Respiratory Disease in Foals
By Wendy Vaala, VMD, dipACVIM
Intervet, Inc.
Most young foals experience some form of respiratory disease before they are weaned at 4 to 6 months of age. Possible causes of lung disease in the age foal include bacterial infection, viral infection, parasite migration through the lings and reactive airway disease as a result of some environmental irritant. It is important to monitor any young foal with a cough or nasal discharge to determine how serious the condition may be and what therapy is indicated. The presence of a fever is usually indicative of infection in contrast to parasite migration or environmental allergies.
The most common signs of respiratory disease are cough, nasal discharge, increased breathing rate (greater than 30 breaths/min at rest) and fever (temp above 101.5 F.) Foals suffering from severe lung disease often exhibit increased work of breathing, nostril flaring and exercise intolerance. Auscultation of the lungs with a stethoscope may reveal moist breath sounds described as rattles or crackles. Foals experiencing allergic lung disease often have varying degrees of bronchospasm and airway narrowing that produces a wheezing sound on auscultation.
Two bacteria capable of causing lung abscesses in foals are Streptococcus and Rhodococcus equi. Viral infections may produce a leucopenia which is an abnormally low white cell count. Heavy parasite infections may cause an increase is a specific type of white cells called eosinophils. Chest radiographs are helpful in determining if abscessation is present. Ultrasound can also be used to detect abscesses along the periphery of the lungs. A transtracheal aspirate is performed to obtain tracheal secretions for culture and microscopic examination.
Bacterial Pneumonia
Bacterial lung infection in the older foal is usually acquitted via inhalation. Some of the bacteria associated with pneumonia include Streptococcus, Pasteurella, Actinobacillus, E coli and Rhodococcus equi. Affected foals usually have a fever and elevated cell count. Broad spectrum antibiotic therapy is the cornerstone of treatment, Some foals with pneumonia experience significant bronchoconstriction and benefit from bronchoilator therapy using inhalers and a face mask. Foals with pneumonia also benefit from good barn ventilation.
Rhodococcus equi Pheumonia
Rhodococcus equi (R.equi) causes a severe, often-fatal bronchopneumonia in foals less than 6 months of age. R. equi pneumonia results in substantial economic losses as a result of fatal respiratory infection, expensive, long-term antibiotic therapy with drugs that have the potential for side effects and decreased value of affected animals due to poor growth and unthriftiness. Morbidity rates vary worldwide between 5 and 20% with mortality rates as high as 80%.
R. equi is a bacterium that is a soil inhabitant and is resistant to desiccation and sunlight. Because horse manure provides the simple organic acid substrates on which the organism thrives, there is progressive development of infection in the soil on well-established horse farms. The number of organisms in the soil increases with the number of years a farm has been used to support horses. The highest number of R.equi organisms is usually isolated from the soil immediately surrounding the stable area as opposed to grassy pastures. Warm, dry conditions favor multiplication and aerosolization of the organism. R. equi multiplies actively within the intestines of foals 3 months of age. When pneumonic foals swallow infected sputum, the virulent bacteria are able to multiply within the intestines and are then passed in the manure. Therefore foals with R.equi pneumonia are a major source of pasture contamination. Under favorable conditions, a gram of contaminated soil can contain millions of virulent R.equi. Inhalation of dust laden with R.equi is the primary route of pneumonic infection.
Clinical signs of respiratory desease due to R.equi include elevated breathing rates (often .60 breaths/min), labored breathing, high fevers (T.104 F) and exercise intolerance. Coughing is variable. Other non-specific signs include lethargy, weight loss and unthriftiness. Some affected foals also develop cool, non-painful swelling of multiple joints, colic, weight loss and variable diarrhea.
No vaccine is available. Therefore attention focuses on decreasing the risk of infection among newborn foals, early disease recognition among infected foals, and the use of new antibiotics to treat established disease. Reducing the risk of infection begins with environmental management. Environmental control strategies for enzootic farms include decreasing dust formation on pastures and paddocks, housing foals in well-ventilated areas, rotating pastures, reducing the size of mare-foal bands, irrigating and planting dirt areas with grass, and removing manure frequently from stalls, paddocks, indoor arenas and pastures. Breeding mares earlier in the season to ensure foaling during colder weather may reduce the number of susceptible foals exposed to dry, dusty summer conditions.
