Research into factors influencing the onset of equine dermatitis in France

Summary of results and observations - July 2017

In 2016, the Animaderm Observatory published an online questionnaire targeting owners whose horses suffer or do not suffer from equine dermatitis. The objective of this survey was to identify the factors influencing the onset and spread of equine skin allergies in France with an epidemiology-based study in the format "cases (= horses suffering from dermatitis) / control group (= healthy horses)". The survey included questions relating to the equine itself (type, breed, age, sex, category of its registration papers) or regarding its environment and the nature of its care (lifestyle, workload in autumn/winter and spring/summer, type and quantity of supplementary feed, type of feed and feeding frequency, access to a salt lick, administration of VMS, body condition score and dental treatment) as well as questions relating to how long horses with dermatitis spend scratching and the extent of lesions.

We would like to thank all the participants, especially those owners whose horses are not affected by dermatitis; the key potential weakness of this study was sufficient participation to enable statistically meaningful results. 

We were able to collect 1959 completed and verified surveys, of which 50.01% related to healthy horses and 49.99% to horses with dermatitis. This is the first major study conducted on the subject of equine dermatitis. For the first time, information relating to dietary practice and physical condition has been taken into account at an international level.

According to the statistical analyses based upon the data gathered, the occurrence of dermatitis is not linked to age, to sex, or to the animal's life style (field/stable/paddock, part of a herd or on its own). On the other hand, there is a strong statistical correlation between the occurrence of dermatitis and the type of horse, its work load throughout the year, access to hay in spring/summer and its distribution during the year, its body condition score and regular dental treatment. In the same way, there are noticeable differences between breeds, with some appearing more susceptible than others.

These differences are explained in more detail in the paragraphs below.

I - Impact due to the type of equine (riding horse, draft horse, pony, donkey)

Of the 1959 responses from the survey, 67.9% referred to riding horses, 27.3% to ponies, 3.5% to draft horses and 1.3% to donkeys. Overall, the proportions of different equine types recorded in the study almost exactly mirrored those recorded at a national level by the IFCE (French Institute of the Horse and Riding). This suggests that the survey data accurately represents the proportions of each equine type for the entire equine population in France. 

Looking at the survey results, the occurrence of dermatitis varies significantly across the different types of equines: ponies and draft horses seem to be much more susceptible than riding horses. The occurrence of dermatitis amongst the donkey population is the highest but this statistic should be viewed with caution, given the low number of owners who took part in the survey.

II - Impact due to the amount of physical exercise

There is a higher occurrence of dermatitis amongst inactive animals or those that do little work (less than three hours/week) compared with those that have a heavy workload, regardless of the season and the type of equine. 

There are two possible explanations for these findings : 
- physical exertion has a beneficial effect on dermatitis: work effectively acts as protection against dermatitis, 
- on the other hand, dermatitis has a direct impact on the equine's ability to work: it may be that horses suffering from dermatitis are considered less able to work or be ridden (as a result of the sores and skin lesions) and therefore are worked less than their healthy counterparts. 

Both possibilities may be true and it is not possible to decide between them from the study. Care must be taken and allowances made when looking at the analysis and the interpretation of the other factors in the study (using levels of activity as a norm, and studying interactions).

III - Impact due to body condition score / fatness

In order to estimate the equines' body condition, we have developed a system which uses a scale of 1 to 5 based on the size of the horse's heartgirth:

1 : a poor horse (All ribs are visible. You can feel them very easily when stroking the horse)
2 : a thin horse (you can just about make out the ribs and they can be easily felt)
3 : ideal size (you cannot see the ribs but they may be felt one by one)
4 : an overweight horse (you cannot see the ribs and only the last two pairs can be felt easily)
5 : an obese horse (you cannot see the ribs nor can they be felt). 

There is a very significant correlation between the body condition score and dermatitis, with instances of dermatitis appearing much more frequently in animals with a high body condition score (between 4 and 5/5 on our scale) than those with a lower body condition score (1 or 2 on our scale). The ratio of horses with dermatitis/healthy horses increases in direct proportion with the body condition score. 

Draft horses, donkeys and ponies, which seem more vulnerable to dermatitis, also have a higher average body condition score than riding horses, which in reality seem less vulnerable.

In this study, the body condition score is not related (or only marginally) to the intensity of physical activity of the equines, whatever the season and whatever the equine type (donkey, draft horse, riding horse or pony).
The variation in the different body condition scores observed between healthy animals and those with dermatitis has not therefore been linked directly to the differences in work over the year by the animals in these two groups. In addition, for groups with equal work levels, the link between dermatitis and the body condition score remains statistically the same. Given these results, it seems that being overweight is a contributory factor in the development of equine dermatitis. 

IV - The impact of diet

The proportion of horses affected by dermatitis is significantly higher amongst those that receive no feed in spring/summer. The method used to distribute feed to the horses also makes a difference; the number of horses afflicted by dermatitis was significantly lower amongst those that foraged for food at will compared with those that were fed a quantity of hay throughout the year.

There was no correlation in the research between dermatitis and the type of supplementary feed given (processed grain/oats, cereals and mixed feed). As a general rule, the level of work increased in line with the amount of supplementary food given, whether for healthy animals or those affected by dermatitis. 

Also, owners seemed rather good at judging the amount of feed to be given, based on the condition of their animal: the higher the body condition score, the less feed was given to the animals. Thinner horses received more food than those that were overweight. 

