​Respiratory Disease in Weanlings with Compromised Immune Systems

During their first year of life, the immune systems of young horses are developing to protect them from a myriad of bacterial and viral pathogens in the world around them.

While scientific research into the development of equine immunity is ongoing, researchers have learned much in recent years about the complexities of the immune response and its evolution as a foetus becomes a newborn foal, and eventually an adult horse.

From birth, several innate immune responses are fully functional, while other adaptive immune responses mature slowly as young horses reach adulthood. It is for this reason that young horses, including weanlings, are more vulnerable to certain diseases.

In this article, we examine our current understanding of equine immunity, the common respiratory diseases of weanlings with compromised immune systems, and most importantly, the treatment and prevention of respiratory disease in young horses.

Equine Immunity

In horses, the immune system comprises a complex network of white blood cells, antibodies, and other substances, including antigens, as well as several organs. Organs such as the thymus gland, spleen, lymph nodes, and liver are involved in the immune response.

It is the immune system’s sole responsibility to defend the body against foreign invaders, such as bacteria, viruses, and microorganisms, in addition to abnormal or cancerous cells. It does this through three lines of defence:

  • Physical barriers
  • Nonspecific immunity
  • Specific immunity

Just like their owners, a horse’s first line of defence is the physical and mechanical barriers of the body. These include the skin, the cornea of the eye, and the lining of the respiratory, digestive, urinary, and reproductive tracts. These barriers are protected by “good” bacteria.

Nonspecific or innate immunity is present at birth. The most important process involved in nonspecific immunity is acute inflammation, whereby white blood cells destroy invaders.

Specific or adaptive immunity is acquired and subsequently improves over time as the immune system learns the best way to respond after initial exposure to a new antigen.

Routine vaccination is one of the most effective ways to enhance the immune response in young horses. The role of most vaccines is to stimulate the body’s natural defences and they work by prompting the development of specific immunity.

Respiratory Disease in Young Horses

Respiratory diseases of the upper and lower respiratory tracts are common in weanlings under eight months of age due to their susceptible immune systems and several environmental factors. They are often bacterial or viral in nature. They include:

Bacterial –

  • Streptococcus zooepidemicus
  • Rhodococcus equi
  • Streptococcus equi
  • Parasitic pneumonia

Viral –

  • Equine influenza
  • Equine herpesvirus 1 and 4
  • Equine herpesvirus 2
  • Acute respiratory distress syndrome

Streptococcus Zooepidemicus

Streptococcus zooepidemicus is the most common opportunistic bacterial pneumonia. However, other opportunistic microorganisms include Strep pneumoniae, Actinobacillus spp, Pasturella spp, Bordetella bronchoseptica, Klebsiella ssp, and E. coli.

Opportunistic bacterial pneumonia normally develops secondary to other issues, with stress caused by weaning being one of the catalysts.

Rhodococcus Equi

The Rhodococcus equi bacterium is found in dry and dusty soil. It thrives in warm temperatures at around 37°C. Weanlings are exposed to the pneumonia-causing pathogen by inhaling dust particles laden with the virulent form of Rhodococcus equi.

Farm management factors are key to decreasing the risks of infection, including avoiding overcrowding, pasture rotation, dust-free housing, and reducing dirt or sandy areas.

Streptococcus Equi

Streptococcus equi is the bacterium that causes strangles. Highly contagious, it can be transferred directly by nose or mouth contact or aerosol particles, or indirectly by contaminated surfaces, shared equipment, feed, pasture, and even flies.

Vaccination against strangles is imperative for any young horse over three months of age. Three vaccinations are given initially one month apart, followed by a booster every year.

Parasitic Pneumonia

The migration of large parasite burdens can also play a role in the development of bacterial pneumonia in foals and weanlings as they weaken the immune system. Pulmonary migration of roundworms can result in infection, leading to bacterial pneumonia.

Routine de-worming is the best method to prevent these types of infections. It is recommended to start worming after two months of age, with a faecal egg count carried out at weaning, and every three months thereafter, until one year of age.

Equine Influenza

Equine influenza is an orthomyxovirus that has caused numerous outbreaks around the world. The most recognised subtypes are H7N7 and H3N8. Weanlings are susceptible when stressed or housed in overcrowded and poorly ventilated conditions.

The stress induced by weaning supresses the immune system, combined with the confinement and concentration of young horses, allows for easy transmission of the virus.

Equine Herpesvirus 1 and 4

Also known as Rhinopneumonitis, the equine herpesvirus is an alpha herpes virus that manifests in the respiratory tract, causing a respiratory infection consistent with pneumonia. It is most common in foals, weanlings, and yearlings.

While there is no vaccine for the neurologic form of the disease, six-monthly vaccination is vital for horses under six years of age to protect against respiratory complications.

Equine Herpesvirus 2

Equine herpesvirus 2 is also called equine cytomegalovirus. While less is known about EHV-2 than its counterparts EHV-1 and EHV-4, the virus may be associated with lower respiratory disease and possibly involved in inflammatory airway disease in young horses.

It is thought that EHV-2 suppresses the immune system, allowing other respiratory infections — bacterial or viral — to take hold. Currently, there is no vaccine available.

Acute Respiratory Distress Syndrome

A potentially fatal disease, acute respiratory distress syndrome is most common in foals and weanlings up to eight months of age. It typically manifests as interstitial or bronchointerstitial pneumonia that in many cases results in death.

Many bacterial and viral pathogens may be implicated in the development of the disease, and EHV-2 is also suspected. Prevention is key as treatment is usually unsuccessful.

Veterinary and Alternative Prevention

It is clear there are some common threads that connect the most prevalent respiratory diseases seen in foals, weanlings, and yearlings. Protecting the young immune system from compromise through routine vaccination, de-worming, and good husbandry is essential.

In addition, utilising equipment, such as the Flexineb® E3 Complete System, to deliver veterinary and natural remedies to treat respiratory disease, remove mucous, and promote optimum respiratory health is also recommended.

The Flexineb® E3 System is a portable, silent equine nebuliser that delivers aerosolised drugs and natural therapies to the airways of the horse by converting them into a fine mist that is easily inhaled and acts directly within the respiratory tracts.

Disclaimer: This article is not intended to replace professional veterinary advice. Always consult your veterinarian if your horse as any health-related issue or is exhibiting symptoms.

Targeted Keywords:

  • Equine immunity
  • Streptococcus zooepidemicus
  • Rhodococcus equi
  • Streptococcus equi
  • Parasitic pneumonia
  • Equine influenza
  • Equine herpesvirus 1 and 4
  • Equine herpesvirus 2
  • Acute respiratory distress syndrome




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