The interaction between humans, animals and the surrounding environment is particularly close in many Low-and-Middle-Income Countries (LMICs), where animals provide transportation, draft power, fuel, clothing and are a source of protein (i.e. meat, eggs, and milk). In this context, the zoonoses, those infectious diseases transmitted between animals and humans, can lead to serious public health risks which translate into huge economic consequences and obstacles to development efforts (WHO, 2010). For example, just rabies, tuberculosis, brucellosis and anthrax alone are responsible for 2.2 million human deaths and 2.4 billion illnesses each year (Grace et al., 2012). Furthermore, 75 percent of emerging pathogens fall within the category of zoonotic diseases (WHO, 2020) and although arising from wildlife mainly, livestock may serve as an intermediary. Many of these diseases have become major global health threats in the recent past (e.g. Nipah, H5N1 and H1N1 influenza, MERS-CoV, Rift Valley fever) and continue to undermine our global health system as seen during the COVID-19 pandemic.
Along with the public health impact, zoonoses also impose considerable economic losses in the livestock sector, which are associated both directly and indirectly with livestock health and production. The presence of zoonosis in livestock can lead to trade barriers, control costs, increased costs due to processing and monitoring to ensure food safety, as well losses due to lack of consumer confidence. The burden of zoonoses tends to fall most heavily on developing countries and contribute to an already burdened public health system.
Both the public and animal health impacts underscore the connectivity between human and animal health, as well as the roles of ecosystem alteration and disruption, climate change, globalization of trade and travel or inadequate biosecurity, among others, that are the main drivers for disease emergence and spread. From 2000–2030, demographic pressures are projected to lead to progressive expansion of densely populated land-use systems. While croplands will encroach on pastoral systems, pastoral systems will expand at the cost of forested systems. Projections to 2030 are that forested systems will be replaced by croplands on 1.5 million square kilometers and by ruminant livestock systems on 2.7 million sq. km (FAO, 2015). Epidemiology states that the transmission of a pathogen tends to increase with host density (Kilpatrick and Altizer, 2012) but this risk of transmission to humans is not only determined by host abundance, but also by the indirect transmission path, largely controlled by climatic factors and soil characteristics influencing the pathogen’s survival in the environment or reservoir (Lambin et al., 2010).
While the emergence of zoonotic pathogens and the scale of their effect cannot be predicted, many links between the two are obvious. Therefore, staying alert as well as prevention, early detection, and response are key components of global health security.
Points of Consideration
In livestock production the risk of zoonosis always exists, whether it is through livestock-human transmission, or transmission from wild animals to humans. Any livestock project should, therefore, follow the concept of One Health, which emphasizes the relationship between its three main elements: animal, human, and environmental health. It can help to tackle challenges in a more comprehensive, cross-sectoral, collaborative manner that strengthens overall systems to prevent, prepare for, and respond to infectious diseases.
To start, project design should consider the human-livestock-ecosystem interfaces (Hassell et al. 2017) and implement a risk-based approach that addresses existing zoonoses, potential zoonotic infections, and the risk of spillover.
In project design there should also exist:
✓ General awareness and knowledge about the presence, burden, and impact of zoonotic diseases in the project area for the establishment of a feasible and successful strategy.
✓ Priority analysis of known zoonostic diseases in the area, whether that is due to public health concerns (e.g. dog-mediated rabies, tuberculosis, brucellosis), livestock economic issues (e.g. Bovine spongiform encephalopathy, H1N1), and others where the concern is evenly shared between public and animal health (e.g. highly pathogenic avian influenza, anthrax)
✓ Based on the project area and priorities, decide which key interventions for prevention are needed in project design to tackle reservoirs, prevent transmission, and establish early on-site detection systems.
Decisions around production methods can have serious impact on managing the risk of zoonosis. Therefore, decisions need to take into account the capacity to manage any risks that could arise.
✓ Assess institutional capacity to deal with an outbreak, including early detection, rapid response, as well as extension services programs that could sustainability support prevention methods
✓ Take into account institutional capacity when designing projects and choose approaches at the project level that aim to prevent risk where possible.
✓ Establish good practices for overall project management (i.e. avoid choosing high-density farming if responsible farm management, prevention, and response cannot be upheld long-term)
✓ Public health interventions for occupational diseases of workers involved in the animal food chains (i.e. farmers, abattoir workers, butchers, truck drivers, meat handlers, or market sellers) should also involve zoonosis that can affect workers.
✓ Minimize disruption to the surrounding environment and ecosystem where possible, and limit interactions between wildlife and livestock (and humans).
