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Abbreviations:
DNA-PK

deoxyribonucleic acid-protein kinase

DNA-PKsc

deoxyribonucleic acid-protein kinase catalytic subunit

EFS

Equine Field Services

GGT

gamma-glutamyl transferase

IL

interleukin

SCID

severe combined immunodeficiency

A 4-week-old female Caspian filly was examined by Equine Field Services (EFS) at the Virginia-Maryland Regional College of Veterinary Medicine for a 1-week history of bilateral mucopurulent nasal discharge and stertorous breathing. The filly had been bright with a good appetite and was not observed coughing. The owner reported a normal foaling, and the filly had risen and suckled fairly quickly after birth, but the foal's immunoglobulin G (IgG) status was unknown and the mare was not current on vaccinations. There were 17 other horses on the farm, including 2 other mare and foal pairs, all reportedly normal.

On presentation, the filly was bright, with an estimated body weight of 35 kg and body condition score of 4 of 9. Physical examination findings included a rectal temperature of 39°C, heart rate of 104/min, and respiratory rate of 18/min. Mucous membranes were slightly injected and capillary refill time was <2 seconds. Increased bronchovesicular sounds were heard bilaterally on auscultation of the lungs; sounds were accentuated on rebreathing exam. All joints and the umbilicus palpated normally. Diagnostic testing was limited because of owner financial constraints, and included a CBC, thoracic ultrasound examination, and upper airway endoscopy. Ultrasonography identified pleural roughening bilaterally. Endoscopy identified pharyngeal edema and copious white purulent material in the trachea (score 5/51) and pharynx, which was collected for culture and cytology. Treatment initiated on the farm, based on a working diagnosis of bacterial pneumonia and pending laboratory results, included ceftiofura (4 mg/kg SC q12h × 2 days, then 4 mg/kg SC q24h) for suspected pneumonia, flunixin meglumineb (1 mg/kg IV, once) to reduce inflammation and pyrexia, clenbuterolc (0.8 μg/kg PO q12h) as a bronchodilator, and guaifenesind (3 mg/kg PO q12 h) as an expectorant.

Lymphopenia (268/μL; reference range, 1,730–4,850/μL2) and monocytosis (1,208/μL; reference range, 50–630/μL2) were present on the CBC. Cytology identified numerous leukocytes, cuboidal and columnar epithelial cells, and extracellular and intracellular cocci. Culture yielded Staphylococcus intermedius, Streptococcus suis, and Streptococcus zooepidemicus, all sensitive to most antibiotics, including ceftiofur. The marked lymphopenia and monocytosis, cytology findings, and culture results all supported a diagnosis of bacterial pneumonia. There was a concern that the filly could have an immunodeficiency disorder based on the profound lymphopenia.

Four days later, the filly's attitude was still bright, but she had a neutrophilia (10,788/μL; reference range, 2,760–9,270/μL2) with a left shift (116/μL bands), and lymphopenia (348/μL). The persistent abnormalities were attributed to the severity of the foal's disease. Based on evidence of clinical improvement, the initial treatment plan was continued with the addition of flunixin meglumine (1 mg/kg PO q24h) if the temperature remained above 38.9°C.

Over the next 4 days, the filly had episodes of pyrexia. She was rechecked on day 8 of treatment, and had a temperature of 39.8°C and increased bronchovesicular sounds bilaterally. To identify the cause of the filly's persistent disease and suspicion of immunodeficiency, diagnostic tests included thoracic radiographs, thoracic ultrasound examination, endoscopy, CBC, and biochemical profile, and serum IgM concentration. Testing the filly for the Arabian severe combined immunodeficiency (SCID) mutation was discussed with but declined by the owner, because of expense and the lack of any other known SCID Caspian horses.

