INTRODUCTION TO PARASITOLOGY

Pamanatasan ng Lungsod ng MaynilaModified by: Maria Cielo B. Malijan, MD, DPPS, FPSDBP

Biological Relationships

• Symbiosis: – is the living together of unlike organisms– may also involve protection or other advantages to

one or both partners

• Different forms of symbiosis: – distinguished on whether or not the association is

detrimental to one of the 2 partners.• Commensalism• Mutualism• Parasitism

Biological RelationshipsBiological Relationships

Commensalism Mutualism Parastitism

one species benefits from the relationship without harming or benefiting the other

e.g. Entamoeba coli in the intestinal lumen is supplied with nourishment and is protected from harm, while it does not cause any damage to the tissues of its host.

Organism 1 = (+)Organism 2 = (0)

Biological RelationshipsBiological Relationships

Commensalism Mutualism Parasitism

2 organisms mutually benefit from each other

e.g. termites and the flagellates in their digestive system, which synthesize cellulase to aid in the breakdown of ingested wood

Organism 1 = (+)Organism 2 = (+)

Biological RelationshipsBiological Relationships

Commensalism Mutualism Parastitism

one organism, the parasite, lives in or another (the host), depending on the latter for its survival and usually at the expense of the host

e.g. Entamoeba histolytica derives nutrition from the human host and causes amebic dysentery

Organism 1 = (+)Organism 2 = (-)

Biological RelationshipsBiological Relationships

Commensalism Mutualism Parasitism

Organism 1 = (+)Organism 2 = (+)

Organism 1 = (+)Organism 2 = (-)

Organism 1 = (+)Organism 2 = (0)

What is a parasite?

• a pathogen that simultaneously injures and derives sustenance from its host

• Parasitology– area of biology concerned with the

phenomenon of dependence of one living organism on another

• Medical Parasitology– Concerned primarily with the animal

parasites of humans and their medical significance, as well as their importance in human communities

PARASITOLOGY

• Parasite• Host• Vector

PARASITESParasites

According to Habitat

EctoparasiteEndoparasite

lives INSIDE the body of a

host

Lives OUTSIDE the body of a

host

Erratic

when it is found in an organ which is not its

usual habitat

Presence of an

endoparasite in a host is called an INFECTION

presence of an ectoparasite inside the

host is called an INFESTATION

PARASITEParasites

According to Depemdence on Host

Obligate Parasite Facultative Parasite

need a host at some stage of their life cycle to complete their development and to propagate their species

e.g. tapeworms depend entirely upon their host for existence

may exist in a free-living state or may become parasitic when the need arises

• Accidental or Incidental parasite: establishes itself in a host where it does not ordinarily live

• Permanent parasite: remains on or in the body of the host for its entire life cycle

• Temporary parasite: lives on the host only for a short period

• Spurious parasite: a free-living organism that passes through the digestive tract without infecting the host

Host: classified based on their role in the life cycle of the

parasite• Definitive or final host: is one

in which the parasite attains sexual maturity– Humans are the definitive host

in taeniasis

• Intermediate host: harbors the asexual or larval stage of the parasite– Pigs or cattle as intermediate

hosts of Taenia spp. while snails are hosts of Schistosoma spp.

• Paratenic host: one in which the parasite does not develop further to later stages. However, the parasite remains alive and is able to infect another susceptible host– Paragonimus metacercariae

in raw wild boar meat can pass through the intestinal wall of humans and complete its development

Host• Reservoir hosts: • allow the parasite’s life cycle to

continue and become additional sources of human infection– Pigs are reservoirs of Balantidium coli– Field rats of Paragonimus westermani– Cats of Brugia malayi

• Humans are not always the final host

• Man may be the most important host in the spread of the disease or an incidental host of parasites prevalent in other animals

• Humans are not always the final host

• Man may be the most important host in the spread of the disease or an incidental host of parasites prevalent in other animals

Vectors: Responsible for transmitting the parasite from one host to another

• Biologic vector: transmits the parasite only after the latter has completed its development within the host

