That stress affects the defenses is a fact agreed by the scientific community. However, some recent studies of the Faculty of Veterinary Sciences of the National University of the Litoral (UNL) (Santa Fe, Argentina) and the CONICET (National Council of Scientific and Technical Research) contributed with data that contradict this general idea. The results of the experiment were published in the Journal of Experimental Zoology – Part A, Ecological and Integrative Physiology.

PREVIOUS STUDIES

Previous studies carried out by researchers from CONICET and the Faculty of Veterinary Sciences showed that prolonged exposure to stressors determines an increase in some elements of the immune system of autochthonous species.

This suggests two things:

1 – As the immune system is very complex, it cannot be generalized that stress affects it in its entirety;

2- The ways of regulating immune function in terms of stress depend on the species, since each one has evolved to adapt to its particular life history.

Taking this background into account, the researchers conducted an experiment in laboratory rats to test the hypothesis that in that species chronic stress only affects specific parts of the immune system, while stimulating more generic ones.

APPROACH

This hypothesis was constructed taking into account the following:

1. During the evolution of vertebrates (animals with spinal column: fish, amphibians, reptiles, birds and mammals) mechanisms of physiological regulation arose to adapt to difficult situations. These responses tend to maximize the chances of surviving bad times. The physiological plan changes and prioritizes some more necessary functions to cope with the crisis to the detriment of others that are less. It is assumed that the decrease in defenses due to stress is part of this adjustment mechanism.

2. However, if this response to stress seeks to maximize the chances of surviving and thriving in difficult times, does it make sense to reduce the totality of the immune system? The logical answer is no, especially if it is critical moments in which there is also greater exposure to agents that produce diseases.

3. Wild animals go through periods where there are many stressors associated with increased exposure to diseases. For example, when a species drastically increases its number, or when its habitat is scarce, population density increases. This brings with it several conflicts: the availability of food per capita is reduced, social stress increases (more fights and competition), and there is a greater risk of pathogen transmission because the individuals being closer to each other increases the contact rate between them.

4. Therefore, from the evolutionary point of view, an adequate response to stressful situations that are associated with an increased risk of infection would be to protect against diseases for which the risk increases.

PROPOSED HYPOTHESIS

Taking into account the above, it is proposed that in terms of stress, certain species invest less in ‘specific’ defenses (those that are generated against a particular agent) but promote ‘generic’ (non-specific) defenses. This prediction is based on studies carried out by ICIVET Litoral researchers in capybaras and “yacarés”, where it was seen that stress situations stimulated the production of ‘natural antibodies’.

“Natural antibodies are proteins of the immune system that are part of the constitutive immunity (also called ‘innate’), which means that they are present without the need for prior exposure to the pathogen (natural or by vaccination) and serve to protect against several agents that produce infection (that is, they are not ‘specific’, but ‘generic’), “the researchers say.

This hypothesis needs to be confirmed, and the phenomenon in general better understood, establishing its mechanisms. For this, an experiment was proposed with laboratory rats, an animal model that allows a high degree of control and precision.

STUDY WITH ANIMAL MODEL

The study consisted of subjecting groups of rats to different scenarios that simulate the stress that is perceived when the population density increases: the shortage of food and the instability and social conflict. The groups were the following:

• a) Low density and food at will (control or control group)
• b) Low density and restricted food
• c) High density and food at will
• d) High density and restricted food

This last group combined both stressors, which is what happens in nature at times of great population abundance or habitat retraction. Next, the rats were vaccinated to generate antibodies and these antibodies were measured (specific against the molecule with which they were vaccinated). Natural antibodies were also measured, which represent generic immunity. To support the hypothesis, the rats had to produce less specific antibodies in stress situations, especially when both stressors were combined, but they had to maintain or even increase the levels of natural antibodies.

RESULTS

The results of the experiment were published in the Journal of Experimental Zoology – Part A, Ecological and Integrative Physiology, and were partly “surprising”. As anticipated and in congruence with what was observed in the capybara (another rodent) and in the “yacaré”, the combination of both stressors stimulated the production of natural antibodies. Unexpectedly, the stressors also improved the production of specific antibodies, either acting in combination or each separately.

This finding means that in situations of stress associated with increased risk of infection rats increase their ability to fight infection by antibodies, whether these are specific or natural. This makes sense from the evolutionary point of view, since at times when rats are at high population density, facing lack of food, high social conflict and high risk of infection; it is beneficial for them to prioritize processes to combat these infections.

Given that rats are important sources of diseases for humans (called ‘zoonotic’); knowing these aspects of great influence on infectious processes is of great importance for public health.

“It is of interest to investigate this triangle of relationships between stress – defenses – infection in all species, since this depends on a better understanding of the phenomenon of health / disease in nature, and that understanding depends on improving public health, the conservation of biodiversity and animal production”, point from the research team.

RESEARCH TEAM

The team of researchers is composed of: LECEN Team: Pablo Cuervo (veterinarian, Doctor of Veterinary Sciences); Andrea Racca, (biochemistry, PhD in Biological Sciences); Pablo Beldomenico (veterinarian, PhD). Of the FCV-UNL: Veterinary. Esp. Amorina Sánchez. From the IMBECU (CONICET CCT-MENDOZA): Eliza Pietrobon (Lic. In Biology) and Susana Valdez (Biochemistry, Doctor in Biochemistry).