V - Impact of regular dental treatment

An extremely significant correlation was noted between dermatitis and regular dental treatment. Thus the proportion of horses afflicted by dermatitis is higher for those which do not receive regular dental treatment compared with horses which are seen annually by a dentist.

VI - Breed of horse

There appears to be no statistical correlation between dermatitis and the breed registration papers (Purebred, crossbred, unknown origin). All breed categories seem to be affected in equal proportions. However, in order to assess the impact of bloodline on dermatitis, we focused on horses with full registration papers (Purebred); those with crossbred and unknown origin papers being less reliable. 

Some breeds appear more vulnerable than others: amongst riding horses, Friesians, Andalusians, Irish cobs, Camargue and Menorquin horses are particularly prone to dermatitis; for ponies, Shetlands, French Saddle Ponies and Haflingers are vulnerable. Other breeds, however, seem to be more resistant: these include Thoroughbreds, French Trotters, Selles Francais and Hanoverians. 

VII - Other factors

The results also highlighted a significant but less important correlation between dermatitis and: 

- feeding supplementary vitamins and minerals (VMS); dermatitis is more prevalent amongst animals that do not receive these supplements
- Horses that have a salt lick: dermatitis is less prevalent among horses that have a salt lick

Conclusions :

This observational study has highlighted a link between equine dermatitis and a number of identified factors. These results should be confirmed by studies known as prospective cohort studies, where these different identified factors will be monitored and/or modified in order to track the appearance and development of dermatitis over time in different subject groups.

Although excess weight has long been associated with a number of equine ailments, such as laminitis, diabetes or even Cushing's disease, this is the first time that evidence has been presented to show it also can cause skin allergies. 
The identification of this association in the study fails to show whether the affliction is reversible; in other words, whether weight loss reduces or makes the symptoms disappear when an animal has been overweight. A clinical study with a weight management programme, tracking individual cases should be able to provide the answers to this question.

Lesions where the horse scratches :
It is interesting to note that the principal areas scratched by horses suffering from dermatitis are those where the body has highest reserves of subcutaneous fat: 
- the base of the mane (in 92% of cases), 
- the base of the tail (87%)  
- the croup (31%). 

Our questionnaire did not ask about the shoulders and the withers, which are also areas where the body stores subcutaneous fat and which are often affected by outbreaks of dermatitis. 

It also emerged in the study that amongst those breeds with full registration papers, horses that are most vulnerable are those which appear to the hardiest, also known as "easy-keepers" (good-doers).

Recommendations :

- Wherever possible, prevent your horse from becoming overweightle: limit the amount of cereals or grain that you add to its feed in favour of forage. Never stop feeding your animal in order to make it lose weight; it may develop severe metabolism problems. Equally, there is no point restricting the time it spends grazing as the horse will adapt its speed of ingestion and grazing so that it eats as much (or even more) in less time.
- Don't be afraid to work your horse more, and even if you spot an outbreak of dermatitis, there is no need to let it stay out in the field without doing any work. If this study has failed to provide clear evidence on the benefits of physical activity on an attack of dermatitis, work does not appear to have a detrimental effect on a horse and it has the additional merit of helping the animal control its weight. 
- Give your horse ad-lib hay throughout the year, including horses living in a field that are able to graze to their heart's content. In fact, the nutritional value of grass varies enormously depending on the quality of the grass, the soil and the season. Pasture vegetation is not always best suited to the physiological needs of our horses, especially in springtime. Avoid feeding straw as it is rich in non-digestible fibre, which could lead to digestive problems such as intestinal colic. 
- Choose good-quality hay, ot too fresh (over three months), nor too stale (no more than a year), and containing a good mix of grass with a pale green colour and a sweet smell. It is best to feed hay continuously. Use a hay net: choose one with small holes for greedy horses.
- Make sure your horse has a dental check each year. As we have indicated in the report, there is a correlation between regular dental treatment and dermatitis. Dental problems may be the cause of a number of physiological ailments and may prevent the horse getting proper nutriments. Whatever the age of your animal, it should be checked every year to limit outbreaks.
- Give your horse high-quality mineral and vitamin supplements which are appropriate for its needs, and ensure it has access to a salt lick. Although horses are able to manage their salt intake rather well according to their needs, this is not true for vitamins, minerals and trace elements. Avoid giving the animal mineral and vitamin supplements in buckets or blocks, which makes it impossible to regulate the daily intake. These compounds usually contain molasses or other appetising ingredients which may cause an overdose in greedy horses. 

Future topics of research

Recent research has uncovered the existence of stem cells which are able to differentiate into mast cells in fat tissue. These immune cells are specifically associated with allergic reactions as they release inflammatory molecules and histamine. In humans and mice, the number of mast cells increases within adipose tissue layers in obese individuals and such obesity increases the likelihood of developing respiratory allergies. It would be interesting to see whether this also applies to the equine population by carrying out biopsies on subcutaneous fatty tissue amongst healthy animals and those suffering from dermatitis with varying body condition scores. 

Furthermore, research has also shown that certain breeds, identified as the most vulnerable in our study had naturally above-average levels of insulin which would render them more likely to suffer from diabetes and associated diseases (laminitis and SME). This is certainly the case for Friesians and breeds originating from the Iberian Peninsula, (Andalusians and Menorquins) and many breeds of pony. Studying the correlation between dermatitis and insulin levels, that is to say diabetes, is a line of research we would very much like to follow.

These observations are all research topics that the Animaderm Observatory intends to study in the years to come.