| || |
| Animal Reservoirs |
Scientists have identified more than one hundred species of pathogenic bacteria that can infect both humans and animals. As you might imagine, zoonoses (diseases that can be transmitted to humans from other vertebrate hosts) are harder to eradicate. For example, Lyme disease is a zoonosis that has emerged, in part, because of human alteration of ecosystems. (See the Biodiversity unit.) A recent example of a probable zoonosis is SARS, which has been found in the civet cat and other animals.
An average of about 36,000 Americans die each year as a result of influenza. The "Spanish Flu" of 1918-9 killed more people worldwide than did World War I. This disease involves the interaction of multiple animal hosts; however, the story is more complicated. Variation among influenza viruses occurs at the level of the hemagglutinin (HA) and neuraminidase (NA) spikes, which cover the viruses' outer envelopes. These proteins are important for the attachment, and eventual release, of the virus from host cells. In response to an infection the immune system mounts a response against these proteins. Nonetheless, an individual immune to one subtype of influenza may not be able to mount an immune response to a new subtype with modified hemagglutinin or neuraminidase. Genetic mutations, resulting from the change of one or more amino acids within HA or NA, are responsible for the recurrence of minor epidemics of influenza in two- to three-year cycles. This is referred to as antigenic drift.
More dramatic changes, called antigenic shifts, occur when multiple viruses cause coinfections in animal cells (Fig. 4). For example, aquatic birds serve as reservoirs for the influenza-A virus. Some, but not all, types of bird influenza can infect humans directly. Occasionally, a new form of the virus - a new human pathogen - arises when multiple viruses infect the same cell. The mixing vessel is often the pig, which can be infected by both the bird and human forms of the virus. Influenza is an RNA virus and its genome is oddly segmented. Genes for HA and NA are found among the eight distinct fragments of single-stranded RNA. If a pig cell is infected with viruses from two different sources, RNA segments might be exchanged. Such genetic exchange can dramatically change the nature of the spikes found on the newly derived virus. Major pandemics of influenza, including the 1918 flu and the "Hong Kong" flu of 1968, have occurred immediately after antigenic shifts have taken place. Farms and markets where poultry, pigs, and humans come in close contact are considered important to the emergence of new subtypes of influenza.
Lyme disease and influenza are just two examples of diseases that have emerged because of human contact with animal reservoirs. Understanding the epidemiology of other emerging infections, such as hantavirus and ebola, also depends on an understanding of animal hosts.