Infective Stages of the Parasites (with examples)

Organisms that live on or inside a host organism deriving nutrients from the host and often causing harm, can cause parasitic infections. Parasites can belong to various biological groups, including protozoa, ectoparasites (such as ticks and lice), and helminths (worms). These infections can affect humans and animals, and they are prevalent in different parts of the world, specifically in areas with poor sanitation and limited access to healthcare.

Parasitic infections go through various stages as they progress through their life cycle, often involving both the host organism (where the parasite resides) and the environment. The stages include infestation, entry into the host, colonization/reproduction, development, migration, evading the host immune response, release of infective forms, transmission to a new host, or survival in the environment.

The infective stage of a parasitic organism’s life cycle refers to the developmental stage at which the organism can infect a host and initiate a new cycle of its life. The specific characteristics of the infective stage vary among different parasites, and understanding these stages is crucial for comprehending the transmission dynamics and implementing effective control measures.

Sporozoite (infecive stages)
Sporozoite of Malarial Parasite



Sprozoites have a special predilection towards the salivary glands, so their concentration reaches a maximum in salivary ducts. When an infected female anopheles mosquito bites a susceptible person,  a single bite is capable of transmitting infection.

Entamoeba histolytica

Mature Quadrinucleate Cyst of Entamoeba histolytica
Mature Quadrinucleate Cyst of Entamoeba histolytica

Mature Quadrinucleate Cyst

These cysts are the infective forms of Entamoeba histolytica. When these cysts are swallowed along with the contaminated food and drink by susceptible persons, they are capable of further development and can cause Amoebic dysentery, Amoebic liver abscess, etc.

Leishmania donovani


Transmission of L. donovani from man to man is carried out by a certain species of sandfly of the genera Phlebotomus and Lutzomyia.

Sandfly introduce promastigote in the skin of the mammalian host which is later engulfed by reticuloendothelial cells, where it converts into intracellular amastigote form. Promastigotes are motile, elongated, extracellular stages of Leishmania donovani, having a single free flagellum. Leishmania donovani causes visceral leishmaniasis (kala-azar). Other species of Leishmania cause cutaneous and mucocutaneous leishmaniasis.

Promastigote form of Leishmania
Promastigote form of Leishmania


Filariform larvae

In Hookworm hatched L3  filariform larvae is the infective stage, transmission to humans is mainly via active skin penetration by the larvae or sometimes by oral route. Hyaluronidase activity of larvae facilities breakage of hyaluronic acid and passage through epidermis and dermis.

Toxoplasma gondii

Toxoplasma gondii is a protozoan parasite that has three different infective stages;

  • invasive tachyzoites (rapidly dividing found in groups or clones)
  • the bradyzoites in tissue cysts (slowly dividing), and
  • the sporozoites in oocysts (environmental stage).

Other parasites and their infective stages are: 

  1. Ascaris lumbricoides (GI parasite): Mature fertilized egg (which contains Rhabiditiform larvae inside)
  2. Wuchereria bancrofti (Filariasis): Third larval Stage (L3)


  2. Siński E. (2003). Environmental contamination with protozoan parasite infective stages: biology and risk assessment. Acta microbiologica Polonica, 52 Suppl, 97–107. 
  3. Koprivnikar, J., Thieltges, D. W., & Johnson, P. T. J. (2023). Consumption of trematode parasite infectious stages: from conceptual synthesis to future research agenda. Journal of helminthology, 97, e33.

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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