Veterinary Science Mimeograph
Series No. 55-3
UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATION!
WILLIARD M. FIFIELD, Director
PARASITES OF ANIMALS INFECTIOUS TO
Leonard E. Swanson, Walter R. Dennis and William M. Stone, Jr.
I. General Introduction:
A. Parasitology is the science that deals with parasitism.
1. A parasite is an animal that lives upon or within another
living organism at whose expense it obtains nourishment and
protection without giving anything in return for this relation-
2. Parasitism is the relationship between the host and parasite.
3. Helminth is a defined term for worms.
4. Helminthology is the study of wormd.
B. All animals are parasitized with one form or another of these
C. Wild animals are especially heavily parasitized and quite often
serve as reservoirs of infection for domestic animals and man.
D. Domestic animals are a constant source of possible infection of
parasites to man.
II. Relationship of Parasites:
A. Symbiosis The living together of two dissimilar organisms
to the advantage or disadvantage of one or both.
1. Mutualism is a form of symbiosis in which both symbionts are
in more or less measure, benefited by the association.
Example: Protozoan fauna of the intestinal tract of termites
change wood materials to digestible forms.
2. Commensalisn is a form of symbiosis in which one symbiont is
benefited, but its co-symbiont is neither benefited nor harmed.
Example: Common colon amebia of man which feed on waste products.
3. Helotism is a form of symbiosis in which one organism enslaves
.^ Example: Dermatobia hominis, jSouth American skin maggot. The
adult female fly captures other insects, deposits her ova on
their bodies and they in turn serve as a means of transportation.
The maggot larvae are liberated from the eggs while the insect
carrier is obtaining its blood meal from man or beast.
h. Parasitism is a form of symbiosis whereby the parasite derives
its food and protection, usually at the expense of the host animal,
yet in light infections this association may be of no harm to either
organism. Biologically it is not the intention of parasites to
overparasitize the host to a point whereby impaired health or death
of the host will result in destruction of the parasite itself.
B, In summary, a symbiont receiving the benefit is the parasite and
the one receiving the damage is'the host.
1. Fi-l1 or definite hosts harbor adults or sexually mature parasites.
2. Intermediate hosts harbor larval forms, where an intermediate
host is required to complete the life cycle.
III, Zooparasites are parasites which belong to the animal kingdom:
A. Ectoparasites are those that are parasitic to the surface of the
body' or have direct contact to the skin.
Examples Lice, fleas, ticks, mites, fly larvae, etc.
B. Endoparasites are parasites which enter the body of the host,
inhabiting the alimentary canal, liver, lungs, blood and other
Example: Ascarids, hooks, flukes, amebia, etc.
IV. The influence of parasites upon the host depends upon the number and
age of parasites present, location, the nature of their food, route
through the body of larval forms, environmental conditions, and age
of the host. The effects of parasites on the host also depend on the
balance of minerals, vitamins, proper food intake and general condition
of the host.
V. Parasites Injure the Host:
A. Through mechanical injury such as biting, migration and loss of blood.
Example: 1. Biting-lice.
2. Iigrating-ascarid or fluke larvae causing tissue destruction.
B. Through obstruction as in the case of a mass infection of ascarids
blocking the intestinal tract and/or bile ducts&
C. By injection of toxins suc as occur in extreme hookworm infections.
D. Tissue feeding of maggots.
E. Anaphylactic reaction extreme sensitivity to touch of ascarids.
VI. Internal p-.rasites of man and animal fall into the following phyla
A. Phylum Platyhelminthes
1. Class Trematoda
Examples: Liver fluke
ab Fasciola hepatica and F. gigantica.
b. Clonorchis sinensis.
a. Schistosoma japonicum.
a. Fasciolopsis buski.
