Is D. congolensis Associated with Pastern Dermatitis in Horses?
August 23, 20225 min read
Equine pastern dermatitis is a frustrating and painful skin condition that is commonly known as mud fever, dew poisoning, and scratches. While this condition is well known to have multiple predisposing factors, the bacteria Dermatophilus congolensis has long been thought to play a significant causative role. This opportunistic bacteria is a facultative anaerobic actinomycete that is thought to result in papular to exudative dermatitis with crusting in horses.
What is D. congolensis?
congolensis is a gram-positive, non-acid-fast, facultative anaerobic actinomycete. Its preferred habitat is relatively unknown. It is thought to be a saprophyte living in the soil, but attempts to isolate it have been unsuccessful. As it is restricted to residing in living layers of epidermis, asymptomatic, chronically infected animals are thought to be the primary reservoir. Because of this, researchers believe D.congolensis is spread via direct contact between animals, through contaminated environments, and possibly through biting insects.
D. congolensis & Equine Pastern Dermatitis
While many veterinarians regard D. congolensis as causing equine pastern dermatitis, there is some conflicting evidence. For example, some researchers list it as a secondary infection and complicating/perpetuating factor, not the root cause. Other sources list D.congolensis as causing dermatophilosis, not pastern dermatitis. Some of this confusion may be because dermatophilosis does have similar symptoms to equine pastern dermatitis, i.e. it’s “characterized by an exudative dermatitis with heavy scab formation.”
However, dermatophilosis is more akin to rain rot, than pastern dermatitis. Whether or not D.congolensis causes equine pastern dermatitis specifically, it is clear that there is a link between unsavory skin conditions and this bacteria.
Equine pastern dermatitis progresses similarly no matter its origin, whether it’s from D. congolensis, poor stable management, or a fungal infection. This makes it difficult to track down the root cause. To discover whether or not D. congolensis plays a role in causing equine pastern dermatitis as previously believed, let’s examine the findings of two peer reviewed studies.
Study #1: “Equine pastern dermatitis” by C. M. Colles, K. M. Colles and J. R. Galpin
This UK-based study was published in the educational journal, Equine Veterinary Education circa 2010. It relied on the use of 12 privately owned horses who had presented in the normal course of clinical practice and were diagnosed with equine pastern dermatitis. These horses ranged in age from 5 to 20 (or simply, “aged”) and breeds varied, although cob and thoroughbred were most common.
The severity of each individual case varied widely and all cases were treated as though they were recently acquired. In cases of equine pastern dermatitis where weeping was not present, scab material was scraped and collected. If there was weeping or purulent exudate present, scabs and/or swabs were collected.
Each specimen was examined for inflammatory cells and D. congolensis, as well as other bacteria. If scabs were collected, they were screened for chorioptic mites as well. No dermatophytes were found throughout the course of the study.
Most of the cultures resulted in mixed bacteria. While D. congolensis was found in two cases, it was only able to be isolated in one case out of the 12 total. The three most common bacteria found were actually b-haemolytic streptococci (found four times), S. aureus (found three times), and CN Staphylococcus (found three times).
Despite the fact that D. congolensis was only found twice, the authors of the study concluded that no conclusion could be drawn and that further research was needed. One of their theories is that D. congolensis is the initiating factor in some equine pastern dermatitis infections and does breach the skin, but is quickly replaced by other bacteria, which is why it cannot be cultured. Alternatively, it may simply be one of the opportunistic organisms that cause infections when other factors, such as mud or abrasions, weaken or break the skin barrier.
While this study is not conclusive, it does show that there is more going on in cases of equine pastern dermatitis than a D. congolensis infection, secondary or otherwise.
Study #2: “The prevalence of Dermatophilus congolensis in horses with pastern dermatitis using PCR to diagnose infection in a population of horses in southern USA”
This study specifically examined horses in the southern United States, which is an extremely different environment from the more moderate climate of the United Kingdom. Researchers were able to study 15 client-owned horses who had been diagnosed with equine pastern dermatitis. Unlike the previous study, this one had a control group made of eight client-owned unaffected horses. This study also differentiated between horses with feathered legs and those without.
Samples were collected and tested for D. congolensis via cytological evaluation and RT-qPCR. Dermatophyte culture and superficial skin scrapings were also performed.
congolensis bacteria was identified in the equine pastern dermatitis of one non-feathered horse. While there was mixed bacterial growth present in all horses with equine pastern dermatitis, D.congolensis was unable to be identified in any other horse in the study.
Chorioptes mites, on the other hand, were present in four out of 15 horses. All four of these horses had feathered legs.
This study concluded that “Dermatophilus congolensis was uncommonly associated with pastern dermatitis in horses in this population.”
It did, however, give us another piece of the puzzle when it comes to managing equine pastern dermatitis. The demonstrated association between chorioptic mange and equine pastern dermatitis in feathered horses could change treatment methods for horses with heavier feathering on the pasterns.
D. Congolensis is More Likely to be Present in Dermatophilosis Infections
While further research is clearly needed, the results of these two small studies point to the idea that D. congolensis does not play as much of a role in equine pastern dermatitis as previously thought. However, this does not mean this bacteria doesn’t play a role in causing other skin conditions. According to the PennState Extension, dermatophilosis, also called rain scald or rain rot, “is a skin infection caused by a bacterium known as Dermatophilus congolensis.”
Unfortunately, few, if any, peer reviewed studies have examined if D. congolensis plays a causative role in dermatophilosis infection in horses. But, there is some evidence that D. congolensis causes dermatophilosis infections in cattle.
In 1915, a Belgian military veterinarian named René Van Saceghem was stationed at a laboratory in the Congo. While there, he was able to culture D. congolensis from rain rot infections in cattle. This was the first time the cause of dermatophilosis in cattle was identified, since the disease was first seen in the area in 1910.
While dermatophilosis and equine pastern dermatitis do have similar symptoms, it is evident based on these studies that the causative agent for these two skin conditions is most likely different.
There are clearly more questions than answers when it comes to the causes of equine pastern dermatitis. As more research becomes available, treatment methods will need to change. It is important to look to the future when managing these skin conditions.
Gone are the days when the use of harsh chemical agents were acceptable to manage equine pastern dermatitis. Harsh chemicals will damage healthy tissues as well as kill bacteria. The focus should now be on using the latest technology to nourish the skin back to health.
Instead of using harsh agents or topicals, choose a topical that is non-toxic, non-steroidal, and antibiotic-free. Over a decade of scientific research has been conducted to perfect the patented, novel technology in Zarasyl Equine.