The best prevention on farms with an increased incidence of R.equi pneumonia is early administration of hyperimmune R.equi plasma to susceptible foals. On farms with a high incidence of disease, one liter of hyperimmune plasma should be administered to foals aged 1-3 days of age and again 21 days later. The intracellular location of this bacteria limits the efficacy of many antibiotics. The ideal treatment involves administration of rifampin with one of the following: crythromycin or clarithromycin or azithromycin. Occasionally this second group of antibiotics may cause diarrhea in certain foals. Antibiotic therapy must often be continued for a minimum of 4-10 weeks.
Viral Pheumonia
Viral pneumonia often presents as diffuse ling disease that is difficult to detect on thoracic auscultation and even more difficult to diagnose with cultures. Viruses incriminated in neonatal respiratory disease include: equine herpes virus (EHV-1 and 4), equine viral arteritis (EVA), invluenza virus, and adenovirus. The role of EHV-2 and equine rhinovirus in foal respiratory disease remains uncertain.
Fatal adenovirus pneumonia is usually associated with Combined Immunodeficiency (CID) Syndrome in Arabian and par Arabian foals. Equine viral arteritis (EVA) typically produces abortion rather than neonatal infection. Equine herpes virus (EHV-1) is a common cause of late term abortion and/or the birth of weak foals dying of acute lung failure. In older weanlings, less than 5-6 months of age, Equine Herpes Virus 1 and 4 is a more likely cause of viral respiratory disease than is influenza virus, which poses a more significant threat to yearlings and 2 and 3 year olds.
Therapy for suspected viral pneumonia is usually rest and symptomatic therapy to control fever spikes.
Parasitic Pheumonia
Pulmonary migration of the larvae form of Parascaris equorum (Ascarudsm also know as round worms) begins 7-14 days after the foal ingests of infective larvae. Since 10-12 weeks are required before Ascarid larvae mature into egg laying adult round worms in the intestinal lumen, many foals may harbor significant Ascarid burdens while remaining negative for eggs in the feces. A careful review of the foal’s deworming history may help confirm suspicions of underlying Ascarid infection. Intrapulmonary parasite migration can cause lung damage that predisposes to secondary bacterial infection. Some foals with Ascarid infection have a cough and purulent nasal discharge. Tracheal aspirates may contain eosinophils.
Foals can pass eggs in the feces within weeks of initial infection. Ascarid eggs are particularly resistant to environmental stresses and may remain viable for up to a decade! Frequent manure removal from stalls and pastures and composting are effective control measures. Hay and concentrates should be fed from containers and not from the ground to reduce the number of Ascarid eggs ingested.
De-wormers used to treat Ascarid infections include Ivermectin (Zimerctrin), pyrantel (Strongid) and fenbendazole (Panacur, SafeGuard). If a heavy Ascarid load is suspected, the foal is at risk to develop an Ascarid impaction if all the intraluminal adult parasites are killed at once. In this case, it is suggested to pre-treat the foal with laxatives (e.g., tube with mineral oil and water with electrolytes and administer half the recommended dose of fenbendazole (5mg/kg) to provide a more gradual elimination of the parasite. This “partial” treatment should be followed in one week with the full dose of fenbendazole or one of the other anthelmintices administered at the label dose.
Summary
Respiratory disease is inevitable in young foals. To reduce the risk of disease, mares and their foals should be isolated from other adult horses on the farm, especially those animals that come and go from shows. Stalls should be well ventilated and the bedding as dust free as possible. A good de-worming program that incorporates dewormers known to be effective against Ascarids will help reduce parasite induced lung inflammation. A vaccination program designed in conjunction with your veterinarian will help reduce the foal’s susceptibility to certain viral and bacterial diseases.
