296 



SCIENCE. 



[N. S. Vol. XXII. No. 558, 



main a source of infection during the re- 

 mainder of their lives to susceptible ani- 

 mals. 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' {Bhi- 

 picephalus 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 immunizing 

 susceptible horses which may probably 

 prove of practical value. Theiler has also 

 made another curious discovery. This dis- 

 ease of horses was found to greatly compli- 

 cate certain immunizing 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 viru- 

 lence for years, whilst the Piroplasma equi 

 dies out in a short time, this danger could 

 be avoided by keeping the horse-sickness 

 serum and virus for some time before using 

 it. 



4. Malignant Jaundice of Dogs. 



This disease is most important to sports- 

 men or to importers 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 {Hcemophysalis leachii). 



Like redwater and biliary fever, the 

 blood of dogs which have recovered re- 

 mains infective. 



The story of the tick infection is a curi- 

 ous 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 pe- 

 culiar phenomenon which may be made 

 use of to confer immunity. The blood of 

 a dog which has recovered from this dis- 

 ease and has been hyper-immunized 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 infect- 

 ivity and no disease results. 



II. DISEASES CAUSED BY PARASITES BELONG- 

 ING TO THE GENUS TRYPANOSOME. 



1. 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 Piroplasm^a, 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. 



The first of this group I would draw your 

 attention to is that disease called nagana 

 or the tsetse-fly disease. 



This fly renders thousands of square 

 miles of Africa uninhabitable. No horses, 

 cattle, nor 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 poi- 

 son into them, in the same way as a snake 

 kills its prey. Nothing was Imown as to 

 the nature of this poison in 1894. In 1895, 

 on account of serious losses among the na- 

 tive 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 discovery that 

 tsetse-fly disease was not caused by a simple 

 poison elaborated by the fly, as formerly 



