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Abbreviations
CBC

complete blood count

EMPF

equine multinodular pulmonary fibrosis

PCR

polymerase chain reaction

A 12-year-old Quarter Horse mare was presented to Kansas State University Veterinary Health Center with a 3-year history of enlarged submandibular lymph nodes that would occasionally open and drain purulent exudate. Several courses of antimicrobials, including enrofloxacin and penicillin at unknown doses, were administered over the course of the 3 years, with temporary response to penicillin. There was no report of fever, nasal discharge, or coughing. No diagnostics had been performed by the owner's regular veterinarian. The horse had a normal appetite, was drinking normally, and had no other problems identified by the owner. The horse was a successful barrel racer and had performed well within a month of presentation.

Physical examination parameters were within normal limits. A “V-shaped” chain of nonpainful, enlarged lymph nodes was noted in the intermandibular space. No other external lymph nodes could be palpated. Examination of the mouth revealed severe gingivitis and gingival recession of the maxillary and mandibular incisors. Rebreathing examination revealed no abnormal breath sounds, no cough was elicited, and appropriate recovery was noted. Differentials included reactive lymphadenitis secondary to severe gingivitis, bacterial infection such as Streptococcus equi subsp. equi, or neoplasia such as lymphoma. Upper airway and guttural pouch endoscopy was normal. The mare was sedated with detomidinea (0.01 mg/kg IV) and a punch biopsy of the enlarged lymph node was performed and submitted for histopathology. The owner declined lab analysis such as complete blood count (CBC), fibrinogen, and serum chemistry during the initial examination. The mare was administered a tetanus toxoid vaccination and was discharged with instructions to administer phenylbutazoneb (2.2 mg/kg PO q12h) for 3 days.

Histopathology revealed T-cell rich, B-cell lymphoma with 1 to 2 mitotic figures per high power field. Surgery was scheduled 7 days later to remove the entire chain of lymph nodes in preparation for chemotherapy. Before surgery, complete blood count and fibrinogen were within reference ranges. Serum chemistry was within reference ranges except for globulin concentration of 2.1 g/dL (ref. range: 2.5–4.7 g/dL). Protein electrophoresis revealed a panhypoglobulinemia. Thoracic ultrasound and radiographs did not reveal evidence of pleural effusion or cranial thoracic masses. Abdominal examination per rectum did not reveal lymphadenopathy. Immunophenotyping of whole blood revealed an increase in CD8+ lymphocytes (32%; ref. range: 13–20%). An increased aneuploid or apoptotic population was present on DNA cell cycle analysis, consistent with a population of neoplastic cells. Immunoglobulin M concentrations were low (25 mg/dL; ref. range: 63–143 mg/dL). The submandibular lymph node chain was removed by sharp and blunt dissection under general anesthesia. Because of the vessels and nerves in close proximity to the chain of lymph nodes, extensive margins were not obtainable. Histopathologic examination confirmed T-cell rich, B-cell lymphoma with 5 to 7 mitotic figures per high power field. Additionally, intranuclear inclusion bodies were also noted in many neoplastic cells. Formalin-fixed, paraffin-embedded samples were submitted to Michigan State University for polymerase chain reaction (PCR) testing for EHV-5 and were positive. Gingival biopsies revealed mucosal hyperplasia with infiltration of plasma cells and lymphocytes. No neoplastic characteristics were noted. Procaine penicillin Gc (22,000 IU/kg IM q12h), gentamicind (6.6 mg/kg IV q24h), and flunixin megluminee (1.1 mg/kg IV q12h) were administered for 3 days postoperatively and the mare was discharged to receive trimethoprim sulfamethoxazolef (20 mg/kg PO q12h) for 5 days and suture removal in 10 days.

Chemotherapy was initiated 1 month after surgery and was administered by the owner's regular veterinarian (doses listed in Table 1). Financial constraints limited chemotherapy administration to 1 month. Complete blood count was performed before each chemotherapy dose and the horse tolerated the medications well with no adverse effects reported.

Table 1. Chemotherapeutic drugs and dosages used to treat a 12-year-old Quarter Horse mare with T-cell rich, B-cell lymphoma
DrugDosageFrequency
Cyclophosphamide200 mg/m2 IVDay 1
Vincristine0.5 mg/m2 IVDay 7
Vincristine0.5 mg/m2 IVDay 14
Cyclophosphamide200 mg/m2 IVDay 21
Vincristine0.5 mg/m2 IVDay 28
Dexamethasone0.04 mg/kg PODay 1
Dexamethasone0.02 mg/kg PODay 2–30

Examination 1 month after chemotherapy was completed (2 months after presentation) revealed a normal physical examination, CBC, and fibrinogen. T-cell rich, B-cell lymphoma was found on aspirate of the submandibular lymph node area. The gingival lesions remained unchanged. The owner decided to forego further treatment and to continue using the horse in barrel races and was advised to monitor the area.

