11.1 How are animal and human diseases interconnected?

11.1.1 What are the common factors in animal and human disease?

In both animals and humans, infectious diseases are caused by microorganisms known as pathogens.

Types of pathogen include:

  • Fungi
  • Viruses
  • Parasites
  • Bacteria

Many pathogens are intrinsically able or have developed the ability to infect both animals and humans, and so may be transmitted between them (Figure 1). These are collectively known as ‘zoonotic’ diseases and make up more than 60% of all known human infectious diseases.

Figure 1: Examples of zoonotic diseases and their affected populations. Redrawn from United States Government Accountability Office. 2011.

Through the biology and ecology of their shared pathogens, human and animal health are interconnected. In turn, humans, animals and pathogens are also influenced by wider changes in ecosystems and the natural environment in which they exist.

In recent years, a growing trend has been the adoption of more interdisciplinary and transdisciplinary approaches to managing health risks. These approaches include One Health, Planetary Health, and Ecohealth (Figure 2), which look beyond human medicine, and also incorporate the biology and health of non-human species, ecosystems functioning, and the effects of environmental change.

Figure 2: The one health perspective takes an integrative systems perspective on health risks, including human, animal, and environmental health within one framework. Reproduced from Davos Global Risk Forum. 2017.

11.1.2 How are diseases transmitted between animal species, and then on to humans?

If pathogens are biologically able to infect more than one type of animal species (including humans), when contact is made between such animals, the potential for the pathogen to be transferred to a new host species exists. Occasions where this takes place are known as ‘spill over’ events.

Various contact routes for disease transmission are possible:

  • Direct contact with an animal’s bodily fluids (e.g. saliva, faeces and blood), via, for example:

    • Touching an infected animal’s skin;
    • Being bitten by an infected animal.
  • Indirect contact within areas where infected animals live and roam, including:
    • Breathing in dust particles and small droplets of saliva;
    • Consuming contaminated food products;
    • Contact with contaminated water, soil, objects or clothing.
  • Disease vectors: organisms that transmit infectious disease between animals, and between animals and humans. These are typically insects that feed on the blood of humans and animals, and in so doing, transmit pathogens between them.

Direct contact and food consumption are the most common modes of transmission for diseases associated with food systems. Airborne transmission of viruses are also common routes for infection.

The risk of transmission to humans is higher for those with high level of occupational exposure to livestock and livestock products, such as those working with livestock, in abattoirs, and in the meat processing industry.

Figure 3: Some common pathways for zoonotic disease transmission. Reproduced from Thornton. 2017.

Table 1 gives some common examples of zoonotic diseases and their modes of transmission.

Table 1: Examples of zoonotic diseases, their infectious agents and modes of transmission.

Infectious agent

Disease

Main reservoir

Usual mode of transmission to humans

Fungi

Ringworm

Wildlife, livestock and domestic animals

Direct contact with infected animals.

Virus

Ebola

Wild animals: chimpanzees, gorillas, fruit bats, monkeys, forest antelope, porcupines

Direct contact with animal Secretions/bush meat.

Rabies

Dogs, cats, bats, foxes

Direct contact via animal bite.

Bacteria

Campylobacter; Salmonella

Livestock: poultry, dairy products, pigs

Direct contact via ingestion of animal products.

Bovine tuberculosis

Cattle

Direct contact via ingestion of milk.

Lyme disease

Ticks, rodents, sheep, deer, small mammals

Indirect contact via a bite from infected tics (vector host).

Anthrax

Livestock, wild animals, environment

Direct contact via ingestion or indirect via environment.

Parasites

Leishmaniosis

Dogs, rodents, cattle

Indirect contact via bite from infected sand-flies.

While the type of contact is what directly enables disease transmission to take place in each case, many other factors may promote or reduce the likelihood that a contact event occurs (i.e. degree of exposure to an infection), and that if it does, this then leads to a human infection (i.e. degree of susceptibility to infection).

These include natural and human induced changes in ecosystems, changes in food and agriculture systems, and changes in human living environments and consumption practices (Figure 4).

Figure 4: Overview of the disease interactions at the human-livestock-ecosystem interface that contribute to the emergence of infectious zoonotic diseases. Redrawn from World Bank. 2010.

In particular, the degree of spatial overlap and interaction – and so exposure – between humans, animals (wild and domesticated) and vector species is important because it determines what steps in the chain of infection from one species to another are physically possible.

Over human history, human interaction with animals has changed considerably (Figure 5). This has been driven by the domestication of animals, expansion of agriculture into natural ecosystems (which now covers almost 40% of the earth’s terrestrial surface), urbanisation, and by the industrialisation of food systems.

Figure 5: Schematic (non-quantitative) representation of the changes in the nature and level of human-animal interaction over time, linked to changes in the size and organisation of human societies. Redrawn based on Reperant, et al. 2012.

Increasing global travel of humans and of animals (via trade), means that in the absence of specific measures to control the spread of disease, localised disease sources can rapidly come into contact with new populations of animals and humans worldwide.

11.1.3 How do animal diseases become human diseases?

Animals and their pathogens often co-evolve mutually sustainable relationships, such that the host animal may experience few or no symptoms, while the rate of infection within the population of an animal species as a whole is persistent and widespread. This means that many animals can act as consistent sources (or ‘reservoirs’) of disease, that can then be passed on to other species that they come into contact with (Figure 6).

Figure 6: Example of interactions between disease reservoir species, intermediate host species, and humans. The severity of infection for viruses often increases with each step away from the original host species. Reproduced from Bean, et al. 2013.
 

