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1
Chapter
Introductory Chapter: Ticks and
Tick-Borne Pathogens
MuhammadAbubakar, Piyumali K.Perera, AbdullahIqbal
and ShumailaManzoor
1. Introduction
Ticks are obligate ectoparasites that feed on the blood of their hosts. Ticks
belong to the phylum Arthropoda, class Arachnida, subclass Acari, order
Parasitiformes, and suborder Ixodida [1, 2]. There are three families of ticks clas-
sified as Ixodidae (hard ticks), Argasidae (soft ticks), and Nuttalliellidae (limited
to Tanzania and South Africa) [3, 4]. More than 900 species of ticks have been
classified in the world. Ticks not only cause physical damage to their hosts by
sucking blood and injuring skin, but many of these tick species also have the ability
to transmit pathogens to their host. The population of ticks in any region depends
upon various factors such as climate, the presence of predators, and competitor
species [5].
According to an estimate, every year, ticks and tick-borne pathogens cause the US
$13.9–18.7 billion loss. Annually, tick infestations result in a loss of almost 3 billion
hides of cattle [6]. Ticks transfer pathogens from their gut to host bloodstream by their
saliva [7]. Ticks transmit a range of pathogens including viruses, bacteria, and protists
to vertebrate hosts, including humans, domestic, and wild animals. These pathogens
cause many viral diseases (e.g., Crimean-Congo hemorrhagic fever, West Nile fever,
Omsk hemorrhagic fever, and Colorado tick fever), bacterial diseases (Lyme disease,
Q fever, borreliosis, and relapsing fever), fungal diseases (dermatophilosis), protozoal
diseases (theileriosis and babesiosis), and rickettsial diseases (anaplasmosis, ehrlichio-
sis, Brazilian spotted fever, and Rocky Mountain spotted fever) [7– 11]. Tick-borne
diseases (TBDs) of domestic animals (e.g., cattle, sheep, and goats) can substantially
affect livestock production, food supply, and economy of many regions worldwide.
TBDs cause production losses mainly as a consequence of infertility, abortions,
reduced weight gain, decreased milk production, lower quality of milk, and mortality.
In addition, costs associated with control and preventive measures, such as dipping
with acaricides, vaccination, chemotherapy, veterinary services, and monitoring, also
contribute considerably to economic losses (Brown, 1997). In addition, most of these
pathogens are very serious zoonotic threats due to the worldwide distribution of ticks
and lack of vaccine availability against these viruses and other pathogens [ 12].
Tick-borne pathogens are not the only problem due to tick infestation. When
ticks feed on their host, they draw blood and cause damage to the skin. Injury of
skin and subcutaneous tissues leads to edema, pruritus, erythema, scaling, and
ulceration [13]. Excoriation can result in secondary bacterial infections. Along
with these physical damages, ticks affect the productivity of animals by disturb-
ing their normal behavior [ 14].
Ticks and Tick-Borne Pathogens
2
2. Tick-borne viruses
Tick-borne viruses (TBV) are specifically named as tiboviruses, and all of
them belong to a group of arboviruses [15]. These viruses require ticks and
vertebrate host to complete their life cycle. Combined evolution of ticks and
tiboviruses results in the development of such a life cycle that totally matches the
feeding cycle of ticks. These viruses belong to nine families of viruses. Among
nine tiboviruses families, eight are RNA families (Flaviviridae, Reoviridae,
Rhabdoviridae, Orthomyxoviridae, Nyamiviridae, Phenuiviridae, Nairoviridae, and
Peribunyaviridae) and one DNA family (Asfarviridae) [13, 16].
To date, almost 19 diseases of livestock and 16 diseases of humans have been
reported by TBV [17 , 18]. Flaviviridae viruses are most common tiboviruses that
include tick-borne encephalitis virus, West Nile virus, louping ill virus, Powassan
virus, and Kyasanur Forest disease virus that are transmitted by Dermacentor reticu-
latus, Ornithodoros moubata, Ixodes ricinus, Ixodes scapularis, and Haemaphysalis
punctata , respectively [19, 20]. West Nile virus is endemic in many African and
European countries [21]. African swine fever virus is also a tick-borne virus that
belongs to family Asfarviridae and transmitted by Ornithodoros porcinus. African
swine fever disease is a very serious threat for pigs due to its high mortality rate
[22 ]. Thogoto virus of family Orthomyxoviridae is transmitted by tick species
such as Rhipicephalus appendiculatus, Boophilus microplus, Hyalomma dromedarii,
Rhipicephalus evertsi, and Amblyomma variegatum [23 ].
