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1.1.1 PARACITE
A parasite is any organism, either animal or plant, which during the whole or part of its life, lives on or in the body of another organism of different species from which it derives its sustenance, at the expense of the host, for the whole or part of its existence.
1.1.2 Parasitology
Parasitology is the area of biology concerned with the phenomena of dependence of one living organism on another. Parasitism is the form of association between two organisms in which one (parasite) lives inside or on an other associate (host). Parasitism is only one of many animal associations. In most instances of parasitism, the association is for the benefit of the parasite and to the detriment of the host. It must be emphasized that parasitism is a condition of life normal to and necessary for parasites and because they feed at the expense of their hosts, their aim is not to destroy the host.
1.1.3 Types of Parasites:
a) Ectoparasites:
Parasites which live on body surfaces of the host; e.g ., lice, fleas, ticks, etc.
b) Endoparasites:
Parasites which live inside the host body in tissues or organs such as blood, peritoneal cavity, brain, etc.; e.g., liver fluke, Ascaris, malaria parasites, etc.
c) Facultative parasite:
An organism which is able to live either a free living or parasitic existence; e.g., Strongyloids Stercoralis of man.
d) Obligatory parasite:
Organism which has become completely dependent upon its host for existence.
e) Aberrant parasite:
Found in locations in the host where they normally do not occur; e.g., Ascaris larvae may migrate to the brain
f) Insidental parasite:
Occurs in hosts where it does not normally occur; e.g., Fasciola normally does not occur in man but is incidental if found in man’s liver.
g) Periodic parasite:
Feeds on host but does not live on host; e.g., blood sucking flies.
h) Hyperparasite:
Parasitizes another parasite; e.g., Histomonas meleagridis (a protozoan) is hyperprsitic on the nematode worm Heterakis gallinae.
i) Monoxenous parasites:
Those with direct life cycles (i.e., with one host).
j) Heteroxenous parasites:
Those with inderect life cycles requiring an intermediate host (i.e., involves 2 or more hosts).
k) Heterogenetic Parasites:
One with alteration of generations e.g., Coccidial parasites and Strongyloides.
l) Euryxenous parasites:
Those with a broad host range.
m) Stenoxenous parasites:
Those with a narrow host range; e.g., host specific coccidia.
n) Parasitiasis:
Refers to the disease state associated with the presence of parasites in the host - an unbalanced association in which the host shows symptoms of infection.
o) Parasitosis:
Refers to the disease state associated with the presence of parasites in the host - an unbalanced association in which the host shows symptoms of infection.
p) Life Cycle:
Refers to the development of a parasite through its various forms - which may be from egg to adult stage for helminths or either asexual or sexual stages or both for protozoa.
q) Other terms include principal host, supplementary host, incidental host and experimental host, - given in terms of importance and utility purpose.
It must be noted that most animals live independently in their natural habitats. Between some of them, however , a number of patterns of association have evolved. These associations are either homogenic, i.e., between individual of the same genotype; or heterogenic, i.e., between individuals of different genotype. Parasitism and associations related to it belong to the heterogenic type and these are:
1.2.1 Commensalism:
This is a type of loose association in which two organisms of different species live together without either being metabolically dependent on the other although one or both organisms may receive benefit from the association. The organism which benefits from the association is the commensal while the other is the host; e.g., Crustacea and corals live together in the sea with the former using the coral colony as a hunting ground.
Commensalism is further subdivided into:
a. Mutualistic commensalism or Mutualism:
One in whioh species share common benefit from the association; e.g., Sea anemone attaching to the shell of hermit crab and Hydra Viridis on convoluta, a turbellarian worm.
b. Phoresy or epizoic commensalism:
One in which an organism merely provides shelter, support, or transport for another organism of a different species; e.g., peritrichus ciliates attached to the body of fishes.
c. Obligatory Commensalism:
One in Which the two organisms must always live in association; each of them cannot live separately.
1.2.2 Symbiosis:
This is an association between two dissimilar organisms in which there is complete mutual benefit. This term is normally restricted to associations of a special kind in which the participating species are dependent on each other for existence. The association is als0 permanent: e.g., wood eating termites and hyperflagellates in their intestine or association between intestinal ciliates and their ruminant host.
Symbiosis should be differentiated from mutualism in which the association is not obligatory to existence; e.g., Hydraviridis and the alga Zoochlorella. In symbiosis, the associates are termed "symbiotes". If there is a large discrepancy in the size of the symbiotes, they are referred to as macro- and microsymbiotes.
