Cardiovascular and Lymphatic System Diseases

Bacterial Cardiovascular Diseases

Bacteria that infect the cardiovascular system may exploit the life cycles of blood-feeding insects to spread from host to host or may result from inadequately treated infections from elsewhere in the body.
Some infections either directly target red blood cells or rely on the circulatory system to spread the infection across the many tissues of the body. As such, some pathogens have adapted to spread among humans through the bites of infected blood-feeding insects. One example of this is Lyme disease, which is caused by the spirochete bacterium Borrelia burgdorferi. B. burgdorferi is transmitted by the bite of an infected tick. When the tick bites, bacteria are dispersed into the skin, where they can spread via the bloodstream to the joints, the central nervous system, and the heart. Symptoms include fever, headache, and erythema migrans, a characteristic skin rash. However, B. burgdorferi bacteria follow a complex, three-stage life cycle, and symptoms of Lyme disease can vary at different times in that cycle. Lyme disease is treatable with the antibiotics deoxycycline and amoxicillin, though if treatment is delayed beyond 30 days, the tissue damage caused by infection can result in permanent health problems.
The Borrelia burgdorferi (dark field microscope, 400x) bacteria, a spirochete, is transmitted by tick bite and can cause Lyme disease. The "bull's-eye" rash (also known as erythema migrans) is a common symptom.
Credit: CDC (left), CDC/James Gathany (right)
The gram-positive coccus bacteria Streptococcus pyogenes may cause diverse diseases in humans that typically involve mucosa or skin infections. Systemic infections may result from inadequately treated S. pyogenes throat infections in a minority of patients. In these cases a type II hypersensitivity (allergic) reaction can happen after B cells present S. pyogenes cell-wall antigens and T cells produce antibodies that cross-react with the cells of the heart-wall muscles or joints. This condition is known as acute rheumatic fever. Antibody attack on heart valves causes inflammation and irreversible damage that can cause a heart murmur. Subsequent infections with S. pyogenes require rapid and aggressive treatment to avoid further damage. Benzathine benzylpenicillin is the antibiotic of choice.

Tularemia, also known as rabbit fever and deer fly fever, is caused by Francisella tularensis, a gram-negative, rod-shaped, intracellular bacterium that is hardy, aerobic, and highly infectious. There are four subspecies of F. tularensis. The infectiousness of these subspecies varies, as does the virulence of the disease they produce in humans. One or more of these subspecies have been found throughout most of North America, Europe, and Asia. The disease is spread via the bites of ticks and deer flies, who pick up bacteria when they bite infected animals. Hunters and taxidermists may also become infected when handling the carcasses of killed, infected animals because the bacteria can be transmitted via the skin. Such an infected animal is known as an animal reservoir, an organism in which a pathogen can replicate, often without causing disease, and that serves as a source of infection for humans.

The main clinical types of tularemia include glandular, ulceroglandular, oculoglandular, pneumonic, oropharyngeal, typhoidal, and septic forms. Once F. tularensis enters the bloodstream, it typically infects macrophages, hiding inside cells to prevent destruction by the immune system. Infected macrophages then travel through the lymphatic system to the lymph nodes and organs, where they can spread to other cells. Signs and symptoms of infection with F. tularensis vary depending upon the route of transmission and the clinical type(s) of infection that develops. These can include skin ulcers, eye infections, lymph node swelling, sore throat, coughing, chest pain, and difficult breathing. Of the several clinical infection types, the pneumonic form is the most severe in terms of morbidity and potential mortality if not properly treated.
The deer fly (Chrysops brunneus) is a common disease vector for Francisella tularensis (scanning electron microscope), the bacteria that causes tularemia. Symptoms vary depending on the type of tularemia the victim suffers.
Credit: CDC/Dr. Gary Alpert (left), NIAID (right)License: CC BY 2.0 (right)

Plague

The three forms of plague include bubonic, pneumonic, and septicemic.

Plague is an infectious disease caused by the bacillus bacterium Yersinia pestis and has three clinical forms: bubonic, pneumonic, and septicemic. Yersinia pestis is a gram-negative rod-shaped bacterium capable of reproducing in any cell type, including lymphocytes. Bubonic plague is a form of plague in which infection by the virulent bacterium Yersinia pestis spreads through the lymphatic vessels and the lymph nodes swell, forming large, colored nodules called buboes. Septicemic plague form of plague in which the bloodstream is so severely infected by the virulent bacterium Yersinia pestis that tiny clots form throughout the body, potentially causing death by blocking blood vessels and causing organ failure.