Two major tick-borne viruses of Nyamiviridae are Nyamanini virus and
Midway nyavirus. Reoviruses include tiboviruses such as Colorado tick fever virus,
Great Island virus, and Chobar Gorge virus. Colorado tick fever virus is prevalent
in the United States and Canada. This virus is transmitted to mammals by a tick
Dermacentor andersoni. Fever, meningitis, rash, and conjunctivitis are typical clini-
cal signs of Colorado tick fever [9].
Rhabdoviridae includes tick-borne viruses such as Isfahan vesiculovirus,
Connecticut virus, and Barur ledantevirus [13]. Nairoviridae contains two major
tick-borne viruses; those are Crimean-Congo hemorrhagic fever virus and
Nairobi sheep disease virus. Crimean-Congo hemorrhagic fever outbreaks have
been reported from many African, Asian, and European countries in the last two
decades. This virus is mainly transmitted by Hyalomma marginatum, H. lusitanicum,
H. truncatum, Rhipicephalus bursa, and Dermacentor marginatus [24].
3. Tick-borne bacteria
Tick-borne bacterial (TBB) diseases not only affect the productivity of animals
but also have zoonotic importance. Lyme disease is one of the major tick-borne
bacterial diseases that is caused by Borrelia burgdorferi [25]. These bacteria are
transmitted to mammal host by I. ricinus, I. hexagonus, I. pacificus, I. scapularis, and
I. persulcatus. Lyme disease is rapidly spreading in Europe. It is estimated that about
10% of the total population of ticks are positive for B. burgdorferi in Europe, and
annually more than 85,000 human cases of Lyme are reported from the European
countries [ 26]. Lyme disease also affects domestic animals. Clinical signs and
symptoms of Lyme disease in animals include lethargy, anorexia, lameness, and
urinary disorder [25].
Another TBB is Francisella tularensis that causes tularemia. Ticks of species I. ricinus ,
D. andersoni, D. variabilis, D. marginatus, and A. americanum act as biological vectors
for Francisella tularensis. These bacteria can cause disease in humans, rodents, rabbits,
and rarely sheep [27]. Q fever is also a tick-borne zoonotic bacterial disease that is
3
Introductory Chapter: Ticks and Tick-Borne Pathogens
DOI: http://dx.doi.org/10.5772/intechopen.82510
caused by Coxiella burnetii. Ticks of species Haemaphysalis bispinosa and I. holocyclus can
also act as reservoir hosts and biological vectors [28 ].
4. Tick-borne Rickettsiae
Tick-borne Rickettsiae (TBR) can spread to new geographic areas and sus-
ceptible population by ticks. Anaplasmosis is an eminent tick-borne rickettsial
disease of cattle that is caused by Anaplasma marginale. This is transmitted by
Rhipicephalus microplus. The mortality rate of anaplasmosis in cattle varies from
30 to 50%. Another tick-borne rickettsia is Rickettsia rickettsii that causes spotted
fever [29]. In the USA, R. rickettsii causes Rocky Mountain spotted fever, and in
Brazil, it causes Brazilian spotted fever. Rocky Mountain spotted fever spreads
mainly by ticks of species Amblyomma americanum, A. cajennense, D. andersoni,
D. variabilis, and R. sanguineus sensu lato [30 ]. Medically significant vectors of
Brazilian spotted fever include A. aureolatum and A. cajennense. The mortality rate
of Rocky Mountain spotted fever in the USA and Brazil has been reported 10%
and 30–40%, respectively. African tick bite fever is another tick-borne rickettsial
disease that is caused by Rickettsia africae. Major vectors of Rickettsia africae are
ticks of A. variegatum and A. hebraeum species [31].
Heartwater or cowdriosis is another tick-borne rickettsial disease that is caused
by Ehrlichia ruminantium . E. ruminantium is mainly transmitted by Amblyomma
variegatum, A. pomposum, and A. hebraeum [32]. This disease is limited to Africa
and South Africa. Cowdriosis is a serious threat to ruminants in sub-Saharan Africa,
where up to 90% mortality rate has been reported [33]. In dogs, Ehrlichia canis
causes Ehrlichiosis. E. canis has been reported from the many Asian, European, and
American countries and transmitted from one dog to another dog by Rhipicephalus
sanguineus sensu lato. Clinical signs of this disease include high fever, lethargy,
anemia, and nose bleeding [34].