1.2.3 Helotism:
Aform of symiosis in which one organism enslaves the other; e.g., Dermatobia hominis and mosquitoes.
1.2.4 Inquilinigm:
A form of an animal association, especially in marine animals, in Which one partner (inquiline) lives within certain sheltered areas (nest, hole, or home) provided by the host; e.g., shrimps and gobiid fish in tropical seas. This type of association is rare in the terrestrial environment.
1.2.5 Predator:
This is a carnivorous animal Yhich hunts and eats other animals. The term "predator" describes what an animal eats and how it obtains its food and should be differentiated with herbivore and omnivore which describes what an animal eats.
Even though hematophagous arthropods are usually referred to as "parasitic arthropods,U" they do not fall into the category of parasitism because they are not adapted to a parasitic environment. They can be considered "micropredators" because they are very such smaller than their prey and do not usually
kill as does for example a fox which can be considered a "macropredator."
Parasites are adapted to exploit the rather unusual environment provided by the interior of another living organism which is referred to as the host. There are different types of hosts:
1.3.1 Definitive host or Primary host:
Harbors the adult or sexually mature stage of the parasite (mosquito - Plasmodum).
1.3.2 Intermdiate host/Secondary host:
This is the host which harbors the developing larval, immature, and juvenile young
stages of a parasite. The intermediate host may harbor many immature stages of a parasite; e.g., Cercaria, Redia and Sporocysts which are all immature stages of Fasciola in the
snail intermediate host. Some parasites require more than one intermediate host which are then designated as first, second intermediate, etc.
It should be noted that parasitic disease which require internediate hosts for transmission are restricted to the distribution of those hosts. The distribution of intermediate hosts is deternined by many factors such as climate, vegetation, historical factors, etc.
1.3.3 Paratenic/Transport host:
This is a host in which the immature stage of the parasite does not normally undergo developnent but only remains encysted until the paratenic host is consumed by the final host; e.g., second stage larva of Toxocara in chickens, lambs, rodent, invertebrates, etc.
1.3.4 Reservoir host:
This is a vertebrate host in which a parasite or disease occurs naturally and which is a source of infection for man or domestic animals. The parasite usually exists in massive numbers in a reservoir host and man or domestic animal may become infected when they enter the locality where the parasites or diseases exist. Such a locality is known as "ridus" and is used primarily in connection with vector-borne diseases.
1.3.5 Tangenital/Incidental host:
This is a host which is only infected with a parasite circumstantially.
1.3.6 Carrier:
A carrier is an animal or man which has a light infection with a parasite but is not harmed by it, usually due to immunity resulting from previous infection, but which serves as a source of infection for susceptible animals or men. For example:
- a. Adult sheep and cattle may be lightly infected with gastrointestinal nematodes without noticeable effect, but
- their lambs and calves may become heavily parasitized from grazing with them. The condition in the adults is called parasitiasis and in the young, parasitosis.
- b. Chickens rarely suffer from coccidiosis because they have recovered from a clinical or -subclinical attack when they were young. However, they still harbor a light infection and continue to shed a fee oocysts (coccidiasis) which is a source of infection to the young chicks.
- c. Cattle previously exposed to anaplasmosis become pre-immune to the infection (few parasites in the blood) but continue to be a source of infection to the ticks.
1.3.7 Vector:
This is an organism (usually an arthropod) which transfers infective forms of a parasite from one host to the other. We differentiate between a mechanical and a biological vector. In a mechanical vector, the parasite does not undergo development before the transfer to another host is effected. Biological vector is characterized by the development of the parasite before its transfer to another host. The following are the distinguishing characteristics of biological and mechanical vectors:
Character Mechanical Vector Biological Vector
Development Does not take place Takes place of parasite Period of Immediately after No infectivity of infectivity ingestion of parasite vector after ingestion, and lost shortly but developmental afterwards phage sets in. Infectivity of Yector is acquired after developmental phase which is called Lag phase Length of Very short For life; hence life Infectivity expectation of vector is important Specificity Not specific Specific How Do Parasites Cause Iniury to Their Host? (Degree of injury, produced)
The pathogenicity of parasites varies greatly, depening on the number present, their habitats, their migratory habits during larval development, their degree of adaptation to the host, resistance of the host, etc. When multiplication of the parasite occurs in or on the host, as in lice, mites, and many protozoa, the increase in number is an important factor in producing detrimental effects. It seems to be true that a few parasites ally do not affect the health of the host but this may be an apparent fact" only, on account of our inability or lack of equipment to measure and , therefore, that in almost all instances there is a certain ‘threshold value," or number of parasites that can be harbored without so called ill effects. This number would vary greatly for different species of hosts, for different parasitic species, and for individuals within a host species.