Seven months after last examination, the horse returned. The masses in the submandibular area were multilobulated and had increased to 1–2 cm in size. Punch biopsy revealed T-cell lymphoma with no evidence of intranuclear inclusion bodies in the small sample. Low CD4+ lymphocytes and increased CD8+ lymphocytes were found on peripheral blood immunophenotyping, suggesting potential recruitment of circulating cytotoxic lymphocytes during viral infection. The sample was not retested for EHV-5; however, because of the increase in CD8+ lymphocytes and the persistent and latent nature of the virus, it was presumed that the horse was still positive for the virus. At this time, antiviral therapy with acyclovir was initiated at 20 mg/kg PO q8h for 4 months.[1]

One month after the course of acyclovir was completed, an aspirate of the lymph node area was performed and a cytologic examination was negative for neoplastic cells. Because it was a relatively small sample, the owner was advised to return the horse in 2 months for a punch biopsy of the area. At the recheck examination, the size of the masses remained unchanged. Histopathology revealed that there were small numbers of small, mature lymphocytes with normal appearance in the sample obtained; in previous samples, no normal lymphoid tissue could be identified. Fibrous scar tissue was present and expected in response to the original surgical procedure performed 11 months previously. Complete blood count, fibrinogen, and chemistry were within normal limits except for continued low but unchanged globulin level (2.1g/dL; ref. range: 2.5–4.7 g/dL). Throughout the treatment cycle, the horse continued to be an active and successful barrel racer.

One year following the last examination, the mare was healthy and in foal. The lymph node area was stable with no change in size.

This report describes apparent induction of remission of peripheral lymphoma associated with EHV-5 by administration of acyclovir.g

Equine lymphoma is an often fatal disease that might affect horses of all ages. To date, there has been no known etiology. A recent study performed by the authors of this article evaluated the association of equine lymphoma and EHV-5 and found that 67% of lymphoma positive tissues were positive for the virus on PCR analysis, compared to 14% of normal, healthy controls.[2] EHV-5 is a gamma herpesvirus and has been recently associated with equine multinodular pulmonary fibrosis (EMPF).[3] Additionally, a horse with T-cell leukemia and EMPF was recently reported.[4] In humans, gamma herpesviruses such as Epstein–Barr virus and Kaposi's sarcoma-associated herpes virus have been associated with malignant and nonmalignant lymphoproliferative conditions such as Burkitt's lymphoma, multicentric Castleman's disease, and Kaposi's sarcoma.[5-7]

The horse is the natural host of 5 recognized herpes viruses: EHV-1 (alpha herpesvirus), EHV-2 (gamma), EHV-3 (alpha), EHV-4 (alpha), and EHV-5 (gamma). EHV-5 was first isolated in Australia in 1987[8] and its prevalence in the equine population is variable depending on the specific region tested, ranging from 3% to 64%.[9, 10] In addition, lower prevalence rates have been found in older versus younger horses.[11] Although EHV-5 has recently been found to be associated with equine multinodular pulmonary fibrosis (EMPF)[3] and there have been an increasing number of reported cases of EHV-5-associated fibrotic lung diseases, there has only been 1 report of lymphoproliferative disease associated with EMPF.[4] It is unknown why some horses that are positive for the virus develop EMPF, some develop lymphoma, and some have no evidence of disease at all. The same is true for humans who are infected with latent herpesviruses and might or might not develop lymphoproliferative disorders.

Three recent case reports[12-14] describe human patients with primary effusion lymphoma, associated with HHV-8, unresponsive to traditional intensive chemotherapy, who were finally moved to remission with intracavitary infusion of cidofovir, an antiviral medication. In these cases, the drug cannot reach proapoptotic levels when given systemically and therefore must be given locally into the pleural cavity.

Acyclovir is an acyclic guanine nucleoside analog that is structurally similar to DNA and RNA nucleosides. These drugs are incorporated into viral DNA and therefore terminate the growing viral DNA chain. Although the pharmacokinetics of acyclovir indicate that the bioavailability of the drug is low (2.8%),[15] successful treatment of EMPF has been achieved using this medication,[1] suggesting that accumulation occurs over time.[16] Although valacyclovir, the prodrug of acyclovir, results in 10-times-higher Cmax and 8-times-higher bioavailability,[17] the administration of this drug over a significant amount of time was cost prohibitive.

There are few reports of successful chemotherapeutic regimens for equine lymphoma.[18, 19] The CHOP protocol (cyclophosphamide, hydroxyaunorubicin [doxorubicin], vincristine [Oncovin®], and prednisone) is one of the most commonly used chemotherapy protocols for lymphoma in dogs and humans. There are limited reports on the use of doxorubicin in horses; it is associated with significant adverse effects in humans and dogs and was cost prohibitive for use in this horse. Cyclophosphamide is a nitrogen mustard alkylating agent from the oxazophorines group. It adds an alkyl group to the guanine base of DNA. It is a prodrug that is converted in the liver into 2 active chemicals, acrolein and phosphoramide. Vincristine is a vinca alkaloid from the Catharanthus roseus (Madagascar periwinkle). It is a mitotic inhibitor that binds to tubulin dimers, inhibiting assembly of microtubular structures. This leads to arrest of mitosis in metaphase. Adverse effects of the medications include gastrointestinal upset and leukopenia. Neither of these was seen in this patient.

In conclusion, this is a report of apparently successful treatment of equine lymphoma with acyclovir. This case demonstrates that this alternative therapy might be useful in cases of lymphoma that are associated with EHV-5. Additional cases are necessary to determine if this treatment will be universally effective.

Conflict of Interest: Authors disclose no conflict of interest.

Footnotes
  1. 1

    Dormosedan, Pfizer Animal Health, New York, NY

  2. 2

    Pributazone, First Priority, Inc., Elgin, IL

  3. 3

    Pfizerpen, Roerig (Pfizer), New York, NY

  4. 4

    GentaFuse, Butler Schein Animal Health, Dublin, OH

  5. 5

    Banamine-S, Merck Animal Health, Summit, NJ

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