Leishmania

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

• Enlarged lymph nodes and spleen
  • Causes lymphoid exhaustion

• Anaemia
  • Red blood cells removed from circulation

• Degeneration and inflammation of multiple organs
  • E.g. Skeletal muscle, myocardium and CNS

Epidemiology

• Vector distribution
  • Tsetse flies found in riverine, savannah and forest habitats
  • Up to 20% flies infected
  • Flies infected for life

• 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

• Tsetse fly control
  • Spraying and trapping

• 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

• Leishmania spp. are intracellular parasites of macrophages

• Are closely related to Trypanosoma spp.

• Cause diseases in humans, dogs and wild animals

• Present in southern Europe, Africa, Asia and South America

Life Cycle

• Transmitted by blood sucking sand flies
  • Phlebotomus spp. in the Old World
  • Lutzomyia spp. in the New World

• The amastigote (morphological form) in found in vertebrate macrophages

• Ingested by sand fly during feeding
  • Transforms in insect gut

• Multiplies and migrates to insect proboscis
  • Innoculated during feeding
  • Can be transmitted percutaneously if sand fly crushed on skin

• Invades macrophages and reverts to amastigote

Pathogenesis

• Infection of vertebrate host
  • Produces foci of proliferating Leishmania-infected macrophages in skin (cutaneous) or internal organs (visceral)

• Very long incubation periods
  • Months to years

• Many infected dogs are asymptomatic

• Cutaneous form
  • Produces areas of ulceration on pinnae of ears

• Visceral form causes chronic wasting condition
  • Generalised eczema
    • Loss of hair around eyes producing 'spectacle' effect
  • Intermittent fever
  • Generalised lymphadenopathy

• Involved in skin infections

Epidemiology

• Disease dependent on sand fly vectors
  • E.g. Common in dogs around the Mediterranean coast, foci around southern Europe and around Madrid

• Reservoirs of infection
  • E.g. Wild animals such as rodents and stray dogs

• Mechanisms of transmission
  • Direct contact
  • sand fly bite

• Leishmaniasis in British dogs
  • Susceptible to infection if exposed whilst abroad in endemic areas as have no immunity
  • No sand flies in Britain but dogs have become infected whilst in contact with infected imported animals

Diagnosis

• Demonstrate Leishmania organisms
  • In skin scraping or smears
  • In lymph node or bone marrow biopsies

Treatment and Control

• Chemotherapy
  • Prolonged treatment, expensive, suppresses infection
  • Does not cure infection

• Prevent sand flies biting
  • Collars, sprays containing insecticide with repellent effect

• Destruction of infected and stray dogs
  • Sand flies biting infected dogs may spread the disease to to other dogs, humans and wildlife
  • There is a slight possibility of transmission to humans by direct contact