Rocky Mountain Spotted Fever

Rocky Mountain spotted fever was first recognized in the Rocky Mountain area of the United States—thus its name. The disease is representative of a group of serious rickettsial diseases that occur worldwide and are transmitted by certain species of ticks, mites, or lice.

Symptoms

Rocky Mountain spotted fever generally begins suddenly with a headache, pains in the muscles and joints, and fever. Within a few days, a rash consisting of faint pink spots appears on the palms, wrists, ankles, and soles. This rash spreads up the arms and legs to the rest of the body and becomes raised and hemorrhagic (figure 22.8), meaning that it is due to blood leaking from damaged blood vessels. Bleeding may occur at various other sites, such as the mouth and nose. Involvement of the heart, kidneys, and other body tissues can result in shock and death unless treatment is given promptly.

Table 22.6 Impetigo

Symptoms

Blisters that break and "weep" plasma and pus; formation of golden-colored crusts; lymph node enlargement

Incubation period

2 to 5 days

Causative organisms

Streptococcus pyogenes, Staphylococcus aureus

Pathogenesis

Initiated by organisms entering the skin through minor breaks; certain strains of S. pyogenes are prone to cause impetigo; some S. aureus strains that make exfoliatin produce large blisters called bullae. Glomerulonephritis is a potential complication

Epidemiology

Spread by direct contact with carriers or patients with impetigo, insects, and fomites

Prevention and treatment

Cleanliness; care of skin injuries. Oral penicillin if cause is known to be S. pyogenes; otherwise, an anti-staphylococcal antibiotic orally or topically

Figure 22.8 Rash Caused by Rocky Mountain Spotted Fever

Characteristically, the rash begins on the arms and legs, spreads centrally, and as shown in this photo, becomes hemorrhagic.

Rodent cell

Figure 22.8 Rash Caused by Rocky Mountain Spotted Fever

Characteristically, the rash begins on the arms and legs, spreads centrally, and as shown in this photo, becomes hemorrhagic.

Causative Agent

Rocky Mountain spotted fever is caused by Rickettsia rickettsii (figure 22.9), an obligate intracellular bacterium. The organisms are tiny, Gram-negative, non-motile coccobacilli. Rickettsia rickettsii is difficult to see well in Gram-stained smears but can be seen using special stains such as Giemsa. Rickettsia rickettsii can sometimes be identified early in an infection by demonstrating the organisms in biopsies—bits of tissue removed surgically—of skin lesions. Also, their DNA can be magnified by the polymerase chain reaction (PCR) and identified with a probe. ■ PCR, pp. 229,239

Pathogenesis

Rocky Mountain spotted fever is acquired from the bite of a tick infected with R. rickettsii. The bite is usually painless and unnoticed; the tick remains attached for hours while it feeds on capillary blood. Rickettsias are not immediately released into tick saliva from the tick's salivary glands. Therefore, the infection is not usually transmitted until the tick has fed for 4 to 10 hours. When the organisms are released into capillary blood with the tick saliva, they are taken up preferentially by the cells lining the small blood vessels. Following attachment to host cells, R. rickettsii is taken into the cells by endocytosis. Inside the cell, the bacteria leave their phago-some and multiply in both the cytoplasm and nucleus without being enclosed in vacuoles. Early in the infection the bacteria enter and then lyse fingerlike host cell cytoplasmic projections. Eventually, the cell membrane is so damaged by this process, the cell takes in water, lyses, and releases the remaining rickettsias. These seed the bloodstream, infecting even more cells. Infection can also extend into the walls of the small blood vessels, causing an inflammatory reaction, clotting of the blood vessels, and small areas of necrosis, or death of tissue. This process is readily apparent in the skin as a hemor-rhagic rash but, more ominously, occurs throughout the body,

Rodent cell nucleus

Rodent cell

10 mm

Figure 22.9 Rickettsia rickettsii Growing Within a Rodent Cell

10 mm

Figure 22.9 Rickettsia rickettsii Growing Within a Rodent Cell resulting in damage to vital organs such as the kidneys and heart. Potentially even more serious is the release of endotoxin into the bloodstream from the rickettsial cell walls, causing shock and generalized bleeding because of disseminated intravascular coagulation. ■ endotoxin, pp. 59,475 ■ disseminated intravascular coagulation, p. 720

Epidemiology

Rocky Mountain spotted fever is an example of a zoonosis, a disease that exists primarily in animals other than humans. It occurs in a spotty distribution across the contiguous United States and extends into Canada, Mexico, and a few countries of South America. The involved areas change over time, but despite the name of the disease, in the United States the highest incidence has generally been in the south Atlantic and south-central states (figure 22.10). Rocky Mountain spotted fever is maintained in nature in various species of ticks and mammals. Generally, little or no illness develops in these natural hosts, but humans, being an accidental host, often develop severe disease. Several species of ticks transmit the disease to humans. The main vector in the western United States is the wood tick, Dermacentor andersoni (figure 22.11), while in the East it is the dog tick, Dermacentor variabilis. Once infected, ticks remain infected for life, transmitting R. rick-ettsii from one generation to the next through their eggs. Ticks are most active from April to September, and it is during this time period that most cases of Rocky Mountain spotted fever occur.

Prevention and Treatment

No vaccine against Rocky Mountain spotted fever is currently available to the public, although promising genetically engineered vaccines are under development. The disease can be prevented if people take the following precautions: (1) avoid tick-infested areas when possible; (2) use protective clothing; (3) use tick repellents such as dimethyltoluamide; (4) carefully inspect their bodies, especially the scalp, armpits, and groin, for ticks several times daily; and (5) remove attached ticks carefully

22.3 Bacterial Skin Diseases

Figure 22.10 Total Reported Cases of Rocky Mountain Spotted Fever by State and Region, 1994-1998

□ Pacific 7 □ W.S. Central 364 □ Mid Atlantic 200

□ Mountain 62 □ E.S. Central 401 ■ New England 40 D W.N. Central 161 □ E.N. Central 148 □ S. Atlantic 1260

□ Pacific 7 □ W.S. Central 364 □ Mid Atlantic 200

□ Mountain 62 □ E.S. Central 401 ■ New England 40 D W.N. Central 161 □ E.N. Central 148 □ S. Atlantic 1260

to avoid crushing them and thereby contaminating the bite wound with their infected tissue fluids. Gentle traction with blunt tweezers applied at the mouthparts is the safest method of removal. Touching the tick with a hot object, gasoline, or whiskey is ineffectual. After removal of the tick, the site of the bite should be treated with an antiseptic.

The antibiotics tetracycline and chloramphenicol are highly effective in treating Rocky Mountain spotted fever if given early in the disease, before irreversible damage to vital organs has occurred. Without treatment, the overall mortality from the disease is about 20%, but it can be considerably higher in elderly patients. With early diagnosis and treatment, the mortality rate is less than 5%.

The main features of Rocky Mountain spotted fever are summarized in table 22.7.

Dr. Atkins New Diet Revolution

Dr. Atkins New Diet Revolution

Wanting to lose weight and dont know where to start? Dr Atkins will help you out and lose weight fast. Learn more...

Get My Free Ebook


Post a comment