Radiographs disclosed a slight interstitial pattern with multifocal alveolar pattern, and ultrasonography identified pleural roughening bilaterally (unchanged), all consistent with pneumonia. Endoscopy identified rhinitis, pharyngitis, and mucopurulent debris, which was cultured, draining from both guttural pouches and pooling in the trachea. The guttural pouches were not visualized because the scope diameter was too large to access the pouch. Evaluation of another CBC disclosed leukocytosis (18,150/μL; reference range, 5,400–13,500/μL2) characterized by a neutrophilia (17,061/μL), lymphopenia (363/μL), monocytosis (726/μL), hyperfibrinogenemia (1,000 mg/dL; reference range, 150–650 mg/dL2), hypoalbuminemia (2.6 g/dL; reference range, 2.7–3.5 g/dL), and increased gamma-glutamyl transferase (GGT) (48 U/L; reference range, 8–38 U/L3). The serum IgM concentration was below detectable limits. The persistent leukocytosis (neutrophilia), monocytosis, and increasing fibrinogen concentration all suggested continuing infection and lack of efficacy of current therapy. The persistent lymphopenia and undetectable IgM concentration supported a diagnosis of immunodeficiency. Differential diagnoses for the increased GGT activity included cholestasis or hepatitis from a hepatic abscess, septicemia, or hematologic spread of bacteria to the liver. Ceftiofur administration was increased (from 4 mg/kg SC q24h to 4 mg/kg SC q12h) while culture results of the tracheal mucus were pending, and flunixin meglumine was continued at 35 mg (1 mg/kg PO q12h). Culture of the tracheal mucus yielded Escherichia coli, Klebsiella oxytoca, and Streptococcus uberis, all sensitive to ceftiofur.

The filly's guttural pouches were assessed the next day with a pediatric endoscope, and were free of debris. Evaluation of another CBC indicated leukocytosis (16,670/μL), neutrophilia (15,336/μL), lymphopenia (333/μL), monocytosis (1,000/μL), and hyperfibrinogenemia (900 mg/dL). GGT activity had increased to 80 U/L, suggesting continuation or worsening of the hepatitis or cholestasis. An immunodeficiency disorder was suspected based on the unresolved bacterial pneumonia despite appropriate treatment, emergence of new problems (possible hepatitis or cholestasis), persistent severe lymphopenia, and undetectable serum IgM concentration. Because of the persistently increased GTT activity, ultrasound examination of the liver was performed. A small, round area of increased echogenicity with slight shadowing was found near midline within liver, increasing suspicion of a hepatic abscess. At this time, the ceftiofur was discontinued and doxycyclinee initiated (10 mg/kg PO q12h) with the hope that the lipophilic doxycycline would achieve higher concentrations in the liver. Omeprazolef (2.5 mg/kg PO q24h) also was initiated as a precaution against gastric ulceration.

To characterize the suspected immunodeficiency, diagnostic testing included serum Ig concentrations, genetic testing for the Arabian SCID gene, and flow cytometry. Phytohemagglutinin intradermal skin testing was considered but not performed because reagents were not available. Serum Ig concentrations were undetectable for IgM and IgA but normal for IgG and IgG(t). Blood submitted to VetGeng for DNA testing for the Arabian SCID gene was NULL, meaning the filly was not affected or a carrier of the Arabian SCID gene but not necessarily negative for SCID. Flow cytometry4 (Fig 1) demonstrated decreased lymphocyte subsets in the affected filly as compared with an age-matched half-sibling (0.05 versus 8.37% CD4, 1.65 versus 4.03% CD8, 1.08 versus 5.16% B cells, as a percentage of total leukocytes).

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Figure 1.  Histograms showing CD4, CD8, and CD5 (B-cell) expression from affected filly and age-matched control sibling. Mononuclear cells were isolated from peripheral blood using percoll. Enriched leukocytes were stained with antibodies (VMRD) for CD4, CD8, or CD5.4 Samples, analyzed using a FACS Aria, were gated on total leukocytes because of the very limited number of lymphocytes present in the affected filly. (A) CD4 positive lymphocytes from affected filly. (B) CD4 positive lymphocytes from age-matched control half-sibling. (C) CD8 positive lymphocytes from affected filly. (D) CD8 positive lymphocytes from age-matched control half-sibling. (E) CD5 positive lymphocytes from affected filly. (F) CD5 positive lymphocytes from age-matched control half-sibling.

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Collectively, these results supported an immunodeficiency disorder (likely SCID). Because an absolute diagnosis could not be confirmed and it was possible the filly had delayed development of functional immunity, treatment was continued. The filly's pneumonia and liver disease continued to progress, GGT activity peaked at 171 U/L, and areas of consolidation developed bilaterally in the lungs.