2. Class Cestoda
Example : tapeworms
a. Taenia saginata
b. Taenia solium
c. Echinococcus granulosus
d. Diphyllobothrium latum
B. Phylum Nemathelminthes
1. Class Nematoda
a. Necator americanus
b. Ancylostoma braziliense
c. Enterobius vermicularis
C. Phyly Protozoa
1. Class Infusoria
Example: a. Balantidium coli
2. Class Sarcodina
Example: a. Endamoeba coli
b. E. histolytica
3. Class Sporozoa
Examples a. Isospora hominis
b. Plasmodium falciparum
4. Class. jtigophora
Example: a. Trichomonas vaginalis
VII. Internal parasites of man listed according to their location in
the body are:
A. Bronchial tubes of the lungs
1. Lung fluke
a. Paragonimus westermanii
1. Liver flukes
a. Fasciola hepatica
b. F. gigantic
a. Echinococcus granulosus
C. Small Intestine
1. Threadworms (microscopic in size)
a. Strongyloides stercoralis
a. Necator americanus
a. Ascaris lumbricoides
a. Taenia saginata
b. Taenia solium
c. Dipylidium caninum (occasional)
d. Diphyllobothrium latum
D. Large Intestine
a. Balantidium coli
b. Endamoeba coli
c. Endamoeba histolytica
1. Filaria Wuchereria bancrofti (elephantiasis)
2. Larval migration
a. Ancylostoma braziliense
b. Ancylostoma caninum
1. Larval migration
a. Trichinella spiralis
VIII. Life Cycles:
A. All parasites of the class Nematoda are what is termed direct
infectors. No intermediate host is required, with the exception of
trichinella, which require an intermediate host, the pig, bear, dog, etc.
1. Direct infectors depend on optimum moisture, temperature and
vegetative conditions to complete their life cycle. The eggs are
passed out of the host animal in the feces and hatch, liberating
a minute larva. This larva, after three or more molts, is known
as the infective stage, which is ingested or enters the body by
skin penetration. Upon reaching the digestive tract the larva
finds its suitable location in the host where it reaches sexual
maturity, mates and begins laying eggs.
2. Filaria depend upon mosquitoes or other biting insects to com-
plete their life cycle. The female worms deposit their larvae in
the host bloodstream and/or tissues. The insect vector, feeding on
the infected animal ingest the microscopic larvae, which further
develop in the insect and at a subsequent feeding these larvae are
injected into another host animal.
B. Parasites belonging to the class Trematoda require an intermediate
1. Liver flukes, and others of the Trematoda class require a fresh
water snail to complete their life cycle*
a. Fasciola hepatica and Fasciola gigantic
1. The young flukes that hatch from the eggs burrow into the
soft tissue of a fresh water snail where they develop and
multiply. After a period of time they are liberated into
water as young free swimming flukes. These young flukes
swim around in water, attach themselves to vegetation and are
subsequently eaten by man. This stage in the life cycle is
known as the infective stage to man or beast.
b. Clonorchis sinensis
1. This fluke life cycle is identical with the exception that
it requires two intermediate hosts, a fresh water snail as the
primary intermediate host and a fresh water fish as the secon-
dary intermediate host. Man may become infected by eating
improperly prepared fish.
2. Blood flukes.
a. Schistosoma japonicum
1. Blood fluke life cycles are identical to the above except
that there is a skin penetration by the cercariae stage instead
of encystment on vegetation and no secondary intermediate host
3. Intestinal flukes
a. Fasciolopsis buski
i. The intestinal fluke life cycle is identical witl other
flukes except that the infective stage encyst on water
chestnuts and other plants eaten raw by tropical man and no
secondary intermediate host is required.
C. Parasites belonging to the class Cestoda requirean intermediate
1. Taenia saginata; Taenia Solium and Echinococcus granulosus.
a. The human tape worm (Taenia saginata) adults are found in
the human intestinal tract. The gravid segments are passed to
the exterior, ingested by cattle and the embryo penetrates the
intestinal wall, is picked up b the vascular system and carried
to the.muscles. They usually locate in the heart and cheek
muscles. In heavy infections they may be found in any muscle
of the body. If man eats raw or partially cooked meats, he
may become infected. In modern meat plants inspecting veter-
inarians examine each and,every animal for these cysts and are
able to reduce the incidence of human tapeworm infections.
b. The human tapeworm (Taenia solium) life cycle is identical
with the above except that swine are the intermediate hosts.
It is also possible for man to serve as both the final and
the intermediate host.
c. The dog tapeworm (Echinococcus granulosus) adult lives in
the intestines of dogs and wild carnivera. The tapeworm
segments are passed from the host with fecal materials which
may contaminate human food and/or water supplies. The in-
fective embryo penetrates the intestinal wall, is picked up by
the vascular system and carried to various parts of the body,
especially the liver where it develops into an infective hydatid
cyst. Sheep, goats and wild ruminating animals also may eat
the contaminated food and become infected as is man. Wild or
domestic canine animals may eat these parasitized intermediate
hosts and become infected with the adult tapeworms.
