The term heat stress describes a state where animals are responding to excessive heat load (EHL). Normal function of various tissues and organs within the body require that body temperature be maintained within a relatively narrow range. If body temperature is raised beyond the level that animals can tolerate then there is a risk of organ dysfunction, and even death. Heat stroke is a term used to describe the life-threatening condition of failure of an animal’s thermoregulatory system in response to EHL. Body temperature is the result of a balance between heat load and heat loss. Livestock normally maintain body temperature within a narrow range mainly by influencing metabolic heat production and evaporative heat loss through the respiratory tract, specifically the lungs and nose.
Endogenous heat production (physical activity, digestion and other processes such as inflammation) is a major source of heat accumulation. Animals may also take in additional heat from solar radiation (sun), from nearby structures that are releasing heat (pen fences, floor, ceiling etc) and from the air (if the temperature is higher than the animal’s body temperature).
On an export vessel, the metabolic heat production from the animals is the major source of heat below decks, accounting for a temperature differential between air inflow and outflow of up to 6°C. Other important heat sources on export vessels include the incoming air being blown out of outlets on each deck (incoming air delivered to the pens will be hotter than environmental air as a result of heat from the supply fans and turbulence and friction from supply ducting), and radiant heat from the vessel itself (particularly engine room bulkheads, fuel oil storage walls, the ceiling on the uppermost deck or the sides of the ship that are heated from solar radiation).
Animals dissipate body heat by convection (when air or water is warmed by contact with skin resulting in a loss of body heat), conduction (when heat is transferred from skin to a cooler surface in contact with the skin), radiation (movement of heat from the body into the surrounding atmosphere), and evaporation of water from the body surface (sweating or panting).
The major means of body heat loss are evaporative cooling through panting and sweating, and convective cooling due to air flow. Mechanical ventilation is used on export vessels to provide air flow and ensure removal of heated air. Panting provides increased airflow across the mucosa of the respiratory tract and allows for convective and evaporative cooling.
Environmental conditions that predispose to EHL include recent rainfall, high ambient temperature, high relative humidity, the absence of cloud cover or shelter with high solar radiation, minimal air movement over several days, and sudden adverse climatic conditions. Minimal airflow past the ship’s ventilation inlets will occur if there is little to no wind and the ship is not moving, or if the apparent wind is minimal, which can result in warm air being drawn from the ship’s exhaust fans back into the supply fans. An export ship must be kept moving wherever possible when faced with heat stress conditions. In feedlot situations, exposure to solar radiation is an important contributor to heat load. On export vessels, animals are typically not directly exposed to solar radiation. The impact of solar radiation is more through indirect heating of the air and the physical structure of the ship. The manure pad will hold moisture and contribute to humidity and wet bulb temperature.
Animal factors that may predispose to EHL include breed, coat type (dark and woolly coats may be more likely to accumulate heat), body condition (fatter animals accumulate more heat), lack of adaptation to heat, and concurrent illness. Bos indicus cattle have a greater natural heat tolerance compared to most Bos taurus breeds. Diet can also influence metabolic heat production, and diets that are high in readily fermentable carbohydrate or protein will generate more metabolic heat. Metabolic heat production will also rise during and soon after feeding and therefore feeding animals in the cooler parts of the day may help avoid excessive heat load.
Flow of heat away from an animal depends on temperature and humidity gradients. If the surrounding environmental temperature is lower than the animal’s body temperature then heat loss mechanisms such as convection, radiation and conduction can all operate to move heat from an animal to the environment.
Evaporative cooling is based on a loss of heat associated with evaporation of water from the body surfaces (skin or respiratory tract). Once environmental temperatures exceed about 21°C, evaporative cooling through sweating and panting becomes the major form of heat loss, and if the environmental temperature equals or exceeds an animal’s body temperature then evaporative heat loss is the only available heat loss mechanism for animals to regulate body temperature.
Heat loss from the skin through convection and evaporation (sweating) is more important in cattle than panting (assuming that a temperature gradient exists where body temperature is higher than the surrounding air temperature). In contrast, panting is the main form of evaporative heat loss for sheep. Mechanical ventilation provides an aid to evaporative heat loss by ensuring continual air flow over the animals. If animals are using evaporative cooling to regulate temperature then forced air flow ensures that the higher humidity air is moved away from animals and replaced with incoming air of lower humidity.
Evaporation of surface water is influenced by temperature and relative humidity of the air. As air temperatures rise, air can retain more water as water vapour, meaning that in hot dry conditions (low relative humidity) evaporative cooling offers important potential for heat loss. However, as relative humidity rises there is less potential for air to absorb more water vapour and when relative humidity is 100 %, the air is saturated and additional evaporation cannot occur.
As temperature and humidity rise, evaporative cooling from the skin is lost earlier than evaporative cooling from the respiratory tract. The reason for this is that air flow into the respiratory tract during panting will increase the temperature of the air slightly and this will raise the amount of water vapour that the air can retain (absolute humidity) and drop the relative humidity. As a result, evaporation may still occur, meaning that panting can function as a heat loss mechanism even when evaporative cooling from the skin is no longer occurring.
In many hot climates, animals may accumulate heat through the hottest part of the day and then dissipate heat in the cooler parts of the day and continue to function normally. In situations where the environmental temperature remains high for most of the day and night, animals have relatively little opportunity to lose heat and they may gradually accumulate excessive heat over time.
During export voyages, the conditions that favour heat stress are long periods where very high environmental temperatures are in place for much of the day and night (providing little or no period of respite where animals can shed heat), and periods where both temperature and humidity are high. Under these conditions, animals have very limited ability to lose heat and severe heat stress conditions may rapidly develop. The physical constraints of pens within the decks of an export vessel also mean that there is limited capacity for animals to use some strategies for dealing with EHL, such as moving to less dense or cooler areas.
Heat stress can result in reduced feed intake along with depression, increased heart and respiratory rate. Heat stress is a significant stressor that in turn may reduce resistance to other pathogens. Panting and open mouth breathing predisposes to pneumonia. A continued rise in body temperature will eventually result in respiratory and circulatory failure and death.
A range of factors other than environmental temperature and humidity may influence the risk of heat stress including physical activity, dehydration, ingestion of rapidly fermentable feed, febrile disease and concurrent diseases of the respiratory tract that interfere with evaporative heat exchange. In the export process, dangerous levels of heat stress occur occasionally in assembly points on extreme summer days, at sea when crossing the equatorial zone or when ventilation systems fail or struggle, and in summers in the Middle East. Export risk may be elevated in unacclimatised animals coming from an Australian winter into a northern hemisphere summer. Heat stress is one of the most important concerns of the livestock export industry. It must be prepared for, recognised, and carefully managed when it occurs in order to avoid mortalities either directly from heat stress or other complications that may result. This applies particularly to British and European breed cattle.