Life is all about the continuity of generations of an existing species. Till today, 1.5 million species of diversified organisms have been identified and many more are on way. Of all identified organisms, many have lost their existence and can never be traced, however, those which were preserved by nature as a fossil are lucky to be known. 

By Dr Abdullah G Arijo

Every organism requires energy resources for its biological systems to work. For this purpose, some organisms (plants) have adopted an autotropic way of life where they use chlorophyll to produce chemical energy from solar energy.

Both parasite and host are living organisms struggling for their existence. In one or another way, both depend on each other and should live in peaceful co-existence. A successful co-existence is one where both are likely to live not harming each other. For example, if a parasite draws too much nutrient from its host, ultimately host will lose its health and may be subjected to death. Since the parasite lives in the host, so with the death of the host, the parasite will certainly die. if the parasite provokes the immune system of its host and is killed, the parasites again lose their battle.

Therefore, a good relationship is one where both parasites and host develop successful co-relation and compatibility and would not harm each other to continue their existence. Parasites don’t kill their hosts because they often require a living host to feed, thrive and reproduce. Since parasites must find a new host when the current one dies, it’s in the parasite’s best interest to keep the host alive to absorb as many nutrients as possible from its system.

Other types of parasites may give off toxins that can negatively affect the host’s health. A few parasites have been known to influence their hosts’ behaviour to further their life cycles, according to Live Science. For example, Toxoplasma gondii, a common parasite found in rats, can alter the mind of infected rats by driving them to areas frequented by cats to move up the food chain, and hairworms can induce grasshoppers to leap into the water so that the parasite young can swim away and continue their growth cycle. For continuity of life and a successful parasite is required.

This all is just survival game, where mostly parasite develops compatibility, however, some off the track hazards may harm the host.

Parasites may harm their host both directly and indirectly.

Direct damages caused by parasites.

  1. Annoyance: Most of the hematophagous arthropod parasites cause severe annoyance to their host because of their biting behavior which may lead to difficulty in feed intake, rest which result in loss of production and loss of condition. The parasite which annoys the animals are bedbugs, fleas, deer fly, horsefly, housefly, tsetse fly, sand-fly, stable fly, mosquitoes, black fly, biting midges, and ricks.
  2. Envenomation: There are many arthropods parasite which causes inoculation of venom in their host while feeding or stinging. E.g.: Spider, ants, bees, wasps etc. there are other parasites like a beetle and caterpillar which produce a toxin which causes problem either touch or ingested.
  3. Myiasis: It is the infestation of the larval stage (Maggots) of the parasite in the body of the animal. Fly having complete metamorphosis, some of the flies like, housefly, black fly, horsefly, deer fly etc. lay their egg on the wound of the host, so they develop into the larvae in the wound which damage the tissues of the host result in the deep hole in the tissue. Blowfly larvae invade the eyes of their host.
  4. Failure in treatment and recovery. Trypanosoma Spp and Leishmania spp having glycoprotein on their surface called variable surface glycoprotein (VSGs). These glycoproteins are the antigenic determination and having the ability to change their surface glycoprotein. Once the host immune system produces the antibodies against that surface glycoprotein, the parasite active their genetic material and changes their surface glycoprotein. So, the antibodies react with that parasite which having former VSGs, followed by the multiplication of parasites with new VSGs which result in Intermittent fever in the host.
  5. Encephalopathy: The larval stage of taenia multistep is coenurus which is developed in the central nervous system (Brain), it causes pressure on the parts of the brain to result in the star grazing, stagger gait in the host.
  6. Intermittent Fever: Fever is not a disease it is the reaction which causes by an immune reaction. It is recurrent in the protozoal infection such as trypanosomiasis, malaria and leishmaniasis which is occur due to the dissemination of a large amount of parasite into the blood by the destruction of the infected cell.
  7. Cyst: Larval stage of Echinococcus Spp is a hydatid cyst that is developed in the visceral organs such as the liver, lungs and intestine which cause pressure on the surrounding organs of the host.
  8. Competition for the host’s nutrients: – Most of the parasites want nutrients from the host to survive. For example, Diphyllobothrium latum absorbs vitamin B-12 and can cause anemia and other tapeworms absorb large amounts of proteins and sugars and use the host’s fluids. Hookworm ingests blood, which can be up to 250 ml/day
  9. Cachexia: African Trypanosomiasis and Leishmaniasis may lead to a severe loss of weight in both animals and man. Helminth infestation also lead to cachexia like Fascioliasis
  10. Destruction of host tissues: – Some parasites (tick, mites, hypodermal flies etc.) injure the host skin while they enter into the host. Some parasites (Schistosoma mansoni) cause Swimmers to itch when cercariae penetrate the skin of a person who is working in water. Intestinal worms (Ascaris worms) cause small lesions in the gut and infection with entamoeba actively digest epithelial cells in the large intestine.
  11. Tissue changes: – There are many examples of the parasite which damage the host tissues due to their activities. Paragonimus westemeniasis, for example, invaded the lung tissues and caused serious damages. Schistosoma mansoni causes many pathological changes in the liver and lungs of the host.
  12. Toxins and secretions: – Some parasites (Mosquito and other blood-sucking insects) may leave toxins at the site of biting. Fleas, which are blood-sucking ectoparasites mostly found in dogs and cats add toxins into the host, causing Flea Allergy Dermatitis
  13. Production of toxic compounds: – It is thought that the African trypanosome when in the central nervous system, produces aromatic amino-acid analogues that may influence brain function.
  14. Super-infections
    In the case of mucocutaneous Leishmaniasis, ulcerations may lead to superinfections with bacteria
  15. Immuno-depression: – Malaria, Schistosomiasis etc., lead to a certain degree of immune suppression which makes the infected host more susceptible to other diseases.

Trypanosoma’s VSGs have the ability of antigenic shift or antigenic variation which lead to immune suppression.

  1. Allergic reactions: In the case of onchocerciasis, commonly called river blindness, the presence of the filarial worms under the skin may lead to depigmentation due to allergic reactions. The sting of arthropods parasites like wasp, bees, ants, spider and scorpion may also lead to an allergic reaction.
  2. Anaphylactic shock: – It may be induced by the sudden release of large amounts of parasite antigens into the bloodstream of the host. For example, in malarial infection, Anaphylactic shock occurs when the merozoites are released from infected erythrocytes and in the case of African Trypanosomiasis or sleeping sickness, this occurs when the immune response of the host leads to the massive killing of the circulating parasites. Sometimes drug treatment leading to massive killing of the parasites may result in anaphylactic shock.
  3. Mechanical damage: – In the case of malaria, the breakdown of erythrocytes leads to hemolysis and anaemia. In the case of Ascaris (a roundworm) infection, the presence of the worms in the small intestine may lead to intestinal occlusions. Filarial worms which cause elephantiasis block the lymphatic system, with a result, lymph does not return to the blood and serious problems may occur. If Tapeworms are present in large numbers, they can block the intestine, and this leads to abdominal pain and indigestion problem. Activities of Plasmodium can cause RBC’s to stick together and clog capillaries. This affects the flow of blood.
  4. Irritative reflexes: – Ascaris worm while move inside the intestine can cause irritation which causes intestinal contractions. This affects the digestion of food. Larvae of Enterobius vermicularis cause irritation inside the anal area of the host due to which the host feels uncomfortable and cannot feed and rest properly.

Indirect damages caused by parasites.

There are so many parasites which harm the host directly and indirectly by act as a vector for different disease.