Approximately 4 weeks after treatment was started, the filly was reassessed by endoscopy. Systemic antibiotics were changed to amikacinh (21 mg/kg IV or IM q24h) after a multidrug resistant E. coli was cultured from the tracheal brushing sample. Adjunctive treatments included Propionibacterium acnesi (0.4 mg IV for 3 treatments), IV plasmaj (950 mL), and ophthalmic preparations for a desmetocele ulcer in the left eye (triple antibiotic ophthalmic ointment,k atropine solution,l serum from the dam, ethylenediaminetetraacetic acidm in saline,n and gentamicin ophthalmic solutiono).

On day 43 after initial evaluation, the filly was euthanized because of severe progression of her pneumonia. Just before euthanasia, she had a temperature of 40.4°C, heart rate of 140/min, respiratory rate of 100/min, marked abdominal effort to the respiratory pattern, loud crackles over both lung fields, lymphopenia (75/μL), and increased GGT activity (59 U/L). Serum submitted for an equine adenovirus serum neutralization antibody titer was negative. Necropsy findings included interstitial pneumonia and thymic hypoplasia. Histological findings included a severe absence of lymphocytes within the tracheobronchial, mediastinal (Fig 2), and mesenteric lymph nodes with no follicular development, a severe absence of lymphocytes within the periarteriolar lymphoid sheaths in the spleen (Fig 2), adequate proportions of erythroid and myeloid precursor cells within the bone marrow but no lymphocytes, and no histologically discernable thymic tissue. The final necropsy diagnoses based on histological and gross findings included severe lymphocyte depletion and hypoplasia, bronchointerstitial pneumonia with type II pneumocyte hyperplasia, and intraepithelial intranuclear inclusions (suggestive of adenovirus), corneal ulcer with suppurative keratitis and suppurative hepatitis and adrenal adenitis, all supporting a clinical diagnosis of severe combined immunodeficiency.

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Figure 2.  Histological sections of mediastinal lymph node (A, × 40 and B, × 100) and spleen (C, × 40 and D, × 100) from the affected filly. (A, B) Lymphocyte population is sparse with complete absence of lymphoid follicles (hematoxylin and eosin, bar in A = 50 μm, B = 25 μm). (C, D) Lymphocytes are absent within the periarterioloar lymphoid sheaths. Arterioles (arrow) are surrounded by reiticuloendothelial cells, macrophages, and fibroblasts, and sparse lymphocytes (hematoxylin and eosin, bar in C = 50 μm, D = 25 μm).

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To our knowledge, this is the 1st report of SCID in a Caspian horse. Until now, SCID has only been reported in Arabian or Arabian crosses.5 Caspians are believed to have originated in Iran, and are one of the genetic foundations for many modern horse breeds, including the Arabian.6,7 Caspians were first imported to the United States from Iran in 1965. They are a critical breed according to the American Livestock Breed Conservancy because there are fewer than 200 annual registrations in the United States and an estimated global population of fewer than 2,000.7

Severe combined immunodeficiency, a lethal inherited condition in which both T-cell and B-cell function is absent, has been described in horses, humans, mice, and dogs.5,8,9 SCID was first described in children in the 1960s and Arabian foals in 1973.5,8,9 SCID in Arabian horses results from a spontaneous mutation in the gene encoding deoxyribonucleic acid-protein kinase catalytic subunit (DNA-PKcs), a subunit of deoxyribonucleic acid-protein kinase (DNA-PK), an enzyme important in DNA repair.5,9 This mutation leads to a 5 base pair deletion in the gene on chromosome 9, leading to nonfunctional DNA-PK. DNA-PK is required in the gene rearrangement that leads to T-lymphocyte antigen-specific and B-lymphocyte surface Ig receptors.5,9 Without these gene rearrangement events, lymphocyte precursors are eliminated. The end result is an absence of mature, functional T and B lymphocytes.5,9 The lack of cell-mediated and antibody-mediated immunity leaves foals vulnerable to infections.5

