There are three ways that disease in animal populations can impact human populations. First, animals can transmit diseases to humans, so monitoring disease prevalence and distribution in animal populations may prevent outbreaks in human populations or contribute to the earlier detection of such outbreaks. Second, disease can cause sickness in both animals and humans, and, in some cases, animals are more susceptible than are humans. The classic example of this circumstance is the sensitivity of canaries to coal gases. Therefore, monitoring animals may provide an early warning of a concurrent outbreak in humans. The monitoring of mass mortality events in birds has demonstrated value as an early warning system for increased risk of West Nile virus (WNV) outbreaks in humans (Mostashari et al., 2003). WNV is an arbovirus residing within bird reservoir hosts, which is by mosquitoes and can result in infection spilling over to humans. Clinical disease in humans involves the central nervous system and can result in the occasional death. Predicting outbreak risk in humans for arbovirus diseases can be difficult because this requires knowledge of the distribution and the infection status of the reservoir and the vector. Although events such as poisoning can result in mass mortality of birds, WNV is a predominant cause of these occurrences in birds. Therefore, an increase in the expected frequency of mass mortality events in birds generally indicates increased viral activity within both reservoir hosts and the vector. Third, animals have economic value; hence, disease in animals can threaten the economic health—and in the extreme case physical health (through starvation)—of human populations.

For example, an eradication program to control an outbreak of contagious bovine pleuropneumonia in cattle in Botswana in 1996 required the slaughter of all cattle within the affected region.This mass slaughter led to a 2.3-fold increase in the risk of malnutrition in children under 5 years of age within the affected region compared with the country as a whole. The malnutrition arose from a reduction in milk and meat consumption and also from an economic decline contributed to by loss of cattle for transport and draught power (Boonstra et al., 2001).

The crossover threats, mentioned above, posed by microbes in animals are partly determined by the degree to which we share DNA with them. For example, the chimpanzee genome contains 99% of the human genome, and consequently, chimpanzees can become sick with many illnesses that also strike humans. The reverse is also true. In contrast, a human is unlikely to be affected by many pathogens that make a lion ill, with the notable exception of enteric pathogens such as salmonella. Diseases that can spread from animals to humans include cat-scratch disease, rat bite fever, rabies, tularemia, and plague. Conversely, other mammals and birds can become ill with the same viruses that affect humans, including encephalitides and WNV. Reptiles have even less in common with humans but can carry salmonella, and humans who handle the animals can become infected. Young children are especially vulnerable when they manifest an all-too-common habit of not washing their hands after handling reptiles.

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