Pneumonia is an infection of the lungs and airways that causes disturbance in respiration and deficiency of oxygen in the blood. It is common in foals up to 6 months of age. The causes of foal pneumonia include viral infections (such as equine herpesvirus infection, discussed earlier in this chapter), bacterial infections (such as Rhodococcus equi infection), parasitic migration, and environmental stresses (such as weather fluctuations, dusty conditions, or overcrowding). Often, a combination of factors appears to be involved in the development of foal pneumonia. Some of the more significant types of foal pneumonia are discussed below.

Rhodococcus equi Pneumonia

Although it is not the most common cause of pneumonia, Rhodococcus equi is the most serious cause of pneumonia in foals 1 to 5 months old. It has significant economic consequences due to death, prolonged treatment, surveillance programs for early detection, and relatively expensive prevention strategies. This type of pneumonia is rare in horses older than 6 months of age.

Rhodococcus equi bacteria are widespread in soil and are likely present on all premises to some degree. However, the incidence of disease varies from farm to farm. High summer temperatures, sandy soil, and dusty conditions favor the multiplication and spread of the organism in the environment. Inhalation of dust particles containing disease-causing strains of the bacteria is the major route of lung infection. Manure from infected foals is a major source of bacteria contaminating the environment. Foals are exposed or infected during the first week of life.

Infection progresses slowly, and signs of disease are difficult to detect until lung lesions reach a critical mass. These lung lesions include pneumonia, abscesses in the lungs, and swelling of lymph nodes. When signs begin, most foals are lethargic, run a fever, and have rapid breathing. Cough is an occasional sign, while pus-containing nasal discharge is less common. The veterinarian can hear crackles and wheezes in the chest.

In addition to lung abscesses, intestinal and abdominal abscesses may occur in Rhodococcus equi infection. Foals with abdominal involvement often show fever, depression, loss of appetite, weight loss, colic, and diarrhea. The outlook for foals with the abdominal form of R. equi is less favorable than for those with the lung form. The bone is another, less common site of infection. If the vertebrae are affected, vertebral fracture and spinal cord compression can result. Other sites of abscesses such as the liver and kidneys have been reported.

Routine blood testing reveals abnormalities consistent with infection and inflammation. Particular lesions seen on chest x‑rays can suggest the presence of R. equi; however, identification of the bacteria in fluid from the airways is needed for a definitive diagnosis.

Treatment and Prevention

For treatment, a combination of antibiotics is more effective in fighting the infection than any antibiotic given alone. The use of combination antibiotic therapy has greatly improved the survival of foals. The length of antibiotic therapy typically ranges from 4 to 9 weeks. Supportive therapy includes a clean, comfortable environment and highly palatable, dust-free feeds. Your veterinarian may use intravenous fluid therapy and saline mist (nebulization) to help your horse cough up lung secretions. Nonsteroidal anti-inflammatory drugs (NSAIDs) may be prescribed when fever is present. Treatment with oxygen is necessary in foals with severe respiratory difficulty.

Prompt veterinary attention and appropriate treatment is critical. The survival rate of foals with R. equi pneumonia is approximately 70 to 90% with appropriate therapy. Without therapy (or with inappropriate therapy), the death rate is about 80%.

To help prevent R. equi pneumonia on farms where the disease is present, foals should be maintained in well-ventilated, dust-free areas, avoiding dirt paddocks and overcrowding. Commercially available plasma with antibodies against R. equi should be administered to foals on the first day of life on at-risk premises. Foals with pneumonia should be isolated and their manure composted. Herd surveillance programs (including periodic physical examinations and blood testing) can be put in place for early detection of sick foals on farms where the disease is present.

Acute Bronchointerstitial Pneumonia in Foals

Acute bronchointerstitial pneumonia is a sporadic, rapidly developing disease of foals characterized by severe respiratory distress and high mortality. This sporadic disease has been reported in North America, Australia, and parts of Europe. The cause is not clear. It is likely that a number of different factors can start a chain of events resulting in severe lung damage and acute respiratory distress. Warm weather (temperatures higher than 85°F [29°C]) is a common factor. Many foals have a history of receiving antibiotics (particularly erythromycin) at the time signs developed. No virus is consistently isolated, and no bacterial agent has been consistently identified in infected foals.

The age of affected foals ranges from 1 week to 8 months. Acute bronchointerstitial pneumonia has a sudden onset and is accompanied by high fever. The disease progresses rapidly and may result in sudden death from respiratory failure. Foals are unable or reluctant to move and usually have bluish mucous membranes from lack of oxygen. Severe respiratory distress is the most striking clinical sign. Veterinary evaluation of foals with respiratory distress typically includes arterial blood gas, blood tests, chest x-rays, and culture of samples for identification of bacteria or viruses. The arterial blood gas findings measure the severity of respiratory impairment and are used to monitor the foal's response to therapy.

Because the cause of bronchointerstitial pneumonia is unknown, treatment is directed toward relieving the signs. Treatment includes anti-inflammatory drugs to control fever and inflammation, antibiotics, drugs to dilate constricted airways, supplemental oxygen, and supportive care. Anti-inflammatory therapy with corticosteroids appears to improve survival. When directed by your veterinarian, measures such as an alcohol bath, an air-conditioned stall, and/or a fan are used in conjunction with nonsteroidal anti-inflammatory drugs to help control fever. Additional supportive therapy includes provision of a clean, comfortable environment, highly palatable, dust-free feeds, and medications to prevent ulcer.

Although the death rate is high, affected foals that receive aggressive medical care have a reasonably favorable outlook for survival (70%). However, some foals will have longterm lung damage that may affect performance.

Last full review/revision July 2011 by Bonnie R. Rush, DVM, MS, DACVIM; Neil W. Dyer, DVM, MS, DACVP; Joe Hauptman, DVM, MS, DACVS; Ned F. Kuehn, DVM, MS, DACVIM; Stuart M. Taylor, PhD, BVMS, MRCVS, DECVP; Wendy E. Vaala, VMD, DACVIM; Maureen H. Milne, BVMS, MVM, DCHP, MRCVS