The most characteristic _diseased changes_ found on post-mortem examination occur in the liver and spleen. The liver is enlarged, and a yellowish, mahogany-brown color. The gallbladder is filled with a very thick bile. The spleen may be several times the normal size and dark colored. When it is cut into, the pulpy tissue may resemble thick, dark blood. The kidneys are pale and the bladder may contain dark or reddish-colored urine. In the northern States and outside of the quarantined area, the direct or indirect exposure of the affected cattle to southern cattle, and the presence of the cattle tick, _Margarophus annulatus_, are sufficient evidence to confirm the diagnosis of this disease. _The prevention and control_ depend on destruction of the cattle tick. In the early history of the disease, shipping and driving of southern cattle into and through the northern States caused outbreaks of tick fever and heavy losses among northern cattle. This finally resulted in the locating of the infected district, and the establishment of the Texas-fever quarantine line in 1891 by Dr. D. E. Salmon. Previous to this time Kilborne and Salmon proved that the cattle tick was essential to the spread and production of the disease. A further study of the life history of the tick resulted in the discovery that it could not mature unless it became a parasite of horses, mules, or cattle. This has led to the eradication of the tick in certain sections of the South, by not allowing cattle access to a pasture or lot for a certain period, and by freeing the animals from ticks by hand-picking, dipping and smearing. The immunization of cattle that are shipped into an infected district for breeding purposes is often practised. Immunity is obtained by introducing the P. bigeminun into the blood, either by placing a few virulent young ticks upon the animal, or by repeated inoculation with a very small quantity of virulent blood. QUESTIONS 1. Give the cause and treatment of actinomycosis. 2. Give the cause and treatment of emphysematous anthrax. 3. Give the cause of tick fever; distribution of the disease and methods of control. CHAPTER XXXI INFECTIOUS DISEASES OF POULTRY FOWL CHOLERA.--This is a highly infectious disease of all species of poultry, that is characterized by weakness, depression and yellowish colored excrement. The _specific cause_ of fowl cholera is the _Bacillus avisepticus_ (Fig. 123). This microorganism is transmitted to the healthy birds by the feed, or water becoming contaminated with the discharges from the diseased birds. According to Salmon, the period of incubating varies from four to twenty days. _The early symptoms_ are a falling off in appetite, high fever, dulness, diarrhoea and weakness. The affected bird becomes drowsy, the head is drawn toward the body, and it may remain asleep for long periods at a time. Salmon states that the general outline of the sick bird becomes spherical or ball-shaped.