JPC SYSTEMIC PATHOLOGY
Signalment (JPC #822016): Unknown age cow
HISTORY: A cow with chronic diarrhea
HISTOPATHOLOGIC DESCRIPTION: Lymph node: Multifocally, effacing up to 40% of both cortex and medulla are coalescing areas of lytic necrosis, with loss of normal architecture and replacement by eosinophilic cellular and karyorrhectic debris, admixed with fibrin and hemorrhage. Multifocally there is lymphcytolysis and low numbers of viable and degenerate neutrophils, surrounded by aggregates of foamy macrophages and few multinucleate giant cells of both Langhans and foreign body type that extend into and fill cortical and medullary sinuses. Macrophages and giant cells often contain indistinct negative staining 0.3 x 1.5 µm intracytoplasmic bacilli and golden-brown granular pigment (hemosiderin). Sinuses also contain low numbers of plasma cells and neutrophils and individual necrotic cells.
Acid fast stain: There are numerous acid fast 0.3 x 2 µm bacilli within macrophages and free within sinuses and necrotic debris.
MORPHOLOGIC DIAGNOSIS: Lymph node: Lymphadenitis, necrotizing and pyogranulomatous, multifocal, marked with numerous acid fast intrahistiocytic bacilli, breed unspecified, bovine.
ETIOLOGIC DIAGNOSIS: Mycobacterial lymphadenitis
CAUSE: Mycobacterium avium subsp. Paratuberculosis (Map)
CONDITION: Johne's disease, paratuberculosis
- Small, aerobic, nonspore-forming, nonmotile, weakly gram-positive, acid-fast (specifically Ziehl-Neelsen) bacilli (acid-fast due to mycolic acid in cell wall); widely distributed saprophytes and may survive in the soil for years
- Mycobacterium avium paratuberculosis is the causative agent of Johne’s disease; a chronic, irreversible wasting disease of ruminants
- Infection of newborn and young animals is followed by a clinically in apparent period (with intermittent shedding) that may last for years before clinical manifestation > these animals are an important source of infection
- Lesions can be found in many ruminant species and typically consist of granulomatous enteritis and mesenteric lymphadenitis
- Adults are relatively resistant
- Younger animals more susceptible
- Disease causes intestinal malabsorption and presents as chronic wasting with diarrhea; young animals less than 6 months of age are most susceptible to infection, but clinical signs do not usually occur until animals are over 2 years of age
- Susceptibility increased in Channel Island breeds (Guernsey, Jersey) & Shorthorns
- Transmission is fecal-oral, in utero, and via colostrum or milk > organisms bind to receptors on the luminal surfaces of M-cells > translocated in endocytic vesicles or phagosomes to basal surface > released into Peyers patches > phagocytosed by macrophages
- TLRs, mannose, CD14 and complement receptors are involved in recognition, phagocytosis and cell signaling
- Mycobacteria have protective lipid-rich cell wall and produce peroxidasesà inhibit phagosome acidification, phagosome-lysosome fusion, lysosomal enzymes & oxygen radical formation, disruption of cytokine production (i.e. IFN-γ) and allows bacteria to avoid proteolytic enzymes produced by the host cell> prolonged incubation > draining lymph nodes > thickening of LP with macrophages > sloughing of the mucosal epithelium, villus atrophy, progressive malabsorption> diarrhea, hypoproteinemia, protein-losing enteropathy, ill thrift > emaciation > death
- Virulence factors / mechanisms of modulation of the immune response:
- Complex cell wall: Permeation barrier
- Mannose-capped lipoarabinomannan (Man-LAM): Mycobacterial cell wall receptor; major virulence factor; inhibits macrophage activation and phagosome maturation by inducing marked expression of IL-10
- Toll-like receptor 2 (TLR2): Major signaling receptor binds Map and initiates early production of IL-10 through the mitogen-activated protein kinase (MAPK)-p38 pathway; integrins are not important in Map-induced signaling
- IL-10 suppresses macrophage activation, suppresses TH1 responses (suppresses TNF-α, IL-12, and IL-8), enhances TH2 responses, decreases MHC class II expression, and decreases apoptosis
- Phagosome acidification and phagolysosome fusion blocked by TLR2 signaling, MAPK-p38 activation, and IL-10
- Mycobacteria require iron for growth within phagosomes of tissue macrophages; iron concentration is greatest in macrophages of the ileocecal intestine, resulting in tissue specificity for this region in Johne’s disease
