ParasitologyIntroduction

Introduction to parasitology (1 hour)

• 1. Defines ‘’parasite’’, ’’host’’, ‘’vector’’, ‘’zoonozis’’

• 1.1. Defines protozoon,helminth.• 1.2. Defines cell structure of protozoon and

helminths.• 1.3. Defines the life cycles of parasites.• 1.4. Listts the classification of parasitic

infections.

The term parasite is traditionally applied to several disparate groups of eukaryotic organisms,mainly protozoa and helminths, some arthropods, annelids, and molluscs.

Medical parasitology is concerned with 200 or so species of helminth worms and about 80 species of protozoa that infect humans.

a. Ancient History. The Ebers papyrus, written about 1600 BC, contains references to the presence

of parasitic worms in man. The laws of the Mosaic code prohibiting the consumption of meat

from unclean animals indicate that the early Israelites had knowledge about parasitic

infections. There is also evidence of parasitic recognition among the Roman, Greek, Persian, and

Phoenician scientists. Chinese doctors were aware of parasites as early as 300 BC.b. Modern History. The liver fluke Fasciola hepatica was discovered by Jehan de Brie in 1379 from

sheep. Anton Van Leewenhoek (1632-1723) described the morphology of the protozoan

Giardia lamblia from his own stool. In 1880, Laveran demonstrated the intracellular parasites of malaria.

c. Misconceptions.Many people believed that lice protected children from disease and leeches

were used extensively for blood letting as a cure. For a time, intestinal parasites were

thought to help in cleaning the tract of excess food and waste and until recently, the Chinese

believed that powdered Ascaris was helpful for medicinal treatment of impotency. In France,

the heads of tapeworms(scolices) were once used as weight control measures.

Present Ideology. Modern medicine has recognized the pathogenic effects of parasites and

is searching for drugsuseful in the treatment of parasitic infections.Prevention is the best

method to attack the problem of parasites.

TYPES OF ORGANISM RELATIONSHIPS

a. Normal Flora. Normal flora consists of microorganisms that are normally and

consistently found in or on the body in the absence of disease.b. Symbiosis. This is the close association or living together of two

organismsof different species; each party involved in this relationship is called a

symbiont.c. Mutualism. This is a type of symbiosis in which both organisms (host

andparasite) benefit from the association.d. Commensalism. This is also a type of symbiosis, but in this case, theparasite (commensal) is benefited and the host is neither benefited nor

harmed by therelationship.e. Parasitism. Parasitism is an obligatory relationship in which one

organism,the parasite, is metabolically dependent on another organism, the host.

The host maybe harmed by such a relationship.

Some basic parasite terminology

Ectoparasites: Ectoparasites are true parasites in that they depend on the host

for habitat and nutrition but they live on and not in theirs hosts. While less invasive

some ectoparasites can nonetheless cause serious disease.

Endoparasites: These parasites live in the bodies and tissues of their hosts, some

even live within the cells that make up their hosts. Examples could be the malaria

parasite that develops within the red bloods cells of the circulatory system or the

tape worm that lives in the intestine.

Infection: Infectious diseases are caused by transmissible organisms or biological

agents (in the case of viruses and prions). In its narrower definition infection

implies that the biological agents multiplies within the host. In the case of malaria

for example the mosquito injects 10-100 parasites that within two weeks multiply

into many millions. This multiplication is essential to the development of disease.

Infestation: Some parasites do not multiply within the host and this situation is at

Times referred to as infestation. This is true for most worms. A person who

swallows three tape worm larvae will not develop more than three tape worms.

The severity of disease in infestations therefore is often highly dependent of the

infectious (or infestuous) dose. Note that often infection is used more generally to

describe both phenomena.

Obligate/facultative parasitism: Our very definition of parasitism implies

Complete dependence of the parasite on the host. However, some parasites can

switch between a parasitic and a free-living lifestyle. These are called facultative

parasites. This is often triggered by ecological factors (e.g. the availability of hosts

or nutrition). The vast majority of parasites that cause human disease are obligate

parasites.

Life cycles

Definitive host: This is the host in which the parasite reproduces sexually.

Intermediate host: One or several successive hosts in which the parasite might increase its

number by asexual multiplication.Vector: Hosts that actively transmit the parasite to the next host as opposed to just

being by that next host. A classical example is the mosquito which injects malaria

parasites with itsalvia into humans during a blood meal.