• Aedes sucks blood from a patient with filariasis, the parasite undergoes several stages of development from the first stage larva to 3rd stage larva before it is transmitted to another susceptible host

• Mechanical or phoretic vector: transports the parasite

• Flies and cockroaches that feed on fecal material may carry enteric organisms and transfer these to food

Exposure and Infection• Carrier: harbors a pathogen without manifesting any signs and

symptoms

• Exposure: process of inoculating an infective agent

• Infection: connotes the establishment of the infecting agent in the host

• Incubation period (clinical incubation period): period between infection and evidence of symptoms

• NOTE: Pre-patent period (biologic incubation period): period between infection or acquisition of the parasite and evidence or demonstration of infection

Sources of Infection1. CONTAMINATED SOIL AND WATER:

most common• Lack of sanitary toilets & use of human excreta as fertilizer favor infection with ascaris, hookworm

• Water contaminated with cysts of amebae or flagellates as well as cercariae of Schistosoma

2. FOODmay contain the infective stage of the parasite

3. :ARTHROPODS e.g. mosquitoes as vectors of malaria and filaria parasites

4. OTHER ANIMALS (wild or domesticated) may harbor the parasite– E.g. cats are direct sources of Toxoplasma ifnection

5. OTHER SOURCES– Another person, his beddings and clothing, the immediate environment he has contaminated

– E.g. asymptomatic carriers of Entamoeba histolytica working as food handlers

AUTOINFECTION where the self is the source of infection as seen in the life cycles of E. vermicularis, Hymenolepis nana and Strongyloides stercoralis

Modes of Transmission• Mouth

– the most likely portal of entry– Most cestodes, trematodes and protozoans are food borne

• Skin penetration– : e.g. hookworms & Strongyloides

• Arthropods: – as vectors transmit parasites through bites– Agents of malaria, filaria, leishmaniasis and trypanosomiasis

• Congenital transmission: – Toxoplasma gondii trophozoites can cross the placenta – In transmammary infection with Ancylostoma and Strongyloides, the parasites may be transmitted through the mother’s milk

Modes of Transmission• Inhalation

– of air-borne eggs of Enterobius

• Sexual intercourse – as in the case of Trichomonas vaginalis

Nomenclature• Animal parasites are classified according to the International

Code of Zoological Nomenclature

• Each phylum is subdivided into classes which are further subdivided into orders, families, genera and species– Further subdivisions of suborder, superfamily and subspecies are

employed at times

• Family names are formed by adding ~idae to the stem of the genus type

• Names of the genera and species are italicized or underlined when written

Life Cycle

• May be simple or complicated

• As the life cycle becomes more complicated, the lesser the chances are for the individual parasite to survive

• Perpetuation of a species of parasite depends upon its ability to ensure transmission from one host to the next

Epidemiologic measures• Cumulative prevalence:

– percentage of individuals in a population infected with at least one parasite

• Intensity of infection: – number of worms per infected person– also referred to as worm burden– measured directly by counting expelled worms during treatment; indirectly by counting helminth eggs excreted in feces expressed as eggs per gram (epg)

• Morbidity: – clinical consequences of infections that affect an individual’s well being

Diagnosis

• Demonstration of parasites– E.g. eggs, adults, larvae, cysts, oocysts and trophozoites

• Detection of host immune response to the parasites– E.g. antibodies and antigens– In light infections when parasites are still immature, immunoassays may become useful

Diagnosis

• Specimens for parasitic examination: • Stool• Blood• Urine• Sputum• Cerebrospinal fluid• Tissue aspirate• Tissue biopsies• Orifice swabs

Diagnosis• Microscopy • simple, informative, can establish rapid diagnosis; dependent on skill

of microscopist

– Limited by low density of parasites in specimens; may be affected by stage of the disease, biology of the parasite

– PARASITE CONCENTRATION TECHNIQUES• require additional equipment, supplies and reagents

