Trypanosoma

Introduction

• These parasites are usually found in blood and tissue fluids as intercellular parasite. A few invade tissues and occurs as intracellular parasites like cruzi
• They may be polymorphic or monomorphic.

1. Polymorphic: It has numerous forms as elongated, epimastic and metacyclic form in insect host
2. Monomorphic: Does not change the shape and size throughout their development. Transmitted by insect vectors.

Life cycle of Trypanosoma/ Transmission of infection

• Except, equiperdum of equines, all trypanosomes have arthropod vectors in which transmission is either cyclical or non-cyclical.

Cyclical transmission

• This development requires insects as an intermediate host.
• In insects, trypanosomes multiply and transformed into epimastigote form and then turn into metacyclic infective trypanosomes.
• This transmission occurs in two sites within insects, the fore gut ( technically referred as anterior stain development) or hind gut ( posterior stain development).

I. Anterior stain development

• Multiplication occurs in proboscis and or salivary gland extending upto fore gut. So, these trypanosomes are grouped as ‘salivarian trypanosomes’.
• New infection with metacyclic trypanosome is transmitted by inoculation through fly bite.
• Kinetoplast is small and more terminal or sub-terminal, posterior extremely blunt, may be flagellum free. Undulating membrane is well developed.
• This group of trypanosomes includes:

a. Brucei group

b. Congolense group

c. Vivax group

II. Posterior stain development

• Multiplication and transformation occur in hind gut and infective forms migrate to rectum and passed in faeces. So, these trypanosomes are grouped as ‘stercorarian trypanosome.
• In domestic animals, these are all relatively non-pathogenic.
• Transmission by tabanid flies , reduviid bugs.
• This group of trypanosomes include : theileiri, T. melophagium, T. cervi, T. lwesii, T. musculi, T. cruzi.
• Parasite consist of a large kinetoplast which is not in terminal position.
• Posterior extremity is tapering
• Free flagellum is present
• Undulating membrane is not well developed

Mechanical ( non-cyclical transmission)

• In this transmission, trypanosomes are transferred from one mammalian host to another by interrupted feeding of biting insects.
• Trypanosomes in or on the contaminated proboscis don not multiply and die-quickly. Due to this, cross-transmission is only possible for few hours.

On the basis of type of cyclical development, Trypanosoma is divided into two section:

a. Salivaria:

• Four subgenera i.e. Duttonesia, Nannomonas, Pycnomonas and Trypanozoon are recognized.
• Species in this group complete their development in ‘anterior stain’ and transmission is through inoculation of metacyclic stage.
• These trypanosomes are elongated spindle-shaped protozoa ranging from 8-39 µm long
• Posterior end of the body is usually blunt
• All of this parasite possess flagellum, which arises at the posterior end of trypanosome from a basal body at the foot of flagellar pocket.
• The flagellum runs to the anterior end of the body and is attached along its length to the pellicle to form an undulating membrane.
• Single centrally placed nucleus can be seen when stained adjacent to flagellar pocket, there is small structure called kinetoplast, which contains DNA.

b. Stercoraria:

• Consists of four sub-genera : Megatrypanum, Herpetosoma, Schizotrypanum and Only first three have species of veterinary and medical importance.
• Species in this group complete their development in ‘posterior stain’. Host gets infection by contaminated route.
• Free-flagellum is always present in tryptomastigote.
• Kinetoplast is large and not terminal.
• Posterior end of body is pointed.
• Multiplication in mammalian host is discontinuous. They takes place in epimastogote or amastigote stage.

Developmental stage

a. Inside definitive host

• Host gets infection after inoculation of metacyclic form of parasite into blood.
• After inoculation, they change into trypomastigote form in blood.
• Trypomastigote multiplies by binary fission. First, kinetoplast divides followed by nucleus and then cytoplasm.
• Flagellum do not divide and remains attached to one individual. Second individual develops new flagellum.
• Later trypomastigote form change into intermediate short stumpy form. This is carried by insect host.

b. Development inside vector

• evansi does not undergo cyclical development and transmitted mechanically.
• Cyclical development undergoes in two sites: Anterior station and posterior station.

In case of anterior station

• Three groups of Trypanosome develop in this station
• Brucei develops in midgut, proventriculus and salivary glans.
• Congolence group develops in midgut and proboscis.
• Vivax group develop only in proboscis
• Glossina flies acts as intermediate host.
• Stumpy form of trypanosome carried by insect change into elongated forms of trypnomastigote.
• Trypomastigote then multiplies by binary fission, leave gut and change into epimastigote.
• Epimastigote then change into metacyclic form of trypomastigote after multiplication by binary fission.

In case of posterior station

• Only one group lewis ; develops in this manner.
• Vector differs according to species such as fleas, bed bug, ‘kissing bug’ ( Triatoma), sheep ked (Melophagus ovinus)
• After taking intermediate stumpy form of Trypomastigote, they change into trypomastigote in hind gut.
• Trypomastigote first changes into epimastigote and then into metacyclic trypanosome.
• These metacyclic trypanosome are paddled out in faeces. Infection occurs after contamination of skin wound by faeces.