For all horse owners, having a horse with a runny nose and a cough is a familiar situation. Even without fully understanding the cause of the condition, we are aware that it seldom presents a serious risk to the horse’s life but nonetheless justifies close monitoring. In this article we will take a closer look at infectious conditions of the upper respiratory tract in the horse, how we treat them, and what the outcomes may be.

The upper respiratory tract (URT) of the horse comprises all the structures of the airways from the nostrils to the lower trachea. This includes the sinuses, pharynx (back of the throat), larynx and trachea. The lungs and main airways in the chest are not considered part of the URT. While there is obvious overlap in many of the diseases affecting both the URT and the lungs, these two parts of the respiratory system by and large have their own set of conditions.

VIRAL CAUSES OF URT INFECTION

Most infections which affect the URT are caused by viruses. There are several viruses involved and it is probably worth looking at the most common culprits.

1. Equine Herpes Virus (EHV). The name of the virus ‘Herpes’ will be familiar to many, as the human version of these viruses causes unpleasant skin conditions of various tender parts of the human body, cold sores being the most common. In the horse there are 9 different strains of Herpesvirus, but only 2 are of significance as causes of respiratory infections. These are EHV 1 and EHV 4. These viruses are ubiquitous in the horse population, spreading via nose-to-nose contact, droplets being coughed out by infected animals, and the shared use of buckets and rugs etc. While EHV 4 is restricted purely to respiratory disease, some strains of EHV 1 can also cause spinal infection leading to paralysis, or abortion in pregnant mares. These presentations are thankfully less common than URT infection but are of huge significance, especially in the breeding industry.

There is a commercial vaccine available against EHV 1 and EHV 4

2. Equine Influenza Virus

This is well known to most horse owners as the cause of the disease against which the horse receives an annual vaccination -‘Flu’. Influenza virus causes similar clinical signs as EHV but is much more contagious, spreading easily and causing disease in most of the horses with which it comes into contact.

Control is via herd immunity through vaccination. In the UK and Ireland, it is difficult to estimate how commonly Equine Influenza is seen, as most URT cases will not be swabbed for laboratory identification of the virus involved, though larger outbreaks are identified from time to time. Because of the infectivity of Equine Flu, outbreaks may result in a cessation of equine sporting events.

In Australia, the equine influenza virus has been eradicated from the country and strict procedures are in place to prevent its introduction. However, in 2007, 8 horses were imported from Japan, which managed to introduce and spread the virus. Within 50 days case numbers of influenza were running at 1000 per week, involving police horses, thoroughbreds, and pleasure horses.

3. Minor Viruses

There are some less common viruses such as Rhinovirus and Reovirus which cause mild URT infections.  These have less clinical significance than EHV and Influenza.

BACTERIAL CAUSES OF URT INFECTION

Strep Equi – ‘Strangles’

There is only one bacterium which can cause primary URT infection, and this is Streptococcus equi, the cause of Strangles.

Strangles is a URT infection which initially may resemble the viral conditions but often develops into a more serious disease.

• Severely swollen glands of head and neck which may rupture and leak thick pus.
• The swollen glands may restrict the airways, making it difficult for the horse to breathe easily – hence ‘Strangles’
• Reluctance to eat or drink.
• Thick yellow nasal discharge
• High Temperature
• Very dull and sorry looking horse or pony
• Cough

The bacteria Strep equi is very infectious and can spread around an infected stable yard like wildfire. Buckets, rugs, clothing of workers and vets etc can all carry the bug from one horse to another. If a yard is infected with Strangles then all movement of horses in or out must cease and strict biosecurity measures put in place.

Strangles outbreaks are common in livery yards where horse traffic in and out is high. More than 600 outbreaks are estimated to occur in the UK and Ireland every year.

TREATMENT AND OUTCOME OF URT INFECTIONS

Most viral URT infections will resolve with rest and good care after a couple of weeks. The horse should be allowed to recover in a dust-free stable with good ventilation. If the nasal discharge becomes green and thick, there may be an indication for antibiotics to treat a secondary bacterial infection. This is where the damaged lining of the respiratory tract becomes invaded by the usually harmless bacteria which live in the horse’s throat.

Strangles is a nasty condition. If the glands are swollen and sore, they may need poulticed to encourage them to rupture and drain. While antibiotics (Penicillin) may be used to treat the affected horse, they are not indicated for all cases, and may sometimes even prolong clinical signs and increase the risk of long-term complications.

Written by,

John Henderson BVM&S MRCVS

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