Microbiology

The infectious diseases of the cardiovascular system infect the blood, blood vessels, and heart. In many cases, the infections remain in these areas, but in others, the infections are spread to secondary organs. The diseases of the lymphatic system affect the lymph, lymph vessels, lymph nodes, and lymphoid organs, such as the spleen, tonsils, and thymus.

Streptococcal septicemia. Septicemia is a general expression for microbial infection of the blood and blood vessels. In previous generations, this condition was known asblood poisoning. A common cause of streptococcal septicemia is the Gram-positive streptococcus named Streptococcus pyogenes. This beta-hemolytic streptococcus causes severe fever, malaise, and dropping blood pressure. Shock may accompany the infection, and antibiotic therapy with penicillin is used aggressively. Septicemia may also be caused by a number of Gramnegative rods that release endotoxins.

An important complication of streptococcal septicemia is endocarditis, an infection of the heart valves. This is usually an immune system problem caused by antigen-antibody reactions taking place at the heart valves. Heart valve replacement is sometimes required. The subacute form due to Streptococcus pyogenes is accompanied by fever, weakness, and heart murmur. The acute form is generally due to infection byStaphylococcus aureus and is accompanied by rapid destruction of the heart valves.

Rheumatic fever is an immune reaction taking place in the heart tissues and is usually stimulated by antigens derived from Streptococcus pyogenes. Inflammation of the heart tissues is often accompanied by inflammation and arthritis of the joints, a condition called rheumatoid arthritis. A streptococcal sore throat may precede this condition.

Tularemia. Tularemia is due to a Gram-negative rod called Francisella tularensis.The bacteria enter the body by contact, inhalation, ingestion of contaminated rabbit meat, and the bite of ticks and other arthropods. Patients experience a blood disorder accompanied by fever, malaise, and numerous nonspecific symptoms. Antibiotics such as gentamicin are used in therapy.

Plague. Plague is caused by the Gram-negative rod Yersinia pestis. This organism is similar to the agent of tularemia and is transmitted by its rodent reservoir, the rat flea.The organism enters the lymphatic system and causes swelling of the lymph nodes called buboes. This stage is called bubonic plague. When the bacteria enter the blood, the condition is referred to as septicemic plague, and when the bacteria enter the lungs, the disease is called pneumonic plague. Transmission by airborne droplets is possible at this time. Aggressive antibiotic therapy is necessary to prevent death. The bacteria display a safety-pin appearance due to the accumulation of dye at the poles of the cells. This characteristic is called bipolar staining.

Brucellosis. Brucellosis is also known as undulant fever because it is characterized by alternating periods of high fever and relief. The bacterial agents belong to the genusBrucella. They are small, Gramnegative rods and include B. abortus, B. suis, B. melitensis , and B. canis. In animals, these bacteria cause abortion of the young (contagious abortion) and sterility of the female. They are transmitted to humans by unpasteurized milk and contaminated meat. On entering the bloodstream, the bacteria cause fever, chills, and malaise. Prolonged treatment is required with tetracycline, and vaccines are available for immunizing herds of animals.

Anthrax. Anthrax is due to the Gram-positive, aerobic, sporeforming rod Bacillus anthracis. Spores from this organism are inhaled from the air, or they are acquired during contact with contaminated soil or animals such as sheep and cattle. In the bloodstream, B. anthracis causes severe hemorrhaging, and the spleen, kidneys, and other bloodrich organs become engorged with blood. In the lungs, anthrax is calledwoolsorter's disease and is accompanied by pneumonia. Aggressive antibiotic therapy is necessary to prevent death.

Relapsing fever. Relapsing fever is so named because of the recurrent periods of fever. The etiologic agent is Borrelia recurrentis , which is a spirochete. The organism is transmitted by lice, which are natural parasites of humans. It may also be transmitted among humans by ticks. Jaundice and rose-colored skin spots accompany the infection, which may be treated by antibiotics.

Lyme disease. Lyme disease is caused by Borrelia burgdorferi. This organism is a spirochete transmitted by ticks of the genus Ixodes. First observed in Lyme, Connecticut, Lyme disease is now found throughout the United States.

Among the first symptoms of Lyme disease is a bull's-eye rash occurring on the skin. The rash is called erythema chronicum migrans. It occurs at the site of the tick bite and has a red center and expands over a period of several days. After the rash fades and spirochetes enter the blood, fever and other symptoms appear. In addition, the heart is affected and irregular heartbeat may be observed. On occasion, there is paralysis of the face and meningitis. Some months later, patients display arthritis of the large joints such as hips, ankles, elbows, and knees.

Lyme disease may be treated with a number of antibiotics, including penicillin and tetracycline. A vaccine is currently available for dogs. Diagnosis of the disease depends upon the observance of symptoms and awareness of exposure to ticks.

Rocky Mountain spotted fever. Rocky Mountain spotted fever is caused by the rickettsia Rickettsia rickettsii. This submicroscopic bacterium is transmitted by ticksof the genus Dermacentor. The disease is characterized by a maculopapular skin rash(a “spotted rash”) occurring on the appendages and then spreading to the trunk. The fever is very high, and headaches accompany the disease. Antibiotics such as tetracycline are effective for therapy.

Epidemic typhus. Epidemic typhus is caused by Rickettsia prowazekii , a rickettsia transmitted by the body louse of the genus Pediculus. The organism invades the bloodstream and causes a maculopapular skin rash beginning on the body trunk and spreading to the appendages. The fever is extremely high, and the death rate is significant. Tetracycline antibiotics are effective for therapy, and elimination of lice is essential to stem the spread of the epidemic.

Endemic typhus. Endemic typhus is also called murine typhus because it occurs in mice and other rodents. It is transmitted by the rat flea and is caused by Rickettsia typhi , a submicroscopic rickettsia. The symptoms are similar to those of epidemic typhus but are much milder, and the mortality rate is much lower.

Other rickettsial diseases. Several other rickettsiae are known to cause diseases in humans. One example is rickettsialpox, caused by Rickettsia akari. This organism is transmitted by mites and causes a skin rash that resembles chickenpox. Another disease is tsutsugamushi, also called scrub typhus. This disease is also transmitted by mites. It occurs in Pacific regions and is characterized by a fever and skin rash.

Another rickettsial disease is trench fever, caused by Rochalimaea quintana. This disease is transmitted by lice and was common during World War I, when it affected soldiers in the trenches. Ehrlichiosis is a rickettsial disease due to Ehrlichia canis.Patients suffer headache and fever, but there is no skin rash associated with the disease. A similar disease is human granulocytic ehrlichiosis (HGE), which is also caused by a species of Ehrlichia. Ehrlichia species are transmitted by ticks. The diseases can be treated with tetracycline and other antibiotics.