5. Tick-borne fungi
Ticks are also involved in the transmission of a fungal pathogen, Dermatophilus con-
golensis, to mammals. Dermatophilus congolensis causes a skin disease Dermatophilosis
[35]. This pathogenic fungus is transmitted by a tick vector A. variegatum .
Dermatophilosis causes exudative dermatitis in sheep and cattle which leads to signifi-
cant economic loss due to the devaluation of hide quality [36].
6. Tick-borne protozoa
Ticks can transmit many blood protozoan parasites to their vertebrate hosts.
Among these, two main groups of TBDs are of importance to the livestock: theile-
riosis (i.e., tropical theileriosis and East Coast fever (ECf)) and babesiosis, posing
major health and management problems to cattle and small ruminants, mainly
in tropical and subtropical regions worldwide. The most pathogenic species are
T. annulata and T. parva, the causative agents of tropical or Mediterranean thei-
leriosis and ECf, respectively. Other species, such as Theileria mutans, Theileria
taurotragi, and members of the T. orientalis complex, are usually considered to
cause asymptomatic infections in livestock. Theileria parva is transmitted to cattle
by Rhipicephalus appendiculatus (East Africa) or R. zambeziensis (South Africa)
and causes ECf [35]. Tropical theileriosis is caused by Theileria annulata and
Ticks and Tick-Borne Pathogens
4
© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms
of the Creative Commons Attribution License (http://creativecommons.org/licenses/
by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.
Author details
MuhammadAbubakar 1 *, Piyumali K.Perera2, AbdullahIqbal 1
and ShumailaManzoor1
1 National Veterinary Laboratory, Islamabad, Pakistan
2 Faculty of Science, Department of Zoology, University of Peradeniya, Peradeniya,
Sri Lanka
*Address all correspondence to: mabnvl@gmail.com
transmitted by Hyalomma spp. It is characterized by lymph nodes swelling, high
fever, and dyspnea [37].
Babesiosis is another tick-borne protozoal disease. Bovine babesiosis is
caused by Babesia bovis and B. bigemina. These protozoans are transmitted by
Rhipicephalus microplus and R. annulatus. In bovines, animals having babesiosis
show clinical signs including hemoglobinuria, jaundice, rapid breathing, and high
fever [38]. In canines, Babesia canis causes piroplasmosis. B. canis is transmitted to
dogs by R. sanguineus and D. reticulatus. In humans, B. microti and B. divergens are
responsible for babesiosis [39].
7. Conclusion
Ticks prevalent in the dairy or poultry industries lead to economic losses either
by direct damage to hide and stress to animals or indirectly by pathogens that they
transmit to animals and humans. Prevalence of ticks and tick-borne pathogens is
influenced by environmental factors and quarantine measures. Tick control at any
level can prevent the outbreak of diseases caused by tick-borne pathogens.
5
Introductory Chapter: Ticks and Tick-Borne Pathogens
DOI: http://dx.doi.org/10.5772/intechopen.82510
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... In this regard, ticks are considered vectors of public health and veterinary importance. Some of the clinical conditions caused by tick-borne pathogens are viral diseases which include Crimean-Congo hemorrhagic fever, Omsk hemorrhagic fever, West Nile fever, Colorado tick fever, Congo hemorrhagic fever, and tickborne encephalitis (TBE); bacterial diseases which include Lyme disease or borreliosis, relapsing fever, Q fever, tularemia, and rickettsial diseases (anaplasmosis, ehrlichiosis, Rocky Mountain spotted fever, and Brazilian spotted fever); protozoal diseases which include theileriosis and babesiosis; and fungal disease such as dermatophilosis (Abubakar et al., 2018). In recent years, Severe fever with thrombocytopenia syndrome (SFTS), and Human granulocytic anaplasmosis (HGA) were identified as tick-borne diseases of epidemiological importance (Yu et al., 2015). ...
... Other indirect losses have to do with treatment cost for clinical cases; the cost of inefficiencies associated with the production systems such as the use of less productive but genetically resistant breeds to ticks; incurred expenditures from tick control; confiscation due to residues of acaricides found in milk or meat; and the embargo on animal trade between regions and countries (Hurtado and Giraldo-Ríos, 2018). Almost 3 billion hides of cattle are lost annually to tick infestation (Abubakar et al., 2018). The Australian cattle/beef and dairy industry incur losses of approximately AUS$175 million per annum owing to ticks and tick-borne diseases coupled with the cost of treatments to guarantee compliance with regulatory protocols for livestock movement along regional/interstate and international trade routes (Lew-Tabor and Rodriguez Valle, 2016). ...
