=Trypanosoma dimorphon.=—This trypanosome causes a disease of horses in Gambia. It is also found in horses and cattle in other parts of Africa. The parasite shows marked variation in morphology. =Trypanosoma lewisi.=—Rats in many parts of the world show this infection which is rarely fatal to them. It is transmitted by the rat flea by a process of regurgitation. It can also be transmitted by the rat louse. CHAPTER IV THE TROPICAL RELAPSING FEVERS DEFINITION AND SYNONYMS =Definition.=—There is a group of tropical fevers more or less identical clinically with European relapsing fever and caused by spirochaetes closely allied to _Spironema recurrentis (Spirillum obermeieri)_. It seems probable that the relapsing fevers of East and West Africa are caused by a single species, _S. duttoni_, which is transmitted by a tick, _Ornithodoros moubata_, while that of Northern Africa is caused by another species, _S. berbera_, which is transmitted by lice, either _Pediculus vestimenti_ or _Pediculus capitis_. Another species of spirochaete, _S. carteri_, is supposed to cause the relapsing fever of India and it seems probable that its transmission is brought about by infected lice. Besides the above species of spirochaetes others have been reported, as _S. novyi_ for American and _S. persica_ for Persian relapsing fever. The view taken by Nuttall, that these various names may be of convenience in the study of relapsing fevers but that there is no adequate morphological difference to justify them as species, seems worthy of acceptance. It has been shown that the separation of these spirochaetes on the basis of susceptibility of laboratory animals and cross immunity reactions is untenable. Agglutination of certain strains by their specific sera, however, is a reliable means of separation. As with European relapsing fever, these fevers are characterized by a sudden onset, intense frontal headache, and pain of back and limbs. This fever remains high for three to five days and falls by crisis, to be succeeded by an apyrexial interval of approximately one week. There may be several of these alternating febrile and afebrile periods. The spirochaetes are in the peripheral circulation during the febrile period but not in the afebrile one. The spleen is enlarged and tender. Cases showing jaundice seem more grave. =Synonyms.=—Febris recurrens. Tick fever. French: Typhus recurrent. German: Rückfallfieber. HISTORY AND GEOGRAPHICAL DISTRIBUTION =History.=—Although Hippocrates described the clinical features of relapsing fever quite accurately this knowledge seems to have been lost until about the eighteenth century. The causative spirochaetes were first seen by Obermeier in the blood of a patient in 1868 but he did not publish his discovery until 1873.