September 14, 1905] 



NA TURE 



499 



to South Africa in these days. The native cattle have 

 become naturally immune, and the disease is only fatal 

 to susceptible imported cattle. This, of course, discourages 

 the importation of prize stock ; but with the knowledge 

 we possess it ought to be possible, by good stabling and 

 prevention of contact with tick-infected cattle, to keep the 

 prize stock alive for a reasonable time. The question of 

 the feasibility of immunising the prize stock while calves 

 in England might be considered. 



In regard to methods of conferring immunity on 

 susceptible cattle many have been tried, but none are 

 absolutely free from risk. 



We may sura up in regard to Redwater or Texas Fever 

 by saying that our knowledge of its causation and methods 

 of prevention is much the same as it was ten years 

 ago. The work done by Smith and Kilborne on this 

 disease was of such a brilliant nature, and was done so 

 thoroughly, that little has been left for later workers to do. 



(3) Biliary Fever of Horses, Mules, and Donkeys. 



This is a disease of horses, mules and donkeys very 

 similar to Redwater in cattle, and is caused by a closely 

 allied parasite, the Piroplasma cqui, discovered for the 

 first time in South Africa by Bordet, Danysz, and Theiler, 

 and named by Laveran of Paris. 



It is similar to Redwater, in that animals which have 

 recovered from the disease remain a source of infection 

 during the remainder of their lives to susceptible animals. 

 The native South African horse is, like the cattle, immune 

 to the disease. It is also conveyed by a tick, which has 

 been shown by Theiler to be the " red tick " (Rhipi- 

 cephalus evertsi), the infection being taken in the nymphal 

 and transferred in the adult stage. Theiler has also made 

 the very important observation that if a horse is injected 

 with blood from a donkey which has recovered from the 

 disease, as a rule a mild form of the disease is produced, 

 so that this opens up a method of immunising susceptible 

 horses which mav probably prove of practical value. 

 Theiler has also made another curious discovery. This 

 disease of horses was found to greatly complicate certain 

 immunising experiments he was making against Horse- 

 sickness. He found he was introducing the Piroplasma 

 equi at the same time he injected Horse-sickness virus. 

 But he found out that as the virus of Horse-sickness keeps 

 its virulence for years, whilst the Piroplasma equi dies 

 out in a short time, this danger could be avoided by keep- 

 ing the Horse-sickness serum and virus for some time 

 before using it. 



(4) Malignant Jaundice of Dogs. 



This disease is most important to sportsmen or to im- 

 porters of valuable dogs, as most of these are attacked 

 sooner or later by this disease, and most of them succumb. 

 It is also caused by a species of Piroplasma (Piroplasma 

 canis), and is spread by the dog tick (Haemophysalis 

 leachii). 



Like Redwater and Biliary Fever, the blood of dogs 

 which have recovered remains infective. 



The story of the tick infection is a curious one, and 

 the credit of its discovery is due to Lounsbury. It is only 

 in the adult stage that the tick is capable of producing 

 the disease. It is therefore evident that the Piroplasma 

 must remain latent in the egg, the larval and nymphal 

 stages, and only attain activity in the adult stage. 



According to Theiler there exists a peculiar phenomenon 

 which may be made use of to confer immunity. The 

 blood of a dog which has recovered from this disease and 

 has been hyper-immunised is, as mentioned above, capable 

 of giving rise to the disease in a susceptible dog. Now, 

 if serum be obtained from this blood and a quantity added 

 to a small amount of the blood, this infected blood loses 

 its infectivity and no disease results. 



II. Diseases caused by Parasites belonging to the 



Genus Trypanosoma. 



(i) Nagana or Tsetse-fly Disease. 



We now come to the second group of diseases. These 



are also caused by blood parasites belonging to the same 



class of living things as the Piroplasma, but which are 



free organisms, swimming in the fluid part of the blood, 



and not contained in the red blood corpuscles, as are the 



others. 



NO. 1872, VOL. 72] 



The first of this group I would direct your attention 

 to is that disease called Nagana or the Tsctsc-fiy Disease. 



