Seven-year-old quarter horse mare (Equus ferus caballus).In June 2015,
the horse presented to the referring veterinarian with bilateral conjunctivitis
that progressed to severe anterior uveitis in the left eye. Foot abscesses,
distal limb cellulitis, mandibular lymphadenopathy, nasal discharge, and hives
developed subsequently. Treatments included ceftiofur, oxytetracycline,
dexa-methasone, nonsteroidal anti-inflammatory drugs, and a two-week course of
doxycycline. Despite treatment, the horse remained hyperfibrinogenemic at
800-1300 mg/dL and developed narcolepsy a few months later. Due to health
concerns and the poor prognosis, the horse was euthanized in January 2016 and
submitted to Cornell Animal Health Diagnostic Center for necropsy and tissue
was approximately 200 mL of yellow tinged transparent fluid (serous effusion)
within the peritoneal cavity. The capsular surface of the liver was diffusely
thickened, mottled white to tan to purple to black. There were thousands of
multifocal to coalescing, generalized, white, 1-3 mm, hard white nodules along
the capsular surface with a few dozen similar nodules within the parenchyma.
Similar nodules were present in the thymus and surrounding the mediastinal fat
and in all lung lobes. These nodules were presumed to be parasitic granulomas,
which were confirmed histologically. Evidence of chronic laminitis was present
in both forelimbs. The brain was grossly normal.
cerebrum, and cerebellum: Diffusely, meninges of the cerebellum and cerebrum
are markedly expanded by a dense infiltrate of lymphocytes, macrophages, fewer
neutrophils, and rare plasma cells interspersed with rare wispy spirochetal
bacteria. Within the neuroparenchyma, the Virchow-Robin spaces of blood vessels
are surrounded by a similar inflammatory infiltrate. Blood vessels are often
prominent, characterized by endothelial hypertrophy and have variable
branching. In the white and grey matters are increased numbers of enlarged
glial cells with increased eosinophilic cytoplasm (astrocytes, presumptive).
Multifocally within the choroid plexus are clusters of lymphocytes and
histiocytes, along with few eosinophils.
1. Brain, cerebrum, and cerebellum: Severe,
multifocal to coalescing, chronic lymphohistiocytic neutrophilic meningo-encephalitis
with lymphohistiocytic eosinophilic choroid plexitis, branching blood vessels,
astrocytosis, and rare intralesional spirochetal organisms. Other final
morphologic diagnoses (slides not included):
meningo-myeloencephalitis and radiculoneuritis of the spinal cord and ganglia,
multifocal, chronic lymphocytic hypophysitis
chronic parasitic granulomas in the liver, lung, and thymus
5. Chronic lymphoplasmacytic
portal hepatitis with capsular and bridging fibrosis
laminitis and chronic foot abscess
depletion in spleen and thymus
in early December 2015 revealed elevated gamma glutamyl transferase (GGT) at 61
U/L (normal range 9-24 U/L), rising globulins at 2.3 g/dL from a previous value
of 1.6 g/dL (normal range 2.8-4.7 g/dL), and a lymphocyte count of 1,940
cell/uL, up from a previous count of 1,380 cells/uL (normal range 1,000-4,900 cells/uL;
lymphopenia is <1,500 cells/uL).
B cell concentration was markedly decreased at 19 cells/uL of 1,940 total lymphocytes/uL (0.98% B cells), with 1.0% CD19 B cells (median, CI = 9.0%, 2.0%), 0.2% CD21 B cells (median, CI = 10.2%, 4.2%), and 0.9% IgM B cells (median, CI = 10.2%, 2.1%).
The CD4+ and CD8+ T-cell distributions were slightly increased, and the CD4/CD8 ratio was within the normal reference interval.
Serum IgG concentration was markedly decreased at 423 mg/dL (median, CI = 1,760 mg/dL, 603 mg/dL) and serum IgM concentration was within the normal reference interval at 63 mg/dL (median, CI = 100 mg/dL, 50 mg/dL; deficiency is < 25 mg/dL).
Bacterial cultures and virus isolation of brain tissue were both negative. A quantitative PCR for Borrelia burgdorferi yielded a CT value of 32 (positive result).
Meningoencephalitis/Mutated feline enteric coronavirus (FIP)
stain of a section of brain with Modified Steiner silver stain and
immunohistochemical (IHC) stain for Borrelia burgdorferi confirmed
spiral organisms within areas of inflammation in the meninges of the cerebellum
and cerebrum. IHC stains of section of cerebrum for eastern equine encephalitis
virus, equine herpes virus-1, West Nile virus, and rabies virus yielded no
immunoreactivity. Inflammatory cells were strongly immuno-reactive to CD3 and
IBA1; however, rare CD20 and no Pax5 immunoreactivity were detected, confirming
lack of plasma cells in areas of inflammation and consistent with common
variable immunodeficiency (CVID).
