Lungworm in Horses & Donkeys: Symptoms, Diagnosis & Treatment

Lungworm is a parasitic infection caused by Dictyocaulus arnfieldi, which can lead to serious respiratory issues in horses. While donkeys are the primary carriers, horses can become infected by sharing pastures or being in close contact with infected animals. ^[1]

Lungworm infections are more common in cold, wet climates and are particularly common in horses pastured with donkeys. However, there have been rare cases of infection in horses with no known exposure to donkeys or mules. The severity of the disease depends on the amount of larvae ingested. ^[2][3]

With the widespread use of the dewormer ivermectin, the occurrence of lungworm infections and related pneumonia in horses has significantly decreased. ^[2] This underscores the importance of staying up-to-date with veterinary guidelines for equine parasite control.

Prompt diagnosis and treatment are essential to preventing lung damage and other complications due to lungworm. Read on to learn more about the symptoms of lungworm in horses and donkeys, how the infection spreads, and what you can do to protect your horse from this harmful parasite.

Lungworm in Horses and Donkeys

The lungworm, Dictyocaulus arnfieldi, is a parasite that affects equines all over the world. While lungworms have been found in multiple species of equids, donkeys, burros and mules are the primary hosts for this parasite. ^[1] In fact, studies show that anywhere from 50-80% of donkeys, burros, and mules are infected with lungworm. ^[2]

Interestingly, lungworm does not usually cause significant disease in donkeys, even in animals with a high parasite load. Lungworm infection is less common in horses, but it is more serious, often causing chronic respiratory disease and severe illness. ^[1][2][4]

Adult lungworms range from 3.5 – 7 cm (1.4 –  2.8 in) in length. The males are smaller than the females and have a distinct copulatory bursa that house short, thick spicules (the male sex organ). Lungworm eggs measure up to approximately 100 μm by 60 µm. ^[2][5]

Life Cycle & Transmission

D. arnfieldi have a direct parasitic lifecycle, meaning there is no intermediate host required for it to complete its life cycle. ^[6] The parasitic eggs are passed in the host feces, where they hatch and molt twice into their infective third stage (stage L3). ^[2]

Infective larvae contaminate surrounding grasses, where they are ingested by the horse. Once ingested, D. arnfieldi larvae penetrate the intestinal wall and enter the lymphatic system, which carries them to the lungs. There, the parasites mature over the next two months. ^[2][5]

Adult female worms from the lungs of infected donkeys or, less commonly, horses, are coughed up and then swallowed. Eggs or occasionally first-stage larvae are then passed in the animal’s feces and the cycle begins again. ^[3][5][7]

In the environment, lungworm larvae congregate on or in the sporangiophore (reproductive structure) of fungus growing on the manure. The liquid inside the sporangiophore explodes at midday, spreading fungal spores and larvae up to 3 m (9 ft) away, where they can be ingested by equines. ^[2]

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Infectivity

The first stage larvae can live up to 7 weeks in warm soil. Under the right conditions, they can develop into third stage larvae in as little as 5-7 days. ^[2] However, during periods of drought or very cold conditions, the larvae are vulnerable and may not survive long enough to meet a host. ^[3]

Through various diagnostic methods, the parasites can be detected in donkeys approximately 12-14 weeks after initial infection. Lungworms are rarely detectable in adult horses, though they may be detected in foals and yearlings. ^[5][8][9]

Symptoms

Symptoms of lungworm infection can develop as early as 12 days after the equine ingests third-stage larvae. ^[2]

Clinical signs of lungworm infection are not common in donkeys, even with high parasite loads. However, they can still transmit lungworms to horses or other donkeys. ^[9]

Lungworms in horses can lead to severe respiratory disease such as bronchitis and broncho-pneumonia. ^[9] In some cases, migrating larvae activate the immune system, resulting in immune-mediated bronchopneumonia. ^[10]

Chronic cough is the most notable sign of lungworm infection in horses. For any horse living in a cool, wet climate, parasitic pneumonia should be considered if the animal is coughing consistently. ^[9]

Coughing can be severe, though it may be difficult to distinguish lungworm infection from other types of respiratory disease. ^[3] Secondary bacterial infection may also occur which can cause a wide range of other symptoms. ^[9]

Other clinical signs that can occur with parasitic pneumonia caused by lungworm include: ^[2][4][8]

• Exercise intolerance
• Mucopurulent nasal discharge coming from both nostrils (especially if a secondary bacterial infection is present)
• Increased respiratory rate

Most affected animals still eat well and may appear otherwise healthy. ^[2]

In adult horses, migrating D. arnfieldi are the most common cause of parasitic pneumonia; in weanlings, however, a different parasite, Parascarais equorum, is the most common cause. ^[2][7]

Risk Factors

Lungworm infections are rare in horses, as the primary host for this parasite is donkeys. It is estimated that 50 – 80% of donkeys, mules, and burros carry a lungworm infection. ^[2]

Young horses are most often affected by lungworm infection, with most cases occurring in horses under the age of two. Affected horses of all ages tend to show signs in late summer, fall, and winter. ^[6][9]

