Ve (ký sinh)
Bách khoa toàn thư mở Wikipedia
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Ve hay bét là một số loài côn trùng nhỏ bé, thuộc lớp động vật hình nhện - sống bằng cách bám vào động vật khác để hút máu. Nhiều chứng bệnh nhờ ve mà có thể lây truyền từ người này sang người khác.
Mục lục |
[sửa] Hình thái
Giống như các loại côn trùng khác trong lớp hình nhện, mình ve có hai phần: đầu-ngực và bụng. Ve có tám chân.
Có hai loại ve chính:
- Ve cứng họ Ixodidae: loại này có vỏ bọc thân làm bằng chất chitin. Ve cứng dùng móc ở hàm bám vào thân con vật chứa nhiều ngày, hút máu đến khi no mới nhả ra.
- Ve mềm họ Argasidae: loại này sống trong các hốc kín, chỉ chạy ra kiếm ăn trong thời gian ngắn rồi bỏ về.
Ngoài ra còn có họ Nuttalliellidae, gồm chỉ 1 loại ve rất hiếm thấy ở châu Phi.
[sửa] Lây bệnh
- See main article: Tick-borne disease
Ticks are second only to mosquitoes as vectors of human disease, both infectious and toxic.[1]
Hard ticks can transmit human diseases such as relapsing fever, Lyme disease, Rocky Mountain spotted fever, tularemia, equine encephalitis, Colorado tick fever, and several forms of ehrlichiosis. Additionally, they are responsible for transmitting livestock and pet diseases, including babesiosis, anaplasmosis and cytauxzoonosis. Hard ticks may remain attached to the skin of a host for long periods of time.
Generally, tick-borne diseases correspond to a specific tick-host combination, and are limited in their geographical extent. For example, nearly 90% of all Lyme disease(caused by the Borrelia burgdorferi bacterium) cases have been reported in the Northeastern part of the US,[2] only specific deer ticks carry that disease.[3] According to the Rhode Island Department of Health, roughly 70% of people who develop Lyme disease in that part of North America catch it from ticks in their own yard. [4]
[sửa] Ký sinh
Ticks are blood-sucking parasites that are often found in tall grass, where they will rest themselves at the tip of a blade so as to attach themselves to a passing animal or human. It is a common misconception that the tick can jump from the plant onto the host. Physical contact is the only method of transportation for ticks. They will generally drop off of the animal when full, but this may take several days. Ticks have a harpoon-like structure in their mouth area, known as a hypostome, that allows them to anchor themselves firmly in place while sucking blood.
[sửa] Đề phòng và diệt trừ
A method of reducing deer tick (Ixodes Scapularis/Dammini) populations - Damminix - may be cited. It consists of biodegradable cardboard tubes stuffed with permethrin-treated cotton and works in the following way: Mice collect the cotton for lining their nests. The pesticide on the cotton kills any immature ticks that are feeding on the mice. It is important to put the tubes where mice will find them, such as in dense, dark brush or at the base of a log; mice are unlikely to gather the cotton from an open lawn. Best results are obtained with regular applications early in the spring and again in late summer.The more neighbors who also use Damminix, the better. Damminix appears to help control tick populations, particularly in the year following initial use. Note that it is not effective on the West Coast. See [1] UMM Patient Education Link.
A potential alternative to Damminix is fipronil. It is used in the Maxforce Tick Management system, in which fipronil is painted onto rodents trapped in plastic baitboxes. [2].
The parasitic Ichneumon wasp Ixodiphagus hookeri has long been investigated for its potential to control tick populations. It lays its eggs into ticks; the hatching wasps kill its host.
Another "natural" form of control for ticks is the Guineafowl. They consume mass quantities of ticks. Just 2 birds can clear 2 acres in a single year.
Topical (drops/dust) flea/tick medicines need to be used with care. Phenothrin (85.7%) in combination with Methopren was a popular topical flea/tick therapy for felines. Phenothrin kills adult fleas and ticks. Methoprene is an insect growth regulator that interrupts the insect's life cycle by killing the eggs. However, the US EPA has made at least one manufacturer of these products (Hartz Mountain Corp., Seacaucus, New Jersey, USA), withdraw some products and include strong cautionary statements on others, warning of adverse reactions (http://www.epa.gov/pesticides/factsheets/flea-tick-drops.htm).
[sửa] Chu kỳ sống
[sửa] Deer (black-legged) tick
The deer (or black-legged) tick, and the related western black-legged tick, are the primary known transmitters of Lyme disease in the United States. Both are hard-bodied ticks with a two-year life cycle. Like all species of ticks, deer ticks and their relatives require a blood meal to progress to each successive stage in their life cycles.
The life cycle of the deer tick comprises three growth stages: the larva, nymph and adult. In both the northeastern and mid-western U.S., where Lyme disease has become prevalent, it takes about two years for the tick to hatch from the egg, go through all three stages, reproduce, and then die. A detailed description of this life cycle and the seasonal timing of peak activity, as they occur in these regions, is provided below.
[sửa] Larva
Eggs laid by an adult female deer tick in the spring hatch into larvae later in the summer. These larvae reach their peak activity in August. No bigger than a newsprinted period, a larva will wait on the ground until a small mammal or bird brushes up against it. The larva then attaches itself to its host, begins feeding, and engorges with blood over several days.
