Rocky Mountain spotted fever, typhus-like disease form of tick-borne typhus first described in the Rocky Mountain section of the United States, caused by a specific microorganism (Rickettsia rickettsii) and transmitted to man by ticks. It is identical with a disease known as São Paulo fever in Brazil and with the spotted fever of Colombia. Discovery of the microbe of Rocky Mountain spotted fever in 1906 by H.T. Ricketts led to the understanding of other rickettsiae-caused diseases.The predominance of the disease in western North America among persons exposed to the open range, and its seasonal limitation to late spring and early summer months, were explained when the carrier species was identified as a rickettsial diseases. Despite its name, Rocky Mountain spotted fever is most common on the eastern coast of the United States and has been found in every state. In fact, it is identical with a disease known as São Paulo fever in Brazil and with the spotted fever of Colombia. It is a disease of the summer and early fall, when ticks are active.

In western North America, the carrier species is the wood tick, Dermacentor andersoni, which is widely distributed in the adult form upon on large mammals, particularly cattle and sheep. The occurrence of the human disease in the In the eastern and southern United States was explained when it was discovered that , the common dog tick, Dermacentor variabilis, which attacks manhumans, also acts as a carrier. In the southwestern United States, human cases were are also traced to the lone star tick, Amblyomma americanum. In Brazil the common carrier is Amblyomma cajennense.

The clinical course of the disease is essentially similar to that of typhus fever. In severe cases of spotted fever the rash tends to be more hemorrhagic and to be accentuated on the extremities, particularly about the wrists and ankles. Nervous and mental symptoms are common; restlessness, insomnia, disorientation, and delirium are frequent manifestations of involvement of the central nervous system. Prostration may be marked from the beginning, merging into coma with death possible as early as the sixth or seventh dayillness begins with headache, fever, and chills, soon followed by pains in the bones and joints, weakness, and fatigue. A rash develops in the first week of illness, beginning on the extremities and spreading to the trunk. It is more profuse than the rash of epidemic typhus and affects the face as well as the body. In some people the colour of the rash deepens after a day or two, and in the worst cases it turns purple with blood. By the end of a week in severe cases, the patient shows signs of brain irritation and may be agitated, sleepless, or delirious. Breathing becomes laboured and circulation poor, and areas of gangrene may develop on the hands and feet. In the worst cases, the patient may become comatose and die, but in most cases fever gradually abates and the patient slowly recovers. Convalescence is likely to be slow and may be complicated by visual disturbances, deafness, and mental confusion. Although the patient’s recovery may be delayed, it is usually complete. The case-fatality ratesrate, as in typhus, vary varies directly with age.

Early treatment with antibiotics greatly shortens the disease and decreases the risk of death. Prevention depends primarily upon the exercise of personal care in protection against tick bites. Persons exposed to known infected areas should frequently examine their clothing and body for ticks. Usually the tick does not become attached to its host immediately but crawls about for several hours. The chance of receiving infection from the bite of a tick is directly proportional to the length of time that the tick has fed. The ticks should be removed and the skin area involved swabbed with an antiseptic.

There is a satisfactory vaccination procedure against Rocky Mountain spotted fever: the vaccine is administered in the spring or early summer before the beginning of the tick season and is repeated annually, because the maximum degree of protection conferred is for less than a year. The degree of immunity afforded is relative, but the chance for subsequent infection is lessened, and the risk of death is greatly reduced. See also boutonneuse fever.