SPECIAL PATHOLOGY Jhon-Ryan M. Perez DVM-6

Corynebacterium Gram-positive, pleomorphic bacteria occur in coccoid, club and rod forms (coryneform morphology In stained smears, they occur singly, in pallisades of parallel cells and in angular clusters resembling Chinese letters Fastidious, requiring enriched media Majority are commensals on mucous membranes Cause pyogenic infections Corynebacteriurn species: -non-motile facultative anaerobes -catalase positive, oxidase negative

Differentiation of the corynebacteria Most pathogenic corynebacteria are relatively host specific produce identifiable clinical syndromes The host species and the nature of the disease may suggest the causal agent.

CASEOUS LYMPHADENITIS Caseous lymphadenitis (CL) is a chronic, contagious disease caused by the bacterium Corynebacterium pseudotuberculosis. Although prevalence of CL varies by region and country, it is found worldwide and is of major concern for small ruminant producers in North America. The disease is characterized by abscess formation in or near major peripheral lymph nodes (external form) or within internal organs and lymph nodes (internal form). Although both the external and internal forms of CL occur in sheep and goats, the external form is more common in goats, and the internal form is more common in sheep.

Etiology and Pathogenesis C pseudotuberculosis is a gram-positive, facultative, intracellular coccobacillus. Two biotypes have been identified based on the ability of the bacteria to reduce nitrate: a nitrate-negative group that infects sheep and goats a nitrate-positive group that infects horses. All strains produce an exotoxin called phospholipase D that enhances dissemination of the bacteria by damaging endothelial cells and increasing vascular permeability.

Replication of bacteria occurs in the phagocytes, which then rupture and release bacteria. The ongoing process of bacterial replication, followed by attraction and subsequent death of inflammatory cells, forms the characteristic abscesses associated with CL. To establish infection, C pseudotuberculosis must penetrate skin or mucous membranes. Once the bacteria have entered the body, they move to the lymph nodes via the regional draining lymphatic system. Internally, the bacteria establish infection not only in the lymph nodes but also in the viscera. The incubation period varies from 1 to 3 mo, culminating in development of encapsulated abscesses. C pseudotuberculosis is hardy in the environment and can survive on fomites such as bedding and wood for 2 mo and in soil for 8 mo. The presence of organic material, shade, and moisture favor and enhance survival.

Transmission Sheep become infected through contamination of shearing wounds, by arthropod bites or from contaminated dips. Infection is spread by pus from ruptured abscesses and from nasal and oral secretions. The organism can survive in the environment for several months.

Post-Mortem Lesions Infection results in abscessation and enlargement of superficial or internal lymph nodes. Affected lymph nodes are enlarged and exhibit characteristic encapsulated abscesses which have an 'onion ring' appearance in cross-section. The abscess material is caseous, initially greenish and later putty-coloured. Haematogenous spread can lead to abscessation of internal lymph nodes without obvious superficial lesions. Ill-thrift and pneumonia may be present. The visceral form of the disease may not be detectable antemortem. Goats usually develop the superficial form of the disease with subcutaneous abscesses in the head and neck regions.

Diagnosis The disease may be suspected on clinical grounds or at post-mortem examinations. Smears from lesions may reveal Gram-positive and others from environmental sources. coryneform bacteria. Isolation and identification of C. pseudotuberculosis from abscess material is confirmatory. A sandwich ELISA, which detects circulating antibodies directed against the exotoxin, has been developed for identifying infected sheep (Schreuder et al., 1994).

Treatment Because of the chronic nature of lesions and the ability of the organisms to survive intracellularly, therapy is usually ineffective

Control Appropriate control measures for individual countries are determined by the prevalence of the disease. Exclusion of caseous lymphadenitis from countries free of the disease: -Sheep and goats should be imported only from countries which are either free of the disease or have a low incidence of infection. Animals must be selected from flocks or herds officially certified to be free of infection for 3 years. -Animals should be subjected to pre-importation ELISA testing. -Imported animals should be quarantined for several months and infected animals should be slaughtered.

