Pseudomonas Infection in Horses

Pseudomonas is a bacteria family that can cause many types of illnesses, from loss of sight to reproductive problems to death. Members of the genus Pseudomonas are commonly found in aquatic habitats (wet areas) and in the soil. Pseudomonas aeruginosa is the species of Pseudomonas most commonly associated with infection in animals and people. Pseudomonas aeruginosa is often a part of the normal flora (normal population of bacteria that is there all the time) of the skin, mucous membranes, and intestinal contents of many healthy animals, hence its omnipresence in the environment of animals (and people).

M.A. Ball, DVM

Pseudomonas in the eye.

Pseudomonas is of the Gram negative variety. Remember that the "Gram" status of most bacteria is either Gram positive or negative. This refers to how a particular bacterial genus stains with the special Gram stain--Gram positive is purple and Gram negative is red. This is an important characteristic of bacteria that aids in their identification.

Many of these Pseudomonas infections are considered to be opportunistic. Opportunistic infection means that the organism has a low ability to cause primary disease (is said to be of low "virulence") and takes advantage of an "opportunity" to cause infection. These types of infections are commonly associated with predisposing factors such as wounds, corneal injury in the eye, wind-sucking (pneumovagina), immunosuppressive therapy (such as treatment with high doses of steroids) or diseases, prolonged administration of broad-spectrum antibiotics, poor sanitation, and burns. (Pseudomonas is a common infection encountered by people suffering from burns.)

Pseudomonas infections are classic nosocomial infections. Nosocomial, by definition, means infections that originate in a hospital. Most hospitalized patients, human and animal alike, have some degree of immune system compromise from their primary illness and are therefore more susceptible to infection. Pseudomonas species can be very resistant to disinfection and might develop significant antibiotic resistance; this appears to be a greater problem in human hospitals than equine hospitals, with respect to Pseudomonas infections.

Pseudomonas aeruginosa infections are characterized by a classic greenish discharge or pus and a "grape-like" odor--it is often called the "bacteria of green pus." A grape-like sweet odor to the discharge collected in a wound dressing can be a potential giveaway to a Pseudomonas infection.

One of the more common locations for a Pseudomonas infection in horses is in the cornea (the clear outer surface of the eye). Bacterial corneal ulcers often develop after trauma to the outer surface of the cornea--the epithelium. The cornea is constructed somewhat like an onion; it is composed of multiple layers. At its thickest point, the equine cornea is approximately 0.5 millimeters (slightly less that the thickness of your fingernail). The outer protective layer is called the epithelium, the middle layer is often just called "stroma," and the innermost layer (the one in contact with the fluid within the eye) is called the endothelium. It is the corneal epithelium that creates the protective barrier against bacteria that otherwise can cause infection.

It has been hypothesized that dirt and sand flying into the eye and traumatizing the cornea in racehorses can damage the protective epithelium. As mentioned above, Pseudomonas species are normal bacteria found in soil and, in addition, several reports have demonstrated that Pseudomonas species are part of the normal flora of the eye. The surface of the cornea and the surrounding ocular tissue are not sterile--there is a normal flora (or population) of both bacteria and fungi that inhabit these areas.

Pseudomonas aeruginosa can be a formidable pathogen once an infection takes hold in the cornea. A common presentation for Pseudomonas ulcers is that of a rapidly progressing ulcer that appears to be gelatinous or "melting" in nature. This bacteria can secrete "toxins" and "enzymes" that cause tissue destruction and create the "melting" appearance of the cornea as the tissue is broken down by them. Pseudomonas ulcers can progress so rapidly that sight-threatening damage can occur in as little as 24-48 hours. (Remember how thin the cornea is?)

So, the typical characteristics of a corneal ulcer caused by Pseudomonas aeruginosa are a rapidly progressing ulcer with a "melting" appearance and potentially a greenish hue to the infection; these eyes are also usually very painful.

It is important to obtain a sample from the ulcer for culture, as Pseudomonas aeruginosa can have significant antibiotic resistance, and it is not the only bacteria that can have this presentation.

In the Sept. 1, 1996, issue of the Journal of the American Veterinary Medical Association, Corinne Sweeney, DVM, Diplomate ACVIM, and Nita Irby, DVM, Diplomate American College of Veterinary Ophthalmology, reported on 70 cases of Pseudomonas species infected corneal ulcers seen at the George D. Widener Hospital for Large Animals, University of Pennsylvania.

In that report, the authors noted that aggressive topical treatment was successful in 57 (81%) of the 70 ulcers. Of the remaining 13 ulcers (eyes), eight eyes were removed (enucleation) due to treatment failure, and five eyes were enucleated at the owner's request before medical treatment could be fully evaluated. The average duration of medical treatment after admission until complete resolution of the infection and regeneration of the cornea was 29 days (the range was 6-70 days). The "melting" appearance, often characteristic of Pseudomonas, was noted in 54 (77%) of the 70 ulcers.

The report evaluated the susceptibility of Pseudomonas species isolated from 67 of the 70 ulcers to a number of commonly used ophthalmic antibiotics. The three antibiotics with the best activity were tobramycin, gentomicin, and amikacin with 87%, 85%, and 93% of the 67 isolates being sensitive, respectively. Tobramycin, gentomicin, and amikacin are all related and are members of the aminoglycoside antibiotic family. All other antibiotics tested were determined to be poorly effective.

