Patterns of Disease: Gastrointestinal Tract

Question 1

NORMAL GI TRACT STRUCTURE AND FUNCTION

Answer 1

Mucosa, submucosa, associated glands, muscularis (inner circular, outer longitudinal layer), adventitia/serosa.
Myenteric and Meissner’s (submucosal) plexus make up the enteric nervous system.
Peyer’s patches- lymphoid tissue, undertake immune surveillance.
Mucosa is specialised in different ways depending on location in the tract.
FUNCTION:
-PROVIDE A SELECTIVELY PERMEABLE BARRIER- allowing water, nutrient and electrolyte absorption and excluding pathogens and toxins. Distinguishing harmful and harmless antigens.
-Provided by interactions between epithelial cells in mucosa, mesenchymal cells and leukocytes in lamina propria and enteric nervous system.

Question 2

DEFENCE MECHANISMS AND MUCOSAL IMMUNITY

Answer 2

PHYSICAL BARRIER- epithelial tight junctions, rapid epithelial cell turnover (at villus tips), peristalsis, secreted products (mucins, defensins, lactoferrin*), pancreatic and biliary secretions (bacteriostatic)
COMMENSAL BACTERIA- modulate pH to provide an unfavourable environment for pathogens.
IgA/IgE ANTIBODY SECRETION- adaptive immunity. IgA protects mucosal surfaces.

*Mucins- bind and immobilise pathogens
Defensins- inactivate pathogens
Lactoferrin- iron sequestration.

Question 3

ENTERIC IMMUNITY

Answer 3

M cells are found amongst the enterocytes that make up the mucosal lining of the gut.
They have no mucous covering, so are exposed directly to the GI tract.
They take up large molecules and present them to Peyer’s Patches, allowing release of immune cells.
M cells are a good point of entry for pathogens.

Question 4

ADAPTIVE IMMUNITY

Answer 4

EPITHELIUM- pattern recognition receptors- TLRs on apical membrane, NOD in cytosol. These recognise PAMPs, bind to pathogens and activate cell signalling pathways to release cytokines, chemokines etc.
MHC II expression at basolateral membrane.

MUCOSAL ASSOCIATED LYMPHOID TISSUE

1. Diffuse MALT- intraepithelial and lamina proprial compartments, CD8+ T cells predominate.
2. Peyer’s patches- increase in density in distal small intestine. B cells (lymphocytes) predominate.

DENDRITIC CELLS- present antigen to lymphocytes within Peyer’s patches and mesenteric lymph nodes, resulting in formation of an immune response

Question 5

METHODS OF ENTRY IN TO GI TRACT

Answer 5

1. Pathogens bind to elements in MUCOUS LAYER eg. Brachyspira hyodysenteria
2. Bind to CILIA and alter enterocyte function eg. enterotoxigenic E. coli- toxin production; enteropathogenic E. coli- alter enterocyte structure and function.
3. Infect and thrive within MACROPHAGES eg. Mycobacterium avium sspp. pseudotuberculosis.
4. Enter via infection of DENDRITIC cells.
5. Enter via spread through TIGHT JUNCTIONS eg. Leptospira, feline calicivirus.
6. Enter via M CELL (no mucous coating) macropinocytosis eg. Salmonella.
7. Target MUCOSAL NEURONS and spread to CNS eg. Listeria monocytogenes.

Question 6

MECHANISMS OF PATHOGEN INDUCED GI DISEASE

Answer 6

• NON INFLAMMATORY- pathogen-induced enterocyte hypersecretion eg. ETEC.
• Increased cellular turnover; immature enterocytes are less functional.
• INFLAMMATORY- pathogen induces inflammatory response, enterocytes are damaged as ‘bystanders’ eg. Salmonella.
• direct damage to enterocytes by bacterial toxins eg. Clostridium.

Functional/architectural changes can be present with little or no histological change- clinical signs can still be severe.

