Young foals are at risk for developing severe pneumonia due to infection by bacteria Rhodococcus equi, a soil-based bacterial organism that used to be known as Corynebacterium equi in the past. Rhodococcus equi pneumonia, sometimes known as rattles, represents a potentially devastating disease that necessitates expensive and time-consuming treatment. Now that the treatment is known, however, Rhodococcus equi rarely causes death in foals unless the infection is recognized too late.
This organism is found almost everywhere in the environment, but it is especially concentrated in grounds where horses and cattle have been intensively accommodated. Rhodococcus equi is passed onto the ground in feces by infected foals and adults where it thrives and multiplies. Infection by this organism can cause severe disease in foals aged between 6 to 24 weeks. Rhodococcal pneumonia tends to occur on a seasonal basis in foals; most foals are infected during the warmer months, April through September. As a rule, disease associated with Rhodococcus equi does not effect adult horses.
Of particular concern for horse owners is the fact that symptoms of disease in infected foals are too often not recognized until the disease is advanced. The success of treatment for this condition is highly dependent on early recognition of the clinical signs.
Although a majority of young foals are exposed to Rhodococcus equi in their environment, healthy foals are able to develop good immunity and fight off the infection. In human beings, infection by Rhodococcus equi is almost always recognized as a complication of the immunocompromise associated with HIV infection. Similarly, in foals, Rhodococcus equi is an opportunist that is able to cause infection if the foal's immune system is below par.
Foals are also more likely to become infected if they are exposed to relatively large numbers of Rhodococcus equi in the environment, such as might occur on farms on which large numbers of foals have been raised over many years, which leads to build-up of Rhodococcus equi.
Furthermore, it is now being recognized that certain virulent strains of Rhodococcus equi are more likely to cause infection than nonvirulent strains, and that foals exposed to these virulent strains of Rhodococcus equi are at substantially greater risk of developing disease.
Rhodococcus equi causes infection of the foal's lungs when it gains access through the airways on contaminated dust particles in inspired air. Inspired air is most likely to be contaminated with Rhodococcus equi in hot, dry and dusty conditions. It has been well documented that foals accommodated on dusty dry lots in hot weather are at particular risk for developing Rhodococcus equi pneumonia. Infection is much less likely in foals raised on grassy paddocks in temperate climates.
Reduced Immunity in Foals
Newborn foals acquire their immunity (antibodies) in the mare's first milk or colostrum. Colostral antibodies are able to fight off infection until the foal begins to produce antibodies himself. A natural phase of waning immunity occurs between the ages of 4 and 8 weeks, during which time the foal's total antibody levels are relatively low.
This temporary phase of low immunity is the period in which the antibodies obtained from the mare are being used up and yet the foal's own antibody production has not caught up. During this phase, the foal is at particular risk for infection by opportunistic bacterial pathogens, such as Rhodococcus equi. Foals that receive marginal quantities of antibodies from the mare are at greater risk than foals receiving a plentiful antibody supply.
Other factors that may adversely effect the foal's immunity include the numerous stresses to which young foals are exposed early in their life. Typical and common examples of stressful factors that adversely effect immunity and increase risk of Rhodococcus equi pneumonia include:
Ensuring that the foal obtains a plentiful supply of antibodies against Rhodococcus equi can prevent pneumonia associated with this infection. These antibodies can be injected by the veterinarian if necessary.
Although foals are challenged by and potentially infected with Rhodococcus equi during their first four weeks of life, signs of disease are not usually recognized until at least six weeks of age. Most affected foals are presented to veterinarians between 6 and 12 weeks of age for respiratory symptoms, although other symptoms could prompt evaluation in a foal with Rhodococcus equi.
Foals usually have some evidence of infection of the respiratory system including fever, increased respiratory rate, exercise intolerance, nasal discharge and coughing. However, it should be noted that, with this disease, both nasal discharge and coughing are often mild and infrequent. Some affected foals are mildly diseased, showing subtle signs such as a slightly reduced growth rate, slight lethargy and depression and a poor quality hair coat. However, most foals are in excellent body condition and appear to thrive.
Many affected foals develop clattering breath sounds, due to the presence of inflammatory material in their airway, hence the name rattles. In many respects, Rhodococcus equi pneumonia is similar to tuberculosis in people, which used to be known as consumption. Consumptive lung disease causes insidious, slowly-progressive, extensive and devastating destruction of the lungs over a long period of time. As the lungs gradually become more affected with this disease process, the foal's ability to obtain oxygen from the air is eventually diminished.
