S Mimeographed Report, 59-1 December, 1958
UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATI O HUME LIBRARY
JOSEPH R. BECKENBACH, Director
AUG 4 1972
PARASITES OF ANIMALS INFECTIOUS TO MAN
by I.F.A.S. Univ. of Florida
Leonard E. Swanson, Parasitologis
Department of Veterinary Science
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 worms.
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. Commensalism 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, South 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 4hile the insect
carrier is obtaining its blood meal from ian or beast.
4. 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 parasite'
to overparasitize the host to a point whereby impaired health or
death of the host will result in destruction of the parasite
B. In summary, a symbiont receiving the benefit is the parasite and the
one receiving the damage is the host.
1. Final 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.
- 2 .
Example: Lice, fleas, ticks, mites, fly larvae, etc.
B. Endoparasites are parasite; 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. Migrating-ascarid or fluke larvae causing tissue
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 such as occur in extreme hookworm infections.
D. Tissue feeding of maggots.
E. Anaphylactic reaction extreme sensitivity to touch of ascarids.
VI. Internal parasites of man and animal fall into the following phyla
A. Phylum Platyhelminthes
1. Class Trematoda
Examples: Liver fluke
- 3 -
a. Fasciola hepatica and F. gigantica.
b. Clonorchis sinensis.
a. Schistosoma japonicum.
a. Fasciolopsis buski.
2. Class Cestoda
a. Taenia saginata
b. Taenia solium
c. Echinoooccus granulosus
d. Diphyllobothrium latum
B. Phylum Nemathelminthes
1. Class Nematoda
a. Necator americanus
b. Ancylostoma braziliense
c. Enterobius vermicularis
C. Phylum Protozoa
1. Class Infusoria
Example: a. Balantidium coli
2. Class Sarcodina
Example a. Endamoeba coli
b. E. histolytica
3. Class Sporozoa
Example: a. Isospora hominia
b. Plasmodium falciparum
4. Class Miastigophora
E'-ample: a. Trichomonas vaginalis
VII. Internal parasites of man listed according to their location in the
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
o. 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. Live Cycles:
A. All parasites of the class Nematoda are what is termed direct infec-
tors. No intermediate host is required, with the exception of
trichinella, which require an intermediate host, the pig, bear, dog,
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 gigantica
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 vege-
tation and are subsequently eaten by man. This stage in
the life cycle is known as the infective stage to man or
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 secondary 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 ex-
cept that there is a skin penetration by the cercariae
stage instead of encystment on vegetation and no secon-
dary intermediate host is required.
- 7 -
3. Intestinal flukes
a. Fasciolopsis buski
1) The intestinal fluke life cycle is identical with 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 require an 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 by 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 plants inspecting
veterinarians examine each and every animal for these cysts
and are able to reduce the incidence of human tapeworm
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
c the intestines of dogs and wild carnivera. The tapeworm
segments are passed from the host with fecal materials which
- 8 -
may contaminate human food and/or water supplies. The
infective 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 in-
fected as is man. Wild or domestic canine animals may
eat these parasitized intermediate hosts and become in-
fected 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 infec-
tions. The indirect group depend upon insect vectors to complete
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 done 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.
- 9 -
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 diagnosis
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, un-
sanitary 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 in-
dividuals 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
and subtropic areas should be discouraged as this invites heavy hookworm
infections. On beaches and public gathering places where dogs roam, there is
- 10 -
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 hookworm infection.
The above statements apply equally well to infections with Strongyloidas
sterooralis. Trichinella or tapeworms of humans are only contracted by consamp-
tion of raw or partially cooked meats, especially meats from uninspected source
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 cocked. Beef from inspected establishments may be eaten raw or par-
tially 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 drinking purposes until proper san-
itary measures and examinations are taken to reduce possible human infections.
- 11 -
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. Minneapolis, Minnesota. 400 pp.
3. Underhill, B. M. 1924. Parasites and Parasitosis of the Domestic
Animals. The Macmillan Company, New York, N. Y. 379 pp.