What is a “normal” body temperature?

If you’ve read any of our posts so far you will know we are all about monitoring body temperature, specifically as a means of understanding heatstroke risk in exercising dogs. As our fabulous friend Dr Jackie Boyd likes to remind dog owners:

You cannot manage what you do not measure and monitor.”

Whilst Dr Boyd is referring to measuring food in relation to canine nutrition and weight management, the same applies to body temperature. You can only be sure your dog is not over heating if you measure their body temperature. You can only be sure your temperature management strategy is working if you continue to monitor their temperature. We’ve previously shared some advice on methods of monitoring body temperature in dogs and other animals, and we have an update coming on this topic very soon! Crucially though, if you’re going to measure your animal’s temperature, you need to know what the numbers mean, and how to react appropriately.

The “normal” range

For almost every measurable aspect of an animal, from age, height and weight, to temperature, heart rate or urine production, there are “normal” reference ranges. In theory, these are statistically calculated limits of the normal population; in other words, 95% of healthy animals will fall within this range – there are always extremes that don’t fit the normal limits.

light-sepia-household-thermometer-166493

Google “normal temperature for a human” and you will see the top result, Wikipedia, reports “The normal human body temperature is often stated as 36.5–37.5°C”. It goes on to mention that “In adults a review of the literature has found a wider range of 33.2–38.2°C (91.8–100.8°F) for normal temperatures, depending on the gender and location measured.” A study recently found that the long established “normal” body temperature for humans is too high, highlighting the continued use of out of date normal ranges by medical professionals all over the world. This situation is mirrored in animal species. Textbooks state the normal range for various biological parameters in almost every species, but rarely state where this reference range has come from, how old it is, which animals were used to establish it and how their temperature was measured.

Our primary interest is canine health and welfare, so this all started when we tried to find a reliable source stating the normal resting temperature for a healthy dog. We struggled. Every textbook listed a slightly different range, 37.9-39.9°C on one website, then 38.2-39.2°C in a veterinary nursing textbook. The most recent reference we found was in a paper measuring rectal temperature in 62 dogs presenting to a veterinary hospital for routine health checks, who reported a normal range of 37.2 to 39.2°C. However, this did not report how this range was calculated, so without details of the statistical methods used to determine this range, it can not be considered truly valid.

Where does this leave us? Well, at the moment, we don’t actually have a robust, recent, reliable normal reference range for canine body temperature, when measured with a rectal thermometer. Three different sources state that 39.2°C should be considered the upper limit of normal, so we are fairly happy to accept that as our upper limit. What we do have, is a scientifically calculated normal reference range for canine ear temperature – because we calculated it! We measured ear temperature 416 times, from 157 healthy dogs attending various canine sports events over a two year period. Importantly, these dogs were not at the vets, so whilst they may have been excited, they were not stressed or scared. We calculated the normal range to be 36.6-38.8°C.

FINAL Establishing a reference range for normal canine TMT (you can read our published study here in the un-formatted version)

download
37.2 to 39.2°C is the current best estimate for normal rectal temperature in dogs.

Returning to rectal temperature, we know from our research, and from studies done by various other research teams around the world (Zanghi, 2016; Gomart et al., 2014), that ear temperature tends to measure around 0.4-0.6°C lower than rectal temperature. So, the rectal range of 37.2 to 39.2°C then quite nicely reflects our ear temperature reference range, just 0.4-0.6°C higher. This is the range we would consider to be most appropriate for interpreting canine body temperature, when measured with a rectal thermometer.

Equine body temperature

horse legs

Because we are both slightly mad, some might say obsessive, we couldn’t stop at establishing a normal reference range in just dogs. Oh no, we had to look at horses too. Whereas a dog may try to escape, sit down, or worst case scenario bite should they resent having their rectal temperature measured, a horse can, and if suitably aggrieved will kick you. This can be catastrophic. So if you’re going to risk your life measuring your horse’s temperature, and there are plenty of reasons you should be doing this, we felt it our duty as scientists to ensure you have a means of accurately interpreting the result.