- Mycobacteria remove iron from macrophage ferritin using iron-chelating proteins called exochelins, iron-reductases and possibly siderophores
- These may also block iron-dependent bacteriocidal reactions (i.e. Fe3+ dependent conversion of H2O2 to toxic hydroxyl radicals)
TYPICAL CLINICAL FINDINGS:
- Clinical signs may be intermittent or absent
- Profuse diarrhea, rough hair coat, chronic weight loss
- Cattle retain a good appetite and bright attitude
- In sheep and goats, frank diarrhea is uncommon; weight loss is often the predominant clinical sign
- Hypercalcemia (chronic granulomatous disease); hypoproteinemia
TYPICAL GROSS FINDINGS:
- Emaciation, marked loss of muscle mass, serous atrophy of fat, and intermandibular edema
- Segmental thickening of the ileocecal intestine with transverse corrugation of the mucosa by granulomatous infiltration of the lamina propria and submucosa
- Enlarged/edematous mesenteric lymph nodes
- Lymphangitis is common and may be the only recognizable gross feature; thick serosal cords track through the mesentery to draining lymph nodes
- Subintimal fibrosis and mineralization occur in the thoracic aorta
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Two forms: Lepromatous (multibacillary) or tuberculoid (paucibacillary)
- Lepromatous forms: Intestinal lamina propria expanded by sheets of large macrophages with abundant foamy cytoplasm and variable numbers of multinucleate cells, lymphocytes, plasma cells, and eosinophils, and abundant intrahistiocytic acid fast bacilli
- Tuberculoid forms: Few epithelioid macrophages and numerous lymphocytes, plasma cells, and few bacilli
- Occasional mineralization
- Lymphoplasmacytic to granulomatous lymphangitis
- Mesenteric lymph nodes: Early histiocytosis of the subcapsular sinus, progressing to coalescing areas of histiocytic/granulomatous lymphadenitis that can replace much of the cortex and infiltrate the medullary sinuses
- bovis: classic tubercles with caseous necrosis, nodule formation, and calcification
- avium intracellulare: caseous tubercles in birds and a variety of mammals
- Corynebacterium pseudotuberculosis- caseous lymphadenitis in sheep & cattle
- Infects large range of domestic and exotic ruminants:
- Camels, llamas, yaks, bison, antelope, rhinoceroses, wild goats and big horn sheep
- Sheep, goats and deer tend to have more tuberculoid lesions
- Goats: A recent study found that in kids subclinically infected with MAP, initial bacterial-host interactions occurred within the GALT, with lesions occurring as early as 3 months post-inoculation and most commonly consisting of granulomatous inflammation of jejunal and ileocecal valve Peyers patches
- Uncommon findings included crater-like mucosal ulcerations and granulomatous arteritis of small to medium submucosal vessels
- There was no continuous progression in the severity of lesions and there was evidence that regression of lesions may occur
- Immunohistochemistry was markedly less sensitive than bacterial culture for detecting MAP-positive tissue
- Unlike most goats, pygmy goats can develop severe diarrhea and sudden death
- Sheep: Map can affect both lambs and adults but they need high dose to exhibit clinical lesions and immune response; adults (>2yrs) are more resistant and lesions are generally restricted to intestinal lymphoid tissue, while lambs have more multifocal to diffuse lesions in both lymphoid tissue and intestinal mucosa
- Liver core biopsies have a high sensitivity and specificity for diagnosis of ewes with MAP via presence of hepatic epithelioid macrophage microgranulomas
- Implicated in Crohn's disease in humans, though not proven
- Swine: avium complex (especially M. avium subsp. hominissuis) is important in swine which may be carriers; usually asymptomatic but with granulomatous lymphadenitis/hepatitis (especially mesenteric, with calcification) on necropsy
- Natural infections of rabbits, and horses have been documented; typically gross lesions in these species are absent and histological lesions are mild
- Laboratory animals are generally resistant, but infection has been transmitted to mice, hamsters, guinea pigs, rabbits, and macaques
- Marsupials (koalas, dasyurids, numbats, bandicoots, and possum) are susceptible to Map infections
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