Martinez J. Hewlett - Basic Virology

THE NATURE OF VIRUS RESERVOIRS

Some viruses with human reservoirs

Some viruses with vertebrate reservoirs

VIRUSES IN POPULATIONS

Viral epidemiology in small and large populations

Factors affecting the control of viral disease in populations

ANIMAL MODELS TO STUDY VIRAL PATHOGENESIS

A mouse model for studying poxvirus infection and spread

Rabies: Where is the virus during its long incubation period?

Herpes simplex virus latency Murine models Rabbit models Guinea pig models

QUESTIONS FOR CHAPTER 3

Since viruses must replicate to survive, actively infected populations are the usual source of infection. Still, some viruses such as poxviruses, noroviruses, and coronaviruseshave 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 hantavirusand canine parvoviruscan 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 3.1Some 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.

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