Question 7

ACUTE GI DISEASE

Answer 7

1. Parvoviral enteritis of dogs and cats
2. Equine colitis
3. Malignant catarrhal fever
4. Mucosal disease.

Question 8

PARVOVIRAL ENTERITIS OF DOGS AND CATS

Answer 8

Caused by two closely related viruses- Feline Panleukopaenia Virus (FPV) and Canine Parvovirus 2 (CPV2).
Mink Enteritis Virus is closely related as well.
These viruses replicate most efficiently in actively dividing cells.
Enteritis caused is similar in dogs and cats- infection of crypt epithelium destroys stem cells/proliferative compartment; replication decreases.
Produces ‘radiomimetic’ lesions- mimic effects of radiation, producing large, bizarre cells which take a long time to recover.
Gross lesions- segmental haemorrhagic enteritis (CLASSIC PRESENTATION), granular appearance of serosa +/- serosal petechiae, evidence of dehydration, evidence of Gram negative sepsis (often cause of death).

Question 9

PARVOVIRUS- EXTRAINTESTINAL LESIONS

Answer 9

DOG- 3 syndromes:

1. Generalised disease (intestine, other organs)- seen in neonates.
2. Myocarditis and enteritis (rapid turnover causes systemic disease)
3. Enteritis- older animals.

CAT- Cerebellar hypoplasia, bone marrow suppression, lymphoid necrosis

Question 10

EQUINE COLITIS

Answer 10

Seen in USA, Canada.
Salmonellosis or Clostridiosis (or Neorickettsia- Potomac Horse Fever)
Risk factors are similar for both- asymptomatic carriage, stress or antibiotic treatment producing bacterial overgrowth and thus clinical disease.
Gross and histological lesions do not differentiate between causative bacteria.
GROSS- oedematous, thickened, green fibrinonecrotic/pseudomembranous adherent material.

Question 11

SALMONELLOSIS

Answer 11

S. TYPHIMURIUM is the most commonly isolated.
3 syndromes are associated with infection:
1. PERACUTE- septicaemic, affects animals 1-6 months old. Similar to Dublin necrosis in calves.
2. ACUTE- fibrinonecrotic typhlocolitis (affects caecum and colon)
3. CHRONIC- gross lesions highly variable.

Question 12

SALMONELLOSIS- PATHOGENESIS

Answer 12

Bacteria enter via enterocytes or M cells (specialised cells, can phagocytose large molecules).
Virulence factors include- flagella (motility), fimbriae (epithelial binding), LPS cell wall (resists degradation)
Infect and survive within macrophages, inhibiting phagolysosome fusion and stimulating neutrophil chemotaxis.
Diarrhoea is caused by- mucosal damage by neutrophils, secondary to inflammation; bacterially mediated chloride hypersecretion.

Question 13

SALMONELLOSIS- DIAGNOSIS

Answer 13

Bacterial culture- beware false positives (asymptomatic carrier) and negatives (post antibiotic treatment).
Salmonella have fastidious growth requirements- not seen on normal aerobic culture; request Salmonella culture from lab.
Compatible histopathology in post mortem cases.

Question 14

CLOSTRIDIAL COLITIS- COLITIS X

Answer 14

Adult horses, associated with stress (movement) or antibiotic treatment.
C. perfringens A- adult, C. perfringens C- foal are most commonly associated.
Cause diffuse fibrinonecrotic or haemorrhagic and ulcerative typhlocolitis (adults).
Virulence factors- TOXINS (bacteria alone are apathogenic)- bacteria proliferate within the gut lumen and release endotoxin.
Diarrhoea caused by- direct damage to enterocytes with secondary neutrophil chemotaxis.
C. perfringens- B and B2 toxin- pore forming. B2 toxin has been induced in vitro by aminoglycoside treatment- do NOT treat with antibiotics.
C. difficile- A and B toxin, act to disrupt cytoskeleton.
Different types of C. produce different toxins.

Question 15

CLOSTRIDIAL COLITIS- DIAGNOSIS

Answer 15

• Culture- clostridia can be normal gut commensals; asymptomatic carriage is normal. Positive culture is not significant.
• Toxin detection- IS significant, allows determination of type of Clostridia. Faecal samples are submitted to lab for ELISA.

