4 CIRCULAR 148, U. S. DEPARTMENT OF AGRICULTURE 
parasitic infestation at a minimum and avoid losses from this source, 
it is advisable to adopt a program of regular treatment. The usual 
procedure is to administer treatments twice a year. In the northern 
part of the United States, animals may be treated for worm para- 
sites in the late spring or early summer and in the fall, and for bots 
in late November or December and again in February, if necessary, 
to remove those which have developed since the first treatment. In 
the South, owing to the shorter and milder winter and the more 
abundant warmth and moisture, it may be necessary to treat oftener. 
Judgment, based on experience, is a better basis for procedure than 
any general rule. 
Internal parasites occur in various locations in the body of the 
host animal, such as the alimentary canal, lungs, liver, kidneys, 
blood, and various organs and tissues other than the skin. They 
include various forms of animal life known as Protozoa, worms, the 
larval forms of certain insects, and some forms closely related to 
insects. The various kinds are discussed in detail in the following 
pages: . 
PROTOZOA 
Protozoa are relatively low forms of animal life, microscopic in 
size, and consisting of but a single cell. The parasitic Protozoa of 
domesticated animals are known to occur in various locations, and 
are especially common in the alimentary canal and in the blood. 
Fortunately, horses in the United States are relatively free from 
certain disease-producing protozoan parasites which occur in horses 
in other parts of the world where they constitute a limiting factor 
in horse production. The only known pathogenic protozoan para- 
site of horses in this country is the organism, one of the trypano- 
somes, which produces dourine.* Trypanosomes closely related to 
this organism occur in the blood of horses in South America, Asia, 
the Philippine Islands, and elsewhere and produce serious and fatal 
diseases. Up to the present time these parasites have not become 
established in this country, and quarantine measures to keep them 
out are enforced. 
Protozoan parasites of various kinds often occur in large num-- 
bers in the cecum and the upper colon of horses, but these forms 
are not definitely known to produce any digestive or other disturb- 
ances and are usually regarded as comparatively harmless. 
WORM PARASITES 
As already stated, the worm parasites of horses include flukes, 
tapeworms, and roundworms, the last-mentioned group being the 
most common and the most injurious. 
FLUKES 
Flukes or trematodes are soft, more or less flattened, leaf-shaped 
worms, occurring in various locations, especially in the digestive 
tract and in organs which communicate with the digestive tract. 
Only a few kinds of flukes are known to occur in horses, and several 
’ Information concerning dourine is published in Farmers’ Bulletin 1146, Dourine of 
Horses. 
