Airway patrol: understanding parasitic respiratory disease

By Wendy Talbot on 18 September 2020

Some respiratory diseases can be caused by parasites. Parasites are organisms that live on or within a host. Worms are the most common parasite to affect horses.


Lungworm is the equine parasitic respiratory infection most people have heard of. It is primarily found in donkeys which may not show obvious clinical signs. The parasite is usually specific to donkeys and is not well adapted to completing its life cycle in a horse. However, sometimes horses can be infected if they are grazed alongside their equine cousins, especially if their immune system is compromised in some way. If they are infected horses are more likely to show notable signs of respiratory disease.

Lungworm can cause a persistent cough and increased respiratory rate. In severe cases they may cause secondary pneumonia.


Ascarids are worms that can affect foals of less than 12 months old (and occasionally young horses), causing respiratory disease. The larvae can affect the lungs from around 1 month of age but the most dangerous stage of the parasite is when the larvae subsequently develop into adults in the gut when the foal is between 3-12 months of age.

Foals with ascarids may be underweight with a nasal discharge and a cough.

How are lungworm and ascarids transmitted

The larvae are eaten from the pasture. Lungworm migrate to the lungs where they develop into adults and begin to lay eggs within a month. They cause an irritating cough, which helps the eggs to move up the windpipe where they are swallowed and enter the intestinal tract and are eventually excreted in the faeces of donkeys. Lungworm in horses affect the lungs but are unlikely to be able to produce eggs to continue the cycle.

In the case of ascarids the larvae pass through the lungs, are coughed up in a similar way to lungworm but then develop into adults in the gut where they can cause fatal blockages.

Preventing parasitic respiratory disease

Unless your horse spends time with donkeys lungworm is unlikely to be a problem. It can be treated with specific types of wormers on advice from your vet or registered animal medicine advisor (RAMA).

Ascarids represent the biggest parasitic threat to young foals. To reduce the risk it’s important to make sure your foals and youngsters have clean pasture and that you implement a suitable regular worm control programme in consultation with your vet or RAMA.

Keeping paddocks regularly (at least twice weekly) cleared of droppings will help to reduce the risk of these and other parasitic diseases.

If you think your horse may have any symptoms of respiratory disease it’s sensible to isolate them immediately, into a clean, well-ventilated, dust free area, check temperature, pulse and respiratory rates and contact your vet to discuss the next course of action.


  1. Nielsen MK (2016) Evidence-based considerations for control of Parascaris spp.infections in horses. Equine vet. Educ. (2016) 28 (4) 224-231




Wendy graduated from Bristol University in 1999. She then went on to complete a residency at Liverpool University and holds a European Diploma in Equine Internal Medicine. After working in practice for 13 years, she joined Zoetis in 2012 as the National Equine Veterinary Manager.

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