74 



MISCELLANEOUS PUBLICATION NO. 1065, U.S. DEPARTMENT OF AGRICULTURE 



Blackhead of turkeys and chickens is caused by 

 a microscopic, single-celled animal, Histomonas 

 meleagredis, that attacks the ceca, or blind gut, 

 and the liver. Occasionally the bird ingests the 

 naked organism but usually a second parasite, the 

 cecal worm, Heterahis gallinae, is involved. The 

 protozoan spends part of its life cycle in the eggs 

 and larva, of the cecal worm. The cecal worm egg 

 is sturdy and may survive in the soil for a year or 

 longer. Recently, U.S. Department of Agriculture 

 scientists learned that eggs of cecal worms are often 

 eaten by two or three earthworm species {112). 

 This leads to a concentration of cecal worms and 

 blackhead parasites in the earthworms, which are 

 readily eaten by poultry. 



Turkeys also may get hexamitiasis by consum- 

 ing food, water, or soil contaminated with drop- 

 pings of infected birds. Research has provided no 

 satisfactory treatment. 



A number of worms are parasitic to cattle, swine, 

 and poultry. Many of these can live for long 

 periods in the soil {122). For example, the eggs 

 of large intestinal roundworms of swine can live 

 up to 7 years in soil. Threadworms of sheep may 

 develop into adult free-living males and females 

 that can give rise to a large number of infective 

 larvae in the pasture. Sanitation is the preferred 

 method of control. 



Salmon poisoning is a rickettsial disease of clogs 

 and other canines, such as foxes, that eat uncooked 

 salmon or trout parasitized by the intestinal fluke 

 Nanophyetus salmincola. This fluke carries the 

 rickettsial micro-organism. The disease is usually 

 fatal. The disease-carrying fluke requires two in- 

 termediate hosts to complete its life cycle — a fresh- 

 water snail and a fish. In streams, each fluke egg 

 hatches into a miracidium, a free-swimming form 

 that enters the snail, Oxytreirm plicifer. There the 

 fluke passes through two more stages, emerging 

 from the snail as larva that penetrates the body 

 of salmon or other fish of the same family. In the 

 fish, fluke larva.e encyst in the muscles and internal 

 organs. Research has disclosed that the fluke and 

 infectiA r e rickettsial micro-organisms survived in 

 migrating salmon for as long as 4 years after fish 

 were experimentally infected {US) . 



Tremendous research progress has been made in 

 the development of insecticides and miticides that 

 control arthropods transmitting animal diseases 



in the environment. (See appendix III, "Agricul- 

 tural Chemicals," p. 76.) 



Plant Disease Agents Contaminating the Envi- 

 ronment 



Most plant diseases contaminate the environ- 

 ment in one way or another. This whole area of 

 research is too vast for detailed consideration in 

 this report, but a few examples of how air, water, 

 and soil are contaminated by plant disease agents 

 are in order. 



Black shank of tobacco appeared near Quincy, 

 Fia., about 1915, but was not identified until some 

 years later. The fungus disease, Phytophthora 

 parasitica var. nicotianae, can spread rapidly into 

 new areas and is a serious threat wherever it occurs. 

 It almost always appears first in a low place in 

 the field. About midsummer a few plants wilt. In 

 the early stage one or more of the large lateral 

 roots are blackened and killed. Then the entire 

 root system and the base of the stalk decay and the 

 plant dies. 



Black shank is spread by moving contaminated 

 water, soil, or plants. The disease spores can con- 

 taminate ponds or streams into which infested 

 fields drain. In some areas, once a field became in- 

 fected it remained so, although a rotation of 5 or 6 

 years might reduce the infection to a trace. In 

 another locality the disease has never persisted. 

 The story of developing plants resistant to black 

 shank begins with the work of W. B. Tisclale at the 

 Florida agricultural experiment station when he 

 intercrossed and selected many varieties in order to 

 develop a resistant commercial shade tobacco {24) • 



A combination of planting resistant varieties of 

 tobacco and rotating crops is the best control in 

 many areas where tobacco is grown {J/.6). 



Stem rust of wheat is an example of a plant dis- 

 ease that may be spread, with disastrous results, 

 by contaminated air. For example, in 1953, 60 per- 

 cent of the wheat crop in Minnesota and some 

 neighboring States was lost. The loss that year for 

 the whole country was about a quarter of the wheat 

 crop. 



The fungus, Puccinia graminis tritici, causes 

 stem rust of wheat, barley, and many wild grasses 

 and is itself a plant. As any other plant, the fungus 

 germinates, grows, and fructifies. It multiplies by 

 countless billions, mutates, and hybridizes contin- 

 uously to form types that may attack varieties of 

 grain previously resistant or immune to the rust 



