be (Acari: Ixodidae).

A number of other terms are often encountered when dealing with species. These are the terms sensu stricto from the latin meaning in a narrow sense and sensu lato meaning in the broad sense. The former term is used when a species has been clearly delineated morphologically, phenotypically, or genetically as a well-defined species and is usually denoted as s.s. The later term is commonly used when a group of tentative species, often morphologically similar are grouped together and denoted with s.l. The brown dog tick is sometimes referred to as R. sanguineus s.l. to indicate that the variants under discussion are considered a complex of species and not necessarily a single unified entity (Dantas-Torres et al., 2013). Another term used when describing a specimen is cf or compare with. This indicates that the writer believes that the specimen being described is similar, but not the same, as a known species.

Figure 1.2  A schematic with definitions on the naming of the sheep tick Ixodes ricinus.

Another form of classification that is commonly encountered when researching ticks is that applied to geography in the definition of biogeographic regions. The distribution of tick species is intimately linked with the ecological and climatic conditions that enable the tick to persist, and they do not follow country boundaries but limited by larger climatic regions. A classification of the world regions was developed by Miklos Udvardy of the California State University, USA (http://portals.iucn.org/library/efiles/documents/op-018.pdf Accessed 17/02/2021), who divided the land surfaces of the earth into eight areas that shared similar habitat types (Fig. 1.3). Studies on tick biology and taxonomy often focus on these regions rather than the geopolitical boundaries of countries and continents as this better reflects the influences of climate, vertebrate assemblages, and geographical boundaries such as mountains and oceans (Pereira de Oliveira et al., 2019; Hornok et al., 2021; Estrada-Peña et al., 2021).

The structure of the book

This book has been divided into three sections. The first deals with the biology and ecology of ticks in three chapters. Biology deals with activities of an organism, while ecology studies its relationship with other species and the environment. The three chapters consider the classification and diversity of ticks, the tick life cycle, and finally the challenges of finding a host and then feeding on it. The second section reflects my own interest in tick-borne diseases with chapters on diseases of humans, animals, and of particular relevance for the future with a chapter on emerging diseases of ticks. The third and final section covers topical subjects including control of ticks, the complete microbiome of ticks, and the impact of climate change on ticks and tick-borne diseases.

Figure 1.3  A map of the world showing six of the eight biogeographical regions. The two remaining regions are the Oceanic region covering islands in the Pacific Ocean and Antarctica.

Each chapter is written as a stand-alone essay although information in the first section informs aspects of tick-borne disease and control.

Conclusions

In order to understand the source of disease and attempt to control and prevent transmission, we need to study tick biology, how particular species feed, reproduce, and the adaptations that enable them to survive in often hostile environments. We also need to investigate their ecology, the relationships within and between species, and their environment. This is clearly relevant to understanding the profound effects of climate change. Finally, we need to understand the diseases they cause, what pathogens are related to which ticks species, when and where transmission occurs, and then use this information to develop means to prevent this from occurring or ameliorating the effects if disease does develop.

Developments in molecular biology, and particularly genetic sequencing, have had a dramatic impact on the ability to identify tick species and the diseases they transmit. Tick classification, originally based on morphological discrimination, is constantly evolving in response to new ways of distinguishing specimens. The introduction of genomic sequence data is allowing a reassessment of the classification of hard and soft ticks, often resulting in the taxonomic revision of existing species.

When venturing into almost any group of species, including pathogens, one of the first challenges is to understand the nomenclature and in part the history that has led to that nomenclature. In my own experience, this has been experienced with the genus Babesia, a protozoan tick-transmitted parasite that infects red blood cells. For much of the 20th century, Babesias were described as small or large based on the morphology of the parasite in red blood cells. However, as more species were identified, this means of describing the pathogen was inevitably going to prove limiting. Genomic sequenced-based methods are now helping address this challenge, but even with this support, there is still uncertainty that some are a true babesia species or represent a separate, distinct genera with a different evolutionary history. Also, some diseases are caused by a species complex containing a bewildering array of genotypes, often identified on the basis of a single DNA sequence. Take, for example, Theileria orientalis, found in many locations in Eurasia but only causing disease in cattle in East Asia. As scientists and authors, we need to try and work within what is considered a reasonable interpretation of the current classification. But with the acknowledgment that whatever is written today may be obsolete tomorrow as a result of new discoveries, progress in technology, and novel reinterpretations of existing data.

