130 



THE AORICULTUBAL JOURNAL. 



insect. That such protective influence 

 13 a matter ot kmd as well as degree 

 seems possible, for instances have been 

 brought forward where a locally estab- 

 lished immunity failed to protect in a 

 contiguous district. As an illustration 

 of this point the writer was credibly in- 

 formed of the case of a resident in the 

 northern part of JNatal, who was not 

 troubled by mosquitoes on his own farm 

 —although they abounded there — but 

 who was victimised to a considerable ex- 

 tent whenever he stayed at a friend's 

 farm, distant some twenty miles, and 

 that the owner of this farm, himself ex- 

 empted from trouble from his own mos- 

 (juitoes, nevertheless was attacked when 

 visiting his friend's farm. 



This experience, strange as it may 

 seem, becomes not altogether incredible 

 the more one understands of the various 

 species, their methods of breeding, habits, 

 etc., some propagating their species al- 

 most solely in rain water pools, otliers in 

 slowly flowing watercourses and drains, 

 and others, again, in tubs, tanks, cisterns, 

 etc. 



It does not, therefore, seem irrational 

 to suppose that a somewhat similarity of 

 immunity to attack may be aciiiiii'ed by 

 animals not stabled and always exposed, 

 for in this way it seems possible that a 

 protective mtluence may become estab- 

 lished in such animals, enabling them to 

 render inert, by acquired local reaction of 

 tissue or other defensive process not 

 understood, the attack which otherwise 

 would afford a favourable nidus or entry 

 for the casual organism. 



Such apparent increase of suscepti- 

 bility in animals suddenly turned out 

 may, of course, be held to be due to al- 

 teration of environment, etc., producing 

 in this way a possible "chill" or reduction 

 of resistive power. This action may be 

 argued also to involve risk of producing 

 the disease by other means than that of 

 inoculation by insect-bite. 



Tbe disease, in Natal at least, and 

 probably wherever the malady is known, 

 occurs at a time of the year when atmos- 

 pheric conditions are favourable to the 

 activity and spread of the mosquito and 

 winged insect life generally. The heavy 



rams surcharging the atmosphere with 

 moisture aud raising the level of the sub- 

 soil water, forms with the alternating ex- 

 cessively warm days of summer an es- 

 sential and ideal condition for the breed- 

 ing of the mosquito and allied species of 

 insect, and though the same may be held 

 to be the case with most of the lower 

 forms of life, the constant presence of 

 surface water makes this season of the 

 year one adapted to the propagation of 

 this species of insect in particular. 



That no risk is incurred during the 

 actual time of rain seems a Avide-spread 

 theory in Natal, and is an important fact 

 when considered in this connection. 



Mosquitoes do not fly abroad during 

 rain, and rain is not understood to cheek 

 the spread of zymotic disease generally. 

 That freedom from risk during rainfall 

 should have been noticed to exist reflects 

 credit on those who, without knowledge 

 of periods of incubation, etc., have made 

 so acute an observation ; which, however, 

 remains to be substantiated. 



Cases of horsesickness have been re- 

 ported from parts of Natal as having oc- 

 curred during the winter months, and at 

 a time when the temperature falls during 

 the night to many degrees below zero. 

 This, if the diagnosis is correct, would 

 point to the al)ility of the mosquito to 

 withstand such extremes of temperature, 

 and not become incapacitated from using 

 the warm hours of mid-day in order to 

 commit its specific depredations. 



That mosquito larvae can withstand 

 the rigours of an artic winter seems defi- 

 nitely proved. 



The increase of cases with the advent 

 of the early frosts after a bad horsesick- 

 ness season seems to point to the possi- 

 bility of the disease remaining latent in 

 the "system of an animal and becoming 

 excited into activity by exposure to low 

 temperature.' It is difficult to imagine 

 such causes as those of increased microbic 

 or insect activity giving rise to such in- 

 crease of cases. The effect of chill or 

 reduced temperature in exciting the 

 malarial svm]itoms into activity in man is 

 well recognised, and may furnish a clue 

 to this phenomena in the horse. 



A matter of more difficult solution 

 under this heading is the question of the 



