Introduction to Parasitology

by DrChika

Parasitology is the study of parasites and their interaction with other organisms (i.e. their hosts). It is the study of parasites, their ecology, transmission and life cycle. The study of parasites is as ancient as mankind as man became conscious of his immediate environment. The field of Parasitology was fully established and widely appreciated in the early 1860’s. During this period, scientists as at the time postulated that protozoa and helminthes (which are both parasites) were actually responsible for some diseases of man and animals. Diseases and infections caused by parasites are by far amongst the top-most ever-present malady that man and his domestic animals encounter on almost daily basis aside bacterial, viral or fungal infections.

The disease burden of parasitic infections cannot be over-emphasized, and this menace is experienced most in tropical developing countries where both natural and man-made activities has made it possible for the parasites to continue their devastating effects almost unperturbed. For the purpose of this book, we shall concern ourselves with the medical discipline of Parasitology, and this has to do with parasitic protozoa (such as Plasmodium), helminthes (Tapeworm) and arthropods (Tick). Though some bacteria, fungi and viruses are pathogenic, and cause disease and infections in humans; these microorganisms are not covered in the study of Parasitology even though they can coexist parasitically with man. Not all parasites are microorganisms (such flea and tick) but all microorganisms are parasitic in nature, and they cause harm or disease in their host.

There are about 10,000 species of protozoal parasites with different life cycles, and that cause a variety of diseases in human beings and animals. Parasitic infections (example malaria) accounts for billions of resource waste in most parts of the world where they are a great source of concern and burden to health practitioners because they account to a colossal amount of mortality and morbidity in these regions (in the African and Asian continent where these organisms are most prevalent). Understanding the life cycles of these parasites and the mechanisms of their pathogenesis is critical to the containment of the diseases that they cause. There spread and transmission in any population can also be better controlled; and a reduction in economic wastages due to them will be achieved as well.

Parasites are living organisms that establish a physiological association with the tissues on the surface or inside the body of another organism called the host (example man, animal and host). The association between a parasite and a host is usually one that inflicts some level of injury and discomfort to the host, and this occasionally results to a disease or infection in the affected organism or host. This kind of association or relationship between a parasite and a host is called parasitism. In such relationships, only the parasites benefit while the host is often left diseased in the cause of the relationship. The host does not benefit but rather suffers the sole dependability of the parasite on it for food, water, shelter and protection. The host of a parasite can be definitive host, intermediate host or reservoir host.

Reservoir hosts store or harbour the parasite, and they serve as medium via which other susceptible human or animal hosts become infected by the protozoa. Definitive hosts are hosts that maintain the mature infective form of the parasite; and in some cases some clinically important parasites have humans as their only definitive hosts (example Trichinella spiralis). Sexual development of parasites takes place in the definitive host. Intermediate hosts are the second hostsof a parasite apart from the definitive host; and they are known to substitute with the definitive host and are usually known to harbour some reproductive stages of the parasite like the definitive host. Asexual development of parasites takes place in the intermediate host.

In establishing a disease condition,parasites usually replicate intracellularly (i.e. inside the cells of its human host) as is seen in the Plasmodium species which solely multiply in the blood or extracellularly (outside the cells of its host) as is seen in Giardia species that develop in the lumen of the intestinal tract. There are a wide variety of parasites or protozoa that parasitize humans and cause diseases or parasitic infections (Table 1); and some of these infections are subclinical in nature while the others are symptomatic and are presented with some clinical episodes that make the individual seek for medical help. The parasites that infect man and other animals are usually found all over the world and close to their immediate environments; and such organisms constitute one of the world’s greatest danger to the socioeconomic and wellbeing of humans.

