Ticks: Morphology, Biology and Ecology

What are ticks?

1. Arthropods

2. Arachnids

3. Vectors

4. Mites

5. Blood feeders

6. Parasites

Taxonomy of Ticks

1. Phylum: Arthropods

2. Subphylum: Chelicerata

3. Class: Arachnida

4. Subclass: Acari

Phylum Arthropoda

• invertebrate

• bilateral symmetry

• chitinous skeleton w/ articulating segments and/or appendages

Subphylum Chelicerata

• modified first appendages, chelicerae, used for grasping, piercing, cutting and other functions associated with food gathering and feeding

• The body is divided into two parts: (1) the prosoma and (2) the opisthosoma.

Define Chelicerae

• modified first appendages used for grasping, piercing, and cutting

• similar to knives or scissors

• when they cut, they introduce the hypostome

Define Hypostome

helps the ticks anchor themselves to the host

Class Arachnida

• bear pedipalps, the modified second pair of appendages and four pairs of walking legs, all found on the prosoma (gnathosoma + podosoma).

• arachnids lack a clearly defined head

  • the anterior body segments bearing the chelicerae and palps, are integrated with the posterior segments bearing the walking legs to form the prosoma.

Subclass Acari

• exhibit extreme fusion of the body segments.

• The podosoma and the opisthosoma are fused, forming the idiosoma.

• In contrast with other arachnids, the opisthosomal region lacks distinct segmentation while the mouthparts are joined to a separate body region, the gnathosoma (= capitulum in ticks).

• Ticks and mites belong to this subclass

T or F: Are ticks mites?

True.

Information about Mites

• Mites comprise a vast assemblage of species inhabiting terrestrial, marine and freshwater habitats

• Mites occur in soil, fresh, brackish and saltwater at all depths, in and on vegetation, in nests and burrows of vertebrates, and on the bodies or even in the internal organs of various vertebrate and invertebrate animals

• Many are free-living as herbivores, fungivores, and predators; others are parasitic, including ectoparasitic and endoparasitic adaptations

• About 35000 species have been described, and another 1000000 undescribed species are believed to exist.

What orders is Subclass Acari divided into?

1. Order Parasitiformes

2. Order Acariformes

Order Parasitiformes

• have stigmatal (i.e. respiratory) pores on the podosomal and/or opisthosomal portions of the body

• have well-developed, freely articulated coxae

Order Acariformes

• lack stigmata and typically have the coxae fused with the ventral body wall

Ticks are comprised of three parts:

1. Idiosoma (body)

2. Capitulum

3. Legs

(Visual) Anatomy of Ticks

Ticks as Arthropods: Segmented Body

• reduced segmentation

• each segment on the hypothetical ground plan contains a pair of ganglia internally that innervate a single pair of appendages

• and which are connected form front to back by a ventral nerve cord of variable length

Diagrams illustrating the external anatomy of a representative ixodid tick female (Ixodes cookei)

Differences in Scutum between Males and Females

• females have theirs that partially covered by the idiosoma (body) while the males have theirs that completely cover the idiosoma

• Look at the groove of the anal pore to look at the difference between major tick groups

Define Spiracle

• opening to the system

• typically located after the fourth leg

• NOTE: adult ticks have 4 legs, larvae have 3 legs, and nymphs have 4 legs

List the developmental stages of ticks

1. Egg

2. Larva

3. Nymph

4. Adult

How many times does a tick feed per life cycle?

Each active stage feeds once in its life cycle

Biology of Ixodidae

• Females forcibly detached from the host can reattach

• females cannot feed until repletion without mating

• there are a few parthenogenetic species

• females increase their body mass 100 times

Biology of Ixodidae: Number of Eggs

• depends on the species and can range from a few hundred to 36, 000 in Ambylomma variegatum

  • 1500-2000 Ixodes scapularis

  • 5000 Amblyomma americanum

  • 4000-6000 Dermacentor variabilis.