CVID is a primary immunodeficiency disease of humans and horses that encompass a group of heterogenous disorders characterized by hypo-gammaglobulinemia. Generally, at least two isotypes of antibodies are affected, although IgG deficiency alone is recognized. Human CVID patients often present with recurrent respiratory infections and have a high frequency of autoimmune and lympho-proliferative disease.1,8,9 It is one of the most common primary immuno-deficiencies reported in humans, with an incidence rate of 1 in 25,000 humans. In horses, CVID is a rare condition with relatively few cases reported,1,2,3,10,14 though the Equine Immunology Laboratory at Cornell College of Veterinary Medicine has diagnosed this condition in over 50 horses since 2002 and has been actively investigating potential genetic and epigenetic mechanisms of disease.12 Current research on equine CVID focuses on the disruption of B cell development in the bone marrow, and has identified decreased mRNA expression and incomplete demethylation of the PAX5 gene, required for commitment and differentiation of B cells.12,13
Like human patients, horses clinically manifest with recurrent infections of the respiratory tract. In addition, persistent bacterial meningitis has been associated with infection by common skin contaminants such as Staphylococcus spp.,3,10 while Borrelia burgdorferi has been highly suspected in other cases of meningitis. One case report of CNS and PNS inflammation in a CVID horse documented a positive Western blot analysis result with low to moderate Borrelia burgdorferi antibody response in serum and a positive PCR assay result from CSF using primers for the outer surface protein A (ospA) gene.5 These tests confirm exposure to the bacterium; however, neither test demonstrates active Borrelia burgdorferi infection within areas of CNS inflammation. Lyme neuroborreliosis in horses, as with most species, is characterized by suppurative or non-suppurative, lymphoplasmacytic, histiocytic perivascular to diffuse inflammation most severely affecting the CNS, including the meninges, ganglia, and cranial and spinal nerve roots, with varying degrees of necrosis, fibrosis, and neuro-parenchymal invasion.4
In the present case, the inflammation is predominately lymphocytic and histiocytic and the distribution includes the spinal cord and ganglia, meninges, choroid plexus, pituitary gland, and neuroparenchyma. By histochemistry and immunohistochemistry, rare spirochetal organisms were present within areas of perivascular inflammation, while a quantitative PCR confirmed the presence of Borrelia burgdorferi nucleic acid in the affected cerebrum . The history of uveitis and narcolepsy, the clinical data, histologic findings of severe meningo-myeloencephalitis, choroid plexitis, and hypophysitis, and ancillary testing are consistent with Lyme neuroborreliosis.4,11 The severe inflammation, fibrosis and parasitic granulomas in the liver, lung, and thymus are attributed to massive parasitic migration; a finding consistent with CVID and a lack of antibody response to parasitic antigens.2,14 The lack of humoral immunity, the primary host defense mechanism against Borrelia burgdorferi, likely contributed to chronic Lyme disease in this horse with CVID.
Cerebrum: Chorio-meningoencephalitis, lymphohistiocytic,
multifocal to coalescing, marked, quarter horse, Equus ferus caballus.
Lyme neuro-borreliosis is an uncommon manifestation of Lyme
disease caused by Borrelia burgdorferi sensu lato infection in
the nervous system, and is typically associated with immunosuppression in
horses, humans, and experimental laboratory animal models.4-6 The
contributor provides an outstanding demonstration of that patho-genesis in this
case of natural infection in a horse with common variable immuno-deficiency
(CVID). As mentioned above, CVID is associated with a late-onset B cell
lymphopenia and hypo-gammaglobulinemia with marked decrease in serum IgG. CVID
typically manifests as opportunistic recurrent pneumonia, septicemia, and
a White-footed mice are the principal reservoir host for B. burgdorferi in the endemic Northeastern United States, and the bacteria are transferred to susceptible host species by the Ixodes sp. tick vector. B. burgdorferi localizes in the digestive tract of ixodid ticks via its outer surface protein A (OspA) after feeding on an infected reservoir host.7 When the vector attaches to a susceptible mammalian host and takes a blood meal, there is a subsequent increase in temperature within the tick digestive tract. This change in temperature represses OspA expression and induces OspC synthesis.
This new conformation allows the spirochete to localize to the salivary glands of the tick. Interestingly, this change in conformation can take as long as 48 hours to complete, necessitating the prolonged attachment of the tick to the host. The spirochete then enters the host via the ticks salivary secretions during feeding.7
Previous reports of borreliosis in horses have documented arthritis, uveitis, encephalitis, and ataxia.5 Uveitis, present in this case, is the most common reported extra-neural manifestation of B. burgdorferi infection in horses, but is rarely reported in other species.6 The most common mani-festation of disease in dogs is polyarthritis, with fewer cases of membranoproliferative glomerulonephritis.4-6 Equine neuro-borreliosis is challenging to diagnose clinically due to the wide variability in clinical presentation and current lack of reliable antemortem diagnostic tests; however, the conference moderator instructed that the index of suspicion for Lyme disease should be high in horses that present with neurologic deficits and concurrent uveitis.4-6
Few conference participants included Lyme disease as a differential diagnosis in this case. Most favored a viral encephalitis caused by an alphavirus (EEE, WEE, VEE), rabies, or West Nile virus due to the relatively non-specific lymphohistiocytic inflammation in this case. Others included equine protozoal myelitis caused by Sarcocystis neurona; however, one would expect to see necrotizing granulomatous and eosinophilic lesions, which are not a feature of this case.6 In conjunction with the excellent images provided by the contributor, the Joint Pathology Center ran a Warthin-Starry silver stain, which highlights numerous argyrophilic spirochetes consistent with B. burgdorferi within the inflamed neuroparenchyma. This case demonstrates the importance of including Lyme disease as a differential diagnosis in horses with neurologic disease.
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