How Common Is It?

So, now that some of the specifics regarding ocular infection with Pseudomonas species have been discussed, how common is it? As mentioned before, corneal infection generally is preceded by some type of ocular trauma. This is true for most corneal infections, not just Pseudomonas species. Dirt and sand kicked up in the faces of the trailing horses has been implicated for racehorses. Other potential sources for ocular trauma include riding through thick woods or heavy brush, trailering, and highly hung hay bags or highly mounted hay racks where chaff can easily fall into the eyes.

Another study by Cecil Moore, DVM, MS, from the College of Veterinary Medicine, University of Missouri, in the June, 1988, issue of the American Journal of Veterinary Research, reported on the causative organism in 63 cases of corneal infection. Of the 63 cases, 14 (22%) were confirmed to be caused by a Pseudomonas species.

An earlier study, in a 1983 Journal of the American Veterinary Medical Association, noted that 17 of 123 cases (14%) of corneal ulcers were caused by infection with a Pseudomonas species. The two most common bacteria involved with infection of the cornea in this study were Streptococcus ("strep") species (44%) or Staphylococcus ("staph") species (24%).

The bottom line here is that infection of the cornea (development of a corneal ulcer) can be a rapidly progressing and sight-threatening problem. Depending on the organism involved, the condition of the eye (and hence, the prognosis) can deteriorate very quickly.

The above information is not intended to be a guide for self-treatment of your horses, but to serve as an aid in understanding these problems and raise awareness of the potential seriousness of ocular infections. Should you suspect a corneal infection based on the clinical signs of ocular pain--squinting or frequent blinking, excessive tearing, increased sensitivity to light, and/or any discoloration or spots on the surface of the eye--the eye should be evaluated by a veterinarian or veterinary ophthalmologist as soon as possible. Delay of the appropriate treatment for ocular problems can negatively affect the prognosis.

Reproductive Problems

Now, to completely jump to the other end, another area of potential concern with respect to Pseudomonas infections in the horse is the reproductive system. Once thought to be a more significant problem, Pseudomonas species infections currently are more sporadic and isolated in occurrence. In the list of predisposing factors, pneumo-vagina (air in the vagina) or "wind-sucking" is mentioned, but this can predispose a mare to infection with many other organisms, too. The potential for Pseudomonas species infection is considered in infertility workups and will show up on cultures from cervical or uterine swabs.

Robert Hillman, DVM, Diplomate ACT, College of Veterinary Medicine, Cornell University, indicates that: "Pseudomonas species infections were commonly associated with the excessive use of disinfectants (chlorhexidine or Betadine) for washing the stallion, but this practice has largely been discontinued."

Pseudomonas can be very resistant to common disinfectants and the use of them on the penis reduces the normal flora (population) of bacteria that reside there--it is that reduction that can allow for an infection or overgrowth of Pseudomonas.

If Pseudomonas species infection becomes a significant secondary infection of a wound, the prominent sign will most likely be the thick, greenish-tinged pus with a sweet "grape-like" odor (if you dare to smell it!) on the wound dressing. If this is suspected, a swab can be obtained and cultured with antibiotic sensitivity testing performed. As mentioned before, some Pseudomonas strains can be highly resistant to many antibiotics. A 1% cream of silver sulfadiazine is often used to aid in the treatment of Pseudomonas-infected burns in people and can be used on infected equine wounds as well.

An interesting disease caused by a Pseudomonas species that is not of great concern in the United States is called glanders. Glanders, or "farcy" as it is commonly referred to, is an infectious (and also contagious) disease of horses, asses, and mules that is caused by the bacteria Pseudomonas mallei. The disease is characterized by nodules, abscesses, and ulcers in the respiratory tract and skin. It is considered a "foreign animal disease" and is subject to USDA regulation; therefore, horses being imported or exported to and from many countries are routinely tested for glanders. The Olympic horses were tested for glanders as they entered the United States.

The disease has a long history, first being described by Aristotle in 330 B.C. Glanders, once being an affliction that was widespread throughout the world, has been eradicated from many countries, including the United States, Canada, and Western Europe. It still persists in parts of Africa, Asia, and South America.

We should be thankful for our government's outstanding efforts to eradicate and continually protect our horses from glanders, as death can occur in 2-3 weeks following infection in acute cases. In addition, the disease is transmissible to people, and it is also often fatal.

Pseudomonas at a glance

  • A type of bacteria with several members of its family causing problems in horses.
  • Pseudomonas often is part of the normal flora of the skin, mucous membranes, and intestinal contents of many healthy animals.
  • Pseudomonas is commonly found in the soil and wet areas around stables.
  • Pseudomonas is a fast-growing bacteria that can cause permanent damage to eyes.
  • Pseudomonas also can invade and cause infection in the reproductive and respiratory systems, as well as in wounds.
  • Pseudomonas is the cause of glanders, a deadly disease seen in other parts of the world.

About the Author

Michael Ball, DVM

Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.

Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at or by calling 800/582-5604.

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