Some infected foals also develop remote immune-mediated signs such as joint-distention, limb swelling and uveitis (inflammation in the eyes). In some of these cases, a veterinarian has been called to examine and treat a young foal for an eye problem or to make a diagnosis of joint distention. However, following a complete physical examination, the veterinarian will recognize that the major problem is in the lungs.
It is particularly interesting that affected foals are commonly presented to veterinarians in a state of severe, life-threatening respiratory distress but that the owners recall that the foal had been unremarkable on the preceding day. Sudden-onset respiratory distress in the face of progressive consumption of the lung occurs when the foal's remarkable compensatory mechanisms have been finally depleted. Often, these foals collapse if they are exercised or if the ambient temperature and humidity are very high. Infection by Rhodococcus equi does sometimes also cause diarrhea and, less commonly, joint and bone infections (osteomyelitis).
Diagnosis of Rhodococcus equi pneumonia is initially based on recognition of the typical signs, especially in particular geographical locations in which rhodococcal disease has been previously confirmed. Particularly, common initial findings include:
Abnormal breath sounds are variable. Although the affected foal may be breathing hard, breath sounds may be difficult to detect in areas of the lung field which have been affected. In other areas of the lung, breath sounds may be harsh or even musical. Your veterinarian may detect the presence of abscesses at the lung surface by tapping the chest and listening for the degree of resonance; tapping the chest over normal lung elicits a resonant (booming) sound and tapping the chest over an abscess elicits a dull thud.
The presence of inflammatory fluids in the airway often causes affected foals to ventilate their lungs with noisy breaths. For this reason, the disease is commonly known as the "rattles," but it should be noted that other infectious lung diseases may also cause similar sounds in the absence of Rhodococcus equi infection.
Further diagnostic tests that support the diagnosis of pneumonia include routine blood tests (white blood cell count and plasma fibrinogen are elevated). Ultrasound scanning of the chest can easily be used to identify abscesses at the surface of the lung. Chest radiographs are valuable in determining the extent and severity of pneumonia.
The specific diagnosis of Rhodococcus equi pneumonia depends on positive isolation of the bacterial pathogen in fluids retrieved from the airway. Airway fluid analysis is routinely undertaken by veterinarians and may be accomplished with minimal difficulty using a transtracheal wash technique. In addition to checking for the presence of Rhodococcus equi in airway fluid, the bacteriological analysis includes investigation for other bacterial organisms that might be operating at the same time as Rhodococcus equi and an evaluation of the antimicrobial susceptibility of all cultured microorganisms. Antimicrobial sensitivity testing enables your veterinarian to select the most effective antibiotic for a given infection.
Although blood testing for antibodies against Rhodococcus equi has become popular recently, a positive blood test result merely implies that the foal has been exposed to Rhodococcus equi and should not necessarily be interpreted as proof of the infective disease process itself.
Treatment of affected foals is most effective if treatment can be started as soon as possible in the course of the infection, which is often advanced at the time a problem is recognized by the foal's owner. Foals affected with pneumonia should be isolated from other foals and the communal environment during treatment because their manure contains very high levels of virulent Rhodococcus equi. Affected foals represent a very important source of Rhodococcus equi contamination for the environment.
It is also important that affected foals should be brought into a well-ventilated, shaded, cool barn environment with minimal excitement. The most appropriate treatment for Rhodococcus equi pneumonia is usually a combination of two antimicrobials, erythromycin and rifampin administered orally. It is important to ensure that erythromycin estolate is the type of erythromycin selected.
The specific dose and duration of this treatment depends to some extent on the severity of the problem in a given foal. In rare cases (less than 10 percent), other antibiotics may be needed because either the isolated variant of Rhodococcus equi is resistant to erythromycin or rifampin or other bacterial pathogens are involved at the same time. Foals must be protected from direct sunlight and high temperatures during treatment because there is risk of death from hyperthermia in affected foals.
In those foals presented with severe respiratory distress, it is sometimes necessary to administer oxygen through a tube passed into the nasal passages. Other treatments that may be considered by your veterinarian include the use of nonsteroidal anti-inflammatory drugs to reduce airway swelling and bronchodilators to increase the airway diameter.
Newer treatments are being evaluated that affect the foal's immune system in such a manner as to increase the defense mechanism's ability to recognize and kill Rhodococcus equi.
Veterinarians usually treat foals affected with Rhodococcus equi pneumonia for two weeks following cessation of clinical signs, which typically takes 6 to 8 weeks. Monitoring the foal's clinical signs, which include respiratory rate and effort at rest, rectal temperature, presence of cough and nasal discharge, presence of abnormal breath sounds detected by the stethoscope, is very important during the treatment period. Clinical improvement can also be evaluated by follow-up radiographic and ultrasonographic examinations of the chest. Repeated blood tests for the white blood cell count and plasma fibrinogen concentration are also used routinely by veterinarians to monitor progress.
