Introduction to infectious agents

Prof.J.Sarath EdirisingheMBBS (Cey), MSc (Lond), PhD (Lond), MD (Col)

Consultant Medical ParasitologistPara-clinical Coordinator

Faculty of Medicine & Allied Sciences,Rajarata University of Sri Lanka

Saliyapura,Anuradhapura

Infectious agents

Prions

Viruses

Bacteria

Micro-organisms

Macro-organisms

Protozoa

Helminths

Arthropod vectors

Some protozoan infections found in Sri Lanka

Malaria

Giardiasis

Amoebiasis

Leishmaniasis

Macro-organisms

Helminths

Parasitism is an association between two organisms of different species which is beneficial to one organism and harmful to the other. .The organism that is benefited derives food (metabolically dependent) and shelter (physically dependent) from the other

Parasitism

Parasite

A parasite is an organism that derives food and shelter through a parasitic relationship and usually is the smallest of the associating pair.

Ectoparasite

Endoparasite

Lives outside the body

Lives inside the body

Facultative parasite

A facultative parasite is one which is not totally dependent on the host and is able to adapt to a free-living existence (depending on the favorability of the environment)

Obligatory parasites

These parasites are totally dependent on the host and are unable to live without the host.

Host

In a parasitic association the larger organism of the associating pair is called a host and is the one that provides food and shelter to the parasite

Definitive host

A definitive host is the one in which sexual multiplication of the parasite takes place

Intermediate host

IH is the one in which the larval stages of the parasite are present or in which asexual multiplication of the parasite takes place

Vector

When one host is involved in the transmission of a parasite from one host to another, it is called a vector

Biological vector

Sometimes a parasite completes a part of its life cycle while being transported by a vector. Such a host is called a biological vector

Mechanical vector

When a parasite is merely transported without any change in the stage of the parasite, the vector is called a mechanical vector

Infection

Infection is defined as the presence of living parasites in the body of the host

Disease

The manifestations the host experiences as a result of infection collectively constitutes a disease

Clinical signClinical symptom

Parasite transmission

1. Active penetration

2. Ingestion of infective stages found in the environment via food or water

3. Transmission by blood sucking vectors

4. Transmission by mechanical vectors

5. By close contact with infected persons and via clothing and bedding

6. By inhalation of the infective stage into posterior pharynx followed by swallowing

7. Via sexual intercourse

8. Transplacental or congenital transmission

9. By way of blood transfusion

Malaria

Lymphatic filariasis

Roundworm infection - Ascariasis

Hookworm disease

Pinworm infection - Enterobiasis

Giardiasis

Cutaneous leishmaniasis

Trichomonas vaginalis infection -trichomoniasis

Whipworm infection - trichuriasis

Scabies

Amoebiasis

Dirofilariasis

Geographical distribution of parasitic infections - Sri Lanka

Malaria- traditionally dry zone <1750mm annual rainfall

Filariasis -South western coastal belt - ‘filarial belt’

Books recommended - • Manson’s Tropical Diseases• Worms and Disease by Ralph Muller• Parasites of Man by Sarath Edirisinghe

These books are available in the Saliyapura library. Therefore do not buy!!