Adaptations can be classified as either progressive or

regressive, or biological.

•Progressive adaptations -

Parasite has developed certain of its parts to a higher level

of functionality.

•Regressive adaptations -

Involve the simplification or disappearance of certain organs

of the parasite.

•Biological adaptationsEnable the parasites to survive better, but cannot be

explained as simply atrophy or development of new organs

for the parasites.

MORPHOLOGICAL ADAPTATIONS OF PARASITES

NATURAL CIRCULATION OF PARASITES

In order to survive, ecto- and endoparasites have evolved to be able

to leave the host and to change hosts. The range o possible hosts

and the way parasites leave and enter their hosts determine the

travel paths of parasites in nature, as well as those of the diseases

they cause.

The typical natural sources of infectious disease-causing agents are

the people and animals already hosting the disease, or carriers of

parasites who spread them in the outside environment.

E. coli

pasiutligės

virusas

gripo sukėlėjas

Bacteriphages penetrate

into E. coli bacterium

• The process by which an organism is entered by disease-

causing agents is known as infection, and the resulting diseases

as infectious diseases.

Cold

snuffle

The process by which parasites (that is protozoa, helminths,

arthropods) enter the host is known as infection, and parasitic diseases

are also termed as infective diseases.

Balantidium coli

Echinococcus

granulosus

Cimex lectularius

Trichomonas vaginalis

Pediculus vestimenti

•An organism (human or animal) in which a

disease-causing parasite lives (viruses, bacteriae,

riketsiae, fungi, protozoa, helminths, arthropods),

develops or reproduces is termed as an infection

source.

Autoinvasion – when individual themselves can become the

sourse for self infection.

Reinvazija – when individual catches repeated infection after

recovery.

It somethimes occurs that a parasitic disease moves from a

symptomatic to an asymptomatic form. A disease without clinical

symptoms is known as a latent disease, and the period it lasts as the

latent period.

If the symptoms of disease, after recovery, take a turn for the

worse, such a situation is termed a relapse of the disease.

An organism in which parasites can accumulate and survive for extended

periods of time is known as a reservoir host.

A natural reservoir is a habitat suitable for sustaining parasite life in the

natural environment.

There are several types of natural reservoir:

• Soil reservoir

• Aquatic reservoir

• Above-ground (terraneous) reservoir

• Technogenic reservoir

Technogenic reservoir

Blatta orientalis

Blatella germanica

Mechanical vectors simply transport

disease-causing agents from host to host.

No development of the parasites occurs in

these vectors.

Vectors are critical to the natural circulation of many

parasites. Most vectors are bloodsucking arthropods

(insects and arachnids). Vectors may be classified into:

•mechanical vectors

•specific vectors.

Musca domestica

Specific vectors are intermediate hosts as well: the

parasites develop in them and are typically passed on at

the end of that development. Consequently, there is

normally a very limited number of spieces (perhaps only a

single spieces) that may serve as specific vectors for any

given parasite

Glossina morsitans Anopheles freeborni

THE CLASSIFICATION OF INFECTIOUS DISEASES

Infectious diseases may be classified by their causing

agents:

1. Viral diseases - caused by viruses

2. Bacterial diseases - caused by bacteria

3. Fungal diseases - caused by fungi

4. Parasitic diseases - caused by parasitic protozoa,

helminths, arachnids and insects.

5. Transmissional diseases are those infectious diseases

which are transferred host-to-host by some transmitting

organism (the vector).

There are two types of transmissional diseases:

obligate transmissional and facultative

transmissional.

•Obligate transmissional are those diseases that

can only be transmitted through a vector,

•Facultative transmissive are those diseases which

may be transmitted with or without a vector, i.e. the

vector is not absolutely necessary.

Infectious diseases may also be classified according to

organism they infect.

1. Anthroponoses are human-only diseases, such as

amoebiasis or trichomoniasis.

2. Anthropozoonoses are diseases of both humans

and animals, such as tick-borne encephalitis,

leishmaniasis, toxoplasmosis.

3. Zoonoses are diseases passed from animal to

animal. Humans can serve as mechanical vectors for

these, as in the examples of cattle lung plague or bird

malaria, or horse breeding disease