Colonic T-cell-rich, large B-cell lymphoma associated with equid herpesvirus 5 infection and secondary trans-colonic fistula in a horse.

The research presents the case of a 17-year-old Rocky Mountain gelding horse suffering from T-cell-rich, large B-cell lymphoma, linked to equid herpesvirus 5 infection. The lymphoma, inturn, resulted in a secondary trans-colonic fistula, a condition that hasn’t been previously reported in horses.

Background

• The horse, initially presented with a four-week history of anorexia, weight loss, lethargy, and fever of unknown origin, was taken to the Virginia-Maryland Veterinary Teaching Hospital for treatment.
• Upon conducting an abdomen ultrasound, the vets observed lymphadenomegaly (enlargement of lymph nodes) of the abdominal and colonic lymph nodes, an abnormal thickening of the wall of the large colon, and a notable mass associated with the large colon.

Findings and Diagnosis

• Due to its poor prognosis, the horse was eventually euthanized. A post-mortem autopsy revealed a roughly 20-cm diameter mass situated within the mesocolon, located between the right ventral and right dorsal colon.
• Tellingly, the mass had infiltrated through the colonic walls and formed a fistula (an abnormal passageway) between the two involved lumina (the interior space within a tubular structure, like the colon).
• Upon further microscopic evaluation, it was found that the mass was largely composed of small numbers of large neoplastic lymphocytes (abnormally growing lymphocytes), numerous small lymphocytes, and many foamy macrophages (a type of white blood cell).

The Lymphoma

• A diagnosis of T-cell-rich, large B-cell lymphoma was made based on immunohistochemical staining for CD79a, CD3, and Iba1. This is a subtype of non-Hodgkin lymphoma that affects B-cells but has a high concentration of T-cells, making it harder to diagnose than other types.

Equid Herpesvirus 5 Infection

• Additionally, an infection with equid herpesvirus 5 was detected concurrently. Equid herpesvirus 5 is known to cause benign lung tumors (fibropapillomas) in horses and is also associated with multicentric lymphoma in horses.
• The virus was confirmed in the horse through in-situ hybridization (ISH), a technique that visualizes specific DNA or RNA sequences in tissue sections.

Significance

• This is the first-ever reported case of a trans-colonic fistula appearing as a result of alimentary lymphoma in a horse, as well as the identification of intralesional equid herpesvirus 5 in equine alimentary lymphoma through ISH.