Diseases in horses : definitions

Definition of health

Before defining a disease, one should look at the definition of health.

According to the World Health Organisation (WHO), in a definition which has remained unchanged since 1946, health is : « a state of physical, mental and social well-being, and not just an absence of illness or infirmity »

Diseases appear when there is an imbalance between these different factors.

Definition of a sickness

An illness is an alteration of the state of health, i.e. an imbalance occurring following an aggression, either directly due to the aggressive agent, or due to an excessive physiological reaction of the organisme being attacked.

• Biological : virus, bacteria, parasites, fugii… These are infectious diseases.
• Feeding : deficiencies, excess, intoxication…. These are metabolic illnesses.
• Genetic : abnormalities…
• Traumatic : like a fall for instance which can cause fractures or trauma…
• Environnemental : noise, stress…
• Physical or chemical : burns, ingestion or contact with pesticides, heavy metals...

The origin of a disease is not necessarily single, it can be divided into two categories :

• Monofactorial diseases which hava a single cause (e.g. Equine infectious anaemia or EIA, equine viral arteritis or EVA...).
• Multifactorial or plurifactorial diseases in which the onset is linked to the combined action of several factors (lameness, colic...).

Infectious diseases

They are caused by bacteria (glanders, tetanos…) or viruses (EVA, EIA, influenza, equine herpes virus or EHV….) These diseases are known as infections.

Parasitical disease

These are due to the presence of parasites such as worms (strongles, ascarids, tapeworm….), or to protozoa (equine myeloencephalitis, piroplasmosis….) . These diseases are termed infestations.

Nutritional and metabolic diseases

They are linked to the horse’s feed (laminitis, colic, Equine metabolic syndrome or EMS...).

Diseases due to prions

These are due to the presence of abnormal proteins, and have been noted in cattle (bovine spongiform encephalitis BSE) or in sheep (scrapie) but not observed in horses.

Two categories are to be distinguished :

Transmissible diseases

These can be spread from an affected animal to a healthy animal. They can be biological, parasitical or genetic.

Contamination is through direct contact, and can be :
    • Vertical - from the mare to the foetus.
NB : Genetic or hereditary diseases. Are sometimes passed on according to the laws of genetics, from the affected parent or parents to their descendants. An individual does not transmit the disease itself, but the faulty gene which causes the disorder.
    • Horizontal – through contact :
        ◦ from one animal to another ;
        ◦ Airborne contact (the animals are close to one another).

Contamination can also occur indirectly, it then can be spread via:
    • Objects, infected elements in the surroundings, these are termed inanimate carriers, such as the air, water, the ground, equipment, buildings...;
    • Living organisms which are known as live or animate carriers, such as Man, ticks, or intermediary hosts….

particularity of zoonosis

Zoonoses are diseases which can be transmitted from vetebrate animals to Man and vice versa. They are mostly parasitical (ringworm, toxoplasmosis…), infectious (rabies, tetanos, tuberculosis…) or caused by prions (BSE Creutzfeld Jakob).

Non transmissible diseases

Unlike the above, these diseases do not spread from one individual to another, they are non-communicable between individuals and non infectious.

This categorie includes metabolic, nutritional, accidental diseases as well as malformations.

Through the skin

The skin, which is the animal’s protective barrier, when damaged, can allow pathogen agents to penetrate the organism.

Digestive pathway

The digestive system contains billions of bacteria necessary for the animal. Any serious imbalance in this flora can lead to disease. This penetration pathway mainly involves parasites.

Through the respiratory system

In this case, germs penetrate with the air breathed in, therefore good ventilation and respect of the density of animals to guarantee good quality air are essential.

Other pathways

Eyes, genitalia...

A disease expresses itself differently depending on the individual and on the husbandry mode. There are therefore two main factors which favour the appearance of diseases :

• Those linked to the animal ;
• Those linked to husbandry management.

Chronic form

The disease evolves slowly and sometimes gives rise to intense bouts (laminitis)

Subacute form

The disease evolves over a few weeks, either due to the fact that the animal is resistant, or because the pathogen is not very aggressive

Acute form

The disease evolves over a few days

Super acute form

The disease evolves in a few hours and is often fatal

Morbidity rate

The morbidity rate enables to measure the frequency of a disease and its impact on a herd.

Morbidity rate = number of sick animals / total number in the herd

Mortality rate

The mortality rate enables to measure the seriousness of a disease in a herd

Mortality rate = number of deaths / total number in the herd

Lethality rate

The lethality rate enables to measure the number of deaths with regard to the total number of sick animals.

Lethality rate = Number of deaths / number of sick animals

Other definitions

• Etiology = study of the causes of diseases.
• Pathology = study of diseases.
• Pathogenesis = study of the individual development of a disease. It includes contamination, multiplication and excretion of the germ, development of immunity and possibilities for diagnosis. The appropriateness and possibilities of fighting the disease stem from this study.
• Prevalence = Measure of the state of health of a given population at a given time, for a given affliction.
• Incidence = Measure of the risk for an individual to contract the disease over a given period of time. It enables to assess the frequency of the pathology.