We started by trying to find the evidence behind the “normal temperature range” for horses. We failed miserably. Once again, whilst plenty of textbooks, websites and other studies investigating equine temperature all state what the normal temperature range for a horse is, none of them reported where this range came from. Similar to the situation in dogs, if you don’t know where a reference range came from – how many horses were included, what part of the world, what breeds of horse, who took their temperature, how was their temperature taken – how can you be certain it is relevant to your horse? The simple answer is, you can’t.

stabled horse

So we decided to establish our own reference range. The horses at our University Equestrian Centre are cherished; the team caring for them will (and have!) battle blizzards, tropical storms and heat waves to ensure these horses are fed, watered, comfortable and loved. Part of their routine husbandry includes temperature monitoring, and yes, being University horses they do have to work for their living, so they are often used in non-invasive research work like our study comparing rectal temperature to eye temperature (measured with a non-contact infra-red thermometer, and coming soon!). Working with Dr Carol Hall and Dr Anne Stevenson, we collated rectal temperatures taken from 41 of the horses on the yard, measured at rest during a number of projects and routine monitoring. This gave us over 600 resting, healthy horse temperatures, all measured with a digital rectal thermometer by a familiar person, in the comfort of their stable. Knowing the horses were relaxed and not stressed is important, as stress can influence temperature in horses and other species.

We used a statistical method to then determine our horse’s “normal” temperature range, basically the middle 95% of the temperature readings. On our yard, this was 36.0-38.0°C. The upper limit is particularly important, as it is around 0.5°C lower than most of the previously published normal temperature ranges for horses. When an animal’s temperature exceeds this upper limit of normal, they are considered to be hyperthermic, too hot. The temperature reading alone can not explain why the animal is too hot, but it is an important indicator that something isn’t quite right (or, the horse has been exercising or in a hot environment). The term pyrexia, or fever, describes an animal that is too hot due to illness, this could be an infection, inflammatory or painful process. A low grade fever is where an animal’s body temperature is just slightly increased above the normal range and can be an early indicator of disease. It is therefore important that the normal reference range is accurate, otherwise these early indicators of illness can go unnoticed.

It is worth noting again, that the method of temperature measurement used is really important to consider when assessing your animal’s health. True core body temperature can only be measured using invasive, or ingestible devices. Rectal temperature remains the most accurate estimate of core temperature in most animal species (thankfully in humans non-contact thermometers appear to be as reliable!), but even the depth of rectal thermometer probe placement can have an impact on the resulting reading. A difference of 1cm can impact the temperature reading in chinchillas, whilst inserting a probe to 15cm or deeper in horses is likely to result in a higher temperature reading being obtained. Whilst we have no published evidence to support our theory, we suspect inserting a rectal probe to 15cm or deeper may also increase your risk of being kicked, so our normal temperature range is based on a 5cm thermometer depth.

If you monitor body temperature in two different ways you will get two different results. We know ear temperature is typically 0.4-0.6°C lower than rectal temperature. So it is important that you use the same method of temperature measurement each time if you are going to monitor your animal for changes in body temperature. That said, if you are ever worried about your animal’s temperature, use a rectal thermometer as this is still the most reliable estimate of core body temperature. Also, see your vet. Both high and very low body temperatures can be fatal in all species, so if your animal is unwell DO NOT delay seeking veterinary treatment. 

Our equine paper is available to read and download (free!) on the publisher’s site until 8th March 2019:

Establishing a yard specific normal rectal temperature reference range for horses. 

After this time, if you do not have access to the Journal of Equine Veterinary Science, then you are of course welcome to read the full article in the unfinished form below (this is the accepted manuscript, just not formatted into the journal’s style).

Establishing a yard specific normal rectal range horses FINAL.

We are hoping to recruit horses from all around the world to our next equine temperature study, so watch this space for further details if you would like your horse to take part!

horse-sleeping

 

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