Equine Placentitis: Common Causes and Newly Emerging Pathogens
During the past six years (Jan. 2, 2002 – Jan. 31, 2008), 1,429 cases of equine placentitis have been diagnosed at the University of Kentucky Livestock Disease Diagnostic Center (LDDC). Of these, 1,189 cases had an infectious agent identified. Various bacteria were isolated from 1,125 cases (Figure 2), and 64 had fungus cultured.
Since the 1998 and 1999 foaling seasons, in which 94 and 144 cases of nocardioform placentitis cases were diagnosed respectively, the number of nocardioform placentitis cases has decreased dramatically, with only 93 cases over the last six foaling seasons (2002-1/2008).
Two bacterial agents have been diagnosed over the past six years that represent potential newly emerging abortigenic pathogens and causes of placentitis: Mycobacterium spp. and Cellulosimicrobium cellulans (formerly called Oerskovia xanthineolytica).
The LDDC received six cases of mycobacterial abortion and placentitis with the diagnosis confirmed by microbiologic, polymerase chain reaction (PCR), and/or histochemical staining procedures. Mycobacterial isolates were cultured and sequenced by the use of PCR analysis identifying isolates of the atypical Runyoun groups. These atypical mycobacteria are within the non-tuberculous group and are classified as saprophytic and opportunistic microorganisms acquired from the environment (soil, water, and/or decaying vegetation).
Gross lesions observed within the submitted cases varied from none to those of a “nocardioformlike” placentitis, which is characterized by the presence of variable amounts of thick, mucoid, viscous exudate and placentitis predominantly located on dependent regions of the chorion, particularly at the base of the horns and body region. Affected fetuses had variable degrees of emaciation and chronic placentitis. Several fetuses had granulomatous to pyogranulomatous pneumonia, and one fetus without pneumonia contained disseminated granulomas in various organs.
Cellulosimicrobium cellulans is a gram-positive, branching bacillus that forms vegetative hyphae on nutrient agar and is an opportunistic microorganism insidious to the environment (particularly the soil). While these organisms have been referred to as “nocardia-like” organisms based on their basic morphology, C. cellulans bacilli are motile and do not form aerial mycelia like Nocardia species. The nine equine cases of C. cellulans abortion and placentitis submitted to the LDDC were noted to produce “nocardioform-like” placentitis lesions as well as granulomatous pneumonia. These findings make it difficult to differentiate a true nocardioform placentitis from a C. cellulans induced placentitis solely on gross examination; therefore, bacteriologic and molecular biologic testing are essential for differentiating these two agents.
Placentitis continues to represent a significant problem and common cause of equine abortions. It is important to recognize that newly identified bacterial causes of placentitis and abortion are being diagnosed and may be emerging as potentially significant abortigenic pathogens. Atypical Mycobacteria and C. cellulans can cause similar gross and histologic lesions within the fetus and/or placenta and should be considered as differential diagnoses when encountering lesions suggestive of nocardioform placentitis.
By Dr. Uneeda K. Bryant
Livestock Disease Diagnostic Center, University of Kentucky
