25.1 Anatomy of the Circulatory and Lymphatic Systems

The circulatory system moves blood throughout the body and has no normal microbiota.
The lymphatic system moves fluids from the interstitial spaces of tissues toward the circulatory system and filters the lymph. It also has no normal microbiota.
The circulatory and lymphatic systems are home to many components of the host immune defenses.
Infections of the circulatory system may occur after a break in the skin barrier or they may enter the bloodstream at the site of a localized infection. Pathogens or toxins in the bloodstream can spread rapidly throughout the body and can provoke systemic and sometimes fatal inflammatory responses such as SIRS, sepsis, and endocarditis.
Infections of the lymphatic system can cause lymphangitis and lymphadenitis.

25.2 Bacterial Infections of the Circulatory and Lymphatic Systems

Bacterial infections of the circulatory system are almost universally serious. Left untreated, most have high mortality rates.
Bacterial pathogens usually require a breach in the immune defenses to colonize the circulatory system. Most often, this involves a wound or the bite of an arthropod vector, but it can also occur in hospital settings and result in nosocomial infections.
Sepsis from both gram-negative and gram-positive bacteria, puerperal fever, rheumatic fever, endocarditis, gas gangrene, osteomyelitis, and toxic shock syndrome are typically a result of injury or introduction of bacteria by medical or surgical intervention.
Tularemia, brucellosis, cat-scratch fever, rat-bite fever, and bubonic plague are zoonotic diseases transmitted by biological vectors
Ehrlichiosis, anaplasmosis, endemic and murine typhus, Rocky Mountain spotted fever, Lyme disease, relapsing fever, and trench fever are transmitted by arthropod vectors.
Because their symptoms are so similar to those of other diseases, many bacterial infections of the circulatory system are difficult to diagnose.
Standard antibiotic therapies are effective for the treatment of most bacterial infections of the circulatory system, unless the bacterium is resistant, in which case synergistic treatment may be required.
The systemic immune response to a bacteremia, which involves the release of excessive amounts of cytokines, can sometimes be more damaging to the host than the infection itself.

Human herpesviruses such Epstein-Barr virus (HHV-4) and cytomegalovirus (HHV-5) are widely distributed. The former is associated with infectious mononucleosis and Burkitt lymphoma, and the latter can cause serious congenital infections as well as serious disease in immunocompromised adults.
Arboviral diseases such as yellow fever, dengue fever, and chikungunya fever are characterized by high fevers and vascular damage that can often be fatal. Ebola virus disease is a highly contagious and often fatal infection spread through contact with bodily fluids.
Although there is a vaccine available for yellow fever, treatments for patients with yellow fever, dengue, chikungunya fever, and Ebola virus disease are limited to supportive therapies.
Patients infected with human immunodeficiency virus (HIV) progress through three stages of disease, culminating in AIDS. Antiretroviral therapy (ART) uses various combinations of drugs to suppress viral loads, extending the period of latency and reducing the likelihood of transmission.
Vector control and animal reservoir control remain the best defenses against most viruses that cause diseases of the circulatory system.

Malaria is a protozoan parasite that remains an important cause of death primarily in the tropics. Several species in the genus Plasmodium are responsible for malaria and all are transmitted by Anopheles mosquitoes. Plasmodium infects and destroys human red blood cells, leading to organ damage, anemia, blood vessel necrosis, and death. Malaria can be treated with various antimalarial drugs and prevented through vector control.
Toxoplasmosis is a widespread protozoal infection that can cause serious infections in the immunocompromised and in developing fetuses. Domestic cats are the definitive host.
Babesiosis is a generally asymptomatic infection of red blood cells that can causes malaria-like symptoms in elderly, immunocompromised, or asplenic patients.
Chagas disease is a tropical disease transmitted by triatomine bugs. The trypanosome infects heart, neural tissues, monocytes, and phagocytes, often remaining latent for many years before causing serious and sometimes fatal damage to the digestive system and heart.
Leishmaniasis is caused by the protozoan Leishmania and is transmitted by sand flies. Symptoms are generally mild, but serious cases may cause organ damage, anemia, and loss of immune competence.
Schistosomiasis is caused by a fluke transmitted by snails. The fluke moves throughout the body in the blood stream and chronically infects various tissues, leading to organ damage.