1. These parasites may be direct or indirect infectors depending
upon the parasite in question. In the direct infector group man
becomes infected by consuming contaminated food, water and direct
contact with the organism. House flies, birds, rats, man, etc.
may serve as mechanical means of transporting these infections.
The indirect group depend upon insect vectors to complete their
a. Those belonging to the malaria group require a mosquito to
complete their life cycle, whereas coccidia which belong to the
same class are direct infectors.
b. Those belonging to the trichomonas, ameba and balantidium
groups are direct infectors.
IX. Damage d6ne to the host:
A. Parasites, depending upon their location in the host and type of
food required for their existence, injure the animal by feeding on
blood, bacteria, tissue, bile, epithelium, lymph, or by mechanical
blocking of the essential physiological functions of the respective
organs or tissues of the body.
Internal parasite infection, whether it be in man or other animal,
cannot be seen in its protected home. The infection may be likened to
a closed book; you do not know of its presence or absence unless accurate
methods of diagnosis are utilized.
Infection may be suggested if man exhibits symptoms of anemia, enlarged
abdomen, irritability and general unthriftyness. The most accurate diag-
nosis universally used for determination of parasitism in man is the
microscopic examination of specimens of feces and/or blood for parasite
ova and/or larval forms of parasites.
In general parasites must live congenially with their host in order
to produce their kind; however, if by accident, overcrowding, improper
diet, unsanitary premises, man may become excessively parasitized.
The most critical point to be emphasized in the control of parasitism
in humans is farm, home and personal sanitation. If modern toilet
facilities are not available, outside toilets should be constructed
so as to exclude fly contamination or possible animal invasion. Locate
the toilet where possible contamination of food or water supply for man
or animal cannot occur and this includes protection from heavy storm
damage. It is realized that average individuals will not normally use
barns, fields and woods for human evacuation but it does occur and should
be discouraged. Fresh vegetables and fruit eaten raw-should be thoroughly
washed, especially those from farmers who may be prone to use night soil
as a fertilizer.
The practice of allowing children to work and play barefoot in tropic
Sand subtropic areas should be discouraged as this invites heavy
hookworm infections. On beaches and public gathering places where
dogs roam, there is always the possibility of picking up creeping
eruption or hookworm infection and this can only be guarded against,
according to our present knowledge, by wearing protective clothing.
Avoid as much as possible these contaminated areas. Dogs that are kept
as pets around homes should be watched closely and treated by a
Veterinarian for the removal of hookworm infection. Also, on your own
premises, it is good common sense to pick up all bowel passages from
dogs and bury or burn same daily. The above not only lessens the
possibility of creeping eruption, but prevents dogs and man from
The above statements apply equally well to infections with Strongyloides
stercoralis. Trichinella or tapeworms of humans are only contracted by
consumption of raw or partially cooked meats, especially meats from
uninspected sources. Pork products even from inspected establishments
should not be eaten unless cooked thoroughly. The meat should have a
greyish appearance and not pink or red when cooked. Beef from inspected
establishments may be eaten raw or partially cooked with relative safety
as the cysts can be located at time of slaughter. If found, proper steps
to eliminate the parasite in the carcass are taken before it is offered
for human consumption. Fish products should only be eaten after proper
preparation and thorough cooking.
As this outline is prepared for public health sanitarians, it is felt
that they can render the greatest service to communities by advising
their people on methods of prevention, and in assisting Physicians and
.Veterinarians in the curtailment and elimination of parasites. Swimming
pools, ponds, lakes or streams should not be used for bathing or drink-
ing purposes until proper sanitary measures and examinations are taken
to reduce possible human infections.
1. Belding, David L. 1942. Textbook of Clinical Parasitology.
Appleton-Century-Crofts, Inc. New York, N.Y. 888 pp.
2. Morgan, Banner B. and P.A. Hawkins. 1951. Veterinary Helminthology.
Burgess Publishing Company. liinneapolis, Minnesota. h00 pp.
3. Underhill, B.il. 1924. Parasites and Parasitosis of the Domestic
Animals. The Iacmillan Company, New York, N.Y. 379 pp.