Parasitology - Ticks

Question 1

Ticks are in the phylum Arthropoda but they are not in the class Insecta as are many other parasites of the skin (flies, fleas, lice).

What class are ticks – and mites – in?

Answer 1

Arachnida - four pairs of legs and chelicerae mouthparts

Question 2

What are chelicerae?

Answer 2

Chelicerae are pointed appendages which are used to grasp food, and are found in place of the chewing mandibles most other arthropods have.

Question 3

How are ticks classified?

Answer 3

Hard ticks & Soft ticks

Question 4

Are you more likely to run into a hard tick or soft tick in the UK?

Answer 4

Hard tick.

Hard ticks are more common in temperate zones such as Western Europe & North America.

Soft ticks are more important in tropical climates (not the UK, although some of them can be found on migrating birds).

Question 5

What are the characteristic features of hard ticks?

Answer 5

1) Presence of scutum - in males, covering whole of dorsum; in females, only small anterior part of dorsum to accommodate body enlargement during engorgement after blood meal; female’s dorsum is comprised of convoluted skin that flattens out when expanded
2) Prominent mouth parts:

Palps - for sensing heat & CO₂ concentration, which helps ticks identify capillaries

Chelicerae - “scissors” for cutting into skin

Hypostome - strawlike structure with backward-facing teeth to help anchor mouthpart while tick sucks blood

3) Festoons, like pie-crust markings, might be present
4) Ornate ticks have coloured patches (some are not ornate)

Question 6

What are the mouth parts of hard ticks?

Answer 6

Palps (feeling, sensing), Chelicerae (cutting), Hypostome (anchoring)

Question 7

What are the charateristic features of soft ticks?

Answer 7

1) Scutum is absent - they can look like flattened leather bags
2) Mouthparts are not visible when tick viewed dorsally
3) Don’t swell as much because they take many meals

Question 8

Describe how ticks feed.

Answer 8

1) Tick stands upright
2) Chelicerae cut through skin → pool of blood (reach capillary)
3) Hypostome inserted deep into skin
4) Mouthparts cemented in place - tick secretes “cement” to help anchor mouthparts in place via a pump separate from blood-sucking tube
5) Tick feeds continuously + injects saliva* (contains substances that ↓ host inflammatory response, ↑ permeability of blood vessels → free flow of
blood)

* ticks feed for 2-3 days & thus must counter blood’s coagulative response

Question 9

Define the terms trans-stadial and transovarian transmission of infection.

Answer 9

Trans-stadial - infection is transmitted to each developmental stage but not eggs (larvae → nymph → adult → NOT EGGS)

Trans-ovarian - infectious agent passed from one generation to next THROUGH the egg, eg., Babesia spp

Question 10

What’s the difference between hard ticks and soft ticks in terms of their hosts?

Answer 10

Hard ticks can have 1, 2 or 3 hosts.

Soft ticks are not classified the same way as they can feed on many different hosts.

Question 11

Describe the life cycle of a one-host tick and give an example.

Answer 11

Larva → Nymph → Adult

• all take one blood meal each on same, single host (but eggs laid & hatched off host)
  eg. Boophilus spp

Question 12

Describe the life cycle of a two-host tick and give an example.

Answer 12

Larva → Nymph both take one blood meal on first host;

Adult feeds on second host;

Eggs laid off host on ground.

eg. Hyalomma sp.

Question 13

Describe the life cycle of a three-host tick and give an example.

Answer 13

• *Larva** - one host
• *Nymph** - second host
• *Adult** - third host

Eggs hatch off host.

Each host can be different species; ie., not species-specific

eg. Ixodes spp. eg. Ixodes ricinis (castor bean tick)

Question 14

Describe the veterinary significance and epidemiology of Ixodes ricinus in the UK.

Answer 14

Ixodes ricinis is a three-host hard tick. It is also known as the “castor bean tick” and is the most common tick in the UK. It affects wild and domestic animals, as well as humans.

