Arthropods: An Overview

Arthropods: An Overview

• Arthropods belong to the kingdom Animalia, phylum Arthropoda, and are classified into different classes such as Insecta, Arachnida, Diplopoda, Chilopoda, Crustacea, and Pentastomida.
• Phylum Arthropoda characteristics:
  • Metameric body (true segmentation with replication of muscles and nerves)
  • Chitinous exoskeleton (nitrogenous polysaccharide)
  • Bilaterally symmetrical
  • Jointed legs
  • Dorsal heart (open circulatory system)
  • Ventral nervous system
  • Striated muscle

Class Arachnida

• Characteristics:
  • 2 body regions or unsegmented
  • Without antenna and wings
  • 4 pairs of legs
  • Stigmata

Class Diplopoda

• Characteristics:
  • Two pairs of legs per segment
  • Two pairs of spiracles per segment
  • Body cylindrical
  • Detritivorous
  • Can secrete toxic substances (e.g., HCl) as a defense

Class Chilopoda

• Characteristics:
  • One pair of legs per segment
  • One pair of spiracles per segment
  • Body flattened dorso-ventrally
  • Carnivorous
  • First pair of legs modified as venomous fangs

Class Crustacea

• Characteristics:
  • One or two segmented body
  • 2 pairs of antenna
  • 5 pairs of legs
  • Without tracheal system

Class Pentastomida

• Characteristics:
  • Segmented slender body
  • Without appendixes

Class Insecta

• Characteristics:
  • 3 distinct body regions: head (feeding, sensory, CNS), thorax (locomotion, respiration), abdomen (feeding, reproduction)
  • 3 pairs of legs
  • 1 pair of antenna
  • Spiracle
  • Exoskeleton
  • Segmented; bilateral
  • Open circulatory system
  • Ventral nervous system

Types of Insect Development

• "A" – (lacking)
• "Hemi" - (incomplete)
• "Holo" - (complete)

Medical Entomology

• Subfields:
  • Public health entomology
  • Veterinary entomology
• Importance:
  • Provide useful medical services/drugs
  • Useful model systems in genetic studies
  • Natural enemies of other medically harmful insects
  • Vectors or hosts of pathogenic organisms
  • Direct causes of disease or distress

Arthropods as Direct Causes of Diseases and/or Distress

• Delusory parasitosis (DP) and entomophobia
• Envenomization - wasps, bees, spiders
• Ectoparasites - ticks, fleas, mites
• Endoparasites - chigoe flea, myiasis
• Allergic reactions - dust mites
• Blister and urtic (itch) inducing insects - blister beetles, some caterpillars
• Annoyance - mosquitoes, black flies

Arthropods as Vectors of Diseases

• Mechanical transfer:
  • Occurs passively and externally from host to host
  • Pathogen does not increase inside vector
  • Examples: House flies and cockroaches transfer bacteria from faeces to human food
• Biological transfer:
  • Specific association between vector, pathogen, and host
  • All three components are essential
  • Pathogen increases inside vector
  • Example: Malaria cycle

Agents Transmitted by Vectors

• Viruses (arboviruses)
• Bacteria (Rickettsia, Francisella)
• Protozoa (Plasmodium)
• Helminths (filarial nematode worms)

Method of Arthropod Vector Transmission

• Bite of vector
• Contaminated food and water
• Direct contact with infected vector

Examples of Arthropod-Borne Diseases

• Scrub typhus (Rickettsia tsutsugamushi)
• Rickettsial pox (Rickettsia akari)
• Tularemia (Francisella tularensis)
• Lyme disease (Borrelia burgdorferi)
• Ehrlichiosis (Ehrlichia canis, etc.)
• Babesiosis (Babesia microti)
• Colorado tick fever (CTF virus, etc.)
• Russian spring-summer encephalitis, Louping ill, etc. (Flaviviridae)
• Nairobi sheep fever, Crimean hemorrhagic fever, etc. (Bunyaviridae)
• Epidemic typhus (Rickettsia prowazekii)
• Trench fever (Bartonella quintana)
• Louse-borne relapsing fever (Borrelia recurrentis)
• Plague (Yersina pestis)
• Murine typhus (Rickettsia typhi)
• Rat tapeworm infection (Hymenolepsis diminuta)
• Dog tapeworm infection (Diphylidium caninum)
• Chaga's disease (Trypanosoma cruzi)
• Hymenolepsis nana
• African trypanosomiasis (Trypanosoma brucei)
• Onchocerciasis (Onchocerca volvulus)
• Leishmaniasis (Leishmania donovani)
• Loaiasis (Loa loa)
• Sandfly fever (Sandfly fever Naples virus, etc.)
• Malaria (Plasmodium falciparum, etc.)

