NOVEMBBB 5, 1915] 



SCIENCE 



659 



Afternoon Session Papers: Geographic Distribution 

 Joseph Grinnell, University of California, pre- 

 siding 

 California as a Testing Ground for Theories of 

 Distributional Control: Joseph Grinnell, Uni- 

 versity of California. 

 Insect Transmission of Swamp Fever : J. W. Scott, 

 University of Wyoming. 



During the summer of 1914, the writer obtained 

 experimentally a well-defined ease of swamp fever, 

 and the conditions of the experiment leave no 

 doubt that the disease was contracted through the 

 agency of certain biting insects. This disease, fre- 

 quently known as ' ' infectious ' ' or pernicious 

 anemia, is a serious and destructive blood disease 

 of the horse. It has been reported from France, 

 Germany and Japan, and is widely distributed in 

 North America, from Texas to the Northwest Ter- 

 ritories, and from the states of Washington and 

 Nevada to the Province of Ontario, Canada. It 

 has an altitudinal distribution from near sea-level 

 to at least 9,000 feet. It is usually found in 

 swampy regions, but has also been reported from 

 rolling, wooded countries. Wherever found, it may 

 become epidemic and cause the loss of a large per- 

 centage of a given herd of horses. The disease 

 shows a seasonal distribution, reaching a maxi- 

 mum number of cases in late summer or early 

 autumn. The disease itself is characterized by 

 progressive emaciation and an intermittent rise in 

 temperature; it is frequently accompanied by 

 anemia, and the rises in temperature are some- 

 times at quite regular intervals. The organism is 

 filterable, for the disease is transmitted by the 

 injection of blood serum after it has been passed 

 through a Berkfeld filter. 



In France, Vallei and Carri concluded that nat- 

 ural transmisson took place through drinking 

 water contaminated with urine or feces from an 

 infected horse. Van Es, of North Dakota, in 

 1911, after a study of several years, thought this 

 the most probable explanation. Swingle, how- 

 ever, in Wyoming, in 1912 and 1913, showed that 

 it was an extremely difficult matter to secure in- 

 fection by way of the alimentary canal, his nu- 

 merous experiments with urine and feces all re- 

 sulting negatively. 



At this point in 1913 the writer began the 

 problem. Since internal transmission was a diffi- 

 cult matter, only one or two cases being known 

 (Van Es, Schlatholter), and since the contamina- 

 tion of drinking water could hardly explain epidem- 

 ics, it was believed that natural transmission must 

 be by means of some external agent. Accordingly 



in the spring of 1914 a screened cage was erected 

 capable of holding five horses. The cage had an 

 entry way 10 feet long, each end closed by a door 

 and the screen had 16 meshes to the inch. 



The first experiment in this cage was with vari- 

 ous kinds of mosquitoes and resulted negatively. 

 A longer and more conclusive experiment with 

 mosquitoes the present summer has had a similar 

 result. There was next introduced into this cage a 

 considerable number of flies; these were house- 

 flies, stable-flies and a few other wild flies, in- 

 cluding one of the smaller species of Tabanids. 

 The house-flies and stable flies thrived and in- 

 creased rapidly in numbers between the first and 

 the twenty-fifth of August; the other flies soon 

 died and were not observed to attack the horses. 

 The stable-flies were observed to feed in large 

 numbers on both infected and well horses. Ex- 

 cept for two or three days infected horses were 

 kept continuously in the cage from July 27 to Au- 

 gust 28. Three well horses were exposed in the 

 cage during this time. On August 28 horse No. 

 22, a healthy strong animal, showed a temperature 

 of 102.8. After two more fever periods this horse 

 died October 5. Subinjection of his blood has 

 produced typical cases, showing that he had the 

 disease. The temperature of this horse had been 

 normal from June 9 to August 28, and he had not 

 been outside of the cage for 25 days, while 10-14 

 days is the ordinary incubation period of the dis- 

 ease. Under these conditions there appears to be 

 no escape from the conclusion that insects trans- 

 mitted the disease. It is believed further that the 

 stable-fly was responsible for the transmission for 

 the following reasons: (1) These flies were ob- 

 served to attack the horses viciously. (2) Nega- 

 tive results of two other experiments show that 

 the mosquitoes were not responsible, even though 

 a few were still in the cage during August. (3) 

 Houseflies do not bite, and the other flies present 

 in the cage did not attack the horses, so far as one 

 could observe, and soon died. 



The experiment, with the stable flies alone, is 

 being repeated, and another experiment involving 

 some of the Tabanidee is also in progress. 



The Big Bears of Western North America, with 

 Special Reference to their Distribution: C. Hart 

 MERRL4.M, United States Biological Survey. 

 The bears are the largest of living carnivores 

 and are widely distributed, being found in both 

 Americas and in Eurasia. The typical genus 

 Ursu^ occurs in both eastern and western hemis- 

 pheres. South America is the home of Tremarctos, 



