M.M. Cooke

Naturally occurring tuberculosis caused by Mycobacterium bovis in brushtail possums (Trichosurus vulpecula): I. An epidemiological analysis of lesion distribution.

Gross and microscopic lesion distributions and culture test results are described for 73 tuberculous possums recovered from a series of cross-sectional studies involving about 500 detailed necropsies. Pathological findings from 11 terminally ill tuberculous possums are also described. Quantitative epidemiological techniques were applied to lesion site data to assess factors influencing the pathogenesis of the disease. In possums with gross lesions, the number of distinct body sites affected varied from one to 10 per animal, with a mean of 4.6. The total number of gross plus microscopic lesions varied from one to 28 per animal with a mean of 11.6, indicating that the degree of generalisation of disease was much greater than appeared grossly. Of 119 possums with no gross lesions which were subjected to additional examinations, tuberculosis was diagnosed in ten (8.4%) by histology or culture of pooled lymph nodes. Among cross-sectional sample tuberculous possums, lesions were found in lungs in 85%, in axillary lymphocentres in 85%, in inguinal lymphocentres in 69%, and in either axillary or inguinal lymphocentres in 95%, indicating that the disease spread rapidly to multiple body sites. More males than females were infected (relative risk = 1.78). When cross-sectionally sampled infected and non-infected possums were compared, no significant associations were found between the presence/absence of disease and either age or indices of body condition, although debility was seen in animals with terminal illness.

Naturally occurring tuberculosis caused by Mycobacterium bovis in brushtail possums (Trichosurus vulpecula): II. Pathology

The gross and microscopic lesions due to Mycobacterium bovis infection are described in 73 brushtail possums (Trichosurus vulpeculu) from population studies and from 11 terminally ill possums. Subcutaneous lesions were suppurative in nature, often discharging through sinuses to the exterior. Histologically, the smallest lesions comprised focal aggregations of macrophages with angulated cytoplasmic boundaries occasionally containing acid-fast organisms. Larger lesions were pyogranulomatous, with extensive necrosis, caseation and large numbers of intra- and extra-cellular acid-fast organisms. Few multinucleate giant cells were seen and fibroplasia was rare. Mineralisation was recorded in tuberculous lymph nodes from only two possums. Histological evidence of haematogenous and lymphatic spread was seen in some cases.

Naturally occurring tuberculosis caused by Mycobacterium bovis in brushtail possums (Trichosurus vulpecula): III. Routes of infection and excretion.

Mycobacterium bovis was cultured from nine of 25 (36%) tracheal washings but not from any of 38 urine and 38 faecal samples from tuberculous possums cross-sectionally sampled from the wild. One of three tracheal washings, one of three urine samples and one of three faecal samples from terminally ill possums were culture-positive. The respiratory route is implicated as the major route of excretion of Mycobacterium bovis from naturally infected possums in horizontal transmission. Tuberculosis was observed in two young possums and was evidence of probable pseudo-vertical transmission via the respiratory route or ingestion of milk. Discharging fistulae were present in 22 of 71 (31%) cross-sectionally sampled tuberculous possums and were associated with relatively advanced disease. Although the frequent involvement of superficial lymphocentres in early stage disease could not be explained satisfactorily, the respiratory route was implicated as the main route of infection from indirect evidence.

A review of infection of wildlife hosts with Mycobacterium bovis and the diagnostic difficulties of the 'no visible lesion' presentation.

Abstract The pathology, frequency and diagnostic implications of ‘no visible lesion’ (NVL) tuberculosis (Tb), i.e. infection with Mycobacterium bovis in the absence of macroscopic lesions, are described in a wide taxonomic range of wildlife hosts. Information collected and evaluated on the definition and occurrence of NVL Tb, histopathological characteristics, post-mortem techniques to detect minimal lesions, and diagnostic difficulties revealed most Tb-infected individuals with NVL had minute tuberculous lesions, which were difficult to see by eye. Acidfast organisms (AFO) were sometimes detected in the lesions. Ideally, mycobacterial culture of pools of lymph nodes and/or oropharyngeal tonsils is necessary for the accurate diagnosis of Tb in the absence of macroscopic lesions. At a very minimum, the diagnostic methods applied for studying the prevalence of Tb in the population should be clearly described, to allow comparison between studies.

Circovirus-like infection in a southern black-backed gull (Larus dominicanus)

A juvenile black-backed gull (Larus dominicanus) from the Manawatu region of New Zealand was found to have chronic airsacculitis due to Aspergillus spp. Histologically, there was moderately severe inflammation in the bursa of Fabricius associated with large, basophilic, intracytoplasmic inclusions, which ultrastructurally had an appearance typical of circovirus inclusions. This finding suggests that circoviruses may be more widespread in avian species than previously recognized and may be responsible for diseases associated with immunosuppression in free-living birds.

