This is the association between two organisms in which one (the parasite) is nutritionally dependent on the other (host). The host is harmed in the process.
Parasites are divided into two categories:Endo-parasites; these are parasites that live inside the body of the host, e.g. plasmodium and HIV

Ectoparasites; these are parasites which live outside the body of the host, e.g. ticks, lice and flea.
Parasites can also be described as:
Obligate parasites; these are parasites which cannot live without their hosts. Examples of obligate parasites are plasmodium and HIV.
Facultative parasites; these are parasites that can spend some time outside the bodies of their hosts. E.g. Ticks.
Incidental parasites; these are organisms that are not usually parasite but may become parasitic due to factors like lack of their normal food, increase in their numbers, etc. an example is Entamoeba gingivalis.

Problems faced by parasites
i) Finding the host may be difficult since most hosts keep on moving from one place to another.
ii) Deficiency of food in case the host has similar deficiency.
iii) They may be killed by the hosts’ immune reactions.
iv) Death of parasites incase the host dies due to starvation.
v) Inabilities to live in a wide range of environment since most of them have low power of locomotion i.e. they are not able to live freely.
To overcome some of these problems, the parasites have a number of adaptations so as to cope up with their mode of life.

General adaptations of parasites

They have means of attachment to the host.

They have penetrative devices for entering and feeding on the host

They show degeneration of unnecessary organs and systems to reduce on their body size in order to fit in the host. e.g. eyes

They produce many eggs, seeds or spores to enhance their survival.

They have vector intermediate hosts

They produce resistant stages to survive in periods when they are outside the host
Types of hosts

Intermediate host:
This is the host in which the larvae stage of parasites develops from (secondary host).

Primary host (infinite host):
This is the host in which sexual reproduction of a parasite occurs from.

This is a protozoan parasite that causes malaria. It is transmitted from one person to another by the female anopheles mosquito. The mosquito acts as the vector.

Life cycle of plasmodium

  • Mosquitoes bite a human and inject saliva to stop blood from clotting in its alimentally canal.
  • In the process hundreds of parasites are moved from the mosquito into the person.
  • The parasites move to the liver through the circulatory system.
  • They burrow in the liver cells and reproduce very fast.
  • Within one to two weeks, the daughter cells break out of the liver and move to invade the red blood cells.
  • In the red blood cells they reproduce rapidly causing the cells to rapture and invade other red blood cells.
  • They then attack new red blood cells causing them to rapture also.
  • If a mosquito sucks blood from an infected person, it will take up these parasites in the red blood cells.
  • The parasites reproduce in the mosquito and migrate to the salivary glands ready to infect the next person when that mosquito bites.
  • Note.
  • Each time the daughter cells of plasmodia are released, thousands of red blood cells rapture and the patient experiences chills accompanied by shivering and sweating. The patient may also become anemic due to loss of red blood cells.

Control of malaria

  • Spraying the walls of dwelling places with insecticides. The insecticide may also be sprayed directly on the mosquito vector.
  • Draining all stagnant water to prevent mosquitoes from breeding there.
  • Removing broken bottles, old tins, old car tyres, e.t.c in which water collects. This also prevents breeding of mosquitoes.
  • Sleeping under mosquito nets
  • Treating the infected people using anti-malarial drugs.
    These are flatworms belonging to phylum platyhelminthes. There are two common species known.
    i) Taenia sagnata (beef tape worm)
    ii) Taenia solium (pork tape worm)
    They live in the small intestine of humans attached to the wall of the small intestine by hooks and suckers. They absorb nutrients from the digested food.
    Life cycle of a tapeworm
    Within the infected human being, the segments containing fertilized eggs break off and pass out in feaces.
    These eggs then tend to become attached to leaf blades of vegetation.
    When the eggs are eaten by the pig or cow depending on the species of the tapeworm, they develop into embryos.
    The released embryos burrow through the intestinal walls into the blood, which transports them to the muscles.
    Within the muscles they develop into bladder worms.
    If uncooked or partially cocked, meat from an infected cow or pig is eaten, the bladder worms are released in the intestines where they develop into tapeworms.
  • Avoid eating raw or half cooked meat.
  • By regular de-worming of infected individuals
  • By proper disposal of wastes
  • Inspection of meat before it is considered fit for human consumption.
    Adaptations of tapeworms to parasitic life
    i) They have lost the alimentally canal hence absorb already digested food over the entire body surface by diffusion.
    ii) They have a thick cuticle to prevent attack by digestive enzymes of the host.
    iii) They produce substances that inactivate the enzymes of the host.
    iv) Each mature proglotids of the tapeworm contains both male and female reproductive organs (hermaphrodites) hence fertilizes itself.
    v) They produce large numbers of eggs to ensure their survival.
    vi) They have suckers for attachment to intestinal walls. This prevents the tape worm from being dislodged by host peristaltic movements
    vii) They have resistant stages in their lifecycles with secondary and intermediate hosts to ensure survival during adverse conditions.
    viii) There is loss of unwanted organs like locomotally organs, eyes, etc. to ensure that they occupy as little space as possible within the host.
    ix) They have the ability to respire anaerobically and can survive in an oxygen free environment.

These are flat worms known as flukes. They are parasites that cause bilharzia (schistosomiasis)

Life cycle

  • Schistosoma requires the use of two hosts to complete its life cycle
  • Eggs are shed in faeces or urine of an infected human
  • Eggs can survive up to a week in dry land
  • If feaces end up in water, larvae called miracidia hatch
  • Find a snail and penetrate its foot
  • Transform into primary sporocysts ( another larval form)
  • Primary sporocysts multiply asexually into secondary sporocysts
  • Travel to snails hepatopancreas
  • Multiply asexually into many cercariae ( another larval form)
  • Cercariae exit the snail into the water (survive for about 48 hours )
  • Swim and attach to human skin with suckers
  • Find a suitable spot (e.g a hair follicle) and penetrate the skin using special enzymes
  • Transform into schistosomulae as they enter(another larval form)
  • Only head parts enter, tails remain behind
  • After a few days in rain, enter into the blood stream through dermal lymphatic vessels or blood venules
  • Travel in blood stream to get to specific blood veins
  • Schistosoma reaches maturity in 6-8 weeks in humans
  • Developed adult male and female find each other and pair up
  • Males make a gynaecophoric channel for longer thinner females to reside
  • The pair travel to rectal and mesenteric veins
  • Attach to venous wall with ventral and oral suckers
  • Females lay eggs on endothelial linning of the venous capillary walls
  • Some eggs are flushed by circulating blood ending up causing inflammation in organs like liver, lungs
  • Most eggs travel to lumen of intestinal tract( for S. mansoni) ureters and bladder(for S. haematobium)
  • Mature eggs produce special enzymes and can penetrate many membranes like renal veins or intestinal walls
  • Eggs get out of the body in faeces or urine
  • Cycle starts again