– PARASITE CULTURE METHODS• require a clean bench and special culture media

– Special staining procedures of fluorescent microcopy available for better visualization

Diagnosis

• Molecular diagnosis: molecular or DNA-based methods– Address problems associated with microscopy and

immunodiagnosis– DNA probes, detection of specific nucleic acid sequences

• Rapid diagnostic tests (RDTs)– Newer and more advanced methods of immunodiagnosis – (antigen-capture tests)– Detects fewer parasites; faster results– Easy to perform and require minimal training of personnel

Diagnosis

Immunodiagnosis• Overcome problems related to microscopic examination

• Detect antigen or antibodies in clinical specimens– Immunofluorescent Assay ELISA, Indirect Hemagglutination Assay, RIA, and dot blot

• Antibody detection tests– Limited in distinguishing active from prior infections

– Useful in detecting exposure and etiology of outbreaks and in mapping foci of infection

– Serve as alternatives when microscopic exam fails

Treatment• Deworming:

– use of antihelminthic drugs

• Cure rate: – no. (expressed as percentage) of previously positive subjects found to be egg-negative on examination of a stool or urine sample at a set time after deworming

– Indicator used to measure reduction in prevalence of infection

• Egg reduction rate (ERR): – percentage fall in egg counts after deworming

– Indicator used to measure reduction in intensity of infection

• Selective treatment: – involves individual-level deworming with selection for treatment based on a diagnosis of infection

• Targeted treatment: – group-level deworming where the (risk) group to be treated (without prior diagnosis) may be defined by age, sex, or social characteristics irrespective of infection status

• Universal treatment: – population-level deworming in which the community is treated irrespective of age, sex, infection status, or other social characteristics

Treatment

Prevention and Control• Morbidity control:

– the avoidance of illness caused by infections

• Information-education-communication (IEC):– a health education strategy that aims to encourage people to adapt and maintain healthy life practices

• Environmental management: – manipulation of environmental factors or their interaction with human beings

– aim: prevent or minimize vector or intermediate host propagation & reduce contact between humans and the infective agent

• Environmental sanitation: • interventions to reduce environmental health risks– Includes safe disposal and hygienic management of human

and animal excreta, refuse and waste water– Involves the control of vectors, intermediate hosts, and

reservoirs of disease– provision of safe drinking water and food safety

Prevention and Control

Eradication versus Elimination

• Disease Eradication: – permanent reduction to zero of the worldwide incidence of infection caused by a specific agent, as a result of deliberate efforts

– continued measures no longer needed

• Disease Elimination: – reduction to zero of the incidence of a specified disease in a defined geographic area as a result of deliberate efforts

– Continued intervention/surveillance still required

Damage Caused By The Parasite To Teh Host

• interference with the vital processes of the host through the enzyme systems (most common) – Secretory and excretory products of parasites allow them to metabolize nutrients from the host

– Ex: E. histolytica trophozoites secrete cysteine proteinases - digest cellular materials & and degrade epithelial basement membrane facilitating tissue invasion

• Invasion and destruction of host tissue– Plasmodium invades rbc, and causes its rupture

• Deprivation of the host of essential nutrients & substances– Heavy hookworm infestation causes iron deficiency anemia

Effects of the Host on the Parasite

• Genetic make-up of the host – influence the interaction with the parasite

– sickle-cell trait confers some protection from Falciparum malaria

• Nutritional status of the host– high protein diet not suitable for the development of intestinal protozoan

• Immune processes– Acquired immunity important in modifying severity of disease in endemic areas

Immune Defense Against Parasitic Infections

• Non Specific Defenses– intact skin: hookworms synthesize a protein that aids in the penetration process

– acidic pH of vaginal secretion and gastric juice: T. vaginalis trohphozoites: unable to survive the acidic environment of the vagina

• Mucus secretions – envelop parasites (G. lamblia) affecting its motility and reducing pathology on the host

• Chemical components of body fluids– Lipase content of breastmilk: toxic go G. lamblia in vitro

Immune Defense Against Parasitic Infections