Parasitic arthropods II

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Parasitic arthropods II

• Arthropods as vectors of disease.

Dr. Jo Hamilton Parasitology BS
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Introduction.

• As we saw in the last lecture arthropods can be
  considered as ectoparasites in their own right
  some arthropods have capacity to become vectors
  of viral, bacterial, protozoal helminth
  diseases.
• During this lecture we will investigate
  arthropods as vectors of disease.

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Objectives and learning outcomes.

• By the end of this session students should be
  able to
• Appreciate that parasitic arthropods can be
  parasites in their own right or vectors of
  disease.
• Describe some of the main vectors of medical and
  veterinary importance the diseases they
  transmit.
• Give named examples of arthropod disease vectors.
• Appreciate the economic importance the
  implications for public health of these arthropod
  vectors.

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Arthropods.

• As we saw previously 3 of the 4 arthropod groups
  contain parasitic species.
• Some of these are important not only as ecto- or
  endo- parasites in their own right but as vectors
  of disease.
• We will consider vectors of medial veterinary
  importance. During this lecture we will consider
• Insecta (Lice, Bugs, Fleas Flies)
• Arachnida (Ticks Mites)
• Information regarding Crustaceans as disease
  vectors can be found in the lecture notes.

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Parasitic arthropods - lice.

• Class Insecta, Order Anoplura, Family Pediculidae
  - the lice.
• Until recently in human history, lice were such
  common companions of Homo sapiens that they were
  considered one of life's inevitable nuisances for
  rich and poor alike.
• Host specific.
• Lice still very common in developing countries.
• Greyish in colour, dorso-ventrally flattened,
  both sexes feed on blood through mouthparts
  designed for piercing and sucking.
• They are apterous (wingless).

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Parasitic arthropods -lice.

• Two species parasitize humans
• Pediculus humanus (head body lice).
• Phthirus pubis (crab louse).
• Pediculus humanus.
• Two forms
• Pediculus humanus humanus (sometimes called
  Pediculus humanus corporis - body louse).
• Pediculus humanus capitis (head louse).

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Parasitic arthropods -lice.

• Body lice.
• Body lice spend most of the time in their host's
  clothing.
• Eggs or nits are cemented to fibres in the
  clothes
• Females lay about 9-10 eggs per day (about 300 in
  her lifetime)
• Crowding conditions facilitate transmission of
  lice (e.g. poverty, refugee situations, mass
  migration, civil unrest etc).

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Parasitic arthropods -lice.

• Head Lice.
• Smaller that body lice.
• Nits are cemented to hairs.
• More prevalent on the back of the neck, behind
  the ears.
• Not life threatening but causes considerable
  discomfort.
• Scratching leads to dermatitis or secondary
  infection.
• Matted hair

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Parasitic arthropods -lice.

• Phthirus pubis - Crab louse or crabs.
• Dwells mainly in the pubic region but it could be
  found in the armpits, beard, moustache, eyebrows
  and eyelashes.
• Nits are cemented to the hair.
• Females deposit about 30 eggs during her life.
• Infection is through contact with bedding or
  other objects, especially in crowded conditions.
• Transmission is often venereal.

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Parasitic arthropods -lice.

• Lice as vectors of disease.
• Typhus.
• Caused by Rickettsia prowazekii.
• Epidemics associated with conditions that favour
  heavy infestations of body lice.
• Played major role in important wars.
• Expulsion of the Moors in Spain, Thirty year war,
  World War I (25 million cases, 3 million
  deaths).1971- over 10,000 cases reported, about
  100 deaths.
• Typhus is fatal for lice.

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Parasitic arthropods -lice.

• Trench fever.
• Caused by another Rickettsia (Rochalimaea
  quintana).
• Debilitating disease transmitted by Pediculus
  humanus humanus.
• Epidemics during World War I and II.
• Long convalescence.

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Parasitic arthropods -lice.

• Relapsing fever.
• Cause by a spirochete, Borrelia recurrentis.
• 1971- 4700 cases, 29 deaths.
• Mostly South America, Africa and Asia.
• Control of lice louse-borne disease
• Insecticides.
• Personal Hygiene.
• Laundering of garments.
• N.B. In conditions that favour heavy louse
  infestation (war, mass migration, refugee
  situations etc.) hygiene laundry may be
  impossible or low priority.