Horses that share pasture with donkeys are at increased risk of lungworm, as the parasite is able to pass freely between the two species without an intermediate host. ^[10] In addition, horses that are not up-to-date on deworming protocols under veterinary guidance are at higher risk. ^[10]

Age is a major factor in the prevalence of lungworm infection. The highest infection rates are found in both old and young age groups, with one analysis putting infection in these age groups at 23% and 18% respectively. This is likely because of decreased immunity that is common in both age groups. ^[11]

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Diagnosis

Diagnosing lungworm infection in horses can be challenging, as not all infected animals shed larvae in their feces. Even when eggs are passed, the number may be minimal, making detection more difficult. A diagnosis is usually based on symptoms and transmission patterns. ^[3]

However, there are several diagnostic tools that your veterinarian may use to aid in diagnosing lungworm infection. These may include clinical examinations, fecal tests, and sometimes a bronchoalveolar lavage or tracheal wash to detect larvae in respiratory secretions.

Diagnostic Tools

If lungworm infection is suspected, your veterinarian may examine your horse’s airways via bronchoscopy and take X-rays of the thoracic area. Bronchoscopy can be used to collect washings from the trachea in order to examine for eggs, larvae, and white blood cells. ^[1][3]

For horses with chronic respiratory disease, large numbers of eosinophils (a type of white blood cell that helps fight infections) in tracheal washings may also indicate lungworm infection, but this finding is not specific to lungworm. ^[1][3]

With secondary infections, the horse may also have inflammatory changes such as mature neutrophils (another type of white blood cell that helps fight infection) and high plasma fibrinogen levels. An increase in liver enzymes may also be evident. ^[2]

Broncho-alveolar lavage (BAL) may show abnormally elevated eosinophil counts as well. However, other conditions such as lower inflammatory airway disease can return the same finding. ^[2]

Since lungworm infection in horses may be infrequent, diagnosis may also be made after failure of antibiotic therapy to improve a respiratory condition. ^[3]

Fecal Egg Counts

Your veterinarian may also perform a fecal egg count using the centrifugal flotation or modified McMaster technique. However, these methods may not show light worm burdens. The Baermann technique, a more detailed method of fecal egg count, is the best way to detect D. arnfieldi infections. ^[2]

With the Baermann technique, approximately 50 grams of feces are wrapped in a double layer of gauze to produce a “teabag”. The Baermann apparatus is a large funnel fitted with a screen to support the fecal teabag. A tube with a clamp connects to the bottom part of the funnel to allow sampling of the sediment. ^[2]

Warm water is poured into the funnel up to the screen and lungworm larvae are collected as they swim away from the fecal material and drop into the stem of the funnel. After 8-12 hours, 5-10 mL of the sediment is drawn off by releasing the clamp.

The sediment is spun at 1500 rpm for 3 minutes, and the clear liquid is drawn off so the remainder can be examined for the presence of larvae. ^[2]

Treatment

Currently, researchers consider macrocyclic lactones (ivermectin and moxidectin) to be the most effective anthelmintics for equine lungworm infection. Ivermectin given at 0.2 mg/kg orally and moxidectin given at 0.4 mg/kg orally both appear to be effective against mature and immature stages of D. arnfieldi in all equines. ^[2][9]

The macrocyclic lactones act by binding to certain receptors in the lungworm’s nerve cells. This causes paralysis in the worms and expulsion from the host’s body. As the concentration of the anthelmintic decreases, worms may regain motility but cannot feed and subsequently die. ^[12]

Prevention

Preventing pasture contact between horses and donkeys, mules, and burros is the best way to prevent lungworm infection in horses. ^[3][5][8] Another prevention strategy for large-scale farm operations is to adhere to robust biosecurity and quarantine protocols for all new or visiting equines. ^[12]

If keeping species together is necessary, donkeys should be treated for lungworm at least yearly and have their feces examined one month later before turnout with horses. ^[2]

With that said, it may still be possible for horses to pick up an infection without the presence of donkeys, burros, or mules. Removing manure from pastures weekly can help to decrease parasite burdens—not only with lungworms but all types of internal parasites. ^[2]

Other prevention strategies include the following: ^[2]

• Elevate all feeders and water sources
• Use gravel as a base for dry lots
• Avoid overgrazing pastures
• Separate yearlings and weanlings

Frequently Asked Questions

Here are some frequently asked questions about lungworm infections in horses and donkeys:

Summary

The following key points highlight important information about lungworm infections, which are common in donkeys, burros, and mules but much rarer in horses:

• Horses infected with lungworms often experience more severe symptoms than donkeys, such as bronchitis and pneumonia.
• A persistent, chronic cough is the most common sign of lungworm infection in horses.
• Diagnosis can be challenging, but tools like bronchoscopy, X-rays, and fecal egg counts (e.g., the Baermann technique) can help detect the parasite.
• Effective treatments include the anthelmintics ivermectin and moxidectin.
• To prevent lungworm infection, it is important to avoid co-grazing horses with donkeys and ensure regular manure removal from pastures.