664 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



under a variety of names for many years. It is known by various 

 names as malarial fever, horse typhoid, "plains" paralysis, and pernicious 

 anaemia, and has been recognized in many portions of the Western United 

 States and Canada. 



This disease is usually of chronic type, but acute cases have been 

 reported. There is usually a long illness extending from a month to a 

 year or more, and marked by periods of fever and debility, alternating 

 with periods of apparent recovery. The phase of apparent illness is 

 characterized by mild fever, general weakness, and staggering gait, and 

 the disease terminates fatally, as a rule. The peculiar features of the 

 disease are the alternating periods of illness and recovery, unthrif tiness in 

 spite of unusually good appetite, pallor of mucous membranes, dropsical 

 swellings of the belly and limbs. 



It has been satisfactorily proved that swamp fever is caused by an 

 invisible, living virus and that the individuals are so small that they are 

 capable of passing readily through laboratory niters. 



Under artificial inoculation with blood, the period of incubation varies 

 from ten to forty days. The natural method of infection is unknown, 

 but there are reasons for believing that infection does not easily occur 

 by way of the digestive organs nor through the respiratory organs. The 

 disease is apparently not communicated by simply stabling diseased 

 animals with healthy animals. 



Distribution in the body is very general, as shown by the wide dis- 

 tribution of characteristic lesions, and as shown by the fact that the blood 

 is infectious. 



The virus which causes swamp fever reduces greatly the number of 

 red blood corpuscles and also produces local haemorrhages which are 

 most frequently small and sharply defined. The reduction of red blood 

 cells produces marked pallor, and there gradually develops noticeable 

 emaciation. 



Post-mortem lesions in many cases are slight. The haemorrhages 

 involve subcutaneous and intermuscular tissues and are rather common 

 on the lungs and heart. Any of the abdominal organs may show the 

 characteristic haemorrhages. The bone marrow has been reported in 

 some cases as distinctly changed in color, the yellow marrow of long bones 

 becoming dark red. In some cases the liver shows degeneration and 

 necrosis or tissue death. 