As a three-host tick, it hatches from eggs off the host, but spends each stage in its life cycle on a different host (larva - nymph - adult).

It is a transmitter of Lyme Disease, which affects humans & dogs, and is also a vector for animal diseases such as Bovine babesiosis (Babesia protozoa), Louping Ill, tickborn fever, tick pyraemia & temporary paralysis brought on by neurotoxins.

Ixodes ricinus life cycle is usually 2-3 years, but can be as long as seven years.

It spends most of its life on the ground, and prefers rough pasture and very high relative humidity (over 90%).

Question 15

Can you use insecticides to kill ticks?

Answer 15

No because they’re not insects. They’re arachnids of the subclass acarid. They can be killed with acaricides, pour-on, spray, dip formulations.

Question 16

What are the general principles of Integrated Pest Control to deal with ticks?

Answer 16

1. Kill ticks on ground - drain, burn, take animals off
2. Kill ticks on animals - acaricides
3. Separate host(s) from infection - rotation, fencing
4. Enhance host resistance - breeding eg. Bos indicus has higher heritability of resistance to ticks than Bost taurus, so X-breed to improve resisitance; vaccination with “hidden vaccine”

Question 17

What is the hidden vaccine against ticks? How does it work?

Answer 17

The hidden vaccine is against the one-host tick, Boophilus microplus.

The vaccine raises antibodies against “hidden antigens” in the tick’s gut by putting these foreign tick antigens into the host. The host makes antibodies against these antigens, and the tick takes up the antibodies when it has a blood meal, killing itself.

Question 18

What are the effects of the hidden tick vaccine on tick burden?

Answer 18

There is no ‘knock down’ effect in the number of ticks, but a drop in the number of ticks feeding, the volume of blood taken in each ‘meal’, the number of eggs laid and larvae emerging onto the pasture.

As such the vaccine is used as a medium- to long-term epidemiological ‘tool’.

Question 19

Discuss the advantages and limitations of this of using the hidden tick vaccine to tick control.

Answer 19

Advantages include:

(i) a valuable alternative tool in the fight to control ticks – remember the concept of ‘integrated parasite control’ and the use of a variety of methods to control ticks; and
(ii) reducing the frequency of acaricidal use long-term should not only reduce the risk of resistance developing (or getting any worse) but also extend the useable life of existing acaricides.

**Limitations/disadvantages include: **

(i) only applicable in situations where the target tick is host specific (Boophilus microplus is cattle-specific) – ticks with a wide host range would not be controlled by such a scheme as they would continue to feed on alternative hosts with little, if any, consequent drop in tick stages on pasture; and
(ii) initial tick control costs will be greater than normal as farmers would have to pay for both vaccination AND acaricidal treatment for the first year at least. Only when tick numbers at pasture began to decline could farmers reduce the frequency (and cost) of acaricidal treatment

Question 20

Why has the Australian hidden-antigen vaccine against Boophilus microplus not been a commercial success?

Answer 20

Difficulties in conveying the concept of vaccination to farmers who might expect the vaccine to have a much faster action (like acaricide sprays, dips, etc), i.e. a marketing nightmare!

Question 26

What is the cause of Lyme Disease and what are the clinical signs?

Answer 26

Lyme disease, also known as borreliosis, is caused by the spirochete bacteria, Borrelia burgdorferi. The vectors that carry this disease to domestic animals and humans from wildlife reservoirs (mainly deer) are hard ticks of the genus Ixodes. In the UK, Ixodes ricinus, also known as the “castor-bean tick”, is the main vector.

Note that Ixodes ricinus is a three-host tick, so the larvae feed on one host, nymphs on another and adults on a third host, such as deer.

Clinical signs in humans are erythema (redness of the skin, caused by hyperemia of the capillaries in the lower layers of the skin) & arthritis. In dogs, acute lameness.