Infections of the Circulatory System

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Infections of the Circulatory System

WHY YOU NEED TO KNOW

HISTORY

Throughout ancient times various civilizations have recognized the importance of blood and understood its role in overall human health. Although blood was recognized as one of the four basic humors of the body, its exact role, composition, and how it moved throughout the body remained mysteries for the ancient physicians. As early as 4 bce physicians discovered the valves of the heart but did not understand their function. From the second century ad to the mid-1200s there had been a series of discoveries that slowly began to clear up the picture about the functioning of the circulatory system. During this time practices such as leeching or bloodletting were practiced in an attempt to “maintain a balance of the body humors.” Imbalances in the humors were believed to be the source of virtually all illnesses and diseases. Ibn al-Nafis in 1242 was the first to describe the process of blood circulation in the human body. Early experimentation in blood transfusion was being conducted as early as the late 1490s, with the first successful transfusion being recorded in the mid-1600s. The 1600s also saw the microscopic examination of blood by Antony van Leeuwenhoek and the subsequent discovery of red blood cells. It was not until William Harvey published his works in 1628 that the modern understanding of the circulatory system was established. Since then our knowledge of the circulatory system has advanced considerably.

IMPACT

Adding to the knowledge that the circulatory system is responsible for carrying nutrients and oxygen to the tissues and cells of the body as well as for taking wastes away from the cells, the importance of the immune system was recognized. The blood and lymph are the delivery systems responsible for transporting various cellular and molecular components throughout the body. In addition, this system is capable of transporting pathogens throughout the body. Therefore, a localized infection can spread and cause a systemic infection affecting multiple organ systems. Such an infection can lead to potentially deadly conditions such as toxic shock syndrome, a condition that can be caused by a number of different bacteria. The damage to heart valves due to bacterial infections that originated in the oral cavity is another example of the complexity of an infection in the circulatory system. The potential for the spread of HIV through blood transfusions also provides a clear example of the impact of circulatory system infections on human health.

FUTURE

Research on organisms responsible for toxic shock syndrome continues as the list of potential pathogens is growing and the “at-risk” group is being expanded. Development of more efficient and effective processes of blood collection, testing, and transfusion screening continues in an effort to minimize the risks involved in the spread of blood-borne diseases. Furthermore, efforts are being made to develop artificial blood to prevent blood-borne diseases. If and when successful, this will have major implications throughout human medicine/healthcare. Whether at the center of an infection or a mere participant in transporting a pathogen through the bloodstream to a target in another body system, the circulatory system remains a central player in the majority of microbial attacks on the body and the subsequent responses.

Introduction

The circulatory system includes the cardiovascular system and the lymphatic system. The lymphatic system consists of the lymph, lymphatic vessels, lymphatic tissue, and lymphatic organs, and is discussed in Chapter 20 (The Immune System). The cardiovascular system consists of the heart, blood, and blood vessels (Figure 14.1). Infection and inflammation of the cardiovascular system frequently cause cardiac and vascular disease. The lymphatic system returns excess tissue fluid back to the cardiovascular system and therefore has direct access to the cardiovascular system. The relationship between the cardiovascular and lymphatic systems is illustrated in Figure 14.2. Once microorganisms gain access to either one of the systems they can spread throughout the body and therefore have the potential to infect any organ system. In general, bacterial and fungal infections will affect the endocardium (tissue lining of the heart chambers), causing endocarditis, whereas viral and parasitic infections affect the myocardium (heart muscle), resulting in myocarditis. Inflammation and infection of the pericardium (membrane surrounding the heart), called pericarditis, can be caused by bacteria, viruses, and rarely by fungi.

Endocarditis

Endocarditis is an inflammation of the endocardium, the lining of the heart or the heart valves (Figure 14.3). The condition can be classified as infective if a microorganism is involved, or as noninfective. Noninfective endocarditis involves the formation of platelet and fibrin thrombi on heart valves and the surrounding endocardium, in response to trauma, circulating immune complexes, vasculitis, or a hypercoagulated state. Infective endocarditis symptoms may develop slowly (subacute) or suddenly and they include the following:

The most common cause of infectious endocarditis is bacterial, but it also can be caused by fungi. In some cases, no causative organism can be identified.

Blood-borne Infectious Diseases

Blood-borne disease is spread by contaminated blood or bodily fluids. Any exposure to blood or other bodily fluids can transmit infectious disease. Although blood-borne diseases can affect anyone exposed to infected blood or bodily fluids, the risk of exposure is greater with certain occupations such as healthcare, emergency response, public safety, teaching, and many others (see Chapter 5, Safety Issues). Pathogens of primary concern for these professions, according to the Centers for Disease Control and Prevention (CDC, Atlanta, GA), are HIV, hepatitis B virus, hepatitis C virus, and those causing viral hemorrhagic fever. These viruses are discussed in detail in Chapter 17 (Sexually Transmitted Infections/Diseases). Diseases that are not transmitted directly by blood or by contact with bodily fluids, but by an insect or other vector, are classified as vector-borne diseases.

