(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
(4) Arthropod-borne. Members of the phylum Arthropoda serve as vectors
(carriers) of parasitic diseases and bacterial and viral infections. The vector is an
integral part of the life cycle of the parasite. In some instances the arthropods are
intermediate hosts while in others they are the definitive host.
(5) Active penetration. In some parasitic infections, the infective form is a
larval stage that has the capability of penetrating the host tissues.
(6) Transfusion. Certain blood and tissue parasites may be present in
donor blood at the time of transfusion. Thus, these parasites may be introduced into the
new host system and cause parasitic infection.
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 animal
wastes can also be used in combating parasites. Treatment of diseased individuals not
only relieves the suffering, but also prevents the spread of the disease.
THE EVOLUTION OF PARASITES
It is believed that at one time parasites were free living and that the loss of
genetic information forced them to adapt to a parasitic existence over many years.
a. Hybrid Vigor. Some parasites replicate by asexual methods. This type of
reproduction tends to concentrate detrimental genetic traits. On the other hand, sexual
reproduction adds vitality to the species.
b. Mutation. Mutations, in the majority of cases, tend to be detrimental to the
species, but occasionally the change may become beneficial by adding or subtracting a
trait that tends to enhance the survival chances of the mutated species.