CHAPTER 26 Person-to-Person Microbial Diseases

Airborne Transmission of Diseases Airborne Pathogens

Many respiratory pathogens are gram- positive Bacteria. Because gram-positive Bacteria are resistant to drying, they are easily transmitted in air. Most respiratory pathogens are transferred from person to person via respiratory aerosols generated by coughing, sneezing, talking, or breathing.

Figure 26.2 illustrates the human respiratory tract.

Streptococcal Diseases Diseases caused by streptococci include streptococcal sore throat and pneumococcal pneumonia. Occasionally, Streptococcus pyogenes infections develop from pharyngitis into serious conditions such as scarlet fever and rheumatic fever. Pneumonia caused by Streptococcus pneumoniae is a serious disease with high mortality.

Definitive diagnosis for both pathogens is by culture. Infections with both pathogens are treatable with antimicrobial drugs, but drug- resistant strains are known, especially for Streptococcus pneumoniae.

Corynebacterium and Diphtheria Diphtheria is an acute respiratory disease caused by the gram-positive bacterium Corynebacterium diphtheriae. Early childhood immunization (DTP) is very effective for preventing this serious respiratory disease.

Bordetella and Whooping Cough In the United States, there has been an increase in the number of annual cases of whooping cough. From an average of less than 2000 pertussis cases per year in the 1970s, the number of cases has now risen to over 8000 per year. Inadequately immunized children are at high risk for acquiring pertussis.

Mycobacterium, Tuberculosis, and Leprosy Tuberculosis is one of the most prevalent and dangerous single diseases in the world. Its incidence is on the increase in developed countries, in part because of the emergence of drug-resistant strains. The pathology of tuberculosis and leprosy is influenced by the cellular immune response.

In most cases of tuberculosis, acute infection does not occur. The infection remains localized, is usually not apparent, and appears to end. But this initial infection hypersensitizes the individual to the bacteria or their products and consequently alters the response of the individual to subsequent M. tuberculosis exposures. A diagnostic test, called the tuberculin test, can be used to measure this hypersensitivity.

Chemotherapy of tuberculosis has been a major factor in control of the disease. Initial success in chemotherapy occurred with the introduction of streptomycin, but the real revolution in tuberculosis treatment came with the discovery of iisoniazid (Figure 26.9).

This drug, virtually specific for mycobacteria, is effective, inexpensive, and is readily absorbed when given orally.

Neisseria meningitidis, Meningitis, and Meningococcemia

Neisseria meningitidis is a common cause of meningococcemia and meningitis in young adults and occasionally occurs in epidemics in closed populations. Bacterial meningitis and meningococcemia are serious diseases with very high mortality rates. Treatment and prevention strategies are in place to deal with epidemic outbreaks, but an effective universal vaccine is not yet available.

Viruses and Respiratory Infections Viral respiratory diseases are highly infectious and may cause serious health problems. However, the common childhood viral diseases measles, mumps, rubella, and chickenpox are all controllable with appropriate immunization procedures.

Although once a common childhood illness, measles generally occurs nowadays in rather isolated outbreaks because of widespread immunization programs begun in the mid-1960s (Figure 26.13).

Colds and Influenza Colds and influenza, or flu, are the most common infectious diseases (Figure 26.16).

Although they are not usually life- threatening by themselves, they can lower resistance and allow serious secondary bacterial infections.

Most antiviral drugs are ineffective against colds, but a pyrazidine derivative (Figure 26.18a) has shown promise for preventing colds after virus exposure.

Antigenic shift occurs when major changes in antigens occur due to gene reassortment in influenza virus. Antigenic drift occurs when minor changes in antigens occur due to gene mutation in influenza virus.

Influenza outbreaks occur annually, and more serious epidemics and pandemics occur periodically (Figure 26.20).

Direct Contact Transmission of Diseases Staphylococcus

Although staphylococci are usually harmless inhabitants of the upper respiratory tract and skin, several serious diseases can result from pyogenic infection, including some caused by staphylococcal superantigens. Staphylococci can cause acne, boils (Figure 26.21), pimples, impetigo, pneumonia, osteomyelitis, carditis, meningitis, and arthritis.

Certain strains of S. aureus have been implicated as the agents responsible for toxic shock syndrome (TSS), a serious outcome of staphylococcal infection characterized by high fever, rash, vomiting, diarrhea, and occasionally death.

