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PARASITES
PROTOZOA



Trypanosoma

  • Protozoal parasites found in the blood and tissues of vertebrates
  • Worldwide distribution
  • Causes sleeping sickness in humans
  • Particularly affect sub-Saharan Africa
    • Affect cattle production
    • Cause Nagana (depression)
  • Divided into two groups depending on the mode of development of the insect vector
    • Salivarian
      • Multiply in the foregut
      • Transmitted via innoculation via feeding
    • Stercorarian
      • Multiply in the hindgut
      • Transmitted via contamination of wounds with insect faeces

Life Cycle

  • Undergo morphological transformations in intermediate host before becoming infective for the next host
  • Blood-sucking flies ingest trypanosomes whilst taking a blood meal from an infected animal
    • Trypanosomes multiply first in the gut of the fly
  • Salivarian trypanosomes are transmitted by Tsetse flies
    • Trypanosomes pass foward to the salivary glands where they transform into the infective stage
    • Innoculated with saliva when Tsetse fly next feeds on a host
  • Stercorarian trypanosomes are transmitted by triatomid bugs, tabanids and keds
    • Trypanosomes pass back to the rectum
    • Next host is infected when skin wounds are contaminated with infected insect faeces

Pathogenesis

  • Salivarian
    • Causes wasting disease in cattle (nagana)
    • Sleeping sickness in humans
  • Stercorarian
    • T. cruzi most important in veterinary medicine
      • Occurs in South America
      • Infects armadillos, possums and humans
      • Causes Chagas Disease
    • Transmitted by a triatomid (kissing) bug
    • Chronic infections are often fatal causing heart failure
    • Non-pathogenic species are transmitted by tabanids and keds
      • T. theileria and T. melophagium
  • Anaemia
    • Red blood cells removed from circulation
  • Degeneration and inflammation of multiple organs
    • E.g. Skeletal muscle, myocardium and CNS

Epidemiology

  • Vector distribution
  • Parasite virulence
    • Some parasitaemic animals survive for long periods of time
      • E.g. T. brucei and T. congolense
      • Increases the opportunity for infection of flies
    • Some trypanosomes kill their host in 1-2 weeks
      • E.g. T. vivax
      • Decreases the chances of fly infection
    • Trypanosomes avoid host immune defences by altering glycoprotein coat (surface antigen) before host antibody response
      • Antigenic variation can occur many times over several months causes relapsing parasitaemia
  • Host response
    • Trypanotolerant wild animals remain parasitaemic for prolpnged periods without showing clincial signs of disease
      • Cause lasting reservoirs of infection
    • Most domestic livestock are susceptible to trypanosomosis
    • Some local breeds of sheep, goats and cattle are trypanotolerant
      • E.g. Bos indicus

Diagnosis

  • Demonstrate trypanosomes in blood
    • Giemsa stained smears
    • Fresh blood films
      • Motile trypanosomes
    • Haematocrit tube
      • Motile trypanosomes at the plasma/buffy coat interface

Control

  • Prophylactic drug treatment
    • Change drug group periodically to decrease the chances of resistance occuring
    • May lead to protective immunity but livestock will still be susceptible to heterologous challenges
  • Barrier fences and buffer zones
    • Separate livestock and wild animals
  • Trypanotolerant livestock

Other trypanosomes

  • Mechanically transmitted by biting flies
    • E.g. Surra affecting horses and camels in North Africa, Asia and South America
    • T. equinum in South America
    • T. evansi in Asia
  • Venerally transmitted
    • E.g. Dourine
      • Transmitted by T. equiperdum
      • Causes genital and abdominal oedema, emaciataion and CNS signs
      • Affects horses and donkeys in Africa, Asia, Central and South America
  • Non-pathogenic species occur in the UK
    • In sheep caused by T. melophagium
    • In cattle caused by T. theileri

Leishmania

Life Cycle

Pathogenesis

Leishmania involved in skin infections

Epidemiology

Diagnosis

Treatment and Control