Infectious Diseases of Animals: Prevention, Treatment or Disaster?

John Ikonomopoulos, Professor of Veterinary Microbiology, Agricultural University of Athens.

Antonia Mataragka, Doctor of Agricultural Sciences, Agricultural University of Athens.

Prevention or Treatment?

The control of infectious diseases is based on a set of measures that fall into two main categories: prevention and treatment.

The relative importance of each of these categories varies depending on the context. For example, the use of therapeutic measures for the treatment of a disease is generally considered more appropriate in companion animals and, of course, in humans. Particularly in the latter case, factors such as the cost of a complete diagnostic work-up or the administration of treatment, aimed either at full recovery or at least at alleviating the patient’s condition, are not substantially taken into account. However, this is not the case for production animals.

When the objective is the management of a disease within a population of animals raised for production purposes, the role of treatment must necessarily be assessed within a cost–benefit framework. In this context, treatment cannot be administered to an animal if it is not economically justified. An important component of this issue is the possibility that the animal may remain a carrier. This is a condition in which the animal shows no clinical signs but sheds the pathogen into the environment in an unpredictable manner, both in terms of timing and quantity (intermittent carrier). The presence of carrier animals represents a major obstacle to disease control within a population, as such animals are extremely difficult to identify in their entirety in order to be removed.

Taking into consideration that the treatment of production animals is subject to the aforementioned limitations, the implementation of preventive measures for the control of infectious diseases becomes highly advantageous, for the following reasons:

• Loss of animal products is prevented.
• The risk of transmission of zoonotic diseases to humans is reduced, while consumer confidence in animal products is enhanced.
• The use of antibiotics is minimized, resulting in:
  • reduced development of antimicrobial resistance in both animals and humans, and
  • lower overall animal management costs.

Before briefly referring to the main systems used to control the spread of infectious diseases, it is necessary to highlight some additional aspects regarding the transmission of the pathogens that cause them.

Infectious diseases are not confined by borders. Their transmission often occurs through contaminated water (e.g., Pseudomonas spp., mycoses, enteric pathogens), through the air (e.g., influenza, foot-and-mouth disease), or via wild animals. These animals, such as martens and badgers (tuberculosis transmission), wild boar (African swine fever), foxes and wolves (rabies), and, of course, migratory birds (avian influenza), cover very long distances on a daily basis and disseminate dangerous pathogens into areas where animals are raised, as these locations typically offer easier access to food.

In this context, it becomes clear that the natural spread of a pathogen is essentially uncontrollable. Measures aimed at limiting it, such as the selective culling of wild animals or their vaccination through baiting, may be effective, but they are subject to numerous constraints. A characteristic example is the decades-long effort of the United Kingdom to limit the spread of mycobacteria in wildlife in order to achieve the eradication of tuberculosis from its livestock population. This effort required enormous resources and would have been unlikely to succeed had the country not been geographically isolated as an island.

Based on the above, it is evident that attempts to control diseases by limiting the natural spread of pathogens require precise planning, substantial resources, and long-term commitment. For such planning to be effective, it must encompass broad ecological communities, which by their nature are not confined within the borders of individual states. Consequently, the adoption of common animal disease control measures by groups of countries, such as the European Union, increases their effectiveness and allows for the relaxation of internal regulations governing the movement of animals and animal products.At the same time, it increases the critical mass and availability of resources, provided that jointly agreed rules are respected. Therefore, deviation from these rules while maintaining the right to the free movement of products would undoubtedly lead to the overall collapse of the disease control system within the defined group of countries.

Control of pathogen spread through culling of animals?

Management measures applied in the context of controlling the spread of infectious diseases in animals are designed with reference to the minimum infectious dose (MID) of the respective pathogens, that is, the smallest quantity of a microorganism required to establish infection.

Reducing the microbial load in the animals’ environment below the MID threshold aims at the eradication of the corresponding disease, which is considered the safest approach for the long-term containment of disease spread.

The implementation of an eradication policy requires the availability of reliable epizootiological and/or epidemiological data (e.g., identification of the natural reservoir of the pathogen, existence of sources of infection, carrier animals, etc.), as well as a reliable diagnostic system. In addition, adequate administrative mechanisms are necessary for the enforcement of control measures (e.g., restrictions on the movement of livestock and animal products, compulsory culling, sanitary disposal, etc.), along with sufficient funding (for diagnostic testing and compensation schemes).Measures aimed at disease eradication are typically based on the implementation of a surveillance system to detect diseased or at least infected animals, followed by their isolation or, more commonly, their culling. These approaches are referred to as “test and removal” or “test and slaughter” strategies.