SCID also has been reported in mice, dogs (Basset Hounds, Cardigan Welsh Corgis, Jack Russell Terriers), and humans.9 SCID mice lack DNA-PK enzyme activity because of a single nucleotide substitution in the DNA-PKcs gene on chromosome 16, resulting in a premature stop codon. Mutations in dogs include a point mutation in DNA-PKcs or in the γc of the interleukin-2 receptor (IL2RG).9,10 In humans, mutation of IL2RG is the most common mechanism of SCID in children.9,10 More than 145 distinct point, insertion, and deletion mutations involving all 8 exons have been reported.9 In each species, SCID manifests early in life as recurrent or chronic infections that fail to respond to therapy.9,10

Affected Arabian SCID foals must be homozygous for the defective SCID gene (autosomal recessive trait). SCID foals generally are normal at birth, but become susceptible to infections once colostral antibodies wane, and acquire infections between birth and 2 months of age, dying before 5 months of age. Age of onset and clinical manifestation of disease depend on adequate passive transfer and type of environmental challenges. Pneumonia from pathogens, including Pneumocystis carinii and adenovirus, often are found in SCID but not in normal foals.5 Although this filly had evidence of an infection with adenovirus based on histopathology, the antibody titer was undetected. The titer could be because of the filly's severely decreased ability to produce antibodies because of low functional B lymphocytes. The amount and quality of colostral antibodies were unknown in this filly because she was not tested for failure of passive transfer, but the amount of colostrum consumed was likely adequate based on her normal IgG concentrations. This filly's lack of functional immune system and type of environmental challenges (17 other horses, several of which traveled) are likely contributing factors as to why the filly became ill at 3 weeks of age.

A definitive diagnosis of SCID in an Arabian foal is based on the presence of the defective SCID gene. Polymerase chain reaction testing was performed by VetGen using DNA from blood. The test was NULL for the defective gene of Arabians. It is possible that this filly had SCID involving a different mutation, which was not identified by VetGen's test. Additional parameters used for definitively diagnosing SCID include (1) lymphopenia (<1,000 lymphocytes/μL), (2) absence of IgM in serum collected presuckle or after 3 weeks of age, and (3) hypoplasia of the spleen or thymus.5 The filly of this report met these criteria, including decreased CD4, CD8, and B cells, thus a diagnosis of SCID was made. Identifying a Caspian filly with SCID will likely impact the breed. Gene sequencing is required to determine the genetic mutation.

Footnotes

  1. Top of page
  2. Footnotes
  3. Acknowledgments
  4. References

a Naxcel, Pfizer Animal Health, New York, NY

b Banamine Injectable Solution, Schering-Plough Animal Health Corp, Union, NJ

c Ventipulmin Syrup, Boehringer Ingelheim Vetmedica Inc, St Joseph, MO

d Robitussin Chest Congestion, Wyeth Consumer Healthcare Inc, Mississauga, ON, Canada

e Doxycycline Hyclate Tablets, USP, West-Ward Pharmaceutical Corp, Eatontown, NJ

f Gastrogard, Merial, Duluth, GA

g VetGen, Ann Arbor, MI, http://www.vetgen.com

h Amikacin Sulfate, Teva Parenteral Medicines Inc, Irvine, CA

i EqStim, Neogen Corporation, Lexington, KY

j Equiplas Plus, Plasvacc USA Inc, Templeton, CA

k Trioptic P, Pfizer Animal Health

l Atropine Sulfate Ophthalmic Solution 1%, Falcon Pharmaceuticals, Alcon Laboratories Inc, Fort Worth, TX

m K3 EDTA 12 mg (purple top blood tube), BD, Franklin Lakes, NJ

n Normal Saline, Vedco Inc, St Joseph, MO

o Gentamicin Sulfate Ophthalmic Solution USP, Falcon Pharmaceuticals, Alcon Laboratories Inc

Acknowledgments

  1. Top of page
  2. Footnotes
  3. Acknowledgments
  4. References

This work was not supported by a grant or otherwise.

The authors thank VetGen for their generosity by donating their services and performing SCID gene testing on our affected filly. The authors also thank Dr Virginia Buechner-Maxwell and Melissa Makris from the flow cytometry laboratory for performing flow cytometry on blood collected from the affected filly and the age-matched half-sibling.

References

  1. Top of page
  2. Footnotes
  3. Acknowledgments
  4. References
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