The endothelium of the heart is altered by infecting pathogens, which causes the deposition of platelets and fibrin at the site of infection. The turbulence of the blood flow around these deposits can lead to further irregularities of the endothelial surfaces, which facilitate the further deposition of platelets and fibrin. Eddies caused by slower blood flow in these areas help circulating pathogens adhere to the fibrin and platelets, which causes an inflammatory response that includes both activation of complement and further damage to the endothelial surfaces. As this process continues, a thrombotic mesh composed of platelets, fibrin, and inflammatory cells forms and leads to the formation of a structure known as mature vegetation. The mature vegetation causes alterations in the cardiac endothelium, which obstructs blood flow and increases turbulence. The increased turbulence can cause part of the mature vegetation to fall off and form a blockage known as an embolus, which can travel to other sites. Movement of the emboli to the brain is a lethal event.
Infectious endocarditis can lead to rupture and perforation of the heart valves and heart failure. The kidneys are also usually affected. If the infection occurs on the left side of the heart, coronary emboli can form, which can be lethal. Infection of the right side of the heart can lead to infection of the lung in addition to the formation of emboli. The infection can either be acute, which presents with high fever and toxicity and can result in death within a few days or weeks, or subacute, which presents with low-grade fever, weight loss and night sweats with death taking weeks to months.
Borrelia burgdorferi is a Gram-negative spirochete that causes Lyme disease. B. Burgdorferi exists as part of a complex life cycle involving Ixodes ticks, mice, and deer. The primary reservoir for B. Burgdorferi are mice, who also serve as the host for the early stages of the Ixodes life cycle. Deer serve as the host for the final stages. In spring, fertile adult female ticks living on the deer become engorged on blood, fall from the deer, and lay their eggs in the soil. When hatched, the tick larvae seek out mice for blood meals. B. burgdorferi is picked up by the tick larvae feeding on the mice and then remains with the ticks throughout their development. During the following spring or summer, the larvae mature to adulthood and parasitize deer. This complex life cycle takes two years, and the deer host is essential to the existence of Lyme disease because this is where Ixodes matures and mates. Humans are infected when ticks feed on people who enter their habitat, but their involvement is incidental. The infection can be controlled by wearing clothing that prevents tick bites when in an area that is a habitat for deer and mice. People should also check for ticks regularly and remove them promptly.
Endemic typhus is caused by Rickettsia typhi. Human infection is incidental and is transmitted by the rat flea (Xenopsylla cheopis). Human infection occurs when people live in close proximity to rats, which is often associated with poor socioeconomic conditions. Epidemic typhus is caused by Rickettsia prowazekii, which is transmitted by the human louse. It is seen in situations, such as overcrowding and infrequent bathing, that favor body lice. It is associated with times of war or natural disasters, when there is mass displacement of people into overcrowded refugee camps. Both forms of typhus are favored by poor socioeconomic conditions.