Rocky Mountain spotted fever (RMSF) is a bacterial disease spread by ticks. It typically begins with a fever and headache, which is followed a few days later with the development of a rash. The rash is generally made up of small spots of bleeding and starts on the wrists and ankles. Other symptoms may include muscle pains and vomiting. Long-term complications following recovery may include hearing loss or loss of part of an arm or leg. The disease is caused by Rickettsia rickettsii, a type of bacterium that is primarily spread to humans by American dog ticks, Rocky Mountain wood ticks, and brown dog ticks. Rarely the disease is spread by blood transfusions. Diagnosis in the early stages is difficult. Several laboratory tests can confirm the diagnosis but treatment should be begun based on symptoms. It is within a group known as spotted fever rickettsiosis, together with Rickettsia parkeri rickettsiosis, Pacific Coast tick fever, and rickettsialpox. Treatment of RMSF is with the antibiotic doxycycline. It works best when started early and is recommended in all age groups, as well as during pregnancy. Antibiotics are not recommended for prevention. Approximately 0.5% of people who are infected die as a result. Before the discovery of tetracycline in the 1940s, more than 10% of those with RMSF died. Fewer than 5,000 cases are reported annually in the United States, usually in June and July. It has been diagnosed throughout the contiguous United States, Western Canada, and parts of Central and South America. Rocky Mountain spotted fever was first identified in the 1800s in the Rocky Mountains.
Signs and symptoms Spotted fever can be challenging to diagnose in its early stages, due to the similarity of symptoms with many different diseases. People infected with R. rickettsii usually notice symptoms following an incubation period of one to two weeks after a tick bite. The early clinical presentation of Rocky Mountain spotted fever is nonspecific and may resemble a variety of other infectious and noninfectious diseases. Initial symptoms:
Fever Nausea Emesis (vomiting) Severe headache Muscle pain Malaise Later signs and symptoms:
Maculopapular rash Petechial rash Abdominal pain Joint pain Conjunctivitis Forgetfulness The classic triad of findings for this disease is fever, rash, and a history of a tick bite. However, this combination is often not identified when patients initially present for care. The rash has a centripetal, or "inward" pattern of spread, meaning it begins at the distal extremities, like ankles and wrists, and courses towards the trunk.
Rash The rash first appears two to five days after the onset of fever, and it is often quite subtle. Younger patients usually develop the rash earlier than older patients. Most often, the rash begins as small, flat, pink, non-itchy spots (macules) on the wrists, forearms, and ankles. These spots turn pale when pressure is applied and eventually become raised on the skin. The characteristic red, spotted (petechial) rash of Rocky Mountain spotted fever is usually not seen until the sixth day or later after the onset of symptoms. This type of rash occurs in only 35-60% of patients with Rocky Mountain spotted fever. The rash involves the palms or soles in as many as 80% of people. However, this distribution may not occur until later on in the course of the disease. As many as 15% of patients may never develop a rash.
Complications People can develop permanent disabilities including "cognitive deficits, ataxia, hemiparesis, blindness, deafness, or amputation following gangrene".
Cause
Ticks are the natural hosts of the disease, serving as both reservoirs and vectors of R. rickettsii. Ticks transmit the bacteria primarily through their bites. Less commonly, infections may occur following exposure to crushed tick tissues, fluids, or tick feces. A female tick can transmit R. rickettsii to her eggs in a process called transovarial transmission. Ticks can also become infected with R. rickettsii while feeding on blood from the host in either the larval or nymphal stage. After the tick develops into the next stage, the R. rickettsii may be transmitted to the second host during the feeding process. Furthermore, male ticks may transfer R. rickettsii to female ticks through body fluids or spermatozoa during the mating process. These types of transmission represent how generations or life stages of infected ticks are maintained. Once infected, the tick can carry the pathogen for life. Rickettsiae are transmitted through saliva injected while a tick is feeding. Unlike Lyme disease and other tick-borne pathogens that require a prolonged attachment period to establish infection, a person can become infected with R. rickettsii in a feeding time as short as 2 hours. In general, about 1-3% of the tick population carries R. rickettsii, even in areas where the majority of human cases are reported. Therefore, the risk of exposure to a tick carrying R. rickettsii is low. The disease is spread by the American dog tick (Dermacentor variabilis), Rocky Mountain wood tick (D. andersoni), brown dog tick (Rhipicephalus sanguineus), and Amblyomma sculptum. Not all of these are of equal importance, and most are restricted to certain geographic areas. The two major vectors of R. rickettsii in the United States are the American dog tick and the Rocky Mountain wood tick. American dog ticks are widely distributed east of the Rocky Mountains, and they also occur in limited areas along the Pacific Coast. Dogs and medium-sized mammals are the preferred hosts of an adult American dog tick, although it feeds readily on other large mammals, including human beings. This tick is the most commonly identified species responsible for transmitting R. rickettsii to humans. Rocky Mountain wood ticks (D. andersoni) are found in the Rocky Mountain states and southwestern Canada. The lifecycle of this tick takes up to three years to complete. Adult ticks feed primarily on large mammals. The larvae and nymphs feed on small rodents. Other tick species are naturally infected with R. rickettsii or serve as experimental vectors in the laboratory. These species are likely to play only a minor role in the ecology of R. rickettsii.
Pathophysiology
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