Bacterial diseases affecting the respiratory tract of horses

Introduction: Infections caused by diverse bacterial pathogens represent a significant health threat to equine populations, frequently resulting in considerable economic repercussions. Horses of all ages and breeds can be affected, with some diseases posing risks to humans as well. In equine infections, bacteria often act as secondary pathogens in the upper respiratory tract following primary viral infections; however, when bacterial infection occurs, it can lead to significant disease. Horses are susceptible to a range of both bacterial and viral pathogens. Bacterial diseases of the respiratory tract are clinically significant and warrant considerable concern. Respiratory tract infections can range from mild to severe pneumonia, significantly impacting horse’s performance and overall well-being. By thoroughly understanding the most prevalent bacterial pathogens in horses like Streptococcus equi subsp. equi, Echerichia coli, Streptococcus equi subsp. zooepidemicus, Pasteurella spp., Actinobacillus equuli, Pseudomonas aeurogenosa, Bacteroides and Fusobacterium: their clinical presentations, transmission mechanisms, and effective prevention methods, equine caretakers can implement robust management and control protocols. These measures are essential for protecting equine health and minimizing economic losses caused by disease. The following are key bacterial diseases that affect the respiratory tract of horses:

1. Strangles:  is a highly contagious and reportable disease of the upper respiratory tract of horses caused by Streptococcus equi subsp equi: a Gram positive bacterium. Clinical form of the disease is caused by the organism in horses, donkeys and mules. Animals in any age may contract the disease, although young and old animals are more susceptible. Transmission primarily occurs through direct contact with exudates such as nasal discharge or pus from infected horses or indirectly through fomites/ contaminated equipment like water, feed, or grooming tools. The bacteria typically enter the body of the animal through the nasal passages or mouth and then infect the tonsils and lymph nodes. In addition, overcrowding, common feeding and watering equipment predispose the animals to disease. Disease outbreaks can occur any time of the year, but mostly occurs in cold weather. Symptoms usually manifest between three and eight days post-infection. Usually within 3 days after initial exposure to the bacteria, first sign of the disease is high fever up to 106 °F. This is followed by thick mucoid to mucopurulent nasal discharge, swollen lymph nodes (especially around the jaw and neck) leading to abscess formation that may or may not be draining.  Infected animals are lethargic, dull and depressed. There is lack of appetite with difficulty in swallowing. Infected animals’ head and neck lymph nodes are affected, resulting in abscess formation that can compress airways, causing difficulty in breathing -hence the name “strangles”. In chronic form of strangles known as “bastard strangles”, abscess formation may occur in other areas of the body, such as the abdomen, lungs, and brain. When this occurs, animals can show signs of colic, neurological abnormalities like head pressing etc. If the abscesses rupture, it can have fatal consequences. Preliminary diagnosis relies on observation of clinical symptoms that may be indicative of strangles, but for definitive diagnosis, isolation of the organism or PCR on samples of nasal swabs, guttural pouch lavage, or abscess material can be carried out. For prevention and control, as the disease is highly contagious, horses suspected of the disease should be immediately quarantined. Nose to nose contact of animals in the barn should be avoided. Vaccination, appropriate cleaning/disinfection of stalls and water troughs, isolating affected and freshly arrived animals, and proper wound care can help avoid the disease.

2. Rhodococcal pneumonia (in foals): is caused by gram-positive, facultative intracellular pathogen Rhodococcus equi and is a major source of illness in young foals. Disease susceptibility is more in foals, particularly those aged between 2 weeks to 6 months. Clinical disease occurs rarely in horses greater than 8 months of age. The organism is transmitted through inhalation or ingestion of contaminated dust or soil, particularly in stables with poor ventilation. Though the prevalence of disease varies from farm to farm but it has been suggested that the risk of infection is higher in large, high density breeding farms with frequent movement of animals on and off site. Clinical indications in the early course of disease are mild and non- specific but become more evident as the pneumonia progresses. Early signs include lethargy, mild fever, decreased exercise tolerance, slight cough and increased respiratory rate. As pneumonia progresses, symptoms include cough, nasal discharge, fever, and difficulty in breathing. Purulent nasal discharge is less common. It can also cause abscesses in the lungs. Extra pulmonary clinical symptoms of R. equi infection may be observed as a result of pulmonary illness or independently and they include pyrexia, depression, loss of appetite, weight loss, colic, and diarrhoea. Other most commonly observed extra pulmonary manifestations include osteomyelitis, polysynovitis (joint pain and swelling), and uveitis. Infected foals can shed the bacteria in their faeces causing the pathogen to persist endemically at the breeding farm. Infections by R. equi often resolve, but the prognosis worsens markedly when abdominal abscesses form in severely ill foals. Diagnosis relies on clinical signs, culture of the organism and identification by PCR from trans- tracheal wash samples. There are no commercially available vaccines that can provide complete protection against R. equi infections, so disease can be prevented by maintaining clean stables, avoiding overcrowding of foals in stables, improved ventilation, and reduced dust exposure. Dirt paddocks should be avoided. Any foal that appears unwell should be separated from others, and strict manure disposal protocols should be applied.

3. Pleuropneumonia: also known as shipping fever is a potentially fatal infection of the lungs and the pleura (space surrounding the lungs) caused most commonly byStreptococcus equi subsp. zooepidemicus. Other bacterial spp. involved include Escherichia coli, Klebsiella pneumoniae,  Actinobacillus, Pasteurella, Bacteroides, Fusobacterium spp. and Peptostreptococcus spp. Anaerobic bacteria are commonly found in chronicor severe cases and may worsen the prognosis. Pleuropneumonia typically develops when an initial condition weakens the lungs’ natural defenses, permitting a secondary bacterial invasion. Long distance transport, recent viral infections and strenuous exercises are common risk factors that predispose the animals to disease. Spread occurs by respiratory droplets, especially in crowded or poorly ventilated stables. Clinical signs of pleuropneumonia can be seen within seven days of transport but can develop in as less as 24 hrs. Symptoms include coughing, nasal discharge, elevated temperature, dyspnoea, anorexia, depression, and lethargy. Affected animals may exhibit exercise intolerance, diminished performance, coughing, or bilateral nasal discharge observed during or immediately following physical exertion. Clinical signs may not be obvious in early stages, but as the disease progresses they may include fever, anorexia, depression, bilateral nasal discharge that is typically mucopurulent though it may occasionally present as haemorrhagic, cough, weight loss, abnormally rapid and shallow breathing, and respiratory distress. Halitosis and an accompanying malodorous nasal discharge may be present. Pleuropneumonia is a life‑threatening illness that carries a very high fatality rate in its most severe forms. Diagnosis is based on clinical signs, culture of the organisms and PCR. Preventive measures include isolating sick horses, maintaining good stable hygiene, and improved airflow in stables.

Effective prevention and control of bacterial respiratory diseases are vital for ensuring the health of individual animals and safeguarding the overall productivity and wellbeing of the herd. This can be achieved through the implementation of measures including strict hygiene maintenance and dryness in barns, feeding areas, and water sources. Ensure that all horses receive timely vaccinations, particularly against diseases such as strangles. Conduct regular health assessments and promptly isolate any animals exhibiting signs of illness to prevent the spread of infectious pathogens. As diseases can lead to both direct and indirect economic losses, it is advisable to implement biosecurity measures to minimize disease transmission and promote overall animal health.