Ticks

Paul R EarlFacultad de Ciencias Biológicas

Universidad Autónoma de Nuevo LeónSan Nicolás, NL, Mexico

TICKS are universaly distributed ectoparasites of vertebrates that take blood and lymph from them. They are related to mites and distantly to spiders. They are vectors of many microbial pathogens. About 850 species have been described, and the most important in Europe, the Far East and the New World are Ixodes species. Try The Ticks of the World (Acarida, Ixodida). Nomenclature, Described stages, Hosts, Distribution (Including new species described before 1/01/96) by JL Camicas, JP Hervy, F.

Adam, PC Morel. Ed Orstom, Paris, 1998.

Ixodes ticks transmit the spirochete Borrelia burgdorferi of Lyme disease in Eurasia and North America, tick-borne encephalitis virus (TBEV) in Europe and Asia, human babesiosis, caused by Babesia microti and B. bovis to man in North America, Babesia divergens in Europe and B. bigemina & B. bovis in Mexico. Human granulocytic ehrlichiosis (HGE) now called human anaplasmosis is caused by Anaplasma phagocytophila, A. equi & HGE agent in North America and Europe. The pathogens Bartonella spp., Francisella tularensis, Coxiella burnetii and Rickettsia spp. cause tularaemia, Q-fever and tick-borne lymphadenopathy (TIBOLA) are importance.

Tick biologyTicks are arachnids that comprise 2 major families: the Ixodidae (ixodid or hard ticks), having a dorsal shield (scutum), and the Argasidae (argasid or soft ticks). These 2 general taxonomic categories differ in form, behavior, pattern of development, and disease relationship. Ixodes ricinus is equipped with long cheliceras and the central hypopharynx: the piercing and sucking mouthparts. The hypostome is equipped with recurved spines. A quick hardening cement in ticks can improve the functioning of the puncture channel.

TICKS rank second to mosquitoes as disease transmissors and disperse a greater variety of pathogens. Persistent painless blood-sucking of large amounts of blood like 8 ml over weeks on mammals and birds allows wide dispersion. Wide host range and tendency to feed on several hosts during their lifetime ensures ample opportunity to acquire and transmit pathogens. Hardiness and longevity allow them to survive periods of unfavorable conditions while waiting for a host. Waiting is called questing. High reproductive potential ensures large populations and a high frequency of host-vector contact. Ticks feed slowly. This allows for pathogen acquisition and transmission, and vector dispersal by host

migration.

With 3-host ticks, the larva, nymph and adult each feed on a different individual vertebrate host, often a wide range of species, for a period of a few days. Hard ticks moult only twice; the egg stage generates a 6-legged larva, which develops into an 8-legged nymph, which in turn moults to produce a similar but larger adult. Hard ixodid ticks increase their body mass 10–200 times during feeding. Larvas feed on their host for an average of 2–4 days and increase their mass 10–20 times. Nymphs feed 3–5 days and increase by 15–40 times and adult females

feed 6–10 days and increase by 100–200 times.

Each stage of I. ricinus takes about 1 year to develop from one to the next, so the shortest life cycle takes 3 years to complete. However, it may vary from 2-6 years depending on the availability of hosts and climatic conditions. I. ricinus spends 99% of its life living free in certain locations with special vegetation.Ticks digest their blood meals intracellularly. Ticks do not pierce blood vessels and are pool-feeders. They feed on lymph and dissolved cells besides blood.

Hard ticks (family Ixodidae) are responsible for transmission of the majority of tick-borne diseases of humans. They are the vectors of babesiosis, Colorado tick fever, Lyme disease, Rocky Mountain spotted fever, and tularemia in the US. While soft ticks (family Argasidae) do not directly transmit these diseases to man, they may be involved in the maintenance of natural cycles among reservoir hosts, as in the case of Colorado tick fever. In addition, soft ticks of the genus Ornithodoros are the vectors of tick-borne relapsing fever.

Developmental StagesSeed tick. Normally, thousands of tiny larvas hatch from the eggs batch and crawl about searching for a host. Lucky ones attach to a mammal or lizard. Feeding time is generally short, a few hours or a day, but up to 3 days for Ixodes scapularis. During feeding the host wanders and the tick is transported to a new location where it drops off when engorged.Nymph. After molting, hard tick nymphs climb grass leaves or plant stems and quest. As they become higher in ground level, they tend to attach to larger hosts. Soft tick nymphs burrow in and await the arrival of a host. After several days of feeding they drop off and again molt.

Adult. Ticks sometimes wait for months or more than a year for a suitable host. When finally engorged, they drop off to oviposit. Ticks that tend to feed on the same host species at each stage are referred to as 1-host ticks, whereas those that feed on different hosts each time are called 3-host ticks.The first pair of legs is extended and used to grasp the host when contact is made. This behavior is known as questing. If warmth or carbon dioxide is detected by the tick, it will seek it out and climb onto the host as it passes. The height at which questing takes place determines the size of the host and therefore, to some extent, the species selected.

The occurrence of tick-borne encephalitis viruses is determined by the distribution of their tick vectors which, in turn, is governed by ecological conditions. The tick hosts and vectors of TBE group viruses are primarily of the genus Ixodes.

In central Europe I. ricinus is the main transmitter of the Western virus subtype. I. ricinus activity starts when the temperatures rises to 5–7 C in March or April and ends late in the year in October and November.

American dog tick (Dermacentor variabilis)

Male A ventral view

Brown dog tick (Rhipicephalus sanguineus)

Rocky Mountain wood tick (Dermacentor andersoni)

Lone star tick (Amblyomma americanum)

Deer tick (Ixodes scapularis)

Ixodes pacificus

Boophilus microphilus

An engorged female of B. microphilus >

Relapsing fever ticks (Ornithodorus spp.)

Fowl tick (Argas persicus)

This soft tick feeds rapidly at night and subsequently oviposits in cracks and crevices. The tick has two or three nymphal stages before molting to the adult stage. Like the relapsing fever tick, these ticks may live for months or years without a blood meal. Argus persicus readily attacks humans but does not transmit human disease. It is a vector of fowl spirochetosis.

Ixodes ricinus

I. ricinus is the most important vector in Europe.

Spinose ear tick (Otobius megnini)

SummaryIn this introductory lecture, most fundamental points are reviewed, and some economically important ticks are illustrated. How ticks, mites and mosquitoes fit into temperate and tropical medicine and veterinary practice is a very long story involving discovery not told here. Who was Howard Taylor Ricketts ?The information here on ticks is a good start for vector control or other fields of interest that are often neglected. All ticks are parasites! Modern medicine or its US version does not concentrate on vector transmitted diseases which are at once tropical and rural, and once upon a time were colonial.