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Parasitic arthropods - flies.

• Class Insecta, Order Diptera the flies.
• Diptera are the most medically important insects
  causing more than a million deaths each year.

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Parasitic arthropods - flies.

• Class Insecta, Order Diptera (flies), Family
  Culicidae (the Mosquitoes).
• Mosquitoes most widespread important vectors of
  human and animal diseases.
• There are two blood-feeding subfamilies of
  mosquitoes, Anophelinae and Culicinae.
• Vectors of protozoa, filarial nematodes,
  arboviruses.
• Only female feeds on blood. Male requires only
  sugar meal for survival.

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Parasitic arthropods - flies.

• Family Culicidae , subfamily Culicinae, Genus
  Culex.
• Members of this genus are the main vectors of
  West Nile Virus.
• Culex pipiens (house mosquito) main vector of
  Western equine encephalitis St. Louis
  encephalitis.
• Culex spp. are vectors of the filarial nematodes
  Dirofilaria immitis Wuchereria bancrofti

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Parasitic arthropods - flies.

• Family Culicidae, subfamily Culicinae, Genus
  Aedes.
• Vector of disease, mainly viruses.
• Aedes aegypti also known as Yellow fever
  mosquito.
• Yellow fever is a very serious zoonosis
  responsible for millions of deaths during
  epidemics.
• Also a vector for Dengue or breakbone fever.
• 4 strains varying severity. Dengue hemorrhagic
  fever severe form of disease - skin haemorrhages
  significant mortality.

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Parasitic arthropods - flies.

• Family Culicidae, subfamily Anophelinae, Genus
  Anopheles - mosquitoes as vectors of protozoal
  parasites.
• Female Anopheles mosquitoes ingest Plasmodium
  gametocytes from infected human hosts and the
  sexual phase of the parasite occurs within the
  gut, resulting in thousands of sporozoites being
  released into the body cavity.
• The sporozoites find their way to the arthropod's
  salivary glands where they are transmitted to
  human while the mosquito takes a blood feed.

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Parasitic arthropods - flies.

• Within the host red cells Plasmodia reproduce
  asexually eventually burst from the erythrocyte
  invade other uninvolved red cells.
• This event produces periodic fever anaemia in
  the host, a disease process known as malaria.
• The four spp. causing malaria in humans are
  Plasmodium vivax, Plasmodium ovale, Plasmodium
  malariae, Plasmodium falciparum.

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Parasitic arthropods - flies.

• Family Psychodidae (the sandflies) - Sandflies as
  vectors of protozoal disease leishmanisis.
• Localized cutaneous lesions or ulcers are
  produced by infection with one of the Leishmania
  sp.
• L. tropica in the Old World L. mexicana in the
  New World.
• L. braziliensis is the cause of American
  mucocutaneous leishmaniasis (espundia), L.
  donovani of kala azar, a disseminated visceral
  disease.

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Parasitic arthropods - flies.

• All four species transmitted by nocturnally
  feeding phlebotomine sandflies.
• These sandflies are small, delicate, short-lived
  biting insects, found in animal burrows
  crevices throughout tropics subtropics.
• The blood meal transfers Leishmania to the flies,
  where the parasites multiply within the gut
  mature.
• The mature promastigotes are injected into skin
  of new mammalian host where they are taken up and
  multiply within macrophages.

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Parasitic arthropods - flies.

• The disease is a zoonotic infection of tropical
  subtropical rodents particularly common in areas
  of China, India, Asia Minor, Africa, around the
  Mediterranean, Central America.
• In the Mediterranean area, southern Russia,
  India, human disease involves urban transmission,
  with the domestic dog serving as the reservoir.
• Direct human-to-human transmission also occurs.
• Sandflies can also be vectors of arboviruses (see
  lecture notes for details).

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Parasitic arthropods - flies.

• Family Simuliidae (the blackflies).
• Certain species of adult blackflies of the genus
  Simulium responsible for transmission of the
  filarial nematode, Onchocerca volvulus, the
  causative agent of river blindness in central
  and western Africa.
• Certain species can also transmit various forms
  of cattle onchocerciases.