The ending of the nineteenth-century was characterized by an escalation of ticks and tick-borne diseases that resulted in the death of many cattle. This necessitated the search for an effective means of tick control. Arsenicals were introduced in Australia in 1895, and arsenic-based dipping vats went on to be used for about 40 years until resistance was found in ticks and more effective alternatives - chemical acaricides - were developed after World War II. However, the development of resistance by ticks, environmental persistence, and mammalian toxicity militated against the sustained use of subsequent chemical acaricides. Furthermore, the development of resistance is a phenomenon that would always evolve, and the multiple mechanisms underlying the synthetic acaricides resistance are of great importance for future integrated control of ticks and tick-borne diseases. Hence, this study retrospectively reviewed the development of synthetic acaricides and the underlying mechanisms of tick resistance against synthetic acaricides in the hope of providing the implications and perspectives for resistance prevention and mitigation for future tick control.
Lyme disease-causing Borrelia burgdorferi has been reported in 10–19% of Ixodes ticks from Alberta, Canada, where the tick vector Ixodes scapularis is at the northwestern edge of its range. However, the presence of Borrelia has not been verified independently, and the bacterial microbiome of these ticks has not been described. We performed 16S rRNA bacterial surveys on female I. scapularis from Alberta that were previously qPCR-tested in a Lyme disease surveillance program. Both 16S and qPCR methods were concordant for the presence of Borrelia. The 16S studies also provided a profile of associated bacteria that showed the microbiome of I. scapularis in Alberta was similar to other areas of North America. Ticks that were qPCR-positive for Borrelia had significantly greater bacterial diversity than Borrelia-negative ticks, on the basis of generalized linear model testing. This study adds value to ongoing tick surveillance and is a foundation for deeper understanding of tick microbial ecology and disease transmission in a region where I. scapularis range expansion, induced by climate and land use changes, is likely to have increasing public health implications.
Ticks are important vectors for the transmission of pathogens including viruses. The viruses carried by ticks also known as tick-borne viruses (TBVs), contain a large group of viruses with diverse genetic properties and are concluded in two orders, nine families, and at least 12 genera. Some members of the TBVs are notorious agents causing severe diseases with high mortality rates in humans and livestock, while some others may pose risks to public health that are still unclear to us. Herein, we review the current knowledge of TBVs with emphases on the history of virus isolation and identification, tick vectors, and potential pathogenicity to humans and animals, including assigned species as well as the recently discovered and unassigned species. All these will promote our understanding of the diversity of TBVs, and will facilitate the further investigation of TBVs in association with both ticks and vertebrate hosts.
Ticks are efficient vectors of arboviruses, although less than 10% of tick species are known to be virus vectors. Most tick-borne viruses (TBV) are RNA viruses some of which cause serious diseases in humans and animals worldwide. Several TBV impacting human or domesticated animal health have been found to emerge or re-emerge recently. In order to survive in nature, TBV must infect and replicate in both vertebrate and tick cells, representing very different physiological environments. Information on molecular mechanisms that allow TBV to switch between infecting and replicating in tick and vertebrate cells is scarce. In general, ticks succeed in completing their blood meal thanks to a plethora of biologically active molecules in their saliva that counteract and modulate different arms of the host defense responses (haemostasis, inflammation, innate and acquired immunity, and wound healing). The transmission of TBV occurs primarily during tick feeding and is a complex process, known to be promoted by tick saliva constituents. However, the underlying molecular mechanisms of TBV transmission are poorly understood. Immunomodulatory properties of tick saliva helping overcome the first line of defense to injury and early interactions at the tick-host skin interface appear to be essential in successful TBV transmission and infection of susceptible vertebrate hosts. The local host skin site of tick attachment, modulated by tick saliva, is an important focus of virus replication. Immunomodulation of the tick attachment site also promotes co-feeding transmission of viruses from infected to non-infected ticks in the absence of host viraemia (non-viraemic transmission). Future research should be aimed at identification of the key tick salivary molecules promoting virus transmission, and a molecular description of tick-host-virus interactions and of tick-mediated skin immunomodulation. Such insights will enable the rationale design of anti-tick vaccines that protect against disease caused by tick-borne viruses.
