The equine placenta is a fetal-derived transient organ that directly interacts with the mare. Throughout gestation, the placenta allows for the metabolic exchange of gases, water, nutrients, and small hormones between the fetus and mare. Placental disease can occur at any stage of development and, in severe cases, can result in death and expulsion (abortion) of the fetus.
A variety of both infectious and non-infectious causes of placental disease are known. Potential infectious causes of placental disease and abortion that occur in Central Kentucky include 1) ascending placentitis caused by various bacteria and fungi, 2) hematogenous placentitis and fetal infection caused by bacteria, fungi, and viruses, and 3) atypical/mucoid placentitis caused by Gram-positive branching nocardioform bacteria and a variety of other less common bacteria. Non-infectious causes that can negatively affect the fetus or perinate include umbilical cord torsion, maternal stress and disease, placental (cervical pole) necrosis, twin pregnancy, premature placental separation (red bag), fescue, hydrops, in utero meconium passage, and other miscellaneous and rare conditions such as cancer. Also of note, the placenta can also develop a variety of changes that look abnormal, but are actually incidental, inconsequential, and non-pathogenic. Common incidental changes include 1) mineral and epithelial plaques on the amnion, umbilical cord, and allantoic surface of the allantochorion, 2) avillous regions of the chorion, 3) slimy tan to brown discolorations associated with autolysis, 4) prominent thick fetal pad with mucus in the distal pregnant horn, 5) persistent yolk sacs, and 6) allantoic vesicles. Many, but not all, causes of placental disease can be determined by placental examination.
The University of Kentucky Veterinary Diagnostic Laboratory (UKVDL) employs veterinary anatomic pathologists to examine equine placentas and perform postmortem examinations of fetoplacental units and animals that die from known and unknown causes. Since 2020, the UKVDL has averaged over 300 equine placental examinations annually. Placental exams are frequently conducted to evaluate abnormal observations, diagnose both infectious and non-infectious causes of placental disease, and answer diagnostic questions from the submitting farms and veterinarians.
Questions occasionally arise from clients about why specific tests are not conducted for certain diseases of concern. Unknown to the client, placentas are tested for the vast majority of diseases of concern, but most individuals aren’t sufficiently familiar with laboratory techniques and assays to know this. This commentary aims to better inform our clients about equine placental examinations.
Once submitted to the lab, a pathologist will first read the farm or veterinarian-generated clinical history. The clinical history typically informs the lab of any concerns the client has about the placenta (e.g., thickened areas, discolored areas, or other potential abnormalities), the estimated gestational age of the fetoplacental unit, and any pertinent clinical information (e.g., the mare had mammary development; the mare was recently sick; the mare was being treated for placentitis, etc.). Please ensure that any specific questions about the placenta are specifically noted in the clinical history.
A gross examination of the placenta is then conducted. This assesses the allantochorion for completeness, determines the umbilical cord length, and evaluates the allantochorion, amnion, and umbilical cord for any significant abnormalities. A standard set of placental samples is taken for bacterial culture, molecular biology analysis, and microscopic evaluation (histopathology). Gross examinations can typically identify evidence of placentitis (regardless of type), umbilical cord abnormalities, twin pregnancies, placental (cervical pole) necrosis, developmental malformations, and incidental/insignificant abnormalities. Placental changes are not always described in the final report if they are deemed incidental. Tissue samples are processed and microscopically evaluated for histopathology. Histopathology is used to identify characteristic patterns typical of certain infections and can be used to microscopically visualize pathogens (e.g., bacteria, fungi, viral inclusions), inflammation, and tissue morphology.
Samples are submitted by the pathologist for routine bacterial culture. Abnormal locations (e.g., placentitis), or a standard set of locations (e.g., cervical star) are tested if abnormalities are not detected on gross examination. Routine culture is a technique capable of growing a very broad range of bacteria that includes most causes of ascending placentitis (e.g., Escherichia coli, Streptococcus spp., Pseudomonas spp., Enterobacter spp., Corynebacterium spp., and others), many causes of hematogenous placentitis, and the causes of mucoid and nocardioform placentitis. The success of culturing a causative bacterium can greatly depend on the degree of autolysis and environmental contamination (e.g., covered in mud or sawdust) of the placenta. If a significant bacterium is cultured, then an antimicrobial susceptibility test is performed on the isolate to potentially aid in treatment.
Samples are submitted for molecular biology analysis, i.e., polymerase chain reaction (PCR). PCR is a genetic-based assay that looks for nucleic acid of a specific microorganism. These highly specific assays are commonly used to quickly detect viruses (e.g., Equine herpes virus-1 and Equine arteritis virus) and difficult-to-grow bacteria (e.g., Leptospira interrogans). Of note, the veterinarian is contacted by VDL support staff if contagious pathogens such as Equine herpes virus-1 or Leptospira interrogans are detected. Although many of the causes of mucoid/nocardioform placentitis can be detected by bacterial culture, the pathologist will request a nocardioform PCR panel, which detects Crossiella equi, Amycolatopsis spp., and Streptomyces sp., if mucoid/nocardioform placentitis is suspected on gross examination.
The pathologist will write a final report once all the testing data becomes available. The diagnosis will include any significant infectious or non-infectious abnormalities, and the report’s comment section may be used to expand on the observed disease processes or directly discuss any concerns that the submitter noted in the clinical history. The final report will be distributed to the individual or organization that paid for the examination; farms that submit placentas through their veterinary clinic need to contact the veterinary clinic to obtain results.
The following comments address frequently asked questions or concerns. Occasionally, specific tests are requested by the submitting farm or veterinarian. Nonetheless, only tests deemed necessary by the pathologist will be performed to keep costs low and minimize unnecessary testing. For example, the nocardioform PCR panel is occasionally requested and not performed. This may occur when the pathologist deems that there is no overt evidence of mucoid or nocardioform placentitis, and the requested PCR results cannot be appropriately interpreted. Regardless, if present, nocardioform bacteria will still be tested for and detected by bacterial culture. Finally, diagnostic reports are a medical document generated by veterinary specialists. Much of the report utilizes medical terminology that may seem callous and be difficult to interpret by non-veterinarians. However, UKVDL laboratory personnel are happy to assist you in interpreting reports, if needed.
In summary, equine placental examinations are an important process to identify potential problems in equine neonates and their mares. Examinations do an excellent job of determining the cause of infectious abnormalities, presence of contagious diseases, and identification of many (but not all) non-infectious causes of placental disease. When submitting a placenta for examination, be sure to list all relevant clinical history and specifically note any questions or diagnostic objectives that you would like answered. Feel free to contact the UKVDL with any questions you may have.