Question

1. Describe why the world is more vulnerable to spread of viral infections today compared to 100 years ago?
2. Using HIV as an example, explain how new human pathogens emerge due to viruses jumping from animals to humans.

Answer 1

There are many reasons why the world has become more vulnerable to infection, some of these are listed below:

urbanization: Cities create ecosystems with higher temperatures and less seasonal changes. This elongates vector transmission seasons, increasing risk of vector-borne diseases. Population growth and urbanization causing increased densities of people have been associated with the evolution of Dengue virus, which prior to this development may have been of minor impact.

Globalization: Globalization has bought along opportunities for knowledge transfer, cultural and scientific exchanges, and rapid aid responses. Also the increasing globalization has been suggested to be a reason for increased transfer of pathogens into new areas. Millions of animals are transported annually, both legally and illegally, in addition to human travels and only a minor portion is subject to disease control.

Deforestation: Deforestation often creates more larval habitats, increasing the number of vectors . Because of the high diversity of vectors, there will always be some species with a preference for the larval habitats created. Mosquitoes are opportunistic, they adapt to new hosts, and the introduction of a disease to a new area may cause other animals to become reservoirs. When mosquitoes adapt to new animals, there may in fact be an increased risk for viral transmission as the mosquitoes require prolonged time for probing and thereby salivate more

Livestock intensification and extensification: Intensified animal production causes high rates of contact between many individual animals, many of which are genetically similar, bred for other purposes than disease resistance, and kept in stressful environments. This increases the risks of spreading diseases, and high numbers of individuals potentially carrying a zoonotic pathogen increases the risk for a host jump . Extensive animal keeping, backyard farming, and mixed production systems have also been associated with disease risks. The outbreaks of highly pathogenic avian influenza in Southeast Asia have been demonstrated to be dependent on rice production, duck densities, and human population density .

B part

Acquired immunodeficiency syndrome in humans is caused by two lentiviruses, human immunodeficiency viruses types 1 and 2 (HIV-1 and HIV-2). Both HIVs are the result of multiple cross-species transmissions of simian immunodeficiency viruses (SIVs) naturally infecting African primates. These transfers resulted in viruses that spread in humans to only a limited extent. However, one transmission event, involving SIVcpz from chimpanzees in southeastern Cameroon, gave rise to HIV-1 group M—the principal cause of the AIDS pandemic. It became clear that HIV-1 and HIV-2 were the result of zoonotic (disease which is naturally transmissible from animal vertebrates to humans) transfers of viruses infecting primates in Africa .

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