Tick Biology


Female Gulf Coast tick in host-
seeking position on grass

Ticks represent a successful group of parasitic arthropods that are more closely related to mites, spiders, and scorpions than to insects. These destructive blood-feeding organisms are capable of persisting within a variety of environments worldwide, and when necessary, can survive for extended periods of time (2-3 yrs) without feeding. The life cycles of ticks consist of four distinct stages; the egg, six-legged larva, eight-legged nymph, and adult.

 
   Ticks utilize many different feeding strategies to complete their life cycles. The most common strategy requires three separate hosts (the so-called three-host ticks), one each for the larval, nymphal, and adult life stages. Host blood provides the nutrition needed for ticks to molt to the next stage, or to produce eggs for the next generation.

  Three-host ticks are normally less host specific than one-host species. The immature larval and nymphal stages generally feed on a wide variety of animal classes (numerous bird or small mammal species), while adult ticks tend to utilize larger animals (deer, cattle, and horses). Three-host ticks generally complete a life cycle in one to two years depending on species. Adult ticks may survive several years without a blood meal.

  One-host ticks attach to their host as unfed larvae where they proceed to feed and molt through larval, nymphal, and adult life stages, all on a single host. One-host ticks may produce 1-4 generations per year, depending on species.

  As ticks feed, blood-meal nutrients are concentrated within their gut. To rid themselves of excess body water (a byproduct of the blood meal), they utilize their salivary glands to “pump” the waste water back into their host. It is through this process of gut-water expulsion that ticks have the potential to transmit a number of bacterial, rickettsial, protozoal, viral, and fungal pathogens to their animal and human hosts.

   However, pathogen transmission via tick-bite is not an immediate nor automatic process, and in general, an infected tick must attach and feed for 1-2 days or longer for pathogen transmission to occur. Moreover, while not all ticks are infected, those that may be are not always successful in transmitting disease agents of sufficient volume to result in illness.

Soft vs. Hard Ticks

  Ticks are separated into two main family groups: the less prevalent “soft ticks” or Argasids, and the more common “hard ticks” or Ixodids. Soft ticks are known to progress through several (2-7) instars in the nymphal stage prior to molting to the adult reproductive phase; the number of nymphal stages is dependent on species, host availability, and climatic and/or environmental factors. There is little difference in appearance between male and female soft ticks.

 Cuticle surfaces of soft tick species vary, but is frequently pebbled in texture with numerous grooves or undulations, appearing leathery-gray to tan in color, and without ornation. The head among soft tick species is ventrally located (not seen from the top/dorsal view), and the absence of a scutum is a key identifying characteristic common to all species. Soft tick females are capable of laying a few to several hundred eggs more than once during their lifetime. The Spinose ear tick, Otobius megnini, is one of the most numerous and wide-spread Argasid pests of cattle and horses in the Southern Region, and elsewhere throughout much of North America.

   Hard ticks are more commonly encountered by animals and people than soft ticks. Male and female hard ticks appear visually distinguishable from each other (sexual dimorphism), and also present different physical characteristics between species than soft ticks.

 

   Male Ixodid species have an inflexible scutum that spans the entire top side or dorsal surface; among Ixodid females the scutum or dorsal shield is less than half the size of males, and limited to a small region directly behind the head (capitulum).

This smaller scutum size facilitates the expansion of the softer cuticle (alloscutum) among feeding females and immatures, allowing it to easily stretch and enlarge (e.g. up to 2-20X the unfed or “flat” size & up to a 250-fold weight gain) as blood-feeding progresses.

 This ability is common to larval, nymphal, and adult life stages of Ixodid ticks, but is limited among the soft (Argasid) tick species.

  Scutum color and ornation in adults of some Ixodid species (particularly several of the Amblyomma and some Dermcentor spp.) can be rather pronounced.

  Male ticks are generally smaller than females and typically attach to hosts and feed prior to females. Fed male Ixodid ticks are not measurably larger than unfed males, as the primary purpose of feeding by male ticks is for attraction and reproduction of females. Attracted female ticks attach to hosts, mate on-host with nearby fed male ticks (except for some Ixodes spp.),

then blood-feed for 7-14 days before detaching and dropping from their host to find a suitable place on the ground to lay their fertilized eggs. Following this single oviposition event, all hard tick females expire as do their mated male consorts. Eggs laid (oviposited) by gravid females will typically incubate and hatch over several weeks to months (depending on temperature and concurrent weather conditions).

  Depending on the influence of environmental cues and host availability, larvae will ascend surrounding vegetation to quest for a passing host and renew the cycle of parasitism.

  Most Ixodid ticks produce one generation per year (univoltine), but some species require 2 years to complete a life cycle (e.g. Ixodes scapularis). Ticks are known for being long-lived, and adults of some species are known to live 2-3 years without a bloodmeal. Three-host tick species have seasonal peaks of host-seeking activity for each stage. Some species and/or stages are known to actively be host seeking year round.