It should be appreciated that the term ‘the virus’ is a generalisation and not a specific reflection of viral disease in horses.  Many diagnoses cover bacterial infections and some non-infectious problems that are, or can be, associated with a loss of performance in individual horses, or in whole yards.

The term is probably most often used in relation to large collections of horses and, most of all then, in relation to yards of racehorses, eventers or other competition horses.  It is also widely used in reference to studfarm infections.  As a rule of thumb, the term is associated with chronic, long-lasting problems that fail to resolve, although there can be acute illness and fatality associated with virulent outbreaks of disease.  In the case of chronic disease in individual animals, ‘the virus’ may indicate any of a number of conditions, but, when there is recurrent infection, the cause may be reduced resistance, for one reason or another.

The Natural Course of Infection

Under normal circumstances, infection for a young, healthy horse is similar to that for an average human.  There is an incubation period, a period of infection, then recovery.  Infection should cover a normal time span, the period from start to end being from one to three weeks, though experience has shown that this is not what happens in all situations.  Some clinical events last a lot longer, even months.

Reasons

With the exception of specific diagnosable conditions like flu (caused by a virus) or strangles (caused by a bacterium), the term ‘the virus’ covers a range of not easily explained symptoms that puzzle clinicians by their persistence or tendency to recur. 

The most likely explanation, in practice, is a combination of management factors that interfere with resistance combined with there being more than one organism, so that there is recurrence in animals already debilitated through prior infection.  While academic sources have been slow to recognise this, it occurs in practice and the presence of a virus like EHV-1 (equine herpes virus), being capable itself of causing serious systemic disease, makes any mixed infection more complex.  EHV-1 causes liver damage and may affect the nervous system, causing recumbency (fatal cases have been reported in the US in 2005).  In already debilitated animals, EHV-1 can have major disease implications and, because of its nature, lends itself to regular repeat episodes in animals previously infected. 

With human herpes simplex infection, disease recurs when resistance is compromised.  The virus is thought to sequester in regional tissues and become clinical when offered the opportunity, as when another organism invades.  While such a scenario is not proven in the case of EHV-1, and would appear unlikely in fully recovered animals, it may feature in some situations where infection is rife.

Diagnosis

Because of the nature of lowgrade equine infection, diagnosis is not a simple matter.  Traditionally, it is most commonly based on blood analysis and viral or bacterial culture.  The problem with blood analysis is that it can be non-specific.  Horses in the incubation, or recovery, phases may not be detected, so there is a possibility of false results on both sides of the systemic phase.  Furthermore, many animals only become seriously affected by an organism when they are worked.  Thus, the influence of work can act as a stress to compound an existing insignificant infection.  It may lead to a recovering horse becoming reinfected, may contribute to bleeding in horses with already compromised lung tissue, or it could conceivably lead to cardiac complications (like fibrillation) in debilitated horses whose infection hasn’t been recognised.

The weakness in relying on tracheal washes and culture of other clinical material is that a negative result isn’t necessarily conclusive and failure to locate or grow an organism (while it can, of course, indicate a horse is disease free, can also be misleading) may mean little if in contact animals are openly transmitting infection.  One of the problems of relying on endoscopy is that pneumonic lung lesions are inclined to be missed, as are sterile conditions reducing total available breathing space.  These are critical problems in competing animals.  

Treatment

In acute viral infections, like EHV-1 with paralysis, treatment may have to be intensive and prolonged. While there is a vaccine, it has recognised limitations. Horses recumbent from this virus may have to be supported in slings and some may not survive. EHV-1 has caused serious disease through the past thirty years and would appear to exploit situations where large numbers of horses are congregated. 

Bacterial infections usually respond well to antibiotics, and horses dehydrated or anaemic after infection can be helped with electrolytes and haematinics. 

However, treatment in complex infectious situations is often as much managemental as therapeutic.

It has been shown that dispersal of badly infected horses to small isolated yards will result in recovery for many in a short period of time. But this may not be what an owner or trainer wants to hear during the course of a training season and it may not be a practical option where there is danger of spreading disease.

In less serious cases where there is little overt illness yet horses are unable to perform, the most important factor is diagnosis, knowing the stage of infection of each horse, having a reasonable idea of the cause. This can only be achieved clinically by reading the symptoms and assessing the effect on the individual horse, a situation technology has so far failed to reach. The effect of illness on circulation and the heart is critical. While this may be reflected in post-exercise heart rates (even with a monitor), simple interpretations are not that reliable and a better assessment may be made while the horse is at rest. Subtle differences in resting heart rates may be significant then, but there has to be a clinical judgement, based on experience. Great harm can be done by subjecting horses to work while suffering from systemic infections, however mild. 

When exercising a sick animal, the heart's reaction to even a short trot can be significant as this reflects ability to cope with work.  The health situation of the lungs (and liver) is also critical. 

There are other steps that can help:

1. The first priority in nursing animals is to stabilise temperatures and make sure no sick animal is cold. The body's reaction to cold is critical (as it is to heat) and defensive mechanisms lose effectiveness as temperatures drop.
2. Feeding has to be assessed and the diet controlled if, for example, there are deficiencies or problems with protein, oils or minerals.
3. Exercise must be strictly gauged to the ability of the individual animal to tolerate it. Any horse in the systemic phases of infection (may not even have a raised temperature) will be seriously compromised by anything more strenuous than a hack. Even this may be too much in given circumstances.

In thirty years of dealing with viral problems, there has always been an answer, but complications can arise because of over-feeding horses with liver damage, for example. Lameness can become another complicating factor that gets attributed to infection, though it generally isn't (there is a possible link in EHV-1 cases). 

Horses moving badly because of nutritional causes can be further impeded by physical muscular damage through injury or because of gallop problems. Many such features can over-ride one another and cloud the picture.

In the case of a single horse with chronic 'viral' problems, it is important to look at all factors that might influence resistance and so lead to what appears to be a long lasting infection. 

Consider the following:

1. What is the feeding situation? Is anything out of balance, are there any deficiencies, like calcium (on a grain diet), copper (on deficient soil), etc.
2. When was the last worming?
3. Is there any possibility of fluke?
4. Is the horse anaemic or dehydrated?
5. Is the horse on sandy soil and could there be sand (or other dirt) in the gut causing malabsorption?
6. Is the stable cold or drafty?

In most such cases, there is a simple answer that can be located and corrected. The infection will then, usually, clear up. If not, blood tests might indicate other pathways. The problem is usually reversible, although there are situations where it might not be (cirrhosis being a typical example).