Influenza

Background
Influenza virus infection, one of the most common infectious diseases, is a highly contagious airborne disease that causes an acute febrile illness and results in variable degrees of systemic symptoms, ranging from mild fatigue to respiratory failure and death. These symptoms contribute to significant loss of workdays, human suffering, mortality, and significant morbidity. The 1918-1919 H1N1 type influenza pandemic killed an estimated 20-50 million persons, with 549,000 deaths in the United States alone.

Accurately diagnosing influenza A or B infection based solely on clinical criteria is difficult because of the overlapping symptoms caused by the various viruses associated with upper respiratory tract infection (URTI). In addition, several serious viruses, including adenoviruses, enteroviruses, and paramyxoviruses, may initially cause influenzalike symptoms. The early presentation of mild or moderate cases of flavivirus infections (eg, dengue) may initially mimic influenza. For example, some cases of West Nile fever acquired in New York in 1999 were clinically misdiagnosed as influenza.

Patients with influenza frequently present with various symptoms shared by many other viral infections. In the northern and southern hemispheres, these symptoms are more common in the winter months. As a result, during the winter, clinics and emergency department waiting rooms fill with patients who have influenza or other URTIs.
Pathophysiology
Influenza results from infection with 1 of 3 basic types of influenza virus—A, B, or C—which are classified within the family Orthomyxoviridae. These single-stranded RNA viruses are structurally and biologically similar but vary antigenically.

The RNA core consists of 8 gene segments surrounded by a coat of 10 (influenza A) or 11 (influenza B) proteins. Immunologically, the most significant surface proteins include hemagglutinin and neuraminidase. The viruses are typed based on these proteins. For example, influenza A subtype H3N2 expresses hemagglutinin 3 and neuraminidase 2.

The most common prevailing influenza A subtypes that infect humans are H1N1 and H3N2. Each year, the trivalent vaccine used worldwide contains A strains from H1N1 and H3N2, along with an influenza B strain.

Influenza virus infection occurs after transfer of respiratory secretions from an infected individual to a person who is immunologically susceptible. If not neutralized by secretory antibodies, the virus invades airway and respiratory tract cells. Once within host cells, cellular dysfunction and degeneration occur, along with viral replication and release of viral progeny. Systemic symptoms result from inflammatory mediators, similar to other viruses. The incubation period of influenza ranges from 18-72 hours.

Influenza A is generally more pathogenic than influenza B. Influenza A is a zoonotic infection, and more than 100 types of influenza A infect most species of birds, pigs, horses, dogs and seals. Indeed, the 1918 pandemic that resulted in millions of human deaths worldwide is believed to have originated from a virulent strain of H1N1 from pigs or birds. Recently, scientists obtained and sequenced the 1918 H1N1 strain from a frozen corpse found in Alaska. The virus was reconstructed at the Centers for Disease Control and Prevention (CDC) laboratory in Atlanta and was found to be highly lethal when tested in mice; the virus was also found to be lethal to chicken embryos. This unique N1 neuraminidase is being studied in order to provide better insight into the N1 found in H5N1, the type responsible for avian influenza (also known as bird flu). Read more »

Bacterial Infections and Pregnancy

This article focuses on common bacterial infections in pregnancy and describes the manifestations of these infections and the therapies used to treat them.

Bacterial infections can affect pregnant women from implantation of the fertilized ovum through the time of delivery and peripartum period. They may also affect the fetus and newborn. Many women with these infections are asymptomatic, necessitating both a high degree of clinical awareness and adequate screening.

Grop B Streptococcus

Group B Streptococcus (GBS) is the most common cause of life-threatening infections in newborns; thus, GBS is the primary focus of any discussion about infections and pregnancy. Infections caused by GBS affect both mother and child. Since the emergence of this pathogen in the 1970s, the increased use of intrapartum prophylaxis has decreased the infection rate by 70%.

Etiology

Streptococcus agalactiae, or GBS, is a facultative, beta-hemolytic, fastidious, gram-positive coccus. GBS can be found as part of normal vaginal, rectal, and oral flora. The virulence of the organism depends largely on the polysaccharide capsule.