Question 16

POTOMAC HORSE FEVER

Answer 16

Neorickettsia risticii- obligate intracellular bacteria. Causes Equine Monocytic Ehrlichiosis aka. Potomac Horse Fever.
Highly geographically localised- river valleys in N.E USA.
Trematode intermediate host, passes to mayfly- accidental ingestion of mayfly by horse can result in infection of monocytes and disease:
Fibrinonecrotic to ulcerative colitis, abortion in pregnant mares, secondary sepsis/laminitis common, mortality up to 30% in untreated cases.

Question 17

BOVINE VIRAL DIARRHOEA VIRUS

Answer 17

Pestivirus, 2 genotypes- BVDV type I (UK) and II (USA).
Non-cytopathic (NCP) and cytopathic (CP) biotypes can be seen within each genotype- assessed by effect on cell culture.
Recombination of non-structural protein NS2-3 results in biotype switch from NCP to CP.
Associated syndromes:
-Non pregnant, immunocompetent animal- transient diarrhoea, leukopaenia, fever. Non-cytopathic.
-Type II disease- acute, fatal ulcerative disease in immunocompetent. Most often associated with genotype II. Indistinguishable from mucosal disease, but is seen in adults.
-Thrombocytopaenic syndrome- seen in calves, superimposed on to or separate from type II disease. Petechiae, bleeding at injection sites etc. seen due to natural infection with BVDV.

Question 18

BOVINE VIRAL DIARRHOEA VIRUS contd.

Answer 18

IN UTERO infection with NCP biotype- outcome depends on stage of pregnancy:
< 4 MONTHS GESTATION- embryonic resorption, abortion or PERSISTENT INFECTION (of calf).
4-6 MONTHS GESTATION- congenital malformations of the eye, CNS- microphthalmia, cerebellar hypoplasia, hydrocephalus etc.
>6 MONTHS GESTATION- calf born alive, clinically normal but seropositive.

Question 19

MUCOSAL DISEASE

Answer 19

BVDV persistently infected (PI) animals are a sourve of infectious virus in a herd.
NCP to CP transformation in the animal causes acute, fatal MUCOSAL DISEASE betwen 6 months and 2 years of age.
BVDV is pancytotropic- it affects many cells- virus can be detected in a wide variety of tissues by immunohistochemistry.
Gross lesions are generally confined to the GI tract (oral cavity to anus), can be associated with Peyer’s patches in the small intestine.

Question 20

MUCOSAL DISEASE- DIAGNOSIS

Answer 20

Disease occurs in virus positive, seronegative animals with compatible gross and histological lesions.
Viral detection- immunohistochemistry- pancytotropic so submission of fixed skin sample is sufficient.

Question 21

MALIGNANT CATARRHAL FEVER

Answer 21

Multisystemic disease, seen in a wide variety of animals; most commonly cattle (deer, bison, water buffalo, pigs also)
Gamma herpes virus- shed by asymptomatic carrier.
Ovine herpesvirus 2- sheep associated malignant catarrhal fever (MCF)
Caprine herpesvirus 2- goat associated MCF
Acelaphine herpesvirus 2- wildebeest associated MCF.

Question 22

MALIGNANT CATARRHAL FEVER- PATHOGENESIS

Answer 22

Virus infects CD8+ (cytotoxic) and large granular T lymphocytes.
Lesions are associated with vasculitis in GI tract, kidney, eye, brain etc; and with lymphoproliferation (vessels filled with activated lymphocytes).

GROSS- wide lesion variation, potentially pansystemic.

• GI tract- oral cavity, forestomachs, less frequent in intestine and colon.
• Genitourinary tract- ulcerative cystitis, nephritis.
• Ophthalmitis and conjunctivitis, pneumonia, portal vasculitis in liver etc.

