The agents in the highest-risk category A can be readily disseminated or transmitted from person to person, typically cause diseases that carry a high mortality rate with potential for major public health impact, may cause pandemics leading to widespread panic and social disruption, and are likely to require special action for public health preparedness. For example, the smallpox virus is a category A agent because of its high transmissibility, case mortality rates of 30% or greater, and the lack of effective therapy. Smallpox readily spreads from person to person, mainly through respiratory secretions and by direct contact with virus in skin lesions. Since routine smallpox vaccination ended in the United States in 1972, immunity has waned, leaving the population highly susceptible. Concern that smallpox could be used for bioterrorism has led to reinstitution of vaccination for some medical and military personnel.
Category B agents are less easy to disseminate, cause disease associated with moderate morbidity but low mortality, and require specific diagnostic and disease surveillance. Many of these agents can be spread in food or water. Category C agents include emerging pathogens that could be engineered for mass dissemination because of ease of availability, production, and dissemination; the potential for high morbidity and mortality; and great impact on health.
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Microbes can enter the host through several body surfaces and, once in the host, can disseminate by different routes.
Microbes can enter the host through breaches in the skin, by inhalation or ingestion, or by sexual transmission. The first defenses against infection are intact skin and mucosal surfaces, which provide physical barriers and produce anti-microbial substances. In general, respiratory, gastrointestinal, or genitourinary tract infections that occur in otherwise healthy individuals are caused by relatively virulent microorganisms that are capable of damaging or penetrating intact epithelial barriers. By contrast, most skin infections in healthy individuals are caused by less virulent organisms that breach the skin through damaged sites (
Table 9.6
The dense, keratinized outer layer of skin is a natural barrier to infection; furthermore, the low pH of the skin (less than 5.5) combined with the presence of fatty acids inhibit growth of microorganisms other than the normal bacteria and fungi. Included among these normal flora are potential opportunists, such as S. aureus and Candida albicans.
Cutaneous infections are typically acquired by entry of microbes through breaks in the skin, including wounds
Table 9.6 Routes of Microbial Infection or surgical incisions (Staphylococci), burns (Pseudomonas aeruginosa), and diabetic and pressure-related foot sores (multibacterial infections). Intravenous catheters in hospitalized patients provide portals for local or systemic infection. Needle sticks can expose the recipient to infected blood and transmit HBV, HCV, or HIV. Some pathogens penetrate the skin via an insect or animal bite. Bites by fleas, ticks, mosquitoes, mites, and lice break the skin and transmit arboviruses (causes of yellow fever and encephalitis), bacteria (plague, Lyme disease, Rocky Mountain spotted fever), protozoa (malaria, leishmaniasis), and helminths (filariasis). Animal bites can lead to infections with bacteria, such as Pasteurella, or viruses, such as rabies. Only a few microorganisms are able to cross the skin barrier directly. For example, Schistosoma larvae released from freshwater snails penetrate swimmers’ skin by releasing enzymes that dissolve the extracellular matrix, traversing unbroken skin. Similarly, certain fungi (dermatophytes) can infect intact stratum corneum of the skin, hair, and nails.