INFECTIOUS ANEMIA OR SWAMP FEVER. 551 



INFECTIOUS ANEMIA OR SWAMP FEVER." 



By John R. Mohleb, V. M. D., 

 Chief of the Pathological Division, Bureau of Aninml Industry. 



Infectious anemia of horses, known also by a number of other 

 names, as swamp fever, American surra, malarial fever, typhoid fever 

 of horses, the unknown disease, no-name disease, plains paralysis, and 

 pernicious anemia, has recently been the subject of much investiga- 

 tion. The cause of the disease has now been definitely determined as 

 an invisible virus, which is capable of passing through the pores of 

 the finest porcelain filters, like the infection of foot-and-mouth dis- 

 ease, rinderpest, hog cholera, and similar diseases. The disease is 

 most prevalent in low-lying and badly drained sections of the coun- 

 try, although it has been found in altitudes as high as 7,500 feet on 

 marshy pastures during wet seasons. Therefore proper drainage of 

 infected pastures is indicated as a preventive. It is also more preva- 

 lent during wet years than in dry seasons. It usually makes its ap- 

 pearance in June and increases in frequency until October, although 

 the chronic cases may be seen in the winter, having been contracted 

 during the warm season. 



Cause. — It has been conclusively proven that infectious anemia is 

 produced by an invisible, filterable organism which is transmissible 

 to horses, mules, and asses by subcutaneous inoculation of blood se- 

 rum. The virus which is present in the blood may be transmitted to 

 a number of equines in a series of inoculations by injecting either the 

 whole blood, the defibrinated blood, or the blood serum which has 

 been passed through a fine Pasteur filter, thus eliminating all the 

 visible forms of organismal life, including bacteria, trypanosoma, 

 piroplasma, etc. This virus has also been found to be active in the 

 carcass of an affected animal twenty-four hours after death. 



Following the injection of the infectious principle there is a period 

 of incubation which may extend from ten days to one and one-half 

 months, at the end of which time the onset of the disease is mani- 

 fested by a rise of temperature. If uncomplicated, the infection runs 

 a chronic course, terminating in death in from two months to one 

 and one-half years, or even longer. The probability of the ^drus 

 being spread by an intermediate host, such as flies, mosquitoes, inter- 

 nal parasites, etc., is now receiving careful investigation. 



From exf)eriments already conducted it appears that this disease, 

 formerly supposed to be confined to Manitoba and Minnesota, is 

 more or less prevalent in Kansas, Nebraska, Colorado, Wyoming, 



<* From Bureau of Animal Industry Circular 138. 