If the host is already infected with the Lyme disease spirochete from previous tick bites, the larva will likely become infected as well. In this way, infected hosts in the wild (primarily white-footed mice, which exist in large numbers in Lyme-endemic areas of the northeast and upper mid-west) serve as spirochete reservoirs, infecting ticks that feed upon them. Other mammals and ground-feeding birds may also serve as reservoirs.
Because deer tick larvae are not born infected, it is believed that they cannot transmit Lyme disease to their human hosts. Instead, "reservoir" hosts, as mentioned above, can infect the larvae. Having already fed, an infected larva will not seek another host, human or otherwise, until after it reaches the next stage in its life cycle. It is not completely known whether larvae, in themselves, pose a threat to humans or their pets.
[sửa] Nymph
Most larvae, after feeding, drop off their hosts and molt, or transform, into nymphs in the fall. The nymphs can remain inactive throughout the winter and early spring.
In May, nymphal activity begins. Host-seeking nymphs wait on vegetation near the ground for a small mammal or bird to approach. The nymph will then latch on to its host and feed for 4 or 5 days, engorging with blood and swelling to many times its original size. If previously infected during its larval stage, the nymph may transmit the Lyme disease spirochete to its host. If not previously infected, the nymph may become infected if its host carries the Lyme disease spirochete from previous infectious tick bites. In highly endemic areas of the northeast, at least 25% of nymphs have been found to harbor the Lyme disease spirochete.
It may happen that humans become the hosts of infected nymphs during their peak spring and summer activity. Although the nymphs' preferred hosts are small mammals and birds, humans and their pets are suitable substitutes. Because nymphs are about the size of a poppy seed, they often go unnoticed until fully engorged, and are therefore responsible for the majority of human Lyme disease cases.
[sửa] Adult
Once engorged, the nymph drops off its host into the leaf litter and molts into an adult. These adults actively seek new hosts throughout the fall, waiting up to 3 feet above the ground on stalks of grass or leaf tips to latch onto deer (its preferred host) or other larger mammals (including humans, dogs, cats, horses, and other domestic animals). Peak activity for adult deer ticks occurs in late October and early November. Of adults sampled in highly endemic areas of the northeast, at least 50% have been found to carry the Lyme disease spirochete.
As winter closes in, adult ticks, unsuccessful in finding hosts, take cover under leaf litter or other surface vegetation, becoming inactive when covered by ice and snow. Generally, winters in the northeast and upper mid-west are cold enough to keep adult ticks at bay until late February or early March but not when temperatures begin to rise. At this time, they resume the quest for hosts in a last-ditch effort to obtain a blood meal allowing them to mate and reproduce. This second activity peak typically occurs in March and early April.
Adult female ticks that attach to deer, whether in the fall or spring, feed for approximately one week. Males feed only intermittently. Mating may take place on or off the host, and is required for the female's successful completion of the blood meal. The females then drop off the host, become gravid, lay their eggs underneath leaf litter in early spring, and die. Each female lays approximately 3,000 eggs. The eggs hatch later in the summer, beginning the two-year cycle anew.
[sửa] Các loài điển hình
- Dermacentor variabilis, the American dog tick, is perhaps the most well-known of the North American hard ticks. This tick does not carry Lyme disease, but can carry Rocky Mountain spotted fever.
- Ixodes scapularis (formerly Ixodes dammini), known as the black-legged tick or deer tick, is common to the eastern part of North America and is known for spreading Lyme disease.
- Ixodes pacificus, the Western black-legged tick, lives in the western part of the continent and is responsible for spreading Lyme disease and the more deadly Rocky Mountain spotted fever. It tends to prefer livestock as its adult host.
- In some parts of Europe, tick-borne meningoencephalitis is a common viral infection.
- Australian tick fauna consists of approximately 75 species, the majority of which fall into the Ixodidae, hard tick, family. The most medically important tick is the Paralysis tick, Ixodes holocyclus. It is found in a 20-kilometre band that follows the eastern coastline of Australia. As this is where much of the human population resides in New South Wales, encounters with these parasites are relatively common. Although most cases of tick bite are uneventful, some can result in life threatening illnesses including paralysis, tick typhus and severe allergic reactions.[5]
[sửa] Tham khảo & Chú thích
- ▲ Edlow, Jonathon A. (2005). Tick-Borne Diseases. Emergency Medicine - Infectious Diseases. www.emedicine.com. Được truy cập ngày 2006-03-14.
- ▲ Lyme Disease. Lyme Disease. Rhode Island Department of Health. Được truy cập ngày 2006-03-14.
- ▲ Ticks and Lyme. Lyme Disease. Rhode Island Department of Health. Được truy cập ngày 2006-03-14.
- ▲ Lyme Disease: Keeping Your Yard Tick-Free. Lyme Disease. Rhode Island Department of Health. Được truy cập ngày 2006-03-14.
- ▲ Ticks. Department of Medical Entomology, University of Sydney (2003). Được truy cập ngày 2006-03-14.
- Muma, Walter: Lyme Disease: Nature Class - March 1997.
- Stafford, Kirby C. III: Tick Bite Prevention, Connecticut Department of Public Health, Feb. 1999.
- Fivaz, B., T. Petney, and I. Horak. 1993. Tick Vector Biology: Medical and Veterinary Aspects. Springer. ISBN 0-387-54045-8.
[sửa] Liên kết ngoài
- National Pest Management Association article on ticks and tick-borne disease
- American Dog Tick Dermacentor variabilis diagnostic photographs
- TickEncounter.org University of Rhode Island site with tick and tick-borne disease information