This fly renders thousands of square miles of Africa un- 

 inhabitable. No horses, cattle, or dogs can venture, even 

 for a day, into the so-called " Fly Country." Now what 

 was our knowledge of this disease ten years ago? At 

 that time it was thought that the tsetse-fly killed animals 

 by injecting a poison into them, in the same way as a 

 snake kills its prey. Nothing was known as to the nature 

 of this poison in 1894. In 1895, on account of serious 

 losses among the native cattle in Zululand from this 

 plague, the then Governor of Natal and Zululand, Sir 

 Walter Hely-Hutchinson, started the investigation of this 

 disease. The result of this investigation was the dis- 

 covery that Tsetse-fly Disease was not caused by a simple 

 poison elaborated by the fly, as formerly believed, but 

 that the cause of the disease was a minute blood parasite 

 which gained entrance to the blood of the animals. This 

 parasite is known by the name Trypanosoma, which 

 signifies a screw-like body. 



Ten years ago two species only had attracted much 

 attention — one living in the blood of healthy rats, dis- 

 covered by Surgeon-IVIajor Lewis in India ; and the other, 

 a trypanosome, found in the blood of horses and mules 

 suffering from a disease known in India as " Surra." 

 As the result of this investigation in Zululand, which 

 lasted two years, it was proved that this trypanosome was 

 undoubtedly the cause of the death of the horses and cattle 

 struck by the fly, and that the tsetse-fly merely acted as 

 a carrier of this blood parasite. 



Here is a representation of the trypanosome of Nagana 

 on the screen. These trypanosomes consist of a single 

 cell ; are sinuous, worm-like creatures, provided with a 

 macronucleus and a micronucleus, a long terminal flagellum, 

 and a narrow fin-like membrane continuous with the 

 flagellum and running the whole length of the body. When 

 alive they are extremely rapid in their movements, con- 

 stantly dashing about, and lashing the red blood corpuscles 

 into motion with their flagellum. They swim equally well 

 with either extremity in front. These organisms multiply 

 in the blood by simple longitudinal division, and often 

 become so numerous as to number several millions in 

 every drop of blood. They are sucked, along with the 

 blood, into the stomach of the fly, live and multiply in 

 the alimentary tract for several days, and, when the fly 

 has its next feed on an animal, take the opportunity of 

 gaining access to the blood of the new host, and so set 

 up the disease. 



Let me now throw on the screen a representation of 

 the tsetse-fly {Glossina morsitans) which does all the 

 mischief. Experiments were made which showed that the 

 fly could convey the parasite from affected to healthy 

 animals for at least forty-eight hours. It is a curious 

 fact that among all the blood-sucking flies the tsetse-fly 

 alone has this power, and up to the present the cause of 

 this has not been thoroughly cleared up. Lately, how- 

 ever, evidence has been brought forward to show that an 

 enormous multiplication and development of the trypano- 

 somes take place in the fly's intestine, a few trvpanosomes 

 multiplying to masses containing numberless parasites 

 within twenty-four hours. Now, if this multiplication only 

 takes place in the intestine of the tsetse-fly, and not in 

 the other kinds of biting flies, this would probably account 

 for the curious connection between the tsetse-fly and the 

 disease. This multiplication of the trypanosomes in the 

 tsetse-fly was discovered by Gray and Tulloch, two young 

 army medical officers, while working in Uganda on 

 " Sleeping Sickness " during the present year. 



Not only was it found that the tsetse-flies could convey 

 the disease from sick to healthy animals, but it was also 

 proved that the wild tsetse-flies brought from the " Fly 

 Country " and straightway placed on healthy animals also 

 gave rise to the disease. The question then arose as to 

 where the tsetse-flies living in the " Fly Country " came 

 bv the trypanosomes. There were no sick horses or cattle 

 in the " Fly Country." Investigation brought to light the 

 curious fact that most of the wild animals — the buffalo, 

 the koodoo, the wildebeeste — carried the trypanosomes in 

 small numbers in their blood, and it was from them that 

 the fly obtained the parasite. The wild animals act as a 

 reservoir of the disease. The trypanosome seems to live in 

 the blood of the wild animals without doing them any 



