PARASITIC ATTRIBUTES THAT INFLUENCE DISEASE
a. Virulence. Virulence refers to the ability of a parasite to establish itself in a
host, maintain that infection, and damage the body of the host. For one reason or
another, one strain of a parasite may cause a more serious disease process than other
strains of the same species.
b. Parasitosis. The more parasites infecting a host, the more severe will be the
effect on that host.
c. Life Span of the Parasite. The longer the parasite lives in or on the host, the
greater the damage that will be caused.
d. Repeated Contact. The more times that a host is parasitized by an
organism, all other factors being equal, the worse the prognosis of the disease.
e. Competition for Food. Intestinal parasites deprive the host of the necessary
nutrients required for survival.
f. Mechanical Interference. Some parasites accumulate (like in the intestines)
in such great numbers that the normal flow of nutrients, waste, or fluid is obstructed.
For example, the microfilariae of some nematodes obstruct the lymphatic system in
elephantiasis. Their presence prevents lymphatic fluid from being circulated out of
lymphatic tissue and the swelling associated with elephantiasis results.
g. Toxic Effects. By-products of metabolism and anatomic parts of the parasite
can be very toxic to the host. In cases of massive infections with Trichinella spiralis, it is
this toxicity that is lethal to the host.
h. Tumor. Tumor formation in the host is a common occurrence with parasites
that invade or irritate the tissue of the host. Pathologists have suggested that repeated
infections with Trichomonas vaginalis could lead to cervical cancer, and recently, some
cases of malignant neoplasms have been reported from Egypt and attributed to
Schistosoma infections. Some benign tumors are caused by the larval stage of the
tapeworm Taenia solium in muscle tissue.
i. Loss of Blood and Body Fluids. The loss of blood and body fluids
caused by parasites is significant. For example, it has been calculated that about 0.5 ml
of blood per worm per day is lost by the host during a hookworm infection. Remember,
these fluids contain nutrients and electrolytes needed by the host.
j. Destruction of Host Tissue. Parasites destroy the host's tissue structures
by the presence and activity of the adults, by the migration of larval forms, and by