Bacterial Infections

Once bacteria have access to the circulatory system they become widely dispersed (bacteremia) and are capable of infecting a wide range of tissues and organs. If the bacteria in the circulatory system are not destroyed by the immune system or by antibiotic treatment, they can multiply in the blood and cause septicemia. Examples of bacterial diseases of the circulatory system are illustrated in Table 14.1.

TABLE 14.1

Bacterial Diseases of the Circulatory System

Disease Organism Target of Infection Transmission Treatment
Rheumatic fever Group A Streptococcus Heart, joints, brain, spinal cord, skin Coughing, sneezing, saliva Penicillin, erythromycin, oral cephalosporin, vancomycin, oxacillin
Gas gangrene Clostridium perfringens Extremities, muscle tissues, and organs Wound contact with soil Removal of necrotic tissue, including amputation

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Bacteremia

Blood is normally sterile, but microorganisms can enter the blood under a variety of conditions. Bacteremia is the term used when bacteria are present in the bloodstream (see Chapter 9, Infection and Disease). Bacteremia has various possible causes including infection during dental procedures, catheterization and the placement of other indwelling devices, surgical procedures, wound infection, and many more. In general, the presence of bacteria in the blood elicits a strong immune response by circulating macrophages, the complement system, and lymphocytes (see Chapter 20, The Immune System), thus preventing bacteria from multiplying. In addition, the blood is relatively low on iron, a requirement for most bacterial growth. If the defenses of the immune and circulatory systems fail microbes can undergo uncontrolled proliferation in the blood, causing a condition called septicemia.

Septicemia

Sepsis is a toxic condition caused by the spread of bacteria or bacterial toxins from the site of infection (see Chapter 9, Infection and Disease). Septicemia is sepsis that occurs when bacteria multiply in the bloodstream. Septicemia is a medical emergency that requires immediate medical treatment. If the condition progresses to septic shock the death rate is as high as 50%, depending on the type of organism involved. Septic shock is the result of hypotension (low blood pressure) despite adequate fluid substitution. Septicemia develops quickly and the patient becomes extremely ill. Although each individual may experience symptoms differently the most common symptoms include the following:

Rheumatic Fever

Rheumatic fever is an inflammatory disease that can develop as a rare complication after a group A streptococcal infection such as strep throat or scarlet fever (see Chapter 11, Medical Highlights: Complications of “Strep Throat”). The condition commonly involves the heart, joints, brain, spinal cord, and skin. In general, rheumatic fever occurs in children between the ages of 4 and 18 years. Symptoms normally begin several weeks after the disappearance of localized throat symptoms and vary greatly between individuals, depending on the parts of the body inflamed. The most common symptoms include the following:

In some children carditis may not be evident, and the inflammation of the heart is recognized only years later, when heart damage is discovered. Treatment involves long-term antibiotic administration to eliminate any residual streptococcal infection, antiinflammatory medication to reduce inflammation, and also the limiting of physical activity that might aggravate the inflamed structures. People who do not take low-dose antibiotics continually, especially during the first 3 to 5 years after the first episodes of the disease, will likely experience recurrence of the condition, resulting in severe heart complications.

Gangrene

Gangrene is a complication of necrosis, the decay and death of tissue, that is often related to wounds. If the blood supply to a tissue is interrupted by an infection or ischemia (restriction of blood supply) gangrene can occur. Gangrene most commonly affects the extremities (Figure 14.4); however, it can also occur in muscle tissue and organs. Enzymes released from the dying cells and tissue will further destroy the surrounding tissues and thus provide a perfect nutrient environment for many bacterial species. Tissues devoid of blood supply become ischemic and provide an environment for anaerobic bacteria. Several species of the genus Clostridium, gram-positive, endospore-forming anaerobes, grow easily under these conditions. Clostridium is commonly found in soil as well as in the intestinal tracts of humans and domestic animals. The most frequent species involved in gangrene is C. perfringens, but other species and several other genera of bacteria can also grow under the conditions mentioned.

Treatment of gangrene usually entails the removal of necrotic tissue, and in many cases amputation may be necessary. Because antibiotics cannot reach the ischemic tissue, antibiotics alone are not effective. In addition to surgery and antibiotics, hyperbaric oxygen therapy can be used to kill the anaerobic Clostridium causing the condition.

There are different types of gangrene, including the following:

• Dry gangrene, due to ischemia and generally beginning at the distal portions of a limb such as in the feet. This condition often occurs in elderly patients with arteriosclerosis, and other persons with impaired peripheral blood flow, such as diabetic patients.

• Internal gangrene, also called white gangrene, is noticeable by the bleaching of internal tissue, and is generally contracted after surgery or trauma.

• Wet gangrene occurs in organs lined by mucous membranes such as the mouth, lower intestinal tract, lungs, and cervix. Although not necessarily associated with moist tissue, bedsores are also categorized as wet gangrene infections. Toxic products formed by the infecting bacteria can be absorbed if the affected tissue is not removed, resulting in septicemia or toxemia, and eventually death.

