(Kelly, 1977)
(Moorhouse, 1991)
By Norbert Fischer B.V.Sc. (Syd)
Feeding animation (new page, Flash format, 580 KB)
Ticks are obligate blood feeders. All active stages (larvae, nymphs and adults) require blood as a source of nutrition (except for a few Argasid genera in which the adult mouthparts are non-functional, ie Antricola, Otobius and Nothoaspis). Adults also require the blood for sperm or egg production. The feeding process of Ixodid ticks has first a slow phase for several days followed by a fast phase in the last 12-24 hours before detachment. There may be a ten fold increase in fed : unfed weights by the end of the slow phase, but there is an additional ten fold increase by the end of the final fast phase. Leaving the full engorgement as late as possible reduces the chances of detection and removal by the host . The process of feeding is called engorging. The hypostome has a groove along its dorsal surface forming a food canal (also known as the preoral canal) through which blood is drawn from the host and passed on to the mouth and pharynx. During blood feeding by Ixodid ticks, the liquid portion of the meal is first concentrated by removal of water and excess ions, which move across the gut epithelium and enter the ticks body cavity (hemocoele). These components are then taken up by the salivary glands which produce a watery saliva that is injected back into the host (Cupp, 1991).
Blood meal digestion in ticks is similar in all species. The digestive system in both Ixodid and argasid ticks is histologically divided into foregut, midgut and hindgut. The foregut comprises the sucking pharynx and the oesophagus. The midgut contains a ventriculus with a valve, a variable number of blind diverticula (caeca), and a rectal tube. The hindgut is divided into a rectal bulb and the rectum itself.
The capitulum (Latin: little head) of the tick is actually comprised of:
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| I. holocyclus capitulum (Kelly, 1977) |
I. holocyclus hypostome (scanning EM) (Moorhouse, 1991) |
The capitulum is not really a true "head" in the sense that the structures one normally associates with the head (primitive brain, salivary glands & eyes) are not contained by it - these structures are located in or on the body of the tick. Incidentally, Ixodes holocyclus has no eyes.
Paired salivary glands are situated laterally in the body cavity of the tick (both argasid and ixodid). They comprise large clusters of acini. A central duct extends posteriorly for the length of the gland and anteriorly into the capitulum where the paired ducts fuse to form the salivarium (Sonenshine,1991)
Once it has chosen a feeding site a tick positions itself with its legs so as to elevate its body at a sharp angle.
Guided by the palps, the mandibular digits (chelicerae) cut into the skin with their horizontal cutting action. These rip and tear at the epidermal layers and a small pool of blood is formed. The hypostome is inserted and this provides the initial attachment strength. In the case of Ixodes holocyclus the hypostome is inserted very deep into the dermis. The palps remain spread apart on the surface. The process by which Ixodid and argasid ticks feed is termed telmophagy (= pool feeding). [This contrasts with the process of solenophagy, used by mosquitos, in which feeding is direct from a small venule.] The resultant pool expands as a result of the anticoagulants released from the salivary glands. In some Ixodid ticks a cement is secreted into the wound within 5-30 minutes of cutting into the skin. This material hardens quickly into a latex-like covering around the mouthparts but excluding the palps that remain flattened out on the skin surface. (Sonenshine, 1991). I. holocyclus, however, is one of the Ixodid ticks which does not produce cementum (Kemp, Stone and Binnington, 1982).
The host reacts against the tick lesion by haemostasis, inflammation and cell mediated immunity (CMI). An array of pharmacologically active substances is injected with the saliva of the tick, including anticoagulants, PGE2, prostacyclin, apyrase and in certain tick species antihistamines. PGE2 and prostacyclin inhibit platelet aggregation and dilate blood vessels. Feeding is almost continuous with pulses of salivation alternating with periods of feeding to ensure continued suppression of host defences.
There is a concentration of saliva and presumably toxin in the granulomatous reaction around the tick mouth parts. It is thought by some experimenters that the residual toxin located in this granuloma is at least partially responsible for the increasing paralysis which occurs after the tick is removed. By comparison, the North American paralysis tick Dermacentor andersoni (found in the Rocky Mtns) does not produce a granuloma at the site of attachment, and in this case the paralysis rapidly regresses after the tick is removed (Jones, 1991). Unlike Dermacentor andersoni, I. holocyclus is a deep feeder with a long hypostome (which may penetrate as deep as 1689 um).