4 CIRCULAR 148, U. S. DEPARTMENT OF AGRICULTURE 



It should be especially noted that some drugs, like oil of chenopodi- 

 um, require preparatory fasting of the animals and that some other 

 drugs, notably phenothiazine, do not require this regimen. If a 

 preliminary fasting period is necessary, one should not neglect to 

 allow water during this time. Moreover, safe and effective medica- 

 tion with any of the drugs that are used against parasites requires 

 close adherence to the recommended dosages and methods of admin- 

 istration. To keep 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 parasites in the late spring or early summer and in 

 the fall. 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, kidueys, 

 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. 3 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 



The worm parasites of horses are flukes, tapeworms, and round- 

 worms, the last-mentioned group being the most common and injuri- 

 ous. 



3 Information concerning dourine is published in Farmers' Bulletin 1146, 

 Dourine of Horses. 