Collectively, wildlife and livestock pathogens represent a vast potential source of human disease, were they all able to infect humans. Yet there is no inevitability about pathogens jumping the species barrier to infect livestock or humans, and relative to the overall amount that exist, few actually do.

Pathogens that have evolved (or have intrinsically) the ability to infect humans, do so to different degrees, and with human health impacts ranging from very minimal to terminal. The ease of a pathogen’s transmission between humans and the duration over which it can survive in the human body are two factors which influence its ability to spread beyond isolated human infections (Figure 7).

Figure 7: The pathogen pyramid. Each level represents a different degree of interaction between pathogens and humans, ranging from exposure through to epidemic spread between humans or livestock. Some pathogens are able to progress from one level to the next (arrows); others are prevented from doing so by biological or ecological barriers (bars). Redrawn from Woolhouse, et al. 2011.

Pathogens may at some point in time, evolve the ability to survive longer or be transmitted more effectively between humans, thereby increasing the level of threat to human health that they pose.

  • Very few zoonotic pathogens (~10%) are able to be transmitted easily between humans or can persist for a long-time period in the human body before either the recovery or the death of the human host from the diseases they cause. Those that can have the potential to form pandemics, where infections spread widely and internationally (e.g. from influenza or AIDs).
  • About a quarter of zoonotic pathogens are capable of some human-to-human transmission but still cannot persist in human populations for very long without repeated reintroductions from wildlife or livestock reservoirs. These may form local or regional disease outbreaks (e.g. Ebola).
  • The vast majority of zoonotic pathogens are either minimally or not transmissible at all between humans, and so their prevalence in human populations depends almost entirely on individuals’ exposure to infected animals or disease vectors (e.g. rabies or lime disease).

11.1.4 How do zoonotic diseases affect public health challenges?

It has been estimated that the top 56 zoonotic diseases are responsible for 2.5 billion cases of human illness and 2.7 million human deaths worldwide each year.

Emerging epidemic / pandemic zoonotic diseases

Global efforts to tackle zoonotic disease have typically been focussed on the minority of diseases capable of forming regional outbreaks or global pandemics, which in a (unlikely but possible) worst-case scenario, could result in tens of millions of deaths and cost trillions of dollars.

This is especially true for so-called ‘emerging’ zoonotic diseases: diseases that existing healthcare systems are either unprepared or unable to treat, because the pathogens that cause them have:

  • Been newly discovered in humans;
  • Appeared in a completely new geographic area;
  • Acquired completely new infectious traits (e.g. suddenly become more infectious or deadly).

Their unpredictability, and the potential for quick and large-scale health impacts (particularly on wealthier population) has made them a high-profile political and public health issue worldwide, receiving large amounts of funding and research.

Most examples of emerging zoonotic diseases with pandemic potential, tend to be viruses, due to their rapid rates of evolution and ability to be transmitted and so spread quickly between hosts (Figure 8). Rates of bacterial evolution is also very fast, and for this reason, the threat to human health from emerging antibiotic resistance is also significant (Section 11.3.3).

Figure 8:  Emergence of zoonotic diseases and resulting deaths over the last century. Based on data from Bean, et al. 2013. Note: the scale is logarithmic.
 

While pandemic outbreaks of new diseases are rare, zoonotic diseases represent the majority of recently emerging infectious diseases recorded in humans (75%) and so are a likely source of new pandemics. Most originate from wildlife sources. However, disease outbreaks originating from livestock tend to occur in areas of dense human population and so also present a considerable risk to public health.

Notably, the number of instances where new diseases have arisen in humans (‘emergence events’) has increased over the 20th century; and of these events, the primary driver in almost half of cases is related to food and farming systems, through changes in land use, agricultural intensification, food industry changes, and bush meat hunting and consumption (Figure 9).

Figure 9: Drivers and locations of emergence events for zoonotic infectious diseases in humans from 1940–2005. Pie chart on the left (A) represents worldwide percentage of emerging events caused by each driver. Map on the right (B) shows, countries in which events took place and the drivers of emergence. Reproduced from Keesing, et al. 2010.

Already existing and endemic zoonotic diseases

Despite affecting the health of billions of people and animals, already existing zoonotic diseases that lack pandemic potential receive far less attention and research than do pandemic ones. Nevertheless, their ongoing costs to society are high: an analysis looking at just 6 major outbreaks of highly fatal diseases, estimated their costs to be at least US$80 billion.

For endemic zoonotic diseases, human cases of infection mainly arise from repeated exposure to disease-hosting animals. Because both wildlife and livestock can act as a constant source for infection and re-infection (Section 11.1.3), these diseases are generally always present in particular geographical areas where animal disease reservoirs exist, causing persistent levels of infection and sickness in local people.

By far, the greatest amount of sickness from all zoonotic diseases, worldwide, is borne by people in low-income countries (98.5% of the total burden), and particularly by poor livestock keepers who often contact endemic zoonotic diseases from their livestock (Figure 10).

Figure 10: The disproportionate impacts of zoonotic disease on poor livestock keepers. Reproduced from Grace, et al. 2012.

Some reasons for this include:

  • Lack of access to effective veterinary care services and national disease control policies;
  • The need to live in very close contact with livestock;
  • Increased susceptibility to infection from poor nutrition and sanitary conditions;
  • Greater proximity and interaction with infected wildlife and disease vectors.

Zoonotic diseases have caused large economic costs in all regions, but in lower-income regions where endemic diseases are not controlled in livestock or wildlife populations, their impact on economic development and human well-being is considerable.

In such areas, poverty tends to increase both exposure and susceptibility to zoonotic diseases, and in turn, the effects of disease can compound poverty through lost income and other costs related to human and animal sickness. In this way, zoonotic diseases can trap people in a cycle of poverty and ill-health.