Transmission of Parasites
It depends upon:
- Source or reservoir of infection
- Mode of transmission.
Sources of Infection
- Man : Man is the source or reservoir for a majority of parasitic infections (e.g., amoebiasis, enterobiasis, etc.) The infection transmitted from one infected man to another man is called as anthroponoses
- Animal: The infection which is transmitted from infected animals to humans is called as zoonoses.
The infection can be transmitted to humans either directly or indirectly via vectors. (e.g., cystic echinococcosis from dogs and toxoplasmosis from cats)
- Vectors: Vector is an agent, usually an arthropod that transmits the infection from one infected human being to another.
NOTE : Vector can be biological or mechanical. An infected blood sucking insect can transmit the parasite directly into the blood during its blood meal.
- Contaminated soil and water: Soil polluted with human excreta containing eggs of the parasites can act as an important source of infection, e.g., hookworm, Ascaris species, Strongyloides species and Trichuris species.
Water contaminated with human excreta containing cysts of E. histolytica or Giardia lamblia, can act as source of infection
- Raw or under cooked meat: Raw beef containing the larvae of Cysticercus bovis and pork containing Cysticercus cellulosae are some of the examples where undercooked meat acts as source of infection
6. Other sources of infection: Include Fish, crab or aquatic plants, etc.
Modes of Transmission
The infective stages of various parasites may be transmitted from one host to another in the following ways:
- Oral or feco-oral route: It is the most common mode of transmission of the parasites. Infection is transmitted orally by ingestion of food, water or vegetables contaminated with feces containing the infective stages of the parasite. (e.g., cysts of E. histolytica, and ova of Ascaris lumbricoides) .
- Penetration of the skin and mucous membranes: Infection is transmitted by the penetration of the larval forms of the parasite through unbroken skin (e.g., filariform larva of Strongyloides stercoralis and hookworm can penetrate through the skin of an individual walking barefooted over fecally contaminated soil), or by introduction of the parasites through bloodsucking insect vectors. (e.g., Plasmodium species, Leishmania species and Wuchereria bancrofti).
- Sexual contact: Trichomonas vaginalis is the most frequent parasite to be transmitted by sexual contact. However, Entamoeba, Giardia and Enterobius are also transmitted rarely by sexual contact among homosexuals.
- Bite of vectors: Many parasitic diseases are transmitted by insect bite such as: malaria (female anopheles mosquito), filariasis (Culex), leishmaniasis (sand-fly), Chagas’ disease (reduviid -bug) and African sleeping sickness (tsetse fly).
- Vertical transmission: Mother to fetus transmission is important for few parasitic infections like Toxoplasma gondii, Plasmodium spp. and Trypanosoma cruzi.
- Blood transfusion: Certain parasites like Plasmodium species, Babesia species, Toxoplasma species, Leishmania species and Trypanosoma species can be transmitted through transfusion of blood or blood products.
- Autoinfection: Few intestinal parasites may be transmitted to the same person by contaminated hand (external autoinfection) or by reverse peristalsis (internal autoinfection). It is observed in Cryptosporidium parvum, Taenia solium, Enterobius vermicularis, Strongyloides stercoralis and Hymenolepis nana.
Life cycle of parasites
The life cycle of the parasite may be direct (simple) or indirect (complex).
1. Direct/simple life cycle: When a parasite requires only one host to complete its development, it is referred as direct/simple life cycle.
2. Indirect/complex life cycle: When a parasite requires two hosts (one
definitive host and another intermediate host) to complete its development, it is referred as indirect/complex life cycle .
NOTE : Some of the helminths require three hosts (one definitive host and two intermediate hosts).
Phylum Protozoa
General Features
- The single protozoal cell performs all functions.
- Most of the protozoa are completely nonpathogenic but few may cause major diseases such as malaria , leishmaniasis, and sleeping sickness. Protozoa like Cryptosporidium parvum and Toxoplasma gondii are being recognized as opportunistic pathogens in patients affected with human immune-deficiency virus (HIV) and in those undergoing immunosuppressive therapy.
- Protozoa exhibit a wide range of a size (1- 150 µm), shape, and structure; yet all possess essential common features.
- Single-celled microorganisms belonging to the animal kingdom are classified as Protozoa (Greek Protos—first; zoon—animal). Within its single cell, the protozoon contains all structures required for performing its various functions.
- Some free-living protozoa resemble plants containing green plastids that enable them to perform photosynthesis. It is believed that these represent the earliest forms of animal life.
- Numerous varieties of protozoa have evolved to suit all manner of environmental conditions. Free-living protozoa are found in all habitats—in the deep ocean or in shallow freshwaters, in hot springs or in ice, under the soil, or in the snow on mountain tops.
- Parasitic protozoa have however adapted to different host species, with more restricted physicochemical requirements.
- Protozoa exhibit a wide range of size, shape, and structure, yet all possess certain essential common features.
- The typical protozoan cell is bounded by a trilaminar unit membrane, supported by a sheet of contractile 6 fibrils that enable the cell to change its shape and to move.
- The cytoplasm can often be differentiated into an outer rim of relatively homogeneous ectoplasm and a more granular inner endoplasm. The ectoplasm serves as the organ of locomotion and for engulfment of food materials by putting forth pseudopodial processes. It also functions in respiration, discharging waste materials, and also as a protective covering for the cell. Within the endoplasm is the nucleus within a tough nuclear membrane.
- The nucleus is usually single but maybe double or multiple, some species having as many as a hundred nuclei in one cell. The nucleus contains one or more nucleoli or a central endosome or karyosome.
The chromatin may be distributed along the inner surface of the nuclear membrane (peripheral chromatin) or as condensed masses around the karyosome.
- The endoplasm shows a number of structures-the endoplasmic reticulum, mitochondria, and Golgi bodies. Contractile vacuoles may be present which serve to regulate the osmotic pressure. Several food vacuoles also may be seen.
- The active feeding and growing stage of the protozoa are called the trophozoite (G. trophos-nourishment).
The cell may obtain nourishment from the environment by diffusion or by active transport across the plasma membrane. Larger food particles are taken in by phagocytosis through pseudopodia.
Some species ingest food through special mouth-like structures or cytostomes.
Minute droplets of food may also enter by pinocytosis. Several species possess a resting or resistant cystic stage which enables prolonged survival under unfavorable conditions.
- The cystic stage may also involve reproduction by the nucleus dividing once or more to give rise to daughter 7 trophozoites on excystation.
- The cyst is usually the infective stage for the vertebrate host. Reproduction is usually asexual.
The most common method is binaryfission by the mitotic division of the nucleus, followed by the division of the cytoplasm. In amoebae, division occurs along any plane, but in flagellates, the division is along the longitudinal axis and in ciliates in the transverse plane.
- Some protozoa, as for instance the malaria parasites exhibit schizogony in which the nucleus undergoes several successive divisions within the schizont to produce a large number of merozoites.
Sexual stages are seen in ciliates and Sporozoa.
- In ciliates, the sexual process is conjugation in which two organisms join together and reciprocally exchange nuclear material.
- In Sporozoa, male and female gametocytes are produced, which after fertilization form the zygote giving rise to numerous sporozoites by sporogony.
