Foal Diseases: Pneumonia

by Dr. Frederick Harper, Extension Horse Specialist and Dennis Geiser, Dept. Large Animal Clinics, University of Tennessee

Foals are more likely to get a respiratory disease in their first six months than any other disease. They are more prone to respiratory diseases than adults. In one study, about one-quarter (22.2 percent) of all foals had a respiratory disease. Pneumonia was responsible for most deaths (16 percent) up to six months of age in one study.

Pneumonia. Foal pneumonia is the major respiratory disease causing economic loss due to death, poor growth and treatment cost. Pneumonia, an inflammation of the lungs, is a common disease in foals of all breeds up to six months of age. A complex disease, pneumonia has many predisposing factors.

In neonates, pneumonia may be due to inadequate lung development, an immature immune system or failure of passive transfer. Failure of passive transfer is when a foal does not acquire adequate immunoglobulin from colostrum to provide ample protection. Such foals are at greater risk of acquiring infection due to common environmental pathogens, especially bacterial infections. In normal foals, there is a rapid and smooth transition from a fluid-filled lung to an air-filled structure. At birth, the respiration rate is 60-90 breaths per minute but declines to 30-45 breaths per minute one to two hours later.

As foals get older, a number of environmental and management factors may predispose them to pneumonia. High ambient temperatures, especially when dry, dusty conditions exist, are related to pneumonia. Foals appear to be less tolerant of extreme temperatures than adult horses. Fluctuations in ambient temperatures with hot days and cool nights are detrimental. Hot weather that demands heat dissipation may stress the respiratory system of foals. Cold weather can also be harmful. Chilling from cold as well as over-protection from cold by excessive use of blankets or reducing ventilation in a barn can also predispose them to pneumonia.

Overcrowding stresses foals and increases the concentration and transmission of microorganisms. Overgrazing pastures results in dusty conditions in dry weather, which can irritate the respiratory tract and compromise respiratory defenses. Most foals probably get pneumonia by breathing dust-borne microorganisms such as Rhodococcus equi (R. equi).

Higher temperatures and humidity promote survival of pathogens in barns. Bedding may be a source of dust and allergens as well as a medium for certain bacteria and fungi. Old, poor-quality bedding should not be used with new-born foals or older foals. Poor stall hygiene with inadequate drainage, improper and irregular mucking and poor ventilation along with high temperatures contributes to the buildup of noxious gases, like ammonia, which can also compromise pulmonary defenses.

Certain handling procedures, especially if foals are handled roughly, such as transportation, showing and weaning can also stress foals. Weaning procedures can result in concentration of foals which aids transmission of disease. Mixing of on-farm mares and foals with visiting ones or show horses increases the probability of foals becoming infected.

Some foals may appear normal with an intermittent cough and a mild nasal discharge of pus, while others may have a high fever, a lack of appetite (go off feed), an excessive purulent nasal discharge, severe respiratory distress, depression and are cyanosis (bluish mucous membranes due to lack of oxygen in the blood). Resting respiratory rates higher than 40 breaths per minute in older foals and weanlings are abnormal under most situations. Resting rates greater than 30 breaths per minute in the cool, early mornings are also cause for concern. Rippling of the rib cage is often an early, subtle sign. Severely affected foals also exhibit flared nostrils and increased efforts of abdominal breathing. Such foals are exercise-intolerant, have an anxious expression, are reluctant to lie down, nurse or even move. If stressed or forced to exercise, these foals may develop signs of severe respiratory distress, cyanosis and disorientation. Foals may be well grown and in good flesh; however, if pneumonia becomes chronic, they may lose weight and be stunted.

Coughing is an important symptom, but it may not be present. In early cases, foals normally cough early in the morning when disturbed or restrained after lying down or after brief exercise. The cough may be intermittent, shallow, deep or hacking. Most foals with pneumonia cough when a rebreathing bag is placed over their nostrils, while normal foals rarely cough when a breathing bag is applied.

Pus coming from both nostrils is frequently observed, but is variable. In some foals, all that is noted is a dry crusting at the opening of the nostrils or dried exudate on the front of the foal’s cannon bones where it wipes its nose. Some foals, especially those with R. equi, normally do not have a nasal discharge.

Foals may have a fever of 102-104 degrees F., that may range as high as 105 degrees F. The early subtle signs of foal pneumonia may be missed by an owner; thus, allowing the condition to progress to advance stages before veterinary assistance is sought.

VIRAL PNEUMONIA.

Viral pneumonia is more commonly observed in neonatal foals and is not usually seen in two- to five-month-old foals. Viruses such as influenza, equine herpesvirus-1 (EHV-1), EHV-4, EHV-2, rhinovirus, adenovirus and possibly others may be important predisposing factors. Viral organisms damage the pulmonary defenses allowing opportunistic bacteria to cause pneumonia.

Foals can be infected by viruses in utero or soon after birth. Those infected in utero can be aborted, born weak or be normal at birth. EHV-1 (rhinopneumonitis) generally causes abortion in late pregnancy but can infect the fetus in utero without an abortion. These foals often appear normal at birth, nurse adequate colostrum, but become feeble and lethargic as respiratory difficulty develops in the first week of life. Foals born with EHV-1 are likely to die in the first week.

Clinical signs along with blood work, x-rays of the thoracic region and isolation of the virus are important diagnostic tools in determining viral pneumonia in foals.

TREATMENT is aimed at preventing secondary bacterial infection, so broad-spectrum antibiotics and general supportive nursing care are employed, including oxygen support and antipyretics.