Mites and Ticks: General

• Characteristics:
  • Sac-like, unsegmented body
  • Highly specialized mouthparts, capitulum/hypostome
  • Phytophagous, predatory, parasitic
  • 4-stage life cycle, egg-larva-nymph-adult
  • Larvae 6 legs, nymph and adult 8 legs### MITES
• Usually less than 1 mm long, with 1-3 nymphal stages
• Oviposition can be via egg or ovoviviparity
• Development from egg to adult takes 8 days to several weeks, with an average of 4 weeks
• Abdomen is joined to cephalothorax, with no segmentation
• Larvae have 3 legs, while adults have 4 legs, but with some species having leg reduction
• Chelicerae are used for tearing or piercing in parasitic species

Effects on Humans and Animals

• Causes dermatitis or other tissue damage
• Leads to loss of blood or other tissue fluids
• Transfers pathogenic agents
• Can cause strong allergic reactions

Scabies

• Caused by Sarcoptes scabei mite
• Common, found worldwide, and affects people of all races and social classes
• Spreads rapidly under crowded conditions, especially through skin-to-skin contact
• Circular body, 0.3-0.45 mm in length, with striped dorsal and ventral surfaces
• Adults have 4 pairs of short legs, with females being larger than males
• Males have long strings on the last two pairs of legs
• Larvae create a moulting pocket

Clinical Presentation

• Burrow or tunnel in the skin, which is pathognomonic
• Mite is visible to the naked eye as a small white dot
• Lesions are initially erythematous, then become papular, and eventually crusty
• Alopecia, skin thickening, and pruritus occur
• Intense itching, especially at night, and over most of the body

Complications

• Secondary bacterial infection
• Nephritogenic streptococcal strains may colonize scabietic lesions, leading to acute glomerulonephritis
• Eczema may be prominent in active scabies and continue after treatment
• Acarophobia

Demodex (Follicle Mites)

• Small, worm-like mites
• Striped abdomen
• 4 pairs of legs, segmented to 5 circles
• Found in hair follicles, feeding on sebum
• Common in the nose, eyebrows, cheeks, and ears

Rosacea

• Chronic inflammatory eruption of the face, especially the central regions
• Affects up to 3% of the world's population
• Microorganisms cover the skin surface and reside deep in hair and glands
• Erythema, papules, pustules, telangectasia, and hypertrophy of the sebaceous glands

House Dust Mites

• Live on skin and sleep furniture
• Feed on dead skin layers
• Proteins, excrements, and allergens can cause allergic reactions
• Approximately 19,000 mites in each gram of house dust
• Control measures include washing bed furniture in hot water, using dehumidifiers, and cleaning the house regularly

Ticks

• Classified into two families: Ixodidae (hard ticks) and Argasidae (soft ticks)
• Segmentation is reduced externally
• Body consists of two parts: gnathostoma (capitulum) and idiosoma
• Capitulum consists of feeding appendages
• Important to humans due to exsanguination, dermatosis, paralysis, and otoacariasis

Medical Importance in Iran

• Tick paralysis: affects talking, walking, breathing, and dysphagia
• Crimean-Congo hemorrhagic fever: transmitted by Hyalomma and Argas spp.
• Tick-borne relapsing fever: caused by Borrelia persica, transmitted by ticks and rodents

Ticks Control

• Repellents
• Removing ticks
• Insecticides: Permethrin, Deltamethrin, Diazinon, and Malathion
• Impregnated ear tags and collars
• Powder spraying and dipping animals

Lice

• Classified into two suborders: Anoplura (sucking lice) and Mallophaga (chewing lice)
• Sucking lice: Pediculidae (human lice) and Pthiridae (crab lice)
• Chewing lice: feed on sloughed epidermal tissues, parts of feathers, and sebaceous secretions
• Lice morphology: all are wingless, body flattened in a dorsoventral axis, short stout legs with strong claws
• Life cycle: egg, nymph, and adult

Sucking Lice

• Pediculidae (human lice): Pediculus humanus (body louse) and Pediculus capitis (head louse)
• Pthiridae (crab lice): Pthirus pubis (pubic lice)
• Body lice: prefer cloth fibers to hair, usually stay on clothing, and can be vectors of pathogens
• Head lice: suck blood from scalp, lay eggs on hair, and are easily spread by close contact
• Pubic lice: typically found in pubic region, armpits, and rarely beards or eyelashes, and are spread by close body contact

Lice Control

• Body lice: change and wash clothing in hot water, and use insecticides like DDT, malathion, and permethrin
• Head lice: soap and water washing, combing to remove nits, and using insecticides
• Pubic lice: insecticidal emulsions and lotions

Fleas

• No details provided in the text### Siphonaptera (Fleas)

• Parasitic on birds and mammals

• All adults have piercing and sucking mouth parts, feeding on blood

• Females require blood to lay eggs

• Most species are not host-specific, feeding on several hosts

• Considered annoying pests due to blood sucking, skin burrowing, and transmission of diseases and parasites

Characteristics of Fleas

• Adults are tan to dark brown in color
• Contain numerous bristles and spines
• Small, laterally flattened, and wingless
• Large hind legs enable spectacular jumps

Importance of Fleas

• Most fleas are occasional biters, causing minor itching and little damage to skin
• Two exceptions:
  • Sticktight flea (Echidnophaga gallinacea) attaches permanently to chicken skin, causing ulcerations
  • Female chigoe flea (Tunga penetrans) penetrates human skin
• Disease transmission is a significant concern

Tungidae (Jigger or Chigger Fleas)

• Only one species: Tunga penetrans
• Introduced from South America in the 17th century
• Very small size