314 VETERINARY HYGIENE 



infected, and fortunately at present in the United Kingdom the 

 ticks are only infected in certain districts. 



Redwater appears in the spring and again in the autumn. The 

 disease occurs chiefly in adult cattle, though calves and yearlings 

 may be attacked. Calves in most cases only show slight symptoms, 

 though they are capable of becoming infected by inoculation. As 

 in the cases of other protozoan diseases, P. bigeminum is only 

 capable of causing infection in the ox or in nearly-related species, 

 e.g., deer and buffaloes. 



The period of incubation is generally from 6 to 10 days follow- 

 ing direct inoculation into the blood, but with natural infection, 

 such as occurs in the field, the period is a fortnight or even longer. 



One attack confers a high degree of immunity, but infected 

 animals harbour parasites in their blood possibly for several years, 

 and herein lies the difficulty in stamping out the disease and prevent- 

 ing its spread. Though such cattle may themselves be apparently 

 healthy they are quite capable of infecting ticks, and so the disease 

 may be perpetuated in an already contaminated locality and may 

 also be spread to fresh pastures. Animals reared on redvvater farms 

 are therefore immune, or more correctly stated, tolerant, but are 

 at the same time dangerous carriers. The mortality varies greatly ; 

 Craig considers it probably less than 5 per cent.* Relapses after 

 apparent recovery from one attack are not infrequent, these occur- 

 ring especially at times at which the vitality may be lowered, e.g., 

 following parturition. 



The ticks responsible for the transmission of piroplasms in 

 this country are the I x odes ricinus (or reduvius) and H&maphy- 

 salis punctata. These ticks in their life history pass through 

 certain stages, namely, egg, larva, nymph and adult. The larval 

 form after leaving the egg attaches itself to a bovine and after 

 sucking blood for a time drops off, moults and becomes a nymph. 

 The latter becomes attached to a second bovine and goes through 

 the same process, the adult form finally making its way on to the 

 body of a third host. These ticks are known as non-continuous 

 feeders. The adult form is sexually differentiated; the males do 

 not suck blood, but after fertilising the females, drop off and die. 

 A proper understanding of the life cycle and habits of these ticks 

 is essential before dealing with preventive measures, since the sup- 

 pression of redwater is necessarily bound up with the eradication 

 of infected ticks. The subject has been largely studied in Great 

 Britain by Stockman. f The life-cycle of I. ricinus occupies at the 



* Wallis-Hoare, System. Vet. Med., 1913, Vol. I., p. 1008. 

 f Annual Report, Proc. Dis. Animals Act, 1910. 