Transport (or paratenic) hosts: In these hosts no parasite development or multiplication

occurs, the parasite just hitches a ride on the food chain. The larva of the fish tape worm can

move in this way from smaller fish to bigger fish and yet bigger fish awaiting the definitive

mammal host to eat a fish.

Reservoir host: Some parasites can infect a variety of mammals in addition to humans.

Biologically these hosts are equal to humans meaning that they occupy the same position in

The life cycle and are not up or downstream. These hosts (often wild or domestic animals living

İn proximity to humans) present a reservoir out of which humans can be re-infected even if the

disease has been cleared from all humans in the area. The presence or absence of a reservoir is

obviously a very important consideration for programs aimed at controlling or eradicating a

disease. while a dead end for the parasite this can still cause disease. We will highlight some

Examples of such disease when we discuss human infection with cat and dog parasites.

Disease Terminology

Prepatency: The phase of the disease between infection and the appearance of Diagnosable parasite stages (e.g. the appearance of tape worm eggs in the feces or malaria parasites in a routine blood smear). Note that this time frame can change with improvements in the sensitivity of diagnostic assays. A patent infectionis thus at an established diagnosable stage.

Incubation period: The incubation period is the time between infection and the development of symptoms (signs of disease like fever, pain or discomfort).

Acute disease: With the onset of symptoms the infection enters its acute (lat. sharp) phase.Acute diseases have a relatively quick onset and short duration. The symptoms worsen into a crisis and then the disease resolves spontaneously into healing, results in the death of the patient, or transforms into chronic disease.

Chronic disease: Chronic diseases (gr. chronos, time) develop slowly

(often over years) are persistent and might show no symptoms once

chronic or worsen over time. In chronic infections the parasite survives or

persists over time (this time can be the rest of the life of the patient).

Convalescence: Convalescence is the period after healing and describes

a phase in which balance is regained and damage repaired. This phase is

characterized by the absence of infectious agents and symptoms, and in

certain cases goes along with the development of long-term immunity to

reinfection.

A typical parasite life cycle diagram

The fish tape worm

Parasite stages are shown in the outer

circle and hosts in the center. The worm reproduces sexually in mammals (definitive host). A motile larva hatches and infects

thefirst intermediate host a waterflea

andtransforms and grows. The waterflea is eaten by a fish

again the parasite transforms and now

livesin the musculature of the fish.

Note That there is no further

development ofthe parasite from trout to pike.The pike is a transport host.

PARASITIC ATTRIBUTES THAT INFLUENCE DISEASE

Virulence. Virulence refers to the ability of a parasite to establish itself in a

host, maintain that infection, and damage the body of the host. For one reason or

another, one strain of a parasite may cause a more serious disease process than other

strains of the same species.

Parasitosis. The more parasites infecting a host, the more severe will be the

effect on that host.

Life Span of the Parasite. The longer the parasite lives in or on the host, the

greater the damage that will be caused.

Repeated Contact. The more times that a host is parasitized by an

organism, all other factors being equal, the worse the prognosis of the

disease.Competition for Food. Intestinal parasites deprive the host of

theNecessary nutrients required for survival.

Mechanical Interference. Some parasites accumulate (like in the

intestines) in such great numbers that the normal flow of nutrients,

waste, or fluid is obstructed. For example, the microfilariae of some

nematodes obstruct the lymphatic system in elephantiasis. Their presence

prevents lymphatic fluid from being circulated out of lymphatic tissue and

the swelling associated with elephantiasis results.

Toxic Effects. By-products of metabolism and anatomic parts of the parasite

can be very toxic to the host. In cases of massive infections with Trichinella spiralis,

it is this toxicity that is lethal to the host.

Tumor. Tumor formation in the host is a common occurrence with parasites

that invade or irritate the tissue of the host. Pathologists have suggested that

Repeated infections with Trichomonas vaginalis could lead to cervical cancer, and

recently, some cases of malignant neoplasms have been reported from Egypt and

attributed to Schistosoma infections. Some benign tumors are caused by the larval

stage of the tapeworm Taenia solium in muscle tissue.Loss of Blood and Body Fluids. The loss of blood and body fluidscaused by parasites is significant. For example, it has been calculated

that about 0.5 ml of blood per worm per day is lost by the host during a hookworm

infection. Remember,these fluids contain nutrients and electrolytes needed by the

host.Destruction of Host Tissue. Parasites destroy the host's tissue

structuresby the presence and activity of the adults, by the migration of larval

forms, and byovipository migrations.Psychological Disorders. The knowledge of parasites crawling inside

thebody can affect anxiety level of the host. But, the more detrimental

psychologicaleffects are those caused by the accumulation of chemicals in the system

of thehost.

