Lungworm
Lungworm- Dictyocaulus arnfieldi
The lungworm, Dictyocaulus arnfieldi, is a parasitic worm which is found in the lungs of infected horses, ponies and donkeys during its adult stage. Donkeys are their preferred host for D.arnfieldi and they can cause significant damage. Horses are typically only infected if grazed alongside donkeys, and infections in horses rarely result in the development of egg-producing adult lungworms.
Although a more significant parasite of donkeys, lungworm can cause clinical symptoms in both donkeys and horses. It should be noted that mules can also contract lungworm but this is quite uncommon.
Ask us what treatment we recommend for your horse
The Parasite:
Lungworms are slender thread-like worms, up to 8cm in length, and their presence in the lungs can cause significant damage. This risk is especially prevalent in unthrifty animals when there is a high worm burden.
Although not particularly associated with a high risk in horses due to the worm’s inability to complete its life-cycle in this species, they can be of concern in the very young, very old, or those with poor immune systems due to other conditions. Lungworm infection in young horses can lead to the development of impaired respiratory function and overall performance in later life and has been indicated as a causative factor in RAO/COPD in horses.
The primary clinical signs are directly correlated to the adult worm’s location within the animal. The infection irritates the lung mucosa resulting in parasitic bronchitis with a resulting production of mucus leading to:
- Difficulty in breathing
- Coughing, ranging to severe coughing
- Loss of appetite
- Increased respiratory rate
- Respiratory distress
The lungworm infection can lead to further complications with bronchitis accompanied by pneumonia and potentially secondary bacterial infections.
It is known that not all infected donkeys exhibit clinical signs, and certainly not the symptomatic cough seen in horses. Donkeys seem able to tolerate even large lungworm infestations without exhibiting adverse clinical signs. Horses, on the other hand, usually express clear and classic symptoms as noted above.
Details and Treatment
Testing for Lungworm
The routine test for suspected lungworm infection is actually performed using a faecal sample, as it is for many other equine worms, but in this case, it utilises a sedimentation test. A routine faecal egg count (FWEC) will not observe lungworm eggs. Why?
Well, the (modified) McMaster technique used in the analysis of other nematode infections works on the principle of flotation in that the eggs/cysts have a lower specific gravity (lighter) than the flotation medium so they appear on the top of it. The lungworm larvae though are comparatively heavy so will not float.
Instead the lungworm faecal test relies on the principle of sedimentation i.e. the heavier material (larvae usually as eggs are not often seen) separates to the bottom of a liquid suspension. Samples of the sedimented material are then observed under the microscope.
The sedimentation test is sometimes referred to as the Baermann testing method (often referred to as the “Baermann technique”). Although this technique is useful for identification of most nematodal larval forms, it is the gold standard method of testing for suspected cases of lungworm infection.
A sedimentation test requires double the amount of faeces when compared to a routine worm egg count and takes somewhere in the region of 3 – 4 days to complete. We work in conjunction with the leading testing laboratory to run lungworm tests so please get in contact to request a kit.
Treating Lungworm
With positively confirmed lungworm cases treatment is often necessary. In the UK we have two antiparasitic active ingredients- moxidectin and ivermectin- which are effective for treatment of lungworms. These are found in a wide range of trade named products, both originator/pioneer and generic products. The example dosing instructions from Eqvalan Oral Paste (correct as of December 2022) are:
- Administer orally to both horses and donkeys at the recommended dose level of 0.2 mg ivermectin per kilogram of bodyweight.
- Bodyweight and dosage should be accurately determined prior to treatment.
- For syringes intended to treat horses up to 600 kg and 1100 kg calibrated markings are provided at 100 kg bodyweight intervals. For the syringe intended to treat horses up to 750 kg, calibrated markings are provided at 125 kg bodyweight intervals.
All animals should be weighed prior to dosing and follow up efficacy testing etc. should be conducted as necessary and recommended by your SQP/Vet.
The de facto current listing of licensed veterinary medicinal products in the UK can be found by consulting the VMD Product Information Database and product information (SPC, product leaflets) should be consulted and adhered to https://www.vmd.defra.gov.uk/productinformationdatabase/
Are all Wormers licensed for Donkeys
No…
Both ivermectin and moxidectin are effective in the treatment of lung worms and both active ingredients are licensed for use in horses. However, not all ivermectin-based wormers are licensed for use in donkeys. At the time of writing, December 2022, the only ivermectin-based wormers available for use in donkeys were:
- Eqvalan Oral Paste for Horses (Ivermectin 18.7 mg/g)- from Boehringer Ingelheim
- Molemec Paste for Horses 18.7 mg/g Oral Paste- Also from Boehringer ingelheim
There are currently no moxidectin-based products licensed in the UK for use in donkeys.
Lungworm Life-cycle
Firstly, there is a commonly held misconception that horse and donkeys should not live and graze together due to the donkey being the lungworm’s preferred host. A study conducted by the Donkey Sanctuary; (2009) Donkeys parasites in the uk: infection levels, treatment intervals and anthelmintic use. abstracts: 22nd international conference of the world association for the advancement of veterinary parasitology, Calgary, canada. pp 39 found that lungworm infection cases in newly presented donkeys were at a level of only 4%.
Accordingly, providing owners are alert to the potential of lungworm infection and address this in their overall parasite control program in consultation with your Vet/SQP, then horse, donkeys and ponies can live together safely.
Lungworms are certainly more prevalent on wet grazing land and can live in the ground for some considerable period. Long dry spells are thought to be effective in reducing environmental burdens, as is poo-picking and cross-grazing with non-permissive hosts which do not allow the life-cycle to complete i.e.a biological dead-end.
Dictyocaulus worms have a direct life cycle, i.e. there are no intermediate hosts involved. Adult females lay eggs in the airways of infected hosts. These eggs are transported to the pharynx within respiratory secretions. From the pharynx these eggs are coughed out, into the mouth to be swallowed or directly to the outside environment. Those eggs that are swallowed release the L1 (first stage) larvae in the gut, which are then shed out into the environment in the faeces.
Once in the environment, L1-larvae develop to infective L3 larvae in about 1 week. These larvae show a low motility and remain close to the droppings. Infective larvae are susceptible to dryness and usually they do not survive longer than about 1 month on pasture.
Horses become infected mainly while grazing, but infection can also happen indoors through contaminated hay or bedding. Once ingested and in the host’s gut infective larvae penetrate into the gut wall and reach the lymphatic nodules . The larvae then use the lymphatic and circulatory system to reach the lungs. Once in the lungs they are stopped in the lung capillaries where they cross the capillary wall and reach the bronchioles, bronchi or the trachea where they complete development to adult worms.
The time between initial infection and first larvae being shed within the faeces is around 4-5 weeks.