Question 23

MALIGNANT CATARRHAL FEVER- DIAGNOSIS

Answer 23

Differentiation from mucosal disease can be difficult.
MCF is characterised by- lesions outwith the GI tract are often present (cf. only GI lesions in MD),
lymphoid hyperplasia IS present in MCF, NOT present in MD (lymphoid atrophy/necrosis)

Question 24

CHRONIC GI DISEASE

Answer 24

1. Johne’s Disease

2. Histiocytic ulcerative colitis of Boxer dogs.

Question 25

JOHNE'S DISEASE

Answer 25

Chronic enteritis of ruminants caused by Mycobacterium avium sspp PSEUDOTUBERCULOSIS. Clinical signs- chronic ill-thrift, weight loss, watery diarrhoea. GROSS- diffuse thickening of mucosa and submucosa, corresponds to a granulomatous enteritis. Granulomatous lymphangitis and lymphadenitis.

Question 26

JOHNE'S DISEASE- PATHOGENESIS

Answer 26

Infection occurs in the neonate- macropinocytosis in the immature gut facilitates infection. Myobacteria are phagocytosed by macrophages. Once inside macrophages, they inhibit phagolysosome function and persist. Persistent macrophage stimulation incites CHRONIC GRANULOMATOUS RESPONSE. Diarrhoea is caused by alteration in mucosal function by inflammatory process, and alteration of lymph flow in occluded lymphatics causing protein losing enteropathy.

Question 27

JOHNE'S DISEASE- DIAGNOSIS

Answer 27

1. Detection of infectious agent- faecal culture (slow growing and fastidious) - faecal or mucosal smear- acid fast stain. Low sensitivity. - PCR- similar sensitivity to culture. 2. Serology- ELISA on milk or blood.

Question 28

HISTIOCYTIC ULCERATIVE COLITIS

Answer 28

Predominantly a disease of Boxers (and Boston terriers), usually less than 2 years old. Rarely reported in non-Boxers. Chronic large intestinal diarrhoea, haematochezia, with or without ill thrift. Similar histologically to Johne's- large, foamy macrophages. GROSS- Marked thickening of the colonic lamina propria and submucosa- corresponds to marked infiltration of foamy macrophages containing abundant Periodic acid-Schiff (PAS) positive granules. Raised, ulcerative nodules in mucosa. Lesions are confined to submucosa, so often require a full thickness biopsy to investigate.

Question 29

HISTIOCYTIC ULCERATIVE COLITIS- PATHOGENESIS

Answer 29

Mycoplasma and Chlamydia previously suspected. PAS positive granules in macrophages identified as phagolysosomes, some containing bacteria. INVASIVE E. COLI detected by FISH (Fluorescent In Situ Hybridisation). Remission achieved with fluoroquinolone treatment, and correlated with eradication of E. coli in mucosa. Macrophage dysfunction also suggested as a cause.

Question 30

EQUINE GRASS SICKNESS

Answer 30

Acute, usually fatal idiopathic acquired dysautonomia (autonomic nervous system derangement). Clinical signs are attributable to a derangement of the ANS, mostly manifested as GI obstruction- functional disturbance. Acute, subacute and chronic presentations can be seen. GROSS- lesions are often subtle to absent on PME. 'Mal de Seco'- dry disease- colonic contents is often very dry and adherent to colonic mucosa.

Question 31

EQUINE GRASS SICKNESS- PATHOGENESIS

Answer 31

Ingested toxin is highly suspected- Clostridium botulinum? Serum from affected horses will cause disease when give IV- serum toxin causes disease. Degenerative changes detected in myenteric and submucosal plexus (neuronal necrosis) and cranial mesenteric and cranial cervical ganglia (non-GI associated ganglia- suggests toxin is not just in the GI tract; it is circulating).

Question 32

EQUINE GRASS SICKNESS- DIAGNOSIS

Answer 32

Clinical diagnosis can be difficult. Definitive diagnosis- identification of chromatolysis of neurons within the submucosal and myenteric plexus in ileal biopsy material (full thickness- surgical procedure). Immunohistochemistry for synaptophysin can be useful in ambiguous cases. Synaptophysin is a neuronal marker; EGS affected neurons will stain differently with synaptophysin. This test can be requested for difficult diagnoses.