Pneumonia in foals is primarily caused by bacterial, viral or parasitic infections. Respiratory diseases are common in young horses, especially in foals between 1 and 6 months of age, who frequently have lower airway infection. Pneumonia is one the main cause of disease and death in foals aged between 32 and 180 days.
One of the major causes of pneumonia in foals aged between 1 and 6 months with most cases occurring before 4 months is Rhodococcus equi bacteria. Inhalation of the soil-borne organism, Rhodococcus equi, can lead to a chronic and severe pneumonia in young horses and people with weak immune system. In addition, ulcerative colitis is a common result to infection in foals, and dissemination from the lung to other body sites is common in either the horse or man. Rhodococcus equi is largely a soil organism but is widespread in the feces of herbivores. Its growth in soil is considerably improved by simple nutrients it obtains from herbivore manure. Studies show that foal management practices, environmental management, and preventative health practices are risk factors for development of Rhodococcus equi pneumonia in foals. Housing foals in stalls with dirt floors may increase the risk for development of Rhodococcus equi pneumonia. Also breeding farms with large acreage, a large number of mares and foals, high foal density, and a population of transient mares and foals are at high risk for foals developing pneumonia caused by the bacteria. The intestinal form of the disease may manifest itself by fever, depression, loss of appetite, weight loss, colic or diarrhea, lameness, nostril flaring and reluctance to move. Heat, pain and severe lameness are characteristics of Rhodococcus equi septic arthritis.
Some horses may be genetically predisposed to Rhodococcus equi pneumonia. Identifying the genetic and biological basis of susceptibility, or perhaps resistance, to this condition in foals is important, because it might lead to the development of diagnostic and therapeutic tools to manage at-risk foals on breeding farms and might shed light on critical host defense mechanisms.1 foals.2
Currently, the treatment of rhodococcosis usually consists of a combined application of macrolides, such as erythromycin, azithromycin or clarithromycin, associated with rifampicin. However, the increasing resistance in bacteria isolates to various antimicrobials, including the macrolides group, has been reported.3 To date, there is no licensed vaccine effective against R. equi pneumonia of foals. In one report, eBeam irradiated R. equi administered enterally produced upper respiratory mucosal immune responses. No evidence of adverse effects was noted among vaccinated
Equine herpes virus 2 (EHV-2). The EHP-2 is ubiquitous in the general equine population. Foals are born free of EHV-2 infection but virtually all acquire the infection during the first months of life. The common signs of EHV-2 infection include uveitis, nasal discharge, pneumonia and colic.
Parasitic pneumonias are not common in horses, especially with today's deworming programs. The typical case involves horses pastured with donkeys infested with the lungworm, Dictyocaulus arnfieldi. In foals, Parascaris equorum is a more common parasite and because its life cycle involves migration through the lung, which can potentially cause signs of respiratory disease. A diagnosis of parasitic pneumonia is often difficult to obtain because it may be difficult to identify larvae in tracheal washes. Parasitic pneumonia could be suspected in foals with a poor deworming history, coughing, nasal discharge and poor response to antimicrobials.
The earlier signs of lower airway infection in foals are probably abnormal lung sounds on auscultation, nasal discharge and/or coughing, coughing when they get up or run, respiratory rate above 30 - 40 breaths per minute, or an increased respiratory effort. Crusting may be found at the nostrils or on the bones where the foal may wipe its nose. Nasal discharge, however, may be absent or undetectable if the lower airway discharge is swallowed.
Foals with lower respiratory tract infection will often cough while forced to take deep breaths, appear restless or distressed because of the rebreathing bag or show a prolonged recovery after the bag is removed. In severe cases, abdominal breathing, nostril flaring, cyanosis and pronounced exercise intolerance will be present. While most foals with early disease continue to be bright and alert, individuals with severe lesions may be depressed or lethargic and anorexic. Some foals with pneumonia will be reluctant to lie down because of increased breathing difficulties and may become very weak.
Foals of 1 to 6 months of age will commonly experience respiratory tract infections, which may not necessarily require therapy. However,when labored breathing, fever, depression or appetite, leukocytosis (an abnormal large increase in the number of white blood cells in the blood during an acute infection) occur, antimicrobial therapy should be initiated without delay. A favorable outcome is expected in most cases of foal pneumonia when an appropriate antimicrobial therapy is initiated early in the course of the disease.