As obligate blood-feeding arthropods, ticks transmit pathogens to humans and domestic animals more often than other arthropod vectors. Livestock farming plays a vital role in the rural economy of Pakistan, and tick infestation causes serious problems with it. However, research on tick species diversity and tick-borne pathogens has rarely been conducted in Pakistan. In this study, a systematic investigation of the tick species infesting livestock in different ecological regions of Pakistan was conducted to determine the microbiome and pathobiome diversity in the indigenous ticks.A total of 3,866 tick specimens were morphologically identified as 19 different tick species representing three important hard ticks, Rhipicephalus, Haemaphysalis and Hyalomma, and two soft ticks, Ornithodorus and Argas. The bacterial diversity across these tick species was assessed by bacterial 16S rRNA gene sequencing using a 454-sequencing platform on 10 of the different tick species infesting livestock. The notable genera detected include Ralstonia, Clostridium, Staphylococcus, Rickettsia, Lactococcus, Lactobacillus, Corynebacterium, Enterobacter, and Enterococcus. A survey of Spotted fever group rickettsia from 514 samples from the 13 different tick species generated rickettsial-specific amplicons in 10% (54) of total ticks tested. Only three tick species Rhipicephalus microplus, Hyalomma anatolicum, and H. dromedarii had evidence of infection with "Candidatus Rickettsia amblyommii" a result further verified using a rompB gene-specific quantitative PCR (qPCR) assay. The Hyalomma ticks also tested positive for the piroplasm, Theileria annulata, using a qPCR assay.This study provides information about tick diversity in Pakistan, and pathogenic bacteria in different tick species. Our results showed evidence for Candidatus R. amblyommii infection in Rhipicephalus microplus, H. anatolicum, and H. dromedarii ticks, which also carried T. annulata.
Nairovirus, one of five bunyaviral genera, includes seven species. Genomic sequence information is limited for members of the Dera Ghazi Khan, Hughes, Qalyub, Sakhalin, and Thiafora nairovirus species. We used next-generation sequencing and historical virus-culture samples to determine 14 complete and nine coding-complete nairoviral genome sequences to further characterize these species. Previously unsequenced viruses include Abu Mina, Clo Mor, Great Saltee, Hughes, Raza, Sakhalin, Soldado, and Tillamook viruses. In addition, we present genomic sequence information on additional isolates of previously sequenced Avalon, Dugbe, Sapphire II, and Zirqa viruses. Finally, we identify Tunis virus, previously thought to be a phlebovirus, as an isolate of Abu Hammad virus. Phylogenetic analyses indicate the need for reassignment of Sapphire II virus to Dera Ghazi Khan nairovirus and reassignment of Hazara, Tofla, and Nairobi sheep disease viruses to novel species. We also propose new species for the Kasokero group (Kasokero, Leopards Hill, Yogue viruses), the Ketarah group (Gossas, Issyk-kul, Keterah/soft tick viruses) and the Burana group (Wēnzhōu tick virus, Huángpí tick virus 1, Tǎchéng tick virus 1). Our analyses emphasize the sister relationship of nairoviruses and arenaviruses, and indicate that several nairo-like viruses (Shāyáng spider virus 1, Xīnzhōu spider virus, Sānxiá water strider virus 1, South Bay virus, Wǔhàn millipede virus 2) require establishment of novel genera in a larger nairovirus-arenavirus supergroup.
The objective of this study was to determine the prevalence of Ehrlichia ruminantium (causative agent of heartwater disease) in ruminants in Al Gezirah State (central Sudan) using PCR technique. A total of 170 blood samples spotted on filter paper were collected, 100 from sheep, 40 from goats, 20 from cattle and 10 from camels. DNA was extracted and amplified using the specific primers (HH1F and HH2R) for amplification of the target sequence (980 bp fragment) of pCS20 gene, the conserved gene region of all E. ruminantium strains. The study revealed a prevalence rate of 3.0% (3/100) among sheep and no one of goats, cattle and camels was positive for E. ruminantium. Two of the three positive sheep had shown clinical signs of heartwater while the third was apparently healthy. This study concluded that the heartwater disease exists in the central Sudan but may be of a low prevalence. More investigations of the disease in other regions of the Sudan using pCS20-PCR are recommended.
Tick-borne viruses (TBVs) belong to the largest biological group known as arboviruses with unique mode of transmission by blood-feeding arthropods (ticks, mosquitoes, sand flies, biting midges, etc.) to a susceptible vertebrate host. Taxonomically, it is a heterogenous group of vertebrate viruses found in several viral families. With only one exception, African swine fever virus, all TBVs have a RNA genome. To date, at least 160 tick-borne viruses are known, some of them pose a significant threat to human and animal health worldwide. Recently, a number of established TBVs has re-emerged and spread to new geographic locations due to the influence of anthropogenic activities and few available vaccines. Moreover, new emerging tick-borne diseases are constantly being reported. Major advances in molecular biotechnologies have led to discoveries of new TBVs and further genetic characterization of unclassified viruses resulting in changes in TBVs classification created by the International Committee for the Taxonomy of Viruses. Although TBVs spend over 95% of their life cycle within tick vectors and the role of ticks as vectors has been known for over 100 years, our knowledge about TBVs and molecular processes involved in the virus-tick interactions is scarce.
