The current rise of average temperatures and the unpredictability of the weather patterns are imposing new challenges to poultry production. The exposure of birds to sudden and/or prolonged hot weather conditions result in heat stress, which has negative effect on the health, productivity, and welfare of birds.

In this series of articles, we will discuss how heat stress affects the productivity of poultry flocks, as well as the strategies that can be used to mitigate its negative impact, especially focusing on gut integrity.

How do chickens respond to heat?

It is essential that the body temperature of birds is maintained between 41 and 42 °C. To achieve that, there should be a balance between the amount of heat a bird produces and the amount of heat it dissipates.

A chicken loses heat by following a difference of temperature between its body surface and the surrounding environment, or through evaporation, following a difference of humidity (Fig. 1). Higher environmental temperatures and/or higher ambient humidity tend to reduce heat dissipation from the bird.

In hot environments, birds have different strategies to reduce the impact on their body temperature. From the behavioural point of view, they tend to reduce feed intake and decrease activity levels to reduce heat production. Also, the water intake increases. Sometimes, birds extend their wings and lay with their breast in direct contact with the ground to help with heat dissipation.

Since birds have the highest proportion of their bodies covered with feathers, they can only dissipate heat from their legs, combs, and wattles. They redirect their blood circulation to these areas to maximise heat loss. Such mechanism has limited effectiveness in very hot environments.

Unlike us, birds cannot sweat, so they evaporate heat from the respiratory tract by panting. In fact, panting is the most relevant method the chicken uses to regulate its body temperature in hot weather (Fig. 1).

The cost of regulating body temperature

Birds pay a cost, sometimes very high, to maintain their body temperature constant under hot environmental conditions.

Firstly, birds lose excessive amounts carbon dioxide (CO2) when they pant for a long time. This makes their body more acidic, resulting in a series of metabolic alterations. In hens, the drop in levels of CO2 reduces the deposition of calcium in the eggshells, resulting in eggs that break easily; in the case of breeder hens, thinner eggshells also result in a reduction of hatchability due to an alteration in the gas and water exchange during incubation, as well as higher chances of embryo contamination.

Secondly, heat stress translates in an abnormal secretion of hormones, which greatly affects carcass composition. Stressed birds have higher amounts of hormones of the cortisol family circulating in the blood, which results in a reduction of protein deposition and an increase in deposition of fat. Also, to reduce the generation of heat, there is an increase in the secretion of hormones that reduce the appetite, which decreases the availability of nutrients for production.

Finally, heat stress has a major negative effect on the integrity of the gut, jeopardising not only the productivity of the flock, but also its health and wellbeing. The following sections explain the concept of gut integrity and how heat stress can compromise it.

Figure 2. A. Transversal section of the intestine showing the mucosa layer in the centre. B. Details of the structure
of the mucosa, showing villi (projections that increase the surface area of digestion and absorption), the crypts
(where the cells of the villi are produced). Note the blood vessels in the centre of the villi; in heat stress, these vessels have a reduced blood flow. C. Detail of the intestinal barrier, constituted by the cells of the villi surface (“lining”), the mucus on the villi surface, the microbiota (microorganisms), the tight junctions keeping cells together (green), and normal distribution of immune cells.

What is gut integrity?

The lining (or mucosa) of a healthy intestine has a series of components forming a barrier that avoids the free passage of microorganisms and toxins into the blood, as well as the loss of proteins and water into the intestine. Such a barrier consists of a layer of cells tightly joined together, a protective layer of mucus, the normal microbiota (microorganism population) of the intestine, as well as immune cells in the gut wall (Fig. 2). The disruption of this barrier results in reduction of poultry performance, with higher chances of occurrence of systemic inflammation and infections.

How does heat stress affect gut integrity?

As mentioned above, in hot conditions, chickens redistribute their blood circulation to the areas of the body with no feathers in order to dissipate heat. In doing so, less blood and, consequently, less oxygen and less nutrients reach the intestinal lining. There is a damage of the cells of the intestinal lining as well as in the tight junction that keep such cells together, resulting in loss of gut integrity. Also, the villi of the lining become shorter and wider. Such alterations result in the impairment of the digestion and absorption, reducing the productive performance of the birds. This is normally accompanied by diarrhoea and feed passage.

The loss of intestinal integrity also allows pathogens to colonise the intestinal wall and pass via blood stream to other organs, producing generalised infections. In fact, there are reports indicating the increase of infections with Salmonella, Campylobacter, and E. coli in heat-stressed flocks. In some of those cases, translocation of bacteria such as salmonella into the reproductive tract of pullets and layers has also been described; this may result in the vertical transmission of the bacteria to the chicks. Large proteins and bacterial toxins can also cross the damaged intestinal wall, producing local and systemic inflammations.

A damaged intestinal barrier also leads to alterations of the conditions inside the gut, resulting in imbalance of the intestinal microbiota (that is the population of microorganism living inside the gut). Studies reported that heat-stressed chickens experienced a reduction in the population of beneficial bacteria (such as Lactobacillus or Bifidobacterium) and an increase in the amount of potentially harmful bacteria (such as Salmonella, E. coli, Campylobacter, and Clostridia). This results in an impaired digestion of the feed and a higher susceptibility to disease.

Managing heat stress

To manage heat stress, it is necessary to facilitate heat dissipation, reduce heat production by the birds, and alleviate the impact of heat stress on productivity, health, and welfare.

Heat dissipation is facilitated by improving the environmental conditions in the poultry shed. Such an improvement depends on the building characteristics (roof insulation, type of ventilation, and presence of cooling systems) and on how the building is managed (such as curtain management and stocking density) (Fig. 1).

Strategies to reduce heat production includes, for example, avoiding bird activity during hot hours by supplying a dark and quiet environment, as well as feeding during the cool part of the day and night.

Birds also should be assisted to reduce the impact of heat stress. It is recommended to supply cool water, administer supplements to compensate for electrolyte losses, and perform dietary adjustments to reduce heat production and compensate for lower feed intake. The effects on the gastrointestinal tract and the metabolism can be ameliorated by administering vitamins A, C, and E, as well as Zinc. Amino acids such as Arginine and Glutamine are important for immunomodulation, microflora balance, and improvement of intestinal integrity.

Particular attention should be given to the utilisation of plant-based products, rich in saponins and terpenoids (Phibro’s Magni-Phi) as well as probiotics (Phibro’s MicroLife Prime). Such products have proven to significantly reduce the occurrence of leaky gut syndrome and improve the composition of the microbiota in the presence of heat stress. Furthermore, the combination of both products resulted in a reduction of the number of E.coli, Salmonella, and Clostridia counts in heat-stressed birds.

In the second article of this series, we will discuss in more detail the implementation of strategies to ameliorate the effect of heat stress on gut integrity.

As usual, please consult your veterinary advisor and/or your nutritionist before implementing any new heat stress management strategies.

(References on request).