• One gonotrophic cycle: Life cycle of feeding and laying eggs

All tick species capable of transmitting a pathogen, do they always have transstadial transmission?

Yes, since they only feed once.

Explanation: Why ticks always have transstadial transmission

• If they lose that pathogen, then they are not vectors.

• To transmit something, they must be able to transmit a pathogen since they only feed once per stage

Define Transovarial transmission

• adult female to offspring

• newly hatched larvae is infected with the disease

Define Transstadial transmission

• passes between developmental stages like larva to nymph and nymph to adult

Names for hard ticks and soft ticks

• Ixodidae – hard ticks

• Argasidae – soft ticks

Classification of Ixodidae

1. Metasriata

2. Prostriata

Define Metastriata

• describing hard ticks in which the anal groove of the adult is behind the anus

• need blood to copulate

EX: Lone star tick (Amblyomma) and American dog tick (Dermacentor)

Define Prostriata

• describing hard ticks in which the anal groove of the adult is above the anus

• do not need to feed

EX: Backlegged tick (Ixodes) —> largest genus

Ixodes ticks are classified into 1, 2, or 3 host ticks

• One host: tick undergoes their entire life cycle on one host

• Two hosts: larvae and nymphs feed on one host and adults feed on a different animal

• Three hosts: each tick stage feeds on three different animals

  • Tend to be more opportunistic than ticks that live on one host or two hosts

(Visual) Ixodidae: One Host Life Cycle

(Visual) Ixodidae: Two Hosts Life Cycle

(Visual) Ixodidae: Three Hosts Life Cycle

• Amblyomma

• Haemaphysalis

• Ixodes

Life Cycle Adaptation to the Environment

• Climate conditions and host types are two main factors influencing the number of hosts in the tick cycle

• Ex: Winter tick

  • Targets moose

  • Spends entire life cycle on moose which provides them the warmth needed to survive

Evolution of the Life Cycle in Ticks

• It is considered that a 3-host life cycle is ancestral for the ixodid ticks.

• All Ixodes species and basal lineages of the metastriata are a three-hosts life cycle

• 1 and 2 host life cycles appeared independently in several genera of the Rhipicephaline

• Advantages of 1 and 2 host cycles: lower mortality/adaptation to cold climates

(Visual) Argasidae Life Cycle

Argasidae Life Cycle

• Life cycle of argasidae is more variable than those of Ixodidae

• Same developmental stages

• The number of instars depends on various factors

  • Degree of engorgement at different stages and temperature

• Life cycle is variable and has more exceptions than hard ticks

Examples of Argasidae Life Cycle Exceptions

1. Larva from some species do not need to feed

2. Adults of the genus Otobius do not feed at all

3. Females of some Ornithodoros species may lay eggs without feeding

Differences between Argasidae and Ixodidae

• Multiple gonotrophic cycles vs one gonotrophic cycle

• Few hundred eggs versus thousands of eggs

• Feed within 15 min. (some larvae species feed for several days) versus longs feeding periods (days)

• Multiple instars stages versus no instars

• Long life span versus 1-2 year life span

• Feeding multiple times per stage versus feeding once per stage

• Strictly nidicolous versus nidicolous and non nidicolous

Evolution of the Life cycle in Soft Ticks

• Phylogenetic relations are not resolved: Difficult to draw definitive conclusions

• BUT we can still make some assumptions based on the biology and life cycle of these ticks

• Nonfeeding instars are probably an adaptation and thus secondary

  • Advantage: less host-dependent

  • Shortened life cycle becoming 1-2 host ticks —> guarantee blood meals

  • Increase the number of instars: Adaptability to harsh/good conditions

Host seeking behavior

• Host-seeking behavior includes specific behavioral reactions that contribute to finding a host (questing)

• Questing periods are seasonal

Questing in nidicolous ticks

• they only drop off when the host is not active

Questing in non-nidicolous ticks

• Seeking a host in open spaces

• To recover moisture:

  • Secrete saliva high on salts to sequester humidity from the environment