Arthropods and Disease Transmission

Although some Arthropods are parasites of humans (e.g., ectoparasites including lice, mosquitoes, and ticks that feed on blood),

other Arthropods play an important role as vectors that transmit a variety of diseases.

Arthropoda—both as ectoparasites and as potential vectors of other disease-causing microorganisms.

Mechanical Transmission

Insects can passively transfer infective organisms from faeces or contaminated soil to food or utensils when they walk across or feed on inadequately protected food.

Flies have long been thought to play a role in the mechanical transmission of those viral diseases in which the organisms are passed in the faeces.

Certain gnats (комары) are mechanical vectors of yaws, and it is possible that trachoma is also spread from person to person in this manner.

Biological Transmission

A biological vector, by contrast, is a required part of the parasite's life cycle because growth and development of the parasite must occur inside the insect.

Arthropods may serve as definitive or intermediate hosts.

Infective parasites that develop in an insect generally are injected directly into a new host or, in a few species, are deposited on the host's skin.

Some pathogens can be passed from an adult Arthropods to its offspring if the pathogen penetrates the egg(s) in utero. This is known as transovarial transmission.

Crustacea are mostly aquatic, breathing by means of gills. Crabs, prawns, shrimps, cyclops are crustaceans.

Crustacea of medical importance

Class Arachnida

They have a head and a thorax fused to form a cephalothorax, which bears simple eyes, jointed and chelate chelicerae and pedipalpi. They have no antennae; there are four pairs of walking legs on adult. Scorpions, spiders, mites and ticks have medical importance.

Order Acarina (Ticks and Mites)

The mouth parts of ticks and mites are adapted for piercing and are attached directly on the body. The adults have four pairs of legs.

Ticks are macroscopic in size, exoparasites. They have long legs and hard or soft exoskeleton. Mites are measured 200 to 400 μm, have short legs.

1. Ixodidae—a family containing (blood sucking hard ticks) that are temporary ectoparasites and are on the host only for feeding during larval, nymphal, and adult stages. Eggs hatch and emerging larvae migrate up blades of grass or twigs to wait for passing hosts. Larvae attach to hosts, feed for a few days, drop off, and then molt into nymphs. Nymphs will attach to new hosts, feed, return to the ground, and then molt to become adults. Adult females feed only once. After mating, eggs are laid and hatch in the environment.

Dermacentor transmit tularemia, Q-fever, Rocky Mountain spotted fever and some other.

Ixodes persulcatus transmit tick-borne viral encephalitis. Three pathogenic organisms—Borrelia burgdorferi (the causative agent of Lyme disease), Babesia microti, and Ehrlichia spp. (a recently described rickettsial pathogen)— are all transmitted to humans through the bite of Ixodes spp. ticks. Mixed infections of B. burgdorferi and B. microti are occurring more frequently.

2. Argasidae—a family containing the soft ticks that are also temporary ectoparasites. Ornithodoros spp., the vector for relapsing fever (Borrelia spp.)

Ornithodorus, a large tick about 1 cm in length belongs to the family Argasidae and transmits relapsing fever.

Ticks can transmit at least one protozoal organism to humans and several bacterial, rickettsial, spirochetal and viral organisms.

3. Sarcoptidae—a family containing mites that are permanent ectoparasites and live and reproduce in burrows in the skin.

These mites attack cats, dogs and humans, causing dermatitis, papules, vesicles, and even tissue necrosis at the bite site. (Ornithonyssus sylviarum)

3. Trombiculidae are small red coloured mites. They transmit scrub typhus (tsutsugamushi disease) in Far East and South West Pacific.

Tick bite wounds may become infected if mouth parts are left behind after tick removal. Ticks remain attached for extended feeding periods (for several hours or days), and the local skin reaction to the mouth parts and the tick's salivary secretions becomes inflamed and is often accompanied by oedema and hemorrhage.

Order Araneidae (Spiders)

Tab. 39.2. Arachnida of medical importance and the pathology they cause.

Date: 2014-12-21; view: 939