Faculty of Allied Medical Science

Parasitology(MLPR-201)

fall 2013/2014

Arthropods as Vectors of the Etiologic Agent of Diseases

Dr. Hamdy  Badie M. El-Wakil

Prof. of Parasitology & Public Health .

Faculty of Allied Medical Sciences.Pharos University In Alexandria,Egypt

Learning Outcomes

• By the end of this lecture the student should

understand Arthropods as Vectors of the Etiologic Agent of Diseases.

INTRODUCTION

Members of the phylum Arthropods are the most abundant and widely

distributed of all animal groups. They may be found in every part of the world

and in every type of environment. Many, particularly those within classes

Insecta and Arachanida, live close association with humans; others, while

primarily parasites of animals, will readily attach or feed upon humans and

some may be specificity adapted as human parasites. They may be of medical

significance simply because of their physical attack and blood- sucking habits,

or they may be of considerable importance as distributors of organisms that

cause disease.

Phylum : Arthropoda

Class : Insecta

(Insects)

Class : Arachnida Class : Crustacea

Sucking lice Ticks Cyclops

True bugs Mites

Flies Scorpiones

Fleas Spiders

Beetles

Bees

Ants

Wasps

Moths

Butter flies

A- Arthropods as vectors of pathogens (Disease transmitters):

• They may act as mechanical vectors either of the etiologic agent or as biological vectors requiring a period of incubation or development in the host through the following methods:-

• I- Mechanical Transmission:• Arthropods transport the various pathogens in or on their

body and drop them on human food, drinks or tissues without any biological changes for them , e.g. when house fly play a role in the transmission of enteric pathogenic, cysts of protozoa and eggs of helminthes.

•

• II- Biological Transmission:• It occurs when the arthropod takes an active

role in the transmission of the pathogenic organism through any of the following ways:

• 1- Propagative:• The organism in the arthropod simply

multiplies in the vector without any cyclical development like in a culture tube e.g. Multiplication of Yersinia pestis in the fleas.

• 2-Cyclodevelopment: The organism undergoes development or morphological changes only without multiplication in the arthropod e.g. development of microfilaria within Culex sp. mosquito.

• 3- Cyclopropagative:• Organisms undergo both developmental

changes and multiplication in the arthropod e.g. Malaria in the female anopheles.

• III- Arthropods can also convey the organisms to man by different methods:-

• 1- Vomit-drop: Non-blood sucking flies may deposit a vomit drop containing the pathogens on human food or drink e.g. Shigella bacilli and cysts of E. histolytica. Others arthropod species may obtain the parasitic organisms in blood meal from an infected person and deposit them in a vomit drop in the puncture wound made in the skin of an uninfected person e.g. Yersina of plague or in faecal pellets near the puncture wound e.g. Rickettsia of epidemic and endemic Typhus.

• 2- Crushing: Contamination from the infected haemolymph may occur when the arthropod is crushed on the skin e.g. epidemic typhus.

• 3- Coxal fluid: during the act of feeding, soft ticks produce infective fluids from the coxal glands that contaminate the bite wound on skin.

• 4- Salivary secretion: Pathogens such as malaria and viruses are discharged through the hypopharynx in minute droplets of salivary secretion at time arthropod takes a blood meal.

• IV- Sources of pathogens transmitted by arthropods:• 1- Human being: as in case of malaria infection.• 2- Rodents: they are considering as a source of infection for

fleas living on them by Yersinia pestis (the causative agent of Plague).

• 3- Monkeys and birds: are sources for numerous viral infections. In some cases, organisms have been so long and so well adjusted to the arthropod that they produce no obvious injury to the vector, to the extent they are transovarially transmitted to their offspring e.g. Rickettsii and Borcellia duttoni in ticks and R. tsutsugamuchi in mites.

B- Arthropods as etiological agent of diseases:

• In addition to their importance in the transmission of pathogens to man, arthropods themselves play a significant part as disease producing:

• 1- Specific parasites: Certain arthropods produce lesions in the human tissue e.g. Myiasis producing flies, and itch mite Sarcoptes scabies producing tunnels in the skin of man.

• 2- Through toxin and venom: Certain ticks produce fatal paralysis in some individuals presumably due to toxins in their secretions. Venoms introduced into the skin by bite of the "black widow" spider, or the sting of a scorpion or bee may at times produce both local reactions and profound systemic shock.

• 3- Allergic reactions: These may be provoked by minute blood sucking flies when depositing droplets of saliva on the skin. House dust mites may cause rhinitis or asthma by serving directly as allergens or by concentrating highly antigenic material.

• 4- Entomophobia: the fear of arthropods acaro-entomophobia can extremely important as a sever neurosis.

• 5- Annoyance and blood loss.• 6- Accidental injury to sense organs.• 7- Dermatosis.

C - General Characters of Arthropods:

• All arthropods, while varying considerably in size and shape, have certain features in common. They are all bilaterally symmetrical and metamerically segmented. They have externally a hard chitinous exoskeleton, sometimes sclerotized or calcified, inside which a hollow blood cavity is containing a clear fluid haemolymph. Arthropods have an excretory system, but a few of them consist of the malpighian tubules (2 or more), which lie in the hemocele and open into the hindgut near its junction with the midgut. They discharge the liquid wastes into the hindgut.

• Respiration may be achieved by a variety of methods e.g. via gills, lung books, gaseous exchange, through the cuticle or by means of spiracles. Circulatory system consisting of heart, aorta, and sometimes-paired blood vessels that opens into the hemocele, which extends into all parts of the body. The blood consists of plasma and amoeboid white cells; respiratory pigment is hemocyanin in the plasma rather than hemoglobin in the case of gill routing and book lung-breathing.

• The forth system, called central nervous system which consists of a dorsally situated composed of dorsal and ventral fused cephalic ganglia "Brain" connected by circumesophageal commissures to the media ventral paired nerve trunk, and to twinned ganglia of each post-cephalic body system. Nerve fibers extend from the central nervous system to all important organs and tissues. Either in many groups, there are light-sensitive organs in the head individual eyespots (ocelli) or compound eyes composed of ommatidia. All arthropods have jointed appendages, which may take the form of legs, antennae, mouthparts or cerci. The sexes are always separates and morphologically distinguishable.

Two different types of development take place:

• 1- Incomplete metamorphosis: Hatched larvae resemble the adults but are smaller and gradually increase in size by successive molting e.g. Ticks.

• 2- Complete metamorphosis: Hatched larvae are completely different morphologically from the adults. They may be worm like, transform to the inactive pupal stage from which finally the adults emerge, e.g. House fly.

D-Arthropods of medical importance:

• They are Included in the Following Classes:• i- Class Insecta or Hexapoda e.g. house fly, fleas, lice, bugs,

mosquitoes and flies (venomous, biological and mechanical vectors, intermediate hosts, and parasitic.

• ii- Class Archanida e.g. scorpion, spiders, ticks and mites.• iii- Class Crustacea e.g. crabs, prawns and copepods.• iv. Class Chilopoda e.g. censpodes (venomous).• v. Class Pentatomida e.g. tongue worms (Endoparasitic).

Assignment

• Cimexlectularis