Preventing Rhodococcus equi pneumonia on farms where it has previously caused disease and fatalities in foals represents one of the important tasks with which veterinarians are faced. Education of horse owners with respect to this important disease is a crucial first step; it is important to emphasize that early recognition of the problem in infected foals will lead to a higher probability for successful treatment.
On specific farms where this disease has already been recognized as a problem, twice-weekly physical examinations of foals at risk is a useful strategy for the early identification of disease and has been shown to be an effective method to increase the success of treatment. The use of regular ultrasonographic examinations of the foals' lungs for abscess development is useful for identification of infected foals prior to development of any clinical signs.
Although blood tests for antibodies against Rhodococcus equi are available, interpretation of these tests does not differentiate infected foals from exposed foals and should not be relied on as the sole method of identifying diseased foals; positive blood test should certainly increase awareness of risk of disease.
Strategies that are directed at reducing the potential level of exposure of foals to Rhodococcus equi include relocating to premises on which the ground has not become severely contaminated with the organism. This option may not be practical, but it is important to recognize that foals raised on contaminated, dusty lots that are devoid of grass are at particular risk compared with foals raised on grassy paddocks. Advice should be sought from county agronomy specialists to encourage grass growth, establishment and maintenance on at-risk premises. Feces should be regarded as a source of Rhodococcus equi contamination in the environment, especially from infected foals, and should be quickly removed and composted.
Although a specific vaccine for Rhodococcus equi has not yet been approved, several immunological strategies are recommended by veterinarians in order to reduce the risk of infection.
The prevention of Rhodococcus equi pneumonia has been significantly impacted in recent years by the availability of a hyper-immune plasma product containing concentrated antibodies against the organism. This plasma product is not useful after infection has occurred, but there is much to support its use in the prevention of pneumonia when administered to at-risk foals during the first two weeks of life.
Your veterinarian must give this plasma product into the vein at one to two weeks of life. It may not be necessary to use the plasma in those foals born early in the season (prior to April) when the risk of infection is lower because of the weather; however, those early foals that are maintained in the same at-risk environment into the warmer weather should probably be protected by the plasma treatment at the beginning of the warmer weather period.
Although not currently available in the United States, a vaccine has been shown to be useful in Argentina; the mare is vaccinated prior to foaling with a view to stimulating antibody production against Rhodococcus equi for natural incorporation into the colostrum. The foal's blood is subsequently tested to ensure that the vaccine was effective at producing antibodies in the colostrum. Foals that failed to receive antibodies through this protocol then receive the hyper-immune plasma product described above.
There is some evidence that rhodococcal infection may be predisposed by prior damage caused by certain viruses. To that end, it is also recommended that mares should be thoroughly immunized against routine respiratory pathogens (influenza, herpes viruses, etc.) on a regular basis to ensure that the foal receives colostral antibodies against those diseases as well. It is important to emphasize that the adequacy of all foals' antibody complements received through colostrum should be routinely checked by a standard antibody blood test at 18 to 24 hours of age. Any foals receiving insufficient antibodies should be identified and treated with a plasma transfusion.
Foals treated early in the course of disease are more likely to respond favorably compared with foals in which the appropriate treatment is delayed. The prognosis also depends on the extent to which the lung fields have been "consumed." Foals presented in a state of extremely severe respiratory distress in which the radiographic appearance of the lung fields indicates that most of the lung has been infected often fail to respond to treatment. Those foals that do recover from the infection can grow up to be healthy, normal adults. In some studies, the fatality rate for Rhodococcus equi pneumonia exceeds 40 percent.
With respect to race horses, surviving Rhodococcus equi pneumonia has been shown to reduce the animal's chance of becoming a racehorse. Racehorses that were affected with Rhodococcus equi pneumonia as foals are less likely to perform successfully compared with non-infected peers.
Approximately two-thirds of foals infected with Rhodococcus equi pneumonia may develop other (extrapulmonary) complications that may affect both the method and the outcome of treatment. During treatment for Rhodococcus equi pneumonia, veterinarians are vigilant for these diverse and sometimes challenging complications. Approximately 50 percent of infected foals develop some degree of intestinal disease (enterocolitis) that may cause diarrhea. Other possible complications include: joint swelling, joint infection and lameness, bone infection, lymph node abscess development, inflammation of the eyes, hyperthermia, peritonitis, anemia, abnormal hair coat, and infection of the guttural pouches.