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Parasitic arthropods - flies.

• Family Simuliidae (the blackflies).
• Important vectors species include
• Simulium damnosum - important vector species in
  Africa.
• Simulium ochraeum - important vector species in
  New World
• Simulium venustum - notorious pest species in
  North America.
• Simulids are also vectors of Leucocytozoon spp.,
  protozoans causing "malaria" in birds. Some
  species cause important diseases of domestic
  poultry, notably turkeys and ducks.

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Parasitic arthropods - flies.

• Suborder Brachycera, Family Tabanidae the
  horseflies deerflies.
• Tabanids are mechanical vectors of important
  bacterial pathogens such as agents of anthrax
  (Bacillus anthracis) tularemia (a disease
  caused by the bacterium Francisella tularensis).
• Pathogens are carried directly on the mouthparts,
  there is no multiplication of bacteria in the
  fly's body (refer to definition of mechanical and
  biological vectors in lecture notes).

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Parasitic arthropods - flies.

• Tabanids are also vectors of Loa loa, a filarial
  nematode.
• Humans are the only host.
• Loa loa is distributed throughout tropical
  Africa. Worms live in subcutaneous tissue and the
  migration of worm causes pain and tissue
  swelling.
• This parasite is often called the "eye worm"
  because worms migrating through the conjunctiva
  of the eye are visible.
• Vectors are species of Chrysops (deerflies). C.
  dimidiata is the most important.

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Parasitic arthropods - flies.

• Family Glossinidae, genus Glossina - the tsetse
  flies.
• Tsetse's are hosts vectors of trypanosome
  protozoans of the genus Trypanosoma brucei
  species complex which cause serious disease in
  humans and cattle.
• These are invasive blood tissue parasites that
  produce highly morbid, frequently lethal diseases
  via insect host transmission.
• The diseases caused by trypanosomes are limited
  to the semitropical tropical niches of the
  tsetse host.

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Parasitic arthropods - flies.

• Trypanosoma brucei gambiense causes "sleeping
  sickness" in humans, an often fatal disease
  characterized by lethargy leading to coma.
• T. brucei rhodesiense is more rapidly pathogenic
  in humans, with death occurring within a few
  weeks before coma ensues.
• T. brucei brucei causes a fatal disease in cattle
  called nagana.
• The human forms cause zoonotic diseases wild
  ungulates are the reservoir hosts are not
  affected by the parasites.

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Parasitic arthropods - flies.

• Tsetse flies are of as much concern
  agriculturally as they are medically.
• Infected flies render millions of square
  kilometres of land unusable for cattle-rearing.
• However, because ranching has been precluded it
  can be argued that the prevalence of tsetse and
  the trypanosomes has protected Africas big
  game animals.

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Parasitic arthropods -bugs.

• Class Insecta, Order Hemiptera (bugs). Family
  Reduviidae (kissing or assassin bugs).
• Vectors of Trypanosoma cruzi (Chagas disease).
• Bugs of the genera Triatoma Rhodinus are
  responsible for transmission of Chagas Disease
  (South American trypanosomiasis) caused by the
  protozoan Trypanosoma cruzi.
• Both sexes feed at night on blood using piercing
  and cutting mouthparts. Some species
  particularly like to bite the face lips - nasty
  considering these bugs can be up to 5 cm in
  length!

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Parasitic arthropods -bugs.

• Up to a third of a gramme of blood can be
  consumed per feed.
• Transmission of T. cruzi to the bug is
  accomplished through the mouthparts, but
  transmission to man occurs by scratching the
  faeces (containing the parasite) of the bug into
  the skin.
• Vectors often defecate while feeding - infective
  protozoans in faeces can enter the wound (bite
  site) when scratched.

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Parasitic arthropods -bugs.

• Infected bugs may remain infectious for up to 2
  years.
• Also possible to contract the disease through
  blood transfusion of poorly monitored blood.
• Endemic areas include Central South America.
• The trypomastigotes disseminate from the site of
  inoculation to circulate in the peripheral blood
  of their human hosts.

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Parasitic arthropods -bugs.

