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 excessive heat load.
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 export vessels 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 and the ceiling on the uppermost deck which is heated from solar radiation).
Heat loss may be 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 sources of heat loss are evaporative cooling through panting and sweating, and convective cooling due to air flow. On board export vessels mechanical ventilation is used to provide certainty of air flow and ensure removal of heated air. Panting provides increased air flow across the mucosa of the respiratory tract and allows for convective cooling and evaporative cooling.
Environmental conditions that predispose to EHL include recent rainfall, high ambient temperature, high relative humidity, absence of cloud cover or shelter with high solar radiation, minimal air movement over several days, and a sudden change to adverse climatic 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.
Animal factors that may predispose to EHL include breed, coat type (dark and wooly 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 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 surface (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 cooloing 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 and increased heart and respiratory rates. Heat stress is a significant stressor that in turn may reduce resistance to other pathogens. Panting and open mouth breathing predispose to pneumonia. 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 risk of heat stress including physical activity, dehydration, febrile disease and concurrent diseases of the respiratory tract that interfere with evaporative heat exchange.
In the live 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 can't cope, and in summers in the Middle East. Export risk may be elevated in unacclimatised animals coming from an Australian winter.
Heat stress is one of the most important concerns of the live 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.
Animals exposed to heat load will attempt to adapt by decreasing heat production and increasing heat loss.
Cattle will sweat, drink more water and a higher respiratory rate. Animals in feedlots or other environments exposed to sunlight, may seek shade, spend more time standing, and generally be more be more restless. On-board export vessels the symptoms are similar but there is no solar radiation to escape from and animals may have less opportunity to move or change behaviour.
If heat load continues, animals will reduce feed intake in an attempt to reduce heat production. Cattle on high concentrate diets may reduce feed intake by up to 25%. Animals will also show a rise in body temperature.
Provided heat loads are not excessive, animals may show the above signs for several days and then adapt and become more tolerant to hot conditions.
Failure of adaptation is more likely to occur when animals are suddenly exposed to excessive head load or when conditions are beyond the capacity of animals to adapt. These conditions can result in severe disease and even death.
Animals that are failing to adapt to EHL will show progressively more severe respiratory signs. They will show increased respiratory rate that progresses from panting to open-mouth breathing and, if stress is sustained, to open-mouthed, laboured breathing with neck extended and tongue extruded. Animals may then collapse, become comatose and die.
Panting scores provide a better indicator of heat load in animals than respiratory rate and should be monitored during periods when heat stress may occur.
Table 1.1: Panting score used in the assessment of heat stress in cattle
In assessing cattle if more than 10% of animals have a panting score of 3.5 or higher, then there is a potential for serious losses if steps are not made quickly to allow animals to dissipate heat.
Wet bulb temperatures of ~28.0oC usually mark the onset of heat stress in Bos taurus cattle. Hot spots on cattle decks are detected during pen inspections by monitoring the level of panting among cattle, and the level of one's own personal discomfort.
Table 1.2: Panting score used in the assessment of heat stress in sheep
Fast panting and open grin
Open mouth panting
Open mouth panting, tongue out
Wet bulb temperatures approaching or exceeding 30oC indicate environmental conditions that favour development of heat stress in small ruminants and preventive measures should be considered under these conditions.
The main differential diagnosis is advanced pneumonia which tends to affect individual animals in a group rather than whole pens. Heat stress and pneumonia may sometimes be difficult to differentiate while the animal is alive. At necropsy, pneumonia is diagnosed by feeling the lungs - firm solid tissue is indicative of pneumonia.
At necropsy the carcass is hot to touch, the eyes sunken, the lungs red to dark red from congestion (but still spongy), and the heart is small and hard from being in a state of strong contracture. The very hot, core temperatures require a cooking thermometer for measurement and are consistently greater than 43.0oC. The muscles are pathognomonically pink (like cooked silver side) and dry rather than red-brown and moist. This latter change is thought to reflect lactic acid leaching of myoglobin from heat denatured cells. In a heat stress event, the first animals to die will be Bos taurus, and particularly animals that are fatter, with long hair coats covered in manure, and those suffering concurrent disease such as lameness or diarrhoea.
Core body temperature normally elevates considerably after death in large, fat, hairy or otherwise well insulated cattle. However it usually takes a few hours before the core temperature may reach the levels that may be seen immediately after death from heat stress. It is important to measure core temperature soon after death to avoid erroneously attributing post-mortem elevation of body temperature to heat stress.
At necropsy, the lungs are red, wet and heavy with congestion. The heart is small and hard in a state of contracture. The muscles are pink rather than the normal brownish-red.
Outbreaks of heat stress must be treated as an emergency.
On land, options are to:
Many of these options are applicable at sea. Additional options at sea incldue:
In severe heat stress situations there may be benefits in spray-wetting cattle. The following issues should be considered:
Livestock exports to the Middle East are subjected to heat stress risk assessment prior to export. This includes application of a computer program called HotStuff which incorporates detailed information on predicted weather conditions, ship design and ventilation capacity, the type and bodyweight of livestock on board, time of year, and the route that will be taken by the vessel. Stocking densities can then be adjusted to minimise heat stress.
At sea, risk periods for heat stress may be anticipated from observations of local conditions and animal behavior, and weather forecasts over the next several days. Preparations can then be made to implement some of the measures listed above under treatment in order to minimize the risk of large scale heat stress occurring.