INFLUENCli OF CLIMATE oN ANIMAL DLSEASE. 481 



Tick-j:urne Diseases. 



When we come to look at the hst of diseases transmitted in 

 this way, we tind that in South Africa they are well represented. 

 Amongst the babesioses or piroplasmoses we find diseases of 

 cattle due lo Babesia b'ujciiiiiniui ( " red\\ ater "" ) and Babesia 

 mutans (producing a form of so-called "gall-sickness"). In 

 the dog Babesia eaiiis is responsible for " biliary fever "' or ■" ma- 

 lignant jaundice " of tliis species. The Theileriases are repre- 

 sented in the luist Coast fever of cattle, and in tlie same sjiecies 

 of animal occurs a disease anaplasmosis, due to a ])arasite 

 described bv Theiler as Aiiaf^lasiiia iiiarcjiiiaie or its variety 

 eciitralc. In ■" biliary fever" of ec|uines. we have Nuttalliosis 

 rei)resented, whilst spirocheetosis is met witli in cattle and other 

 species and in " heart-water" of cattle, shec]), and goats we have 

 a disease due to an organism of the filterable variety. 



In this class of diseases we find from tlie biological point 

 of view an extreme!}- complex state of afi:airs existing in the inter- 

 relationshij)s between the disease-causing parasites, their verte- 

 brate hosts, and their rlefinitive invertebrate transmitting hosts. 

 The disease-causing parasite needs the presence of both types 

 of b.ost for its full development, although the forms met \\ith in 

 one host, sa}' the vertebrate, are capable of existing for long 

 periods in the absence of the other invertebrate host, and viec 

 versa. Hence a study of eftect of climate and local conditions 

 on the spread of these diseases nn'ght include a study of these 

 conditions in their influence on the distribution of both types of 

 host. In this paper, however, we conline our attention solely 

 to the study of the eliect of these conditions on the life history 

 of the invertebrate host, the tick. In order to do this, we nuist 

 first briefly enquire into the life-history itself, the essentials oi 

 which are as follows. The mature female tick having engorged 

 herself with the blood of the vertebrate host, and having been at 

 the same time fertilised by the male, dro])s from her host to the 

 ground, and after a varying ])eriod ( pre-ovi])osition period) lays 

 her eggs. These eggs sooner or later hatch, and from each of 

 them emerges a six-legged or hexapod larva which must undergo 

 a moult or ecdysis before passing to the next or nymj)hal stage. 

 The nymphal stage is represented by an eight-legged or octapod 

 form, which resembles the adult except that it is sexually im- 

 mature, and it must also undergo an ecdysis before it can ])ass 

 to the final stage of the adult or imago. Thirs we recognise four 

 stages in the life-history, namely, the Qgg. the hexajjod larva, 

 the octopod nymph, and the imago or aduh male or female. 



The conditions under which these dift'erent stages occur 

 vary, however, with the different species. In one species, for 

 instance, the larva hatched from the egg having climbed up a 

 grass blade, and thence passed on to its host, the tick may 

 then pass the rest of its life-cycle tip to the adult stage on this 

 same animal. This occurs, for example, in the case of the " one- 

 Iiost " group of ticks, an example of which is found in RJiipiec- 

 pJialiis decoloratus. In another case the larva, gaining its host 