Part of the reason to write this book is to increase my own knowledge on the subject and investigate themes that I would not normally consider. As a source of references, I have tried to use open access publications from the past 10 years to give an as up-to-date perspective as possible and reference material the readers can easily access.

References

1. Arthur D.R. Ticks in Egypt in 1500 B.C. Nature. 1965;206(4988):1060–1061. doi: 10.1038/2061060a0.

2. Benoit J.B, Yoder J.A, Lopez-Martinez G, Elnitsky M.A, Lee R.E, Denlinger D.L.Habitat requirements of the seabird tick, Ixodes uriae (Acari: Ixodidae), from the Antarctic Peninsula in relation to water balance characteristics of eggs, nonfed and engorged stages. Journal of Comparative Physiology B: Biochemical, Systemic, and Environmental Physiology. 2007;177(2):205–215. doi: 10.1007/s00360-006-0122-7.

3. Dantas-Torres F, Latrofa M.S, Annoscia G, Giannelli A, Parisi A, Otranto D.Morphological and genetic diversity of Rhipicephalus sanguineus sensu lato from the New and Old Worlds. Parasites & Vectors. 2013;6(1):213. doi: 10.1186/1756-3305-6-213.

4. Estrada-Peña A, Binder L.C, Nava S, Szabó M.P.J, Labruna M.B. Exploring the ecological and evolutionary relationships between Rickettsia and hard ticks in the Neotropical region. Ticks and Tick-Borne Diseases. 2021;12(5):101754. doi: 10.1016/j.ttbdis.2021.101754. .

5. Hoogstraal H, Kohls G.M. Argas (Microargas) transversus banks (New Subgenus) (Ixodoidea, Argasidae), a diminutive parasite of the galapagos giant tortoise: redescription of the holotype male and description of the larva. Annals of the Entomological Society of America. 1966;59(2):247–252. doi: 10.1093/aesa/59.2.247.

6. Hornok S, Meyer-Kayser E, Kontschán J, Takács N, Plantard O, Cullen S, Gaughran A, Szekeres S, Majoros G, Beck R, Boldogh S.A, Horváth G, Kutasi C, Sándor A.D.Morphology of Pholeoixodes species associated with carnivores in the western Palearctic: pictorial key based on molecularly identified Ixodes (Ph.) canisuga, I. (Ph.) hexagonus and I. (Ph.) kaiseri males, nymphs and larvae. Ticks and Tick-Borne Diseases. 2021;12(4):101715. doi: 10.1016/j.ttbdis.2021.101715.

7. Lee R.E, Baust J.G. Cold-hardiness in the antarctic tick, Ixodes uriae. Physiological Zoology. 1987;60(4):499–506. doi: 10.1086/physzool.60.4.30157912.

8. Muñoz-Leal S, González-Acuña D. The tick Ixodes uriae (Acari: Ixodidae): hosts, geographical distribution, and vector roles. Ticks and Tick-Borne Diseases. 2015;6(6):843–868. doi: 10.1016/j.ttbdis.2015.07.014.

9. Pereira de Oliveira R, Hutet E, Paboeuf F, Duhayon M, Boinas F, Perez de Leon A, Filatov S, Vial L, Le Potier M.-F, Gladue D. Comparative vector competence of the Afrotropical soft tick Ornithodoros moubata and Palearctic species, O. erraticus and O. verrucosus, for African swine fever virus strains circulating in Eurasia. PLoS One. 2019;14(11):e0225657. doi: 10.1371/journal.pone.0225657.

Chapter 2: Tick classification and diversity

Abstract

Ticks are classified into three families, the hard ticks or Ixodidae, the soft ticks or Argasidae, and a third family, the Nuttaliellidae, of which there is a single species of tick found only in Southern Africa, Nuttalliella namaqua. This species shares morphological features of the other two families but is clearly distinct. This chapter gives a brief description of the diversity and nomenclature of ticks focusing on the main approaches to identification and classification of ticks. Historically this has been dominated by morphological structure and has also been the source of much debate on how this leads to the classification of ticks, much of which remains to be resolved. The chapter will also consider alternative means of identifying ticks, particularly the growing field and role of molecular methods for identifying specimens and in turn what this tells us about tick evolution and the relationships between tick species around the world. While offering some benefits, these technologies still have their limitations, mainly in their cost, accessibility, and their dependence on existing