Veterinary Handbook Disease Finder

Heat Stress


Other Names

  • Excessive Heat Load
  • Heat Stroke
  • Hyperthermia




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.

Clinical Signs and Diagnosis

Animals exposed to heat load will attempt to adapt by decreasing heat production and increasing heat loss.

Cattle will sweat, drink more water and increase their respiratory rate. Animals in feedlots or other environments exposed to sunlight may seek shade, spend more time standing, and generally be more restless. On board export vessels the symptoms are similar but animals may have less opportunity to move or change behaviour. They may move towards fans and away from the ship’s structures that radiate heat near pen areas.

Table 4.3: Panting score used in the assessment of heat stress in cattle

Breathing Pattern Panting score (PS) Respiratory rate (per minute)
Normal - No panting, difficult to see chest movement. 0 <40
Slight panting, mouth closed, no drool or foam. Easy to see chest movement. 1 40-70
Fast panting, drool or foam present. No open mouth panting. 2 70-120
As for 2 but without occasional open mouth. Tongue not protruding. 2.5 70-120
Open mouth + some drooling. Neck extended and head usually up. 3 120-160
As for 3 but with tongue out slightly & occasionally fully extended for short periods. Excessive drooling. 3.5 120-160
Open mouth with tongue fully extended for prolonged periods + excessive drooling. Neck extended and head up. 4 >160
As for 4 but with head held down. Cattle 'breath' from flank, drooling may cease 4.5 Variable - RR may decrease

If heat load continues, animals will reduce feed intake in an attempt to reduce heat production due to rumen fermentation. 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 heat 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. 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 taken quickly to allow animals to dissipate heat.

Wet bulb temperatures of ~28ºC 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 4.4: Panting score used in the assessment of heat stress in sheep

Panting score



No panting


Slight panting


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.

In heat stress, at necropsy the carcase 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 core temperatures require a cooking thermometer for measurement and are consistently greater than 43ºC. The muscles are pathognomonically pink (like cooked silverside) and dry rather than red-brown and moist. This latter change is thought to be a result of lactic acid leaching 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 diseases such as lameness, pneumonia 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.


Outbreaks of heat stress must be treated as an emergency.

On land, options are to:
  • Ensure there is unlimited access to clean, cool water for all animals.
  • Minimise handling and disturbance of animals. Essential activities should be conducted at the coolest times of the day, usually early in the morning or late at night.
  • Reduce stocking densities.
  • Use low stress stock handling techniques.
  • Erect shade that encourages air flow including over loading and unloading ramps.
  • Consider moving affected animals to cooler pens with reduced stocking density, shade and better air flow.
  • Remove barriers to wind.
  • Provide fan-forced airflow.
  • Temporarily reduce or cease feeding of concentrate and consider a higher roughage proportion in ration until other emergency measures are implemented.

Many of these options are applicable at sea. Additional options at sea include:

  • Move animals from areas of the vessel that may be particularly prone to heat stress, e.g. areas where mechanical ventilation is less effective or pens are affected by heat radiation. This is best done in preparation for a heat stress event rather than when the conditions are already hot. 
  • Stow higher risk animals in lower risk areas of the ship. 
  • Reduce density in pens by spreading animals between more pens or opening dividers to create larger pens.
  • Minimise disturbance of the animals - all unnecessary physical exertion must be avoided.
  • Increase access to water by emptying feed troughs and filling with water.
  • Add electrolytes to water (particularly for Bos taurus cattle) to ensure supplementation with sodium, potassium and bicarbonate to replenish electrolytes lost through evaporative cooling.
  • Ensure ventilation systems are operating maximally, and check that intake and exhaust fans are unobstructed.
  • Set up portable fans in poorly ventilated areas or on higher risk animals.
  • Maximise the ability for animals to access fans by opening up pen areas or removing gates around fans.
  • Shade top deck areas from direct sunlight.
  • Change course to allow breeze to blow across the ship, especially if there are following winds.
    • Ventilation of open decks is dependent on wind direction and speed. When there is a following breeze, effective ventilation in open decks may be very poor. In a following breeze, minor course deviations (up to 30°), may result in major improvements in effective ventilation with relatively little loss in overall progress. Course deviations may be carried out for days under extreme conditions with the ship zig-zagging around the intended course. If air speed can be increased from close to zero to around 1 m/second, the effect can reduce wet bulb temperature by as much as 4 to 5°C.
  • Keep the ship moving where ever possible in a heat event, this will ensure that the airflow from the exhaust fan outlets is not being drawn back into the supply fan inlets. 
  • Temporarily reduce or stop feeding. Feed chaff in preference to shipping pellets to reduce heat produced by rumen fermentation. 
  • In preparation for a heat event, conduct a deck-wash down and spread absorbent bedding to reduce humidity.

In severe heat stress situations, there may be benefits in spray-wetting cattle. The following issues should be considered:

  • The increase in humidity that will be caused by spray wetting. 
  • Only consider spray wetting if it will improve evaporative cooling rather than adding humidity.
  • Set-up spray wetting equipment when weather forecast predicts risk conditions.
  • Start spraying when 5% show panting score 3, i.e. open mouth breathing and drooling with neck extended.
  • Spray only when there is discernible air movement.
  • Sea or fresh water is ok.
  • Apply spray to the head and neck for maximum benefit.
  • Avoid cold water (<25oC) and high pressure.
  • Don't spray-wet if there is a ventilation system failure, otherwise dangerous increases in humidity may occur.
  • Ensure minimal disturbance of cattle - all unnecessary physical exertion must be avoided.
  • Continue spray-wetting until respiratory distress has eased.


Livestock exports to the Middle East are subjected to heat stress risk assessment prior to export. This includes application of the industry’s heat stress risk assessment software 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 behaviour, 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 minimise the risk of large scale heat stress occurring. 

See Appendix 1 for more information on the assessment of heat stress risk.