Transmission

Twenty to 25% of pregnant women are asymptomatic carriers of vaginal or rectal GBS. Intrapartum transmission occurs via ascending spread or at the time of delivery.

Clinical spectrum

Because only 0.5-1% of mothers who carry GBS develop signs and symptoms of disease, clinical diagnosis of GBS infection can be problematic.

In pregnant women, GBS is a cause of cystitis, amnionitis, endometritis, and stillbirth. Occasionally, GBS has caused endocarditis and meningitis in pregnant women, while, in postpartum women, GBS has been identified as a cause of urinary tract infections (UTIs) and pelvic abscesses. Read more »

Mycoplasma Infections

Background

Mycoplasma species are the smallest free-living organisms. These organisms are unique among prokaryotes in that they lack a cell wall, a feature largely responsible for their biologic properties such as their lack of a reaction to Gram stain and their lack of susceptibility to many commonly prescribed antimicrobial agents, including beta-lactams. Mycoplasmal organisms are usually associated with mucosal surfaces, residing extracellularly in the respiratory and urogenital tracts. They rarely penetrate the submucosa, except in the case of immunosuppression or instrumentation, when they may invade the bloodstream and disseminate to different organs and tissues throughout the body.

Although scientists have isolated at least 17 species of Mycoplasma from humans, 4 types of organisms are responsible for most clinically significant infections that may come to the attention of practicing physicians. These species are Mycoplasma pneumoniae, Mycoplasma hominis, Mycoplasma genitalium, and Ureaplasma species. The focus of this article is infections caused by M pneumoniae; articles on Ureaplasma infections (eg, Ureaplasma Infection) and genital mycoplasmal infections contain discussions of infections caused by other mycoplasmal species.

Pathophysiology

M pneumoniae is perhaps best known as the cause of walking or atypical pneumonia, but the most frequent clinical syndrome caused by this organism is actually tracheobronchitis or bronchiolitis, often accompanied by upper respiratory tract manifestations. Pneumonia develops in only 5%-10% of persons who are infected. Acute pharyngitis and myringitis are less common.

After inhalation of respiratory aerosols, the organism attaches to host cells in the respiratory tract. The P1 adhesin and other accessory proteins mediate attachment, followed by induction of ciliostasis, local inflammation that consists primarily of perivascular and peribronchial infiltration of mononuclear leukocytes, and tissue destruction that may be mediated by liberation of peroxides. Recently, M pneumoniae has been shown to produce an exotoxin that is believed to play a role in the damage to the respiratory epithelium that occurs during acute infection. The organism also has the ability to exist intracellularly. Additionally, acute mycoplasmal respiratory tract infection may be associated with exacerbations of chronic bronchitis and asthma. More extensive information on the pathogenesis of mycoplasmal respiratory infections is available in a recent review article.

Spread of infection throughout households is common, although person-to-person transmission is slower than for many other common bacterial respiratory tract infections; close contact appears necessary. Generally, the incubation period is 2-3 weeks. The organism may persist in the respiratory tract for several months, and sometimes for years in patients who are immunosuppressed, after initial infection. Read more »

Rheumatic Fever

Background

Acute rheumatic fever (ARF) is an autoimmune inflammatory process that develops as a sequela of streptococcal infection. ARF has extremely variable manifestations and remains a clinical syndrome for which no specific diagnostic test exists. Persons who have experienced an episode of ARF are predisposed to recurrence following subsequent (rheumatogenic) group A streptococcal infections. The most significant complication of ARF is rheumatic heart disease, which usually occurs after repeated bouts of acute illness.

Pathophysiology

ARF is characterized by nonsuppurative inflammatory lesions of the joints, heart, subcutaneous tissue, and central nervous system. An extensive literature search has shown that, at least in developed countries, rheumatic fever follows pharyngeal infection with rheumatogenic group A streptococci. The risk of developing rheumatic fever after an episode of streptococcal pharyngitis has been estimated at 0.3-3%. More recent investigations of rheumatic fever occurring in the aboriginal populations of Australia suggest that streptococcal skin infections might also be associated with the development of rheumatic fever. In Oceania and Hawaii, streptococcal strains that are not typically associated with rheumatic fever have been found to cause the disease.