In holdings classified as positive, restrictions are usually imposed on the movement of animals and their products until they are declared disease-free again. In most cases, perhaps with the main exception of the use of DIVA[1] vaccines, vaccination is generally prohibited in areas where an eradication policy is implemented, in order to enhance the effectiveness of these measures. The rationale for avoiding vaccination in regions that are either disease-free or under eradication programs becomes clearer when one considers that, as a rule, vaccination does not prevent infection[2]. This means that a vaccinated animal exposed to a pathogen may not develop clinical disease, but it can still become infected and shed the microorganism into the environment for a period that cannot be accurately predicted.

With regard to certain highly contagious diseases, such as avian influenza or foot-and-mouth disease, the detection of positive animals triggers a series of measures, including the preventive culling of all susceptible animals within a given area, even if they are not infected (stamping out). It should be emphasized that this measure primarily aims to protect public health. However, it has also been repeatedly applied within the European Union, often at considerable cost, for diseases that are not particularly dangerous to humans, such as sheep and goat pox, African swine fever, and foot-and-mouth disease.In such cases, the critical risk lies mainly in the high transmissibility of these diseases: if not controlled immediately, they may spread throughout the entire livestock population and fundamentally alter its health status, as their eradication would then become practically impossible. Such a development may have extremely adverse consequences for animal production across all countries participating in a common control system, as it would significantly increase livestock lossesand also allow other countries or groups of countries to impose trade restrictions on animal products. At the same time, it undermines consumer confidence in these products—something that has proven in practice to be very difficult to restore.

Is the livestock farmer ultimately doomed?

This question is entirely justified. How can a livestock farmer today cope with all these sanitary restrictions, which represent only a small part of the difficulties they face?

Animal feed has effectively become a commodity traded on financial markets, while climate change makes its price highly unpredictable. Competition from countries or groups of countries that produce animal products without being subject to the same sanitary restrictions further undermines the viability of a sector that is inherently vulnerable. To this already challenging context must be added the systematic pressure arising from carbon dioxide emissions, which is gradually fostering the perception that the sector should be abandoned altogether as outdated or even environmentally harmful.

Taking the above into account, it should first be emphasized that animal production must be regarded as a sector of strategic importance for the sustainable development of society. It is clear that without it, the threat of loss of food security and autonomy would be extremely difficult to address. Moreover, it should not be overlooked that animal production functions as one of the most effective means of preventing rural depopulation, something of fundamental importance for environmental protection.

On this basis, one arrives at a conclusion that is now largely accepted even at the level of the European Union: animal production cannot be subject to the same competitiveness rules that apply to other productive sectors, such as the automotive industry. It must be continuously upgraded and substantially supported. This also represents the best investment of the sector for the benefit of the European consumer, who may reasonably prefer lower prices but is now well informed and does not readily resort to products that are unsafe or unreliable.

Naturally, this approach requires strict regulations and significant expertise on the part of farmers, veterinarians, and the competent authorities. While animal production, at least in its intensive form, may no longer be a suitable occupation for individuals who fail to appreciate the importance of such rules, support for all others should be both generous and fair. The promotion of the adoption of strict sanitary measures should be achieved primarily through encouragement and support, rather than through punishment and marginalization.

Unfortunately, the above may currently seem like wishful thinking in our country, especially in light of the recent public discourse on the issue. Nevertheless, one may still remain hopeful without being considered naïve, particularly when taking into account that our country is now competing with others that already have decades of experience in systematic livestock production. In many of these countries, there were indeed issues of inconsistency and inefficiency; however, addressing these problems gradually led to the reform of both sides involved, namely the farmers and the competent authorities.

A characteristic example is that of a Northern European country which voluntarily adopted an extremely strict system for the control of salmonellosis in farmed poultry, primarily aiming to increase productivity and improve the hygiene of the final product. Within this system, which even provided for the complete abandonment of facilities, along with the culling of entire flocks when found positive for the pathogen, producers actively competed to participate, knowing that the state would provide timely support to fully upgrade their production units in accordance with strict control standards.

This approach proved fully effective. Therefore, the path forward is clear!