Animals with obvious lesions should be segregated and culled. -Regular testing of flocks or herds using ELISA should be followed by culling of animals with positive or doubtful results. -Lambs can be removed from seropositive dams at birth and reared artificially. -Contaminated buildings and equipment should be thoroughly disinfected.

Mycobacterium Acid-fast (ZN-positive) rods Cell walls rich in complex lipids and waxes containing mycolic acids Complex egg-enriched media required for growth of pathogenic species , Aerobic, non-motile, non-spore-forming Genus includes obligatc pathogens, opportunistic pathogens and saprophytes Pathogenic species grow slowly, colonies visible after several weeks Some mycobacteria produce carotenoid pigments Resistant to chemical disinfectants and environmental influences but susceptible to heat treatment (pasteurization) Multiply intracellulary and cause chronic, granulomatous infections Major discases include tuberculosis, Johne's disease and feline leprosy

Usual habitat Environmental mycobacteria are found in soil, on vegetation and in water. Obligate pathogens, shed by infected animals, can also survive in the environment for extended periods (Morris et a)., 1994).

Differentiation of pathogenic mycobacteria The ZN staining method is used to differentiate mycobacteria from other bacteria. Differentiation of pathogenic mycobacteria relies on cultural characteristics, biochemical tests, animal inoculation, chromatographic analyses and molecular techniques. Mycobacteria associated with opportunistic infections can be differentiated on the basis of pigment production, optimal incubation temperature and growth rate (Table 17.2).

PARATUBERCULOSIS (JOHNE'S DISEASE) Paratuberculosis is a chronic, contagious granulomatous enteritis characterized in cattle by persistent diarrhea, progressive weight loss, debilitation, and eventually death. It is considered a listed disease by the OIE, meaning it is a priority disease for international trade. The etiologic agent, Mycobacterium paratuberculosis, also known as Mycobacterium avium subsp paratuberculosis, formerly referred to as Mycobacterium johnei, is believed capable of infecting and causing disease in all other ruminants (eg, sheep, goats, llamas, deer) and in captive and free-ranging wildlife.

Epidemiology The epidemiology of the disease has been studied in cattle and the pattern of infection and spread in other species is assumed to be similar. Infection is acquired by calves at an early age through ingestion of organisms shed in the faeces of infected animals. Mycobacterium avium subsp. paratuberculosis may remain viable in the environment for up to one year under suitable conditions.

Clinical signs and Pathology Clinical signs develop in most ruminant species after a prolonged subclinicaI phase of infection. Affected cattle are usually more than 2 years of age when signs are first observed. The main clinical feature in cattle is diarrhoea, initially intermittent but becoming persistent and profuse. Progressive weight loss results without loss of appetite, and affected animals seldom survive for more than a year after initial detection.

Post-mortem Lesions In cattle, the mucosa of affected areas of the terminal small intestine and the large intestine is usually thickened and folded into transverse corrugations. The mesenteric and ileocaecal lymph nodes are enlarged and oedematous. Thickening of the intestinal mucosa is less marked in sheep, and necrosis and caseation may be present in the regional lymph nodes. Lesions in deer are similar to those in sheep.

Diagnosis Specimens for direct microscopy from live animals include scrapings or pinch biopsies from the rectum. Faeces may be submitted for culture and serum for serological tests. Postmortem specimens for histopathological examination from cattle include tissue from affected regions of the intestines and from regional lymph nodes. Specimens for microscopicaI examination should be stained by the ZN technique. Isolation of M. avium subsp. paratuberculosis from faeces or tissues.

Treatment Isoniazid at 20 mg/kg (orally) Rifampin at 20 mg/kg (orally)

Control Animals with clinical signs suggestive of paratuberculosis should be isolated. In problem herds, appropriate hygiene and husbandry measures should be instituted to prevent infection of young susceptible animals.