PREVENTION. Pregnant mares should be immunized for EHV-1 (rhinopneumonitis) in the fifth, seventh and ninth months of pregnancy and isolated from exposure to horses coming onto the farm. Foaling stalls should be clean and adequately ventilated. The naval cord of the newborn foal should be properly treated. Adequate intake of good-quality colostrum decreases the incidence and severity of viral respiratory infections.

BACTERIAL PNEUMONIA.

Bacterial pneumonia may affect newborn foals, but more commonly occurs in foals between one to six months of age, since most foals exhibit clinical signs before four months of age.

Increased vulnerability may be due to a decline in the level of maternal antibodies from colostrum, and the exposure to common pathogens before the foal’s natural immunity develops.

As noted, respiratory viral infections predispose the foal’s lungs to secondary bacterial invasion. Other factors contributing to the foal’s susceptibility to bacterial invasion are stress, poor ventilation with increased ammonia concentration and migration of internal parasite larva. Internal parasites cause unthriftiness and, in the case of ascarid larvae, may damage the lungs causing a direct mild pneumonia as they migrate through the lungs.

Mixed bacteria infections are common. Opportunistic bacteria which normally inhabit the upper respiratory tract, the gastrointestinal tract or are environmental contaminants grow on the epithelium tissue of the respiratory tract when the natural defense mechanisms are overwhelmed by a massive challenge, especially when the foals are compromised by viral infections, stress, dust or noxious gases. The most common bacteria resulting in foal pneumonia are Streptococcus zooepidemicus, Streptococcus equi and Rhodococcus equi (R. equi).

Streptococcus zooepidemicus normally is found in the upper airways of most horses. It is a secondary invader of the lower airways of stressed horses. Streptococcus equi causes strangles. Normally, foals infected with Streptococcus equi have abscesses of the lymph nodes. However, Streptococcus equi can also cause pneumonia in foals.

Rhodococcus equi. R. equi causes the most devastating and fatal form of pneumonia in foals. Some data indicates that foals which have had R. equi pneumonia may not perform as well as those which do not have the disease. R. equi occurs naturally in the digestive tract of horses, including foals. It can survive in equine fecal contaminated soil for 12 months. It also grows rapidly in manure pats, especially in high temperatures with reduced moisture conditions such as occurs in late summer.

The incubation period for R. equi ranges from 10 to 21 days. The primary route of infection is inhaling R. equi on particles of dust.

R. equi causes multiple abscesses of varying sizes in the lungs and pulmonary lymph nodes but also infects the digestive tract, bone and joints. Most cases occur in the summer due to a decrease in passive immunity acquired from the dam’s colostrum and before the foal’s immune system is functional. This is also the time of optimal environmental conditions for R. equi.

Clinical signs may not appear until several weeks after the foal is infected due to the nature of the bacterial growth in the respiratory tract. By the time clinical signs appear, pulmonary lesions are advanced, and the R. equi organism is difficult to treat. Without treatment, foals appear unthrifty and may die after several weeks or months. R. equi foals have a nonproductive cough, no nasal discharge and a marked delay of recovery from using a rebreathing bag.

TREATMENT. For proper treatment of foal pneumonia, contact your equine veterinarian. Treatment can be costly. Rest foals that have pneumonia.

PREVENTION. Due to its nature, it is best to prevent R. equi on farms where it occurs frequently than to treat the disease. A prevention strategy includes: 1) decreasing the size of the challenge; 2) early recognition of the disease; and 3) passive immunization.

Overgrazing and overcrowding of mare and foal should be avoided to reduce contact with dust and manure. Mares and foals should be dispersed in small herds on grass pastures or paddocks rather than be in large herds. Pasture rotation helps prevent dusty areas, especially near water and shade areas. Adequate shade in pastures should be provided in hot weather. Outside mares and foals or show horses should not be mixed with farm mares and foals. Barns must be properly ventilated and relatively dust-free while stalls should be properly bedded and mucked-out. Reduce dust in the foals’s environment by sprinkling dusty areas such as drylots, walkways, etc. Manure removal from pastures, paddocks, stalls, pens, aisles and walkways is important.

Foal mares early in the year, so foals will be older and more immune to challenges of R. equi in the summer when the bacteria is more opportunistic. Mares and foals should be immunized against influenza and rhinopneumonitis. An effective, routine deworming program should be conducted especially for ascarids (roundworms). Foals must obtain adequate colostrum intake in the first 12 hours after birth and be checked for failure of passive transfer. Their umbilical stumps must be properly treated. Limit transportation of foals, but when necessary, use extreme caution. The inside of trailers can be intensely hot in the summer, especially when parked. Make sure trailers are properly and adequately ventilated.

Observe foals closely for early signs of pneumonia. Early detection and treatment are critical if treatment is to be effective and cost efficient. Early detection of R. equi is not easy since clinical signs occur after the disease is well advanced. A twice-weekly physical examination including checking lung sounds is helpful. Foals can be tested from birth to five months of age at two-week intervals with agar-gel immunodiffusion (AGID) assay. This test should not be used as a definite diagnosis or to treat foals, but it is a screening test. Positive foals should be observed carefully and often including daily temperature, periodic physical examination, frequent plasma fibrinogen test and ultrasound if possible. Detection of abscesses of varying size with ultrasound has proven to be rapid, practical and a sensitive test for early detection of R. equi pneumonia.

The administration of hyperimmune (HI) plasma to foals is the most cost-effective preventative technique for farms with high incidence of R. equi pneumonia. A pint of hyperimmune plasma is given at 1 to 2 weeks of age to foals born in the R. equi season or at the beginning of the warm season. It may be advisable to give a second dose 25 to 30 days of age. This practice had reduced R. equi pneumonia on problem farms, while vaccination of mares and/or the foals for R. equi has not been effective.