Basic Virology. Martinez J. Hewlett

viruses must replicate to survive, actively infected populations are the usual source of infection. Still, some viruses such as poxviruses, noroviruses, and coronaviruses have a high resistance to desiccation. In this case a contaminated object such as a desk, pen, book, contaminated clothing, or other inanimate object can be identified as the immediate reservoir. The persistence of some viruses in fecal material is also a potentially long‐lasting, essentially passive reservoir of infection. Noroviruses, which are notorious for causing outbreaks of diarrhea on cruise ships and in nursing homes, are extremely difficult to eliminate from these facilities after an outbreak. Other viruses can even persist for long periods of time in arid environments. The last documented cases of smallpox in Somalia were apparently acquired from contaminated soil. Aerosols of infectious hantavirus and canine parvovirus can be infectious for many months after secretion. Also, some viruses, especially hepatitis A virus, can be isolated from contaminated water sources for several days or even weeks after inoculation.

Figure 3.1 Some transmission routes of specific viruses from their source (reservoir) to humans. The mode of transmission (vector) is also shown.

      Source: Based on Mims, C.A. and White, D.O. (1984). Viral Pathogenesis and Immunology. Boston: Blackwell Science.

Even though infectious virus can sometimes be maintained for a time in the environment in a passive state, viruses are more commonly transmitted via contact with an infectious host. The two most usual reservoirs for human disease are other humans or other animals (zoonosis). Often this transmission requires direct contact between hosts, or may involve an intermediate vector such as a mosquito or tick; modes of spread of some human viruses are illustrated in Figure 3.1, and pathogenic viruses and their reservoirs discussed in this section are listed in Table 3.1.

Some viruses with human reservoirs

A significant number of human viruses leading to either mild or life‐threatening disease are maintained solely in human populations. The list runs the gamut from colds caused mainly by rhinoviruses, warts caused by papillomaviruses, to AIDS caused by HIV. The mode of passage of viruses between humans (i.e., the vector) is intimately linked to human behavior. This behavior can be modified by the disease symptoms themselves. Thus, a respiratory infection leads to irritation of the airways resulting in coughing and sneezing, which spreads an aerosol of droplets containing virus. Herpes simplex virus 1 (HSV‐1) is spread in saliva resulting in transfer of the virus to the recipient's oral epithelial cells; in contrast, the closely related varicella zoster virus (VZV, or chickenpox) is spread by inhalation of a virus‐loaded aerosol or dust containing dead skin flakes. Warts are spread by direct physical contact between the virus‐loaded source (another wart or dead skin from a wart) and layers of the skin below the keratinized epidermis exposed by small cuts or abrasions. Poliovirus and the related coxsackieviruses are spread by virus‐containing feces contaminating food, drinks, or fomites that are ingested; this is termed oral‐fecal spread. In the case of HIV, body fluids – including blood, breast milk, serum, vaginal secretions, and seminal fluid – are sources of infection. The virus can be spread by passive inoculation of, for example, a contaminated hypodermic syringe, or by transfusion, breastfeeding, or sexual activity.

Table 3.1 Some pathogenic viruses, their vectors or routes of spread, and their hosts.

Virus	Vector/Route	Host	Disease
Poliovirus	Human–fecal contamination of water or food	Human	Enteric infection; in rare cases, CNS infection (poliomyelitis)
Western equine encephalitis	Mosquito	Horse	Viral encephalitis in the horse – occasional infection of human
La Crosse encephalitis	Mosquito	Squirrel, fox (reservoir); human	No obvious disease in squirrel or fox; viral encephalitis in human
Sin Nombre virus (hantavirus)	Deer mouse	Deer mouse, other rodents (reservoir); human	Hantavirus hemorrhagic respiratory distress syndrome
HIV	Direct injection of virus‐infected body fluids into blood	Human	AIDS
Measles	Aerosol	Human	Skin rash, neurological involvement
Yellow fever	Mosquito	Tropical monkeys; human	Malaise, jaundice
Chikungunya	Mosquito	Human, primates	Mild to severe hemorrhagic and arthritic disease
Dengue fever	Mosquito	Human, primates	Mild to severe hemorrhagic disease
Ebola	Unknown vector, but nosocomial transmission	Fruit bat; humans and primates	Often fatal hemorrhagic fever
Hepatitis A	Fecal contamination of water or food	Human	Acute hepatitis
Hepatitis B	Direct injection of blood	Human	Chronic hepatitis, liver carcinoma
Hepatitis C	Direct injection of blood	Human	Acute and chronic hepatitis
Hepatitis delta	Blood, requires coinfection with hepatitis B	Human	Acute hepatitis
Hepatitis E	Fecal contamination of water or food	Human	Mild acute hepatitis, except often fatal to pregnant women
Norovirus	Fecal contamination of water or food	Human	Severe diarrhea
Rabies	Bite of infected animal	Vertebrates	Fatal encephalitis
Herpes simplex virus (HSV)	Saliva, other secretions