Molecular mimicry accounts for the tissue injury that occurs in rheumatic fever. Both the humoral and cellular host defenses of a genetically vulnerable host are involved. In this process, the patient’s immune responses (both B- and T-cell mediated) are unable to distinguish between the invading microbe and certain host tissues.The resultant inflammation may persist well beyond the acute infection and produces the protean manifestations of rheumatic fever.

Frequency

United States

The incidence of ARF has declined markedly in the past 50 years in both the United States and Western Europe. Most Western physicians see only the late sequelae of rheumatic heart disease; the diagnosis of an acute case is usually reason enough for a ground rounds presentation. This remarkable decline of rheumatic fever likely reflects improved socioeconomic conditions, as well the decline in prevalence of the classically described rheumatogenic strains of group A streptococci.

Following two decades of almost total absence, a resurgence of ARF occurred in the 1980s among middle-class white children in Salt Lake City, Utah. Clusters were also reported in US Army and Navy training camps during the same period. These limited outbreaks were associated with mucoid rheumatogenic strains that were rarely seen in the preceding 20 years. Today, ARF remains a rarity in most of the United States, although Hawaii and American Samoa continue to see a significant number of cases, many of which are caused by streptococcal strains not usually associated with rheumatic fever in persons of Polynesian descent. Read more »

Typhus

Background

Typhus refers to a group of infectious diseases that are caused by rickettsial organisms and that result in an acute febrile illness. Arthropod vectors transmit the etiologic agents to humans. The principle diseases of this group are epidemic or louse-borne typhus and its recrudescent form known as Brill-Zinsser disease, murine typhus, and scrub typhus. (For more information on pediatric scrub typhus, see the eMedicine article Scrub Typhus in the Pediatric: General Medicine volume.)

Pathophysiology

Epidemic typhus is the prototypical infection of the typhus group of diseases, and the pathophysiology of this illness is representative of the entire category. The arthropod vector of epidemic typhus is the body louse (Pediculus corporis). This is the only vector of the typhus group in which humans are the usual host. Rickettsia prowazekii, which is the etiologic agent of typhus, lives in the alimentary tract of the louse. A Rickettsia- harboring louse bites a human to engage in a blood meal and causes a pruritic reaction on the host’s skin. The louse defecates as it eats; when the host scratches the site, the lice are crushed, and the Rickettsia- laden excrement is inoculated into the bite wound. The Rickettsia travel to the bloodstream and rickettsemia develops.

Rickettsia parasitize the endothelial cells of the small venous, arterial, and capillary vessels. The organisms proliferate and cause endothelial cellular enlargement with resultant multiorgan vasculitis. This process may cause thrombosis, and the deposition of leukocytes, macrophages, and platelets may result in small nodules. Thrombosis of supplying blood vessels may cause gangrene of the distal portions of the extremities, nose, ear lobes, and genitalia. This vasculitic process may also result in loss of intravascular colloid with subsequent hypovolemia and decreased tissue perfusion and, possibly, organ failure. Loss of electrolytes is common.

Some people with a history of typhus may develop a recrudescent type of typhus known as Brill-Zinsser disease. After a patient with typhus is treated with antibiotics and the disease appears to be cured, Rickettsia may linger in the body tissues. Months, years, or even decades after treatment, organisms may reemerge and cause a recurrence of typhus. How the Rickettsia organisms linger silently in a person and by what mechanism recrudescence is mediated are unknown. The presentation of Brill-Zinsser disease is less severe than epidemic typhus, and the associated mortality rate is much lower. Risk factors that may predispose to recrudescent typhus include improper or incomplete antibiotic therapy and malnutrition.

Murine typhus and scrub typhus share the same pathophysiology as epidemic typhus, although they are somewhat milder. The incubation period is approximately 12 days for the typhus group. Prior infection with Rickettsia typhi provides subsequent and long-lasting immunity to reinfection. Read more »