HOST ATTRIBUTES THAT INFLUENCE RESISTANCE TO PATHOGENIC

EFFECTS OF PARASITISM

Fitness of the Host. The nutritional and emotional state of the

host can effect the degree of pathogenicity caused by the parasite.Host Age. Both the very young and the very old are

affected to aGreater degree by a parasitic infection than are young

adults.Specific Factors That Influence Resistance.Immunity. Host resistance to parasitic infection is very similar to theresistance shown against bacteria. The immune system

works bythe formation of antibodies against a limitless amount of substances recognized as foreign antigens by the B

lymphocytes.

Complement system. This is a complex system of enzymes found in normal serum that

aids thelysis (destruction) of the parasite during an antigen-antibody

reaction. Complement is composed of nine components labeled C-1

through C-9.Interferon. Interferon is composed of a group of soluble small proteins

produced byinfected cells that inhibit multiplication of virus. The proteins are

not virusspecific, but they are cell specific in production and effects.Nonspecific Factors That Influence Resistance. There are

some Barriers that affect parasitism. These barriers may be chemical,

physical,or biological (the antagonistic effect of organisms already

present at thesite in question).

(1)  Chemical barriers. The host combats the invasion of micro-organisms

by the secretion of chemicals that are antimicrobial in nature. The acid pH

of the stomach, skin, and vagina; the bile salts of the intestines; and the

lysozymes of the eyes and saliva serve as deterents to the invasion of

microorganisms.(2)  Physical barriers. The intact skin, mucus (sticky lining of the

mucousmembranes), nasal hairs, cilia of the respiratory epithelium,

peristaltic movement, and normal microbial flora (occupying attachment

sites)prevent the entrance of microorganisms.(3)  Microbial antagonism. Antiparasitic factors present in the

serum andcompetition for nutrients from the normal flora also serve as

obstacles forthe possible pathogenic parasites.

EPIDEMIOLOGY

Epidemiology is the science that studies propagation and prevalence of

Human disease. It determines the frequency and distribution of a disease

in a community.

a. Endemic. A disease is endemic when it is present in a community at

All times but is clinically found in a few individuals and with low morbidity.

b. Hyperendemic. A hyperendemic disease is one that is present in a

High frequency in a community.c. Sporadic. A low frequency of disease that is not widely

diffused is referred to as sporadic.d. Epidemic. An epidemic occurs when a disease spreads

rapidly throughA community and affects a large number of individuals.

Mode of Infection. Some parasites can infect the host by direct

Contact while others require a complex mechanism to enter the

host.(1)  Congenital. Some parasites can be transmitted to the

youngergeneration by the older generation. The transmission of

malaria, toxoplasmosis and several other parasitic diseases may

be accomplished by parasitic forms crossing the placental

barrier from the mother's blood to the unborn fetus.(2)  Direct contact. Some parasites can complete their life

cycle byBeing transmitted from an infected site to another site

where there is no disease. Then they can parasitize either the

new or thesame host.

(3)  Ingestion. Food and water are very important in the spread of parasitic

diseases because most parasites inhabit the gastrointestinal tract. The infective

form of the parasite (ova or larvae) may be present in the flesh of the intermediate

host (plant or animal) or may be swimming free in contaminated water. The

organism may gain access to food through contamination with night soil, water, by

the hands of foodhandlers, by mechanical agents or by biological carriers such as

insects. In some instances, the intermediate hosts, like small arthropods, are

consumed with the food or water. (4)  Arthropod-borne. Members of the phylum Arthropoda serve as

vectors(carriers) of parasitic diseases and bacterial and viral infections. The

vector is anintegral part of the life cycle of the parasite. In some instances the

arthropods areintermediate hosts while in others they are the definitive host (5)  Active penetration. In some parasitic infections, the infective form

is alarval stage that has the capability of penetrating the host tissues. (6)  Transfusion. Certain blood and tissue parasites may be present indonor blood at the time of transfusion. Thus, these parasites may be

introduced into the new host system and cause parasitic infection.

PREVENTIVE MEASURES

The individual, as well as the community, must get involved in the

prevention of parasitic infections. Life cycles of parasites may be

interrupted by eradication of the vectors. Education about hygiene, eating habits, and disposal of human and animalwastes can also be used in combating parasites.

