Importance of ticks
- Ticks are second only to mosquitoes in the number of diseases that they transmit.
- Ticks feeding may cause irritation, anemia, hypersensitivity reactions, and toxicosis.
- Ticks may transmit numerous pathogens including helminths, protozoa, viruses, bacteria (including rickettsiae) and fungi.
- Several pathogens transmitted by ticks are potential zoonotic agents.
- Two families of ticks are important in veterinary medicine
Ixodidae (hard ticks)
- Environmental stages found in open areas on vegetation.
- Mouthparts visible from dorsal surface.
- Body capable of limited expansion (dorsal surface covered by leathery scutum [see below]).
- Balance water using salivary secretions.
- Secrete cement from salivary glands for attachment.
- Female hard ticks feed and engorge slowly over long periods.
- Imbibe many times their body weight in host blood.
- Females lay thousands of eggs per oviposition in environment.
Argasidae (soft ticks)
- Environmental stages live in nests and burrows of hosts.
- Mouthparts not visible from dorsal surface.
- Body is soft and easily expandable.
- Life cycle consists of many nymphal stages.
- Females feed intermittently and lay eggs in masses.
- Soft ticks do not produce cement for attachment.
Important ixodid (hard) tick species found on companion animals
Rhipicephalus sanguineus (Brown Dog Tick)
Dermacentor variabilis (American Dog Tick); Dermacentor andersoni (Rocky Mountain Wood Tick)
Ixodes scapularis (Black-legged Tick; Deer Tick); Ixodes pacificus (Western Black-legged Tick)
Amblyomma americanum (Lone Star Tick); Amblyomma maculatum (Gulf Coast Tick)
Ixodid (hard) tick structure and function
- Body is divided into head (capitulum [contains mouthparts]), and body (idiosoma).
- Mouthparts consist of structures that function in tactile sensing (palps), lacerating host skin (chelicerae), and anchoring the tick to the host (hypostome).
- Body is covered by a complex waxy somewhat inflexible exoskeleton (tegument) that protects the tick from water loss and predators.
- A scutum (dorsal shield) covers the entire surface of adult male tick, but only a portion of the dorsal surface of female ticks.
- The scutum is sometimes colored with iridescent white or yellow patches (ornate tick) or may be devoid of such ornamentation (inornate ticks).
- Partial coverage of the dorsal surface of female ticks allow for enormous expansion during feeding and engorgement.
- Ticks have an excretory system (malpighian tubules), "liver" (fat body), an open circulatory system with a blood-like hemolymph, a respiratory system (net-like tracheae with openings called spiracles), a rather complex nervous system and a number of sensory structures that detect chemical, thermal, light and mechanical stimuli. A multifunctional organ located on the legs (Haller's organ) receives stimuli used during questing (crawling on vegetation to gain entry onto a host) and host detection. Some ticks have eyes.
Ixodid tick developmental cycles
• Ticks develop through four distinct life cycle stages.
o Egg, larva, nymph, adult
o Larvae, nymphs, and adults are similar in appearance, but differ in size and numbers of legs (larvae possess 6 legs; nymph and adults possess 8 legs).
o Stages increase in size from larva to adult.
o Larvae and nymphs are without features of sexual dimorphism.
• Ticks that infest companion animals are 3-host ticks.
• Most species utilize a different species of host at each stage; R. sanguineus utilizes the dog during each developmental phase.
• Ixodes scapularis (Black-legged Tick; Deer Tick) may parasitize in excess of 100 host species representing three vertebrate classes (mammals, birds, reptiles).
• Certain ticks (i.e. R. sanguineus) can complete their life cycles in weeks, while others require two years or more.
• Eggs are laid by replete females in sheltered environments off the host.
• Larvae and each subsequent stage seek a host, feeds to repletion, drop from the host and either molts to next stage or deposit eggs (replete females).
Behavior of ticks in the environment
- Free-living ixodid ticks reside in numerous environments including forests, savannahs, fields, shrubs and brush.
- They are capable of developmental arrest (diapause) to allow them to survive periods of environmental stress
- Diapause is triggered by environmental changes such as day length, temperature changes, and seasonal changes.
- Ticks seek hosts by questing on vegetation; types and height of vegetation is determined by the host they seek (lower vegetation for smaller hosts; higher vegetation for larger hosts)
- Ticks utilize vibrations, shadows produced by changing light patterns, body heat and odors and chemicals as host-seeking cues.
- Ticks literally "grab" hosts as they pass in close proximity to questing sites.
Feeding habits, pathogen transmission, vector competence
- After gaining access to the host, ticks search for a desired attachment site.
- Attachment occurs by cleaving the host skin with the chelicerae, insertion of the hypostome (anchor) and production of adhesive cement and salivary secretions.
- Tick saliva contains numerous moieties that inhibit immune responses, coagulation of blood, and exert anti-inflammatory and early analgesic effects.
- The tick body undergoes enlargement and expansion (greatest in female) during imbibition of blood.
- As ticks feed, they ingest pathogens that may be circulating in the host's blood.
- Pathogens undergo replication in tick gut cells and are disseminated to numerous tick organs (i.e. salivary glands).
- Pathogens gain entry into a new host when the next stage (after molting) feeds on susceptible hosts (transstadial transmission) or by larvae that hatch from eggs through which the pathogen was passed (transovarian transmission).
- Successful transmission can occur via salivary secretions, coxal fluid, regurgitated gut contents, contaminated mouthparts, or a combination of these mechanisms.
- Whether a tick will serve as a successful vector depends on several factors
o Size of bloodmeal ingested, virulence of pathogen, susceptibility of host.
o Capability of pathogen to infect and replicate in the tick.
o Successful dissemination of the pathogen throughout the tick's body.
o Transstadial or transovarial habits of the pathogen.
o Interactions between multiple microbes infecting the same tick.
• Treatment of the Animal (numerous available agents; the following are marketed by the veterinary profession) >
o Amitraz (Preventic®, Promeris™ for Dogs)
o Fipronil (Frontline®, Frontline® Plus)'
o Permethrin/Imidacloprid (K9 Advantix™, Vectra 3D™)
o Selamectin (Revolution®)
• Treatment of the environment
o Habitat modification
o Host elimination
o Targeted acaricides
• Integrated Tick Control
o Combination of animal treatment and environment treatment/management.
Vector-borne disease control
• Prevent tick infestation by using appropriate tick avoidance behavior (difficult).
o Avoid tick infested areas
o Wear light-colored clothing (allows you to see tick crawling on clothing)
o Use a chemical repellent such as DEET, (Picaridin??)
• Prevent tick feeding (ticks may gain access to pet host, but do not attach or feed)
o Amitraz (Preventic [collar]), Fipronil (Frontline [spray]), and Permethrin/Imidacloprid (K9 Advantix [spot-on]) can prevent vector transmission.
• Treat host with antimicrobial medications (therapeutic or prophylactic regimens).