Pneumonic plague is a form of plague in which the lungs are heavily invaded/infected by the virulent bacterium Yersinia pestis. Bubonic and septicemic plague are spread via the bite of an infected flea that lives on the skin of rats. Rats that live off of human waste foods in urban environments can pass the fleas on to humans, who become infected when the fleas bite. Pneumonic plague can result from septicemic plague or can be passed from person to person when an infected person coughs and/or sneezes aerosolized bacteria into the air. Y. pestis is endemic in many areas of the world, and a vaccine is used to prevent infection in high-risk areas. The severity of outcomes for Y. pestis infections varies depending on the form of plague considered. Bubonic plague is currently the most common disease form resulting from Y. pestis infection but is the least severe in terms of mortality if promptly and properly treated. Most other modern cases of plague occur as pneumonic plague. However, of the three types of plague, the pneumonic form results in the fastest death and highest mortality rates when is not promptly or properly treated with antibiotics such as streptomycin or tetracycline. Septicemic plague has a mortality rate intermediate between that of bubonic or pneumonic plague, but it is much less common than those plague varieties.
Yersinia pestis (scanning electron microscope) is the cause of bubonic plague. It is transmitted via flea bite, specifically the rat flea, or Xenopsylla cheopis. A common symptom includes lymph nodes swollen to extreme sizes.
Credit: Rocky Mountain Laboratories, NIAID, NIH (left), CDC (right)

Anthrax

Anthrax is an infection caused by bacterial spores and can occur in three forms: skin, lung, and intestinal.
Anthrax is an infection caused by the gram-positive bacterium Bacillus anthracis. All members of the Bacillus genus have evolved to form endospores, dormant cell types that are extremely resistant to damage. Bacillus endospores have been shown to survive UV and gamma radiation, desiccation, high and low temperatures, and antibiotics. These endospores remain stable for very long periods of time, having been shown to survive more than 100 years without nutrients or water. Anthrax infection is most common in wild animals (deer, antelope, etc.) and livestock (cow, goats, sheep). When an herbivore eats or inhales the endospores, the bacteria enters its vegetative form and begins to reproduce, causing an infection. Livestock vaccines are commonly used to prevent outbreaks.
Anthrax can be spread from infected herbivores to humans, where the bacteria can overwhelm the lymphatic system and cause death.
In humans, anthrax is caused when vulnerable tissue, such as an open wound, comes into contact with the endospores of Bacillus anthracis. In a complex process after germinating in host tissues, each B. anthracis bacterium produces two exotoxins. One exotoxin causes edema (swelling due to fluid buildup) in host tissues, while the other exotoxin damages host tissues. Anthrax infections are classically cutaneous, respiratory, or gastrointestinal. Cutaneous anthrax occurs when the endospores enter the skin through a cut or scrape. The infection is localized to the skin surrounding the infection site, resulting in a unique black skin lesion. Cutaneous anthrax is the most common (~90% of worldwide cases) and the least dangerous, but it is still life-threatening if not treated. Breathing in endospores results in respiratory anthrax, which is considered the most deadly form. The bacteria invade the lymph nodes of the chest and quickly spread throughout the body, causing fever, shortness of breath, shock, and, without treatment, death. Gastrointestinal anthrax results from consuming the endospores. Symptoms include diarrhea, abdominal pain, loss of appetite, and vomiting. This is the least common form of anthrax. A fourth form, injection anthrax, has recently been detected among intravenous drug users. It occurs when the injected drugs are contaminated with B. anthracis.

Because the spores of B. anthracis are so hardy and the disease they cause is so deadly, they have been identified as a potential biological weapon, an infective agent administered to people in order to intentionally cause disease. In order to counter this, anthrax vaccines have been developed for humans but because of adverse side-effects are not commonly administered to the general population and are inconsistently protective. Individuals at high risk of exposure to B. anthracis, whether natural or weaponized, and individuals suspected to have come in contact with anthrax recently are prescribed the vaccine as a preventive measure. Typical treatment includes the antibiotics penicillin and ciproflaxin and monitoring of a patient’s symptoms to make sure they do not become worse.