• Gas gangrene is caused by bacteria that produce gas within the infected tissue. Toxins produced by the bacteria will cause necrosis of more tissue, thereby providing further bacterial growth. If untreated, the condition is fatal.

Zoonotic Diseases

Any disease and/or infection that can be transmitted from vertebrate animals to humans is classified as a zoonosis. More than 200 zoonoses have been described, but not all affect the circulatory system (Table 14.2). Zoonoses may be caused by bacteria, viruses, parasites, and unconventional agents such as prions. The emerging field of conservation medicine integrates human medicine, veterinary medicine, and environmental services and is largely concerned with zoonoses.

TABLE 14.2

Zoonotic Infections of the Circulatory System

Disease Organism(s) Target of Infection Transmission Treatment
Brucellosis (undulant fever) Species of Brucella Systemic infection with symptoms manifested in the gastrointestinal and respiratory systems Direct contact with sheep, goats, cattle, deer, elk, pigs, dogs, and other animals; contact with animal products contaminated with the bacteria; ingestion of unpasteurized milk or dairy products Doxycycline plus rifampin for nonpregnant adults
Trimethoprim-sulfamethoxazole for pregnant women and children under age 8 yr
Tularemia Francisella tularensis Blood and lymph nodes Direct contact with rabbits, ground squirrels and/or through the bite of an infected tick, deerfly, or other insect Streptomycin, gentamicin fluoroquinolones, and doxycycline
Cat scratch fever Bartonella henselae Lymph nodes, overall systemic infection Cat bites, exposure of eye or lesion to infected saliva Antibiotic therapy is usually not effective but in serious cases cephalosporins, erythromycin, or doxycycline can be used with some success
Rat-bite fever Streptobacillus moniliformis (in United States) Systemic infection with fever, vomiting, joint pain, headaches, and rash Bite or scratch from infected rat, handling infected rat, or ingesting contaminated food or water Penicillin, doxycycline

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Brucellosis (Undulant Fever)

Brucellosis is an infectious disease that occurs worldwide and is caused by various Brucella species. Although the condition is found worldwide, it is most common in areas with insufficient standards for public health and domestic animal health programs. Brucellas are small, gram-negative, aerobic (or capnophilic) coccobacilli that are adapted to intracellular replication. These bacteria affect primarily sheep, goats, cattle, deer, elk, pigs, dogs, and other animals. It is zoonotic and humans become infected by contact with animals or animal products contaminated with the bacteria. Person-to-person spread of brucellosis is extremely rare, but may occur when an infected woman is breastfeeding. High-risk groups include abattoir workers, meat inspectors, animal handlers, veterinarians, and laboratory technicians. In the general United States population, the most common cause of infection is the ingestion of unpasteurized milk or other dairy products.

Clinically, the acute form of brucellosis is nonspecific and represents itself with flulike symptoms including chills, excessive sweating, fever, sweats, weakness, malaise, anorexia, headache, abdominal pain, back pain, and joint pain. Severe infections of the central nervous system or the lining of the heart (endocardium) may occur. Brucellosis can also be chronic, with symptoms that include undulant fevers, arthritis, chronic fatigue syndrome, and depression. Treatment with antibiotics is possible but may be difficult, depending on the timing of the treatment and the severity of the disease. The mortality rate of the disease is low and is usually associated with endocarditis.

Tularemia

Tularemia, also known as “rabbit fever,” is a zoonotic disease caused by Francisella tularensis, a gram-negative bacillus. The organism infects the blood and lymph nodes. The disease is transmitted by contact with infected animals, most commonly rabbits and ground squirrels. People can also contract the illness through an infected tick, deerfly, or other insects. Persons with tularemia do not need to be isolated because the illness is not known to spread from person to person. Although the bacterium can enter the human body at various sites the most common is through a minor abrasion of the skin, where an ulcer will occur. Symptoms of the illness appear 3 to 5 days after exposure, but may take up to 14 days. These symptoms include the following:

The infective dose is very low; only about 10 to 50 organisms are necessary to cause an infection. If not contained the proliferation of F. tularensis can lead to sepsis. Tularemia occurs in many parts of the United States and is on the list for nationally notifiable diseases. Because of the highly infectious nature of the organism, the concern rose that it could be used as a biological weapon and is now included in the bioterrorism preparedness response by the CDC.

Cat Scratch Disease

Cat scratch disease is caused by Bartonella henselae, an aerobic, gram-negative, rod-shaped bacterium that is found in all parts of the world. The disease is transmitted by cat scratches, bites, or even by exposure to saliva on broken skin or contact with the eye. About 40% of cats carry the organism at some point in their lives, without showing symptoms. Kittens are more likely to be infected and pass the bacterium to humans. Because Bartonella is present in the saliva of cats, petting an infected cat with a skin lesion, or touching the eye after contact with the cat’s fur, can also be the point of transmission. Initially a blister or bump may be found at the site of a scratch or bite, which often is mistaken for an insect bite. Within 2 to 3 weeks of the infection lymph nodes close to the area of inoculation will swell. Other than the initial bump and swelling of the lymph nodes, symptoms of infection may include the following:

• Fever

• Fatigue

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