• Clinically, the infection presents as an acute
  febrile illness in children a chronic heart or
  gastrointestinal illness in adults.
• Control of reduviid bugs Chagasdisease.
• Improved construction,
• Insecticides.
• Replacement of dirt floors with cement.

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Parasitic arthropods - ticks.

• These are epidermal parasites, large in size,
  (macroscopic).
• Feed on blood.
• Can be vectors of disease
• Infestation with ticks can result in
• Anaemia- Blood loss due to heavy infection can be
  considerable.
• Dermatitis -Inflammation, swelling, ulcerations
  itching.
• Paralysis -Tick paralysis common in humans when
  they are bitten near the base of the skull.
• Otoacariasis- Infestation of the ear canal.

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Parasitic arthropods - ticks.

• Ticks can transmit viruses, bacteria protozoa.
• Family Ixodidae - Hard ticks.
• Hard ticks are of economic importance in the
  transmission of a variety of diseases.
• The tick genera Rhipicephalus, Hyalomma
  Amblyomma transmit Theileria parva (similar life
  cycle to malaria also called East Coast Fever)
  a parasite of livestock Boophilus transmits
  Babesia (similar to Theileria) to small mammals
  and dogs.

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Parasitic arthropods - ticks.

• Ixodes spp. are the principal vectors of Lyme
  Disease caused by the spirochaete Borrelia
  burgdorferi.
• Common in the Northeastern USA
• Leading arthropod transmitted infection in the
  USA.
• Disease is transmitted to humans by the bite of a
  small tick of the genus Ixodes and is
  characterized by the "bulls-eye" lesions that
  begin at the site of the tick bite.
• Sequelae may include arthralgia, arthritis
  other immune complex deposition manifestations.
• Mice and deer constitute the main animal
  reservoir of B. burgdorferi within ticks.

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Parasitic arthropods - ticks.

• The rickettsial disease Rickettsia rickettsii
  (Rocky Mountain Spotted Fever) is transmitted by
  Dermacentor.
• Decamentor andersoni is the main vector of
  Colorado tick fever virus, Rocky Mountain spotted
  fever.

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Parasitic arthropods - ticks.

• Family Argasidae - Soft ticks.
• The tick Ornithodoros is a vector for the
  spirochaete Borrelia duttoni the plague-like
  bacterium Francisella tularensis that causes
  tularemia.
• Ornithodoros hermsi is the mains vector for
  Borrelia recurrentis (relapsing fever)
• Ornithodoros has been kept for 19 years alive in
  captivity.

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Parasitic arthropods - fleas.

• Order Siphonaptera ( the fleas).
• One of the most important insect pests.
• They have piercing/sucking mouthparts that they
  use to find a small blood vessel.
• Saliva is injected into the area near the
  puncture site.
• Eggs can remain dormant for up to 200 days.
• Fleas can survive long periods as adults without
  food.
• Fleas are not very host specific.

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Parasitic arthropods - fleas.

• Fleas can be vectors of human disease including
• Plague (pest or Black Death).
• Caused by Yersinia pestis
• Bacterium releases potent toxins that act on
  mitochondrial membranes, inhibiting ion uptake
  interfering with the normal functioning of the
  respiratory chain.

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Parasitic arthropods - fleas.

• Murine (Endemic) Typhus.
• Flea-borne typhus.
• Caused by Rickettsia mooseri.
• Similar to louse-borne typhus.

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Parasitic arthropods - fleas.

• Fleas as vectors of helminth diseases.
• Some fleas are intermediate vectors of the
  cestodes Diplylidum caninum (dog parasite) and
  Hymenolepis diminuta (rodent parasite).
• Eggs of these helminths are present in the faeces
  of dogs and are ingested by larval fleas.
• The mammalian host during grooming ingests the
  flea and thus becomes infected.
• Fleas are the primary intermediate host of a very
  common tapeworm of domestic pets, Dipylidium
  caninum.

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Summary.

• By the end of this session you should be able to
• Appreciate that parasitic arthropods can be
  parasites in their own right or vectors of
  disease.
• Describe some of the main vectors of medical and
  veterinary importance the diseases they
  transmit.
• Give named examples of arthropod disease vectors.
• Appreciate the economic importance the
  implications for public health of these arthropod
  vectors.
• Please see the lecture notes for additional
  information suggested additional reading.