Approaches and Tools:
Basic animal disease strategies (prevention, preparedness, control and eradication) are applicable to zoonotic diseases, as indicated in Principle 2. However, the basic principles of zoonoses management and control are focused on breaking the chain of transmission at its epidemiologically weakest link. Three factors are involved where interventions are critical: the reservoir, transmission from the reservoir to the susceptible hosts, and the susceptible hosts. Though no intervention is perfect, below are the general ones available at each key link of transmission.
Interventions at the reservoir:
The primary source of zoonotic infection is the infected reservoir host, which could be either a domesticated or wild animal. Among the main approaches are:
✓ Testing and culling. Infection is controlled by removing the animal or herds found to be infected.
✓ Environmental manipulation. Methods that break the link of transmission by reducing survival of the agent through its vehicle (e.g., water, food, soil, vegetation) wherever the agent may be found outside the host. Also restricted to local situations. For example, proper fecal disposal, or pasture rotation.
✓ Wildlife intervention/population control programs. Broader interventions that control/reduce the population (i.e. control of rabies transmitted from wildlife), mass vaccination, or biological controls such as using natural predators or pathogens of vectors.
Interventions at transmission:
Interrupting the transmission by reducing the opportunity for contact between a potentially infected individual and susceptible host.
✓ Good on-farm management and biosecurity practices. Good on-farm management along with education and awareness of producers and workers are key to preventing transmission of both known and unknown disease-causing agents. For example, provision of adequate and clean toilet facilities - including education and supervision - will prevent the spread of Taenia saginata from feedlot employees to cattle.
✓ Isolation of an infected individual. This reduces the probability of contact and facilitates treatment and disinfection. Depends on early, accurate diagnosis and effective disease control programs.
✓ Quarantine. Based on the segregation of the incoming animal for a period of time, normally defined by two incubation periods of the disease in question to see if the suspected individual develops the disease.
Interventions at the susceptible hosts:
Increasing host resistance is a key intervention for controlling zoonoses at the final link of transmission. Preventing infection (at the reservoir or at transmission) is ideal but not always possible. Increasing host resistance can either prevent or lessen the severity of the disease.
✓ Immunization. Vaccines are used to protect susceptible individuals from infection or from the infection developing into a clinical disease, as well as to prevent transmission by creating an immune population (known as herd immunity) which minimizes the opportunity for transmission between susceptible hosts.
✓ Chemoprophylaxis. Attempts to prevent infection or reduce the severity of disease through a passive means of using medication to increase host resistance, lasting only as long as the drug lasts. This use is vigorously debated because it involves a mass consumption of antimicrobials that favors the emergence of resistant pathogens (Principle 6).
✓ Early detection/enhance surveillance. Target or risk-based strategies, including sero-surveillance.
✓ Raising awareness and education. This can improve sensitivity of the population to potential disease as well as improving laboratory capacity for diagnosis to support surveillance.
Variables to Consider
Each program involving zoonotic diseases will need to be designed according to these disease specifications:
✓ Nature of etiologic agent
✓ Reservoir host
✓ Life cycle of the infecting organism
In the event of an outbreak, mechanisms and education should be in place to deal with consequences:
✓ Frequent testing of on-farm reservoirs where there is regular contact with susceptible hosts
✓ Regular testing of animals
✓ Quarantine facilities appropriate to the size/production methods of the project
✓ Capacity building for good on-farm management and hygiene practices
✓ Access to quality veterinary services and medicines for animals as well as health services of producers and workers
✓ Proper isolation, removal of animal, and disposal of carcasses to avoid pollution
Specific interventions in communication channels and simulation exercises should be made to ensure a One Health approach between public health, environmental and veterinary services, to tackle zoonoses from all sources.
When the risk to human health is either not that high, clear or apparent, an animal disease may often be neglected as a zoonosis and thereby not treated as a public health threat. However, interventions to improve animal health should not be neglected, and can still lead to secondary economic benefits (e.g. brucellosis and tuberculosis).
Likewise, intervention and surveillance for controlling certain diseases in food animals might be driven by public health concerns but are not regarded as a serious problem for livestock production (e.g. non-typhoidal Salmonella, which has a high impact in humans but is normally subclinical in poultry).
Work with public health institutions can be a struggle as they sometimes lack the appropriate platforms and cross-sectoral communication protocols.
Immunization as a method of disease control is generally so effective and commonplace that all too often, the many variables associated with the procedures are ignored. Immunization failures may occur as the result of failure of the delivery system or failure of the immune response, or they may be iatrogenic in origin.
A high cost of proper disposal of carcasses, or inadequate financial compensation for farmers can result in improper disposal that pollute water sources and drinking water, threatening the lives of other livestock, as well as having severe public health consequences.