- Philippe Parola
- Christopher D Paddock
- Cristina Socolovschi
- Didier Raoult
Tick-borne rickettsioses are caused by obligate intracellular bacteria belonging to the spotted fever group of the genus Rickettsia. These zoonoses are among the oldest known vector-borne diseases. However, in the past 25 years, the scope and importance of the recognized tick-associated rickettsial pathogens have increased dramatically, making this complex of diseases an ideal paradigm for the understanding of emerging and reemerging infections. Several species of tick-borne rickettsiae that were considered nonpathogenic for decades are now associated with human infections, and novel Rickettsia species of undetermined pathogenicity continue to be detected in or isolated from ticks around the world. This remarkable expansion of information has been driven largely by the use of molecular techniques that have facilitated the identification of novel and previously recognized rickettsiae in ticks. New approaches, such as swabbing of eschars to obtain material to be tested by PCR, have emerged in recent years and have played a role in describing emerging tick-borne rickettsioses. Here, we present the current knowledge on tick-borne rickettsiae and rickettsioses using a geographic approach toward the epidemiology of these diseases.
Over the past two decades, the northward spread of Ixodes scapularis across Ontario, Canada, has accelerated and the risk of Lyme disease has increased. Active surveillance is a recognized and effective method for detecting reproducing populations of I. scapularis. In this study, we conducted field sampling consistent with an active surveillance approach from May to October 2014 at 104 sites in central, eastern, and southern Ontario to determine the current distribution of I. scapularis and other tick species, and enhance our understanding of the geographic risk associated with Borrelia burgdorferi and other tick-borne pathogens of public health significance in this region. I. scapularis was present at 20 of the 104 sites visited. Individuals of the tick species Dermacentor variabilis, Haemaphysalis leporispalustris, and Ixodes dentatus were also collected. I. scapularis was positive by PCR for B. burgdorferi at five sites. These sites formed a significant spatial cluster in eastern Ontario. No ticks were PCR positive for Borrelia miyamotoi, Anaplasma phagocytophilum, and Babesia microti. This study provides an up-to-date picture of the distribution of I. scapularis and other tick species, and the risk of B. burgdorferi and other pathogens of public health significance in central, eastern, and southern Ontario. This information may allow for more effective surveillance efforts and public health interventions for Lyme disease and other tick-borne diseases in this region.
- M Labuda
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![Pat Nuttall]()
INTRODUCTION Ticks transmit a wide variety of arboviruses (arthropod-borne viruses). Tick-borne viruses are found in six different viral families (Asfarviridae, Reoviridae, Rhabdoviridae, Orthomyxoviridae, Bunyaviridae, Flaviviridae) and at least nine genera. Some as yet unassigned tick-borne viruses may belong to a seventh family, the Arenaviridae. With only one exception (African swine fever virus) all tick-borne viruses (as well as all other arboviruses) are RNA viruses. Some tick-borne viruses pose a significant threat to the health of humans (tick-borne encephalitis virus, Crimean–Congo haemorrhagic fever virus) or livestock (African swine fever virus, Nairobi sheep disease virus). This chapter first considers the characteristics of ticks important in virus transmission and then presents an overview of the tick-borne members of different virus families. TICKS AS VECTORS OF ARBOVIRUSES Ticks are not insects. The significance of this statement is considered in a review of the marked contrasts between the biology of ticks and that of insects, and the consequences for their potential to transmit micro-organisms (Randolph, 1998). Interestingly, tick-borne viruses are found in all the RNA virus families in which insect-borne members are found, with the exception of the family Togaviridae. Virus–tick–vertebrate host relationships are highly specific, and fewer than 10% of all tick species (Argasidae and Ixodidae) are known to play a role as vectors of arboviruses. However, a few tick species transmit several (e.g. Ixodes ricinus, Amblyomma variegatum) or many (I. uriae) tick-borne viruses.
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Source: https://www.researchgate.net/publication/330948603_Introductory_Chapter_Ticks_and_Tick-Borne_Pathogens
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