Table 1: Some clinically important parasitesand their clinical features

ParasiteGroupMorphological formHostDisease
Ascaris lumbricoidesRoundworm/Intestinal nematodeEggs and adult worms  HumansAscariasis
Ancylostoma duodenaleIntestinal hookwormEggsHumansHookworm disease  
Acanthamoeba species*AmoebaCyst and trophozoiteAmoebiasis  
Balantidium coli*CiliateCyst and trophozoiteHumans and pigBalantidial dysentery
Brugia species*Filarial nematodeMicrofilariaCat, humans and monkeyLymphatic filariasis
Clonorchis sinensis*Trematodes/flukeEggsCat, pig, dog, humans, mouse, fish and snailClonorchiasis
Cyclospora cayetanensis*Intestinal coccidianOocystsHumansDiarrhea in immunocompromised host
Cryptosporidium parvum*Intestinal coccidianOocystsHumans, livestock, domestic and wild animalsCryptosporidiosis and diarrhea in immunocompromised host
Dracunculus medinensis*Guinea worm or filarial nematodeLarvaHumans and CyclopsDracunculiasis  
Diphyllobothrium latum*Tapeworm/cestodeEggsCat, humans, dog, crustaceans and fishDiphyllobothriasis  
Entamoeba histolytica*AmoebaeCyst and trophozoite  HumansAmoebic dysentery  
Enterobius vermicularisThreadworm/intestinal nematodeEggs and adult worms  HumansEnterobiasis
Echinococcus granulosusTapeworm/cestodeLarvaSheep, humans, dog, and wild animalsEchinococcosis
Fasciolopsis buskiTrematodes/flukeEggsSnails, humans and pig  Fasciolopsiasis
Fasciola hepaticaTrematodes/flukeEggs  Snail, humans and cattleFascioliasis
Giardia lamblia*FlagellateTrophozoite and cyst  HumansGiardiasis
Hymenolepis nana TapewormEggsHumansHymenolepiasis  
Isospora belli*Intestinal coccidianOocystsHumans  Isosporiasis and diarrhea in immunocompromised host  
Leishmania species*FlagellatesAmastigoteSandfly, humans, dog and rodent  Leishmaniasis
Loa loaFilarial nematodeMicrofilariaChrysops and humansLoiasis or calabar swelling  
Necator americanusHookworm/intestinal nematode  EggsHumansHookworm disease
Onchocerca volvulusFilarial nematodeMicrofilariaBlackfly and humansRiver blindness or onchocerciasis  
Opisthorchis viverriniTrematodes/flukeEggsSnail, humans, cat and fishOpisthorchiasis
Paragonimus westermaniTrematodes/flukeEggsSnail, humans, carnivores, crab and other crustaceansParagonimiasis
Plasmodium species*Blood/tissue coccidianTrophozoite, schizont and gametocyteMosquitoes and humansMalaria
Schistosoma speciesTrematodes/flukeEggsSnail, humans, cat, rodent and cattleIntestinal & urinary schistosomiasis  
Strongyloides stercoralisIntestinal nematodeLarvaHumansStrongyloidiasis  
Taenia soliumTapewormEggsPig and humansTaeniasis  
Taenia saginataTapewormEggsCattle and humansTaeniasis  
Trichinella spiralisFilarial nematodeLarvaHumans, pig, rodent and wild animalsTrichinellosis
Trichomonas vaginalis*  FlagellateTrophozoiteHumansVaginitis (in females) and urethritis (in males)
Trypanosoma species*Flagellate  TrypomastigoteTsetse fly, humans, triatomine bugs, cat and dogAfrican trypanosomiasis (sleeping sickness) and American trypanosomiasis (Chagas disease)  
Toxoplasma gondii*Blood/tissue coccidianOocystsHumans, cat, pigs, birds and rodents  Toxoplasmosis
Trichuris trichiura Whipworm/intestinal nematode  EggsHumansTrichuriasis
Wuchereria bancroftiFilarial/tissue nematodeMicrofilariaMosquitoes and  humansBancroftian filariasis (elephantiasis)

Most of these parasites are usually located and distributed in the tropical regions of the world because of some peculiar features of these areas. Typically examples include the Plasmodium parasites which are inherently found in tropical regions of the world especially in the African continent (Nigeria for example). Plasmodium which causes malaria in man has over the years even now impacted negatively on the wellbeing and socioeconomic welfare of human beings, and this scenario has resulted to the high morbidity and mortality rates recorded in malaria disease cases across Africa. Most parasitic diseases (malaria) are major diseases of the tropical and subtropical regions; and these diseases which are often known as tropical infections are serious public health problems in these parts of the world where the diseases are endemic. Aside the climatic conditions of the tropical and subtropical regions which make them more prone to most parasitic infections than the temperate world; poor health policies, poor environmental sanitation and poverty amongst other factors are amongst the key reasons why some of these diseases are still widespread in these regions, and the situation is pathetic in most developing parts of Africa.


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