Helicobacter pylori and Gastric Ulcers Helicobacter pylori infection appears to be the most common cause of gastric ulcers. Treatment of gastric ulcers now involves antibiotics, which seem to promote a permanent cure.

Hepatitis Viruses Hepatitis caused by viruses can cause cirrhosis, an acute liver disease.

Symptoms include fever and jaundice (production and release of excess bilirubin by the liver due to destruction of liver cells, resulting in yellowing of the skin), among others.

Table 26.2 lists the hepatitis viruses. HBV and HCV can cause chronic infections leading to liver cancer.

Vaccines are available for HAV and HBV. The overall prevalence of hepatitis has decreased significantly in the last 20 years in the United States (Figure 26.23), but viral hepatitis is still a major public health problem because of the high infectivity of the viruses.

Sexually Transmitted Infections Gonorrhea and Syphilis

Table 26.3 lists sexually transmitted diseases and treatment guidelines.

Gonorrhea and syphilis, caused by Neisseria gonorrhoeae and Treponema pallidum, respectively, are sexually transmitted infections (STIs) with potential serious consequences if not treated. During pregnancy, syphilis can be transmitted from an infected woman to the fetus; the disease acquired by the infant is called congenital syphilis.

Although the incidence of these diseases has generally declined in recent years, there are still over 350,000 cases of gonorrhea and 6000 cases of syphilis annually in the United States (Figure 26.25).

Chlamydia, Herpes, and Trichomoniasis Chlamydia, the most prevalent STI, is caused by infection with the bacterium Chlamydia trachomatis. Untreated chlamydial nongonococcal urethritis causes serious complications in males and females.

Herpes lesions can also be transmitted sexually and are caused by herpes simplex virus type 1 and herpes simplex virus type 2. HSV-2 is generally associated with sexual transmission and infection of the anogenital regions. Genital herpes infections are presently incurable, although a limited number of drugs have been successful in controlling the infectious blister stages.

The guanine analog acyclovir (Figure 26.31), given orally and also applied topically, is particularly effective in limiting the shed of active virus from blisters and promoting the healing of blistering lesions.

Trichomonas vaginalis is a protozoan responsible for trichomoniasis, another STI. In general, these STIs are widespread and are more difficult to diagnose and treat than gonorrhea or syphilis.

Acquired Immunodeficiency Syndrome: AIDS and HIV

AIDS is now one of the most prevalent infectious diseases in the human population. HIV destroys the immune system, and opportunistic pathogens then kill the host. HIV can infect cells displaying the CD4 cell-surface protein. The two cell types most commonly infected are macrophages and T- helper cells.

Infected macrophages and T cells produce and release large numbers of HIV particles, which in turn infect other cells that display CD4 (Figure 26.35).

The HIV infection results in a progressive decline in CD4 cell number.

In a normal human, CD4 cells constitute about 70% of the total T-cell pool; in AIDS patients, the number of CD4 cells steadily decreases, and by the time opportunistic infections become established, CD4 cells may be almost absent (Figure 26.36).

The RT-PCR test estimates the number of viruses present in the blood, or the viral load. After initial discovery of infection, the RT- PCR test is used to monitor progression of AIDS and the effectiveness of chemotherapy (Figure 26.37).

There is still no effective vaccine for HIV. However, several antiviral drugs slow the progress of AIDS (Table 26.4).

AZT and the other nucleoside analogs are nucleoside reverse transcriptase inhibitors (NRTIs), and they stop HIV replication (Figure 26.38a).

The second category of anti-HIV drugs is the nonnucleoside reverse transcriptase inhibitors (NNRTIs) (Figure 26.38b).

These compounds directly inhibit the action of reverse transcriptase by interacting with the protein and altering the conformation of the catalytic site.

Another category of anti-HIV drugs is the protease inhibitors (Figure 26.38c).

The protease inhibitors are computer-designed peptide analogs that inhibit processing of viral polypeptides by binding to the active site of the processing enzyme, HIV protease. A final category of anti-HIV drugs is represented by a single drug, enfuvirtide, a fusion inhibitor composed of a 36–amino acid synthetic peptide that acts by binding to the gp41 membrane protein of HIV. The only prevention for the spread of HIV infection is avoidance of behavior such as intravenous drug use (needle sharing) and unsafe sexual practices.