Treatment of diseased individuals not only relieves the suffering, but

also prevents the spread of the disease.

TAXONOMY OF PARASITES INFECTING HUMANS

INTRODUCTIONAll living organisms have been divided into groups with similar characteristics.These groups have been subdivided further until organisms that have identical

traits areclassified under the same genus and species. It is important that you be familiar

withthe principles of taxonomy pertaining to parasites.

DIVISIONS OF LIVING ORGANISMSa. Kingdom. This is a large group of organisms with similar features. In theliterature, various authors list from two to five kingdoms.(1)  Kingdom PLANTA. This kingdom contains all of the plants. There areno plants parasitic to man.(2)  Kingdom PROTISTA. Members of this kingdom are unicellular (one-celled) organisms. The kingdom is further divided into two subkingdoms.(a) Subkingdom EUCARYOTA. Eucaryotes are characterized by anuclear membrane separating the nucleus from the cytoplasm, DNA that is

grouped intounits called chromosomes, multiplication accomplished by mitosis,and energy produced in structures called mitochondria. Some examples areprotozoans and fungi.

(b) Subkingdom PROCARYOTA. The procaryotes are characterized

by no nuclear membrane (therefore, there is no organized nucleus); no chromosomes (the DNA is not separated but

is acontinuous strand); no mitosis (multiplication is

accomplished by simple cell division); and no mitochondria (energy is

produced at The mesosomes). Some examples are bacteria and

bluegreen algae.(3)  Kingdom ANIMALIA. The higher animals, including

man, areplaced in this kingdom. There are many parasites that

infect manin this kingdom.

Phylum. A phylum is a major division of a kingdom. There are

four phyla (plural of phylum) that contain human parasites:

PROTOZOO from the subkingdom EUCARYOTA, and phyla PLATYHELMINTHES, ASCHELMINTHES, and ACANTHOCEPHAHELMINTHES from the kingdom ANIMALIA.c. Class. A phylum is divided into classes. The name of

the class should end in "a." Some examples are Cestoda and

Nematoda.d. Order. Several orders may be contained within a class.

This name ends in "ea." Some examples are Filaroidea, Pseudophyllidea.e. Family. An order may be subdivided into families. The

family's ending is "ae." Some examples are Heterophyidae and Endamoebidae.

f. Genus. Each family is made up of various genera (plural of Genus). The

genus name may have various endings. The first letter of the genus is capitalized

And the name is underlined. The first letter may be used as an abbreviation. Some

examples are Macracanthorhynchus and Entamoeba.g. Species. Each genus is composed of species. The species' name may

alsohave various endings. All letters are small case and the name is

underlined. However,the species' name should never be abbreviated. Some

examples are E. histolytica and M. hirudinaceus.h. Identification. Parasitic organisms identified by laboratory

procedures arereported by using the genus and species names. Some examples are

Giardia Lamblia and Enterobius vermicularis.

Classification

Protozoa Sarcomastigophora (flagellates and amebas) Apicomplexa (sporozoans) Ciliophora (ciliates) Microspora

Helminths Plathyhelminthes (flatworms)

Cestoda (flukes) Trematoda (flukes)

Nemathelminthes (worm like, seperate-sexed, unsegmented roundworms)

Flagellates

Intestinal flagellates Giardia intestinalis Trichomonas spp Dientamoeba fragilis

Trichomonas vaginalis

Hemoflagellates Leishmania donovani Trypanosoma cruzi

Amebas

Intestinal amebas Entamoeba histolytica Entamoeba coli Balantidium coli

Free-living amebas Naegleria fowleri Acanthamoeba castellani

Sporozoans

Blood sporozoans Plasmodium vivax Plasmodium malariae Plasmodium ovale Plasmodium falciparum Babesia microti

Other Isospora belli Sarcocystis bovihumanis Cryptosporidium parvum Toxoplasma gondii

Nematodes

Ascaris lumbricoides Dracunculus medinensis Enterobius vermicularis Wuchereria bacrofti Ancylostoma duodenale Toxocara spp. Loa loa Strongyloides stercoralis Trichinella spiralis Trichuris trichiura

Trematodes

Clonorchis sinensis Fasciola hepatica Paragonimus westermani Schistosoma haemotobium Schistosoma japonicum Schistosoma mansoni

Cestodes

Taenia solium Taenia saginata Echinococcus granulosus Echinococcus multilocularis Diphyllobothrium latum Hymenolepsis nana