Parasites may do harm to their host in a number of ways, some of which are listed below:
In other words, the injury is brought about by interference in some way by the parasite, possibly by its bulk, activity, or physiological products associated with its metabolism.
All living things are classified according to the International Code of Zoological Nomenclature. Each organism belongs to a phylum (or subphylum), class, order,family, genus, and species. At times, further divisions of suborder, superfamily, subfamily, and subspecies is employed. The family name ends in "idae," the superfamily in "oidea," and the subfamily in "inae". The names are latinized and the scientific designation is binomial for species and rinomial for subspecies. The first name is the generic name or genus which begins with a capitol letter. The second name is the species or specific name which begins with a small letter. When in print, the complete name is in italics or underlined Ascaris lumbricoides, if the typewriter does not provide the former. (Brown, H.W.)
The survival of, or perpetuation of parasites in nature, usually depends upon several factors; the presence and habits of suitable hosts, environmental influence upon stages which exit the host, method of transfer, etc. Warm moist conditions favor parasite transfer or transmission in nature when the life cycle is direct or indirect, thus permitting parasite forms (egg, larvae, etc) to develop to the infective stage faster, either on ground or in the intermediate host. On the other hand, parasites which grow best under the above conditions would die out in dry, hot or freezing conditions. The intermediate hosts may not exists in some areas when two hosts are involved, for example, the intermediate host for African trypansosomiasis does not exist in America ad USA. Economy, living standards, education, religion, customs or traditions play important roles in the endemicity of many parasites. Endemic refers to the occurrence of a disease or parasite in a relative small number of individuals in a given locality. (Hookworm in persons of the Southeast was a problem of this magnitude at one time). Epidemic refers to the occurrence of a rapid spreading disease involving a larger number of prsons (term used in human medicine). Epizootic is the comparable term used for animals. Pandemic involves a larger area and rapid spread. Infection refers to the presence or establishment of parasites (pathogens) within the host. Infestation refers to the establishment of parasites on th skin surface, mostly involving ectoparasites such as mites, ticks, lice, fleas, flies, etc.
A number of parasites are transmitted from animals to man or vice versa. Parasitism or diseases of this nature are forms of zoonosis.
The animal parasites of humans and most vertebrates are contained in five or more major divisions or phyla. The single cell Protozoa, long considered one phylum, have recently been divided into a number of groups assigned phylum rank. Those containing organisms that can parasitize man include the Sarcomastigophora (subphyla Mtigohora, the flagellates; and Sarcodina, the amebas), Ciliophora, Apicomplexa, and rarely, the Microsphora. Other phyla containing parasitic species include the Platyhelminthes or flatworms; the Aschelminthes, of which the important group to us is the class Nematoda or roundworms; the Acanthocepbala, or thorny-headed worms; and the Arthrodpods, which includes the insects, spiders, nites. ticks, etc. With the exception of the apicomplexa, Microspora, and Acanthocephala, all of these phyla contain both parasitic and free-living forms. Within each phyla, only those groups that include species of medical importance will be discussed here and in the order qiven above.
The parasite lives in an abnormal environment, so it must adapt to the internal environment of its host through morphological and physiological modifications. The following adaptations should be noted:
In this course, we shall be dealing with three broad groups, namely:
(GK. protos = first; and zoon = animal)
The protozoan parasites we are interested in fall into the following groups:
(GK. helminth or helminthos = worm)
The helminth parasites fall into three groups:
The helminths of medical and veterinary importance belong to:
There are 4 classes in the phylum Platyhelminthes. Two of thes; i.e. Tubellaria and Monogenea, are of no medical importance. The two classes of medical importance are Trematoda and Cestoda.
Of the three orders within Class Trematoda only the order Digenea is of medical importance. The order Monogena is mostly ectoparasites of cold-blooded aquatic vertebrates while the order Aspidogastrea is mostly endoparasites of aquatic hosts.
These constitute the flukes which parasitize the domestic animals and man. Characteristics of digenic trematodes:
In a group of trematodes (the holostomes), an additional large adhesive organ call "holdfast" occurs behind the acetabulum.
The Arthropods we are interested in, fall into three groups:
