Tick (atel) any of the members Argasidae and Ixodidae of two families of the order Acarina, class Arachnida that are ectoparasitic on vertebrate animals other than fishes. They are more common in tropical countries and are most abundant on mammals. However, some parasitize birds and reptiles, and occasionally some feed on amphibians; about 300 species are known. The members of the family Ixodidae are commonly known as hard ticks, and those of Argasidae, the soft ticks. They are easily distinguished from insects in that the body is not definitely divided, ie, there is a strong fusion of the thorax and abdomen producing a sac-like leathery appearance. A discrete head is lacking, but the mouthparts often form a structure known as capitulum. Like most other Arachnida, adult ticks and nymphs bear four pair of legs; the larvae are hexapod. In the ixodid or hard ticks, the dorsum of the adult male is largely or wholly covered by a plate known as scutum. In immature ticks and females, the scutum is restricted to the anterior part of the dorsum, behind the capitulum. In certain hard ticks, there is a row of more or less rectangular areas, separated from adjacent ones by grooves, along the posterior sub-marginal area of the dorsum. These are known as festoons and may be present in both sexes. The soft ticks or argasids lack scutum and festoons; sexual dimorphism is not marked. Most ticks are only few mm in length (2-9 mm), but an engorged female of certain genera may reach up to 14 mm.

Ticks constitute one of the most important groups of animal ectoparasites and vectors of both animal and human diseases. As a transmitter of human diseases alone, they run a close second next to mosquitoes.

More than a dozen of tick species have so far been recorded from Bangladesh infesting cattle, goat, sheep, dog, wild mammals, bird, and lizard. The cattle tick, Boophilus microplus is predominant, followed by Haemaphysalis bispinosa, Hyaloma truncatum and H. anatolicum. The other common species are Aponoma gervaisi, H. canestrini, H. kinneari, Rhipicephalus sanguineus, R. evertsi, and Amblyomma variegatum among the hard ticks, and Argas persicus, the fowl tick, among the soft ticks infesting poultry. B. microplus has occasionally been found to infest human beings.

Fowl tick Common name of the soft tick species Argas persicus of the family Argasidae, order Acarina. It is practically cosmopolitan in its distribution. Although essentially a bird tick, it mostly attacks chickens. It is a relatively large, brownish red, oval tick, the female measuring 7-10 mm, and the male 4-5 mm. The tick normally feeds at night. Since it spends the day in cracks and crevices of the chicken';s house, it is seldom seen. The larval tick (seed tick) remains for several days on the host and is seen more frequently.

Fowl tick is more prevalent in domestic chickens in rural areas of Bangladesh than in poultry farms. Infestation of this tick may cause anaemia and consequently reduction in egg production. It is the vector of fowl spirochaetosis and other avian diseases. [SM Humayun Kabir]

Tick paralysis A disease of man and animals characterized by an acute ascending flaccid motor paralysis caused by the injection of a toxin by certain ticks.

Adult female ticks, and often nymphs, while engorging inject toxins, usually at the later stages of feeding blood. In particular, ticks which attach to the head and neck of the host usually inject toxins. The toxin production is concomitant with egg production in females. Toxin has also been detected in suspension of crushed eggs which when injected in animals also produce paralysis. The degree of paralysis is proportionate to the length of time the tick has been feeding. Frequently, a number of ticks attach themselves to host. Removal of the ticks is usually followed by recovery, provided the cardiac and respiratory centres have not been affected. Tick paralysis is generally observed in isolated cases of individual animals/man but it can also affect herds of animals and can cause severe losses if untreated.

Tick paralysis affecting cattle, sheep, goat, dog, pig, wild ruminants, and guineapig has been widely documented in Africa, Asia, Europe and Australia. Tick species which are increminated to tick paralysis are Argas persicus and Ornithodoros lahorensis among the soft ticks (Argasidae) and several species of Ixodes, Demacentor, Rhipicephalus, Haemaphysalis, Hyalomma, and Amblyomma among the hard ticks (Ixodids).

Out of the above mentioned ticks, only Argas persicus and Rhipicephalus sanguineus have been recorded in Bangladesh. A. persicus has been reported from pigeons, whereas R. sanguineus is a common tick of dog occasionally attacking cattle and man. Although Otobius lahorensis and Hyalomma kutchensis are reported from Pakistan and India, they have not been recorded in Bangladesh. However, in many occasions 'tick toxicosis' which is distinct from “tick paralysis” and is characterized by sweating, profuse moist eczema, and hyperanemia of the mucous memberane, has been reported from man and cattle from the forests of Madhupur and Haluaghat of Tangail and Mymensingh districts. Larva of Boophilus microplus, a common tick of cattle, sheep and goat, occasionally attacks people and has been detected as the cause of this problem in the above mentioned areas. [Md Hafezur Rahman]