Prevalence and spatial distribution of bovine tuberculosis in brushtail possums on a forest-scrub margin

Tuberculosis caused by Mycobacterium bovis was diagnosed in 36 of 68 (53%) brushtail possums (Trichosurus vulpecula) trapped in August 1992 from a population of exceptionally low density (trap catch <3%) on a forest-scrub margin in Westland, New Zealand. The prevalence of tuberculosis in possums, based solely on gross lesions, was at least twice that previously recorded in New Zealand, and was about seven times that recorded from the same population in 1980. More male (66%) than female (33%) possums had grossly visible tuberculous lesions. The distribution of infection appeared continuous along the forest-scrub margin. Both stoats (Mustela erminea) and one of six hares (Lepus europaeus occidentalis) trapped were also infected with M. bovis.

The pathogenesis of experimental endo-bronchial Mycobacterium bovis infection in brushtail possums (Trichosurus vulpecula)

AIM To study the nature and development of experimentally induced respiratory tuberculosis in possums and compare the lesions observed with those seen in the natural disease. METHODS Thirty-three adult possums were inoculated via the endo-bronchial route with 20-100 colony forming units of Mycobacterium bovis. The possums were killed at 1,2,3 and 4 weeks after inoculation and the nature and distribution of the lesions studied in detail histopathologically. Alveolar macrophages recovered from the infected possums were also studied ultrastructurally. RESULTS Macroscopic lesions were largely confined to the respiratory tract, increasing in size and number with time. Histology greatly increased the detection of the total number of lesions. The most common sites affected outside the respiratory tract were the liver and hepatic lymph nodes, but lesions were less common in peripheral lymph nodes than is observed in the natural disease. Intra-pulmonary lesions were centred on blood vessels and their associated lymphatics. Peripheral blood lymphocyte blastogenic responses to M. bovis antigens were first detected at 3 weeks after inoculation, which was 1 week after lymphocyte infiltrations were detected in the lungs, but 1 week before the majority of infections became generalised. CONCLUSIONS Differences in the nature of pulmonary lesions and the distribution of lesions were observed between experimentally induced and the natural disease. Rapid haematogenous and lymphatic spread occurs early in the experimentally induced disease.

Observations on three outbreaks of Leptospira interrogans serovar pomona infection in lambs

Sickness and deaths associated with leptospirosis were reported in three lamb flocks in the Manawatu region. The clinical, pathological and serological features of Leptospiru interrogans serovar pomona infection are described. The mucous membranes of affected animals were pale and jaundiced. Haemoglobinuria, haemoglobinaemia and centrilobular hepatocellular necrosis were also consistent findings. All clinically sick lambs had very high titres in the microscopic agglutination test for L. pomona. Circumstantial evidence of pigs being the source of infection was found in one of the three cases.

Tuberculosis in a free-living brown hare (Lepus europaeus occidentalis)

A free-living brown hare (Lepus europaeus occidentalis) trapped during a survey of tuberculous possums in the Ahaura Valley, Westland, was found to have both microbiological and histopathological evidence of infection with Mycobacterium bovis. The mesenteric lymph nodes were enlarged and showed extensive caseation, tuberculous nodules were present in the liver and kidney, and there was a proliferative pleurisy. Histologically, characteristic tuberculous granulomatous foci were seen in the mesenteric lymph nodes, liver, kidney, pleura and lungs, often in association with small numbers of acid-fast organisms. This is the first report of naturally occurring tuberculosis due to M. bovis in the brown hare.

Temporal patterns in bovine tuberculosis in a brushtail possum population contiguous with infected cattle in the Ahaura Valley, Westland

AIMS To determine temporal and spatial patterns of bovine tuberculosis (Tb) in a population of brushtail possums (Trichosurus vulpecula) free from commercial and recreational hunting and living contiguously with livestock, and to relate these patterns to measures of possum health and density. METHODS Possums were trapped on 11 lines located on a forest-rough grazing margin in the Ahaura Valley in Westland in 1979-80 and each August from 1992 to 1996, and were examined post mortem for gross lesions indicative of Tb. Infection levels in possums were compared with Th test data from cattle grazing in the same area. RESULTS Tuberculosis was identified from the trapped sample of possums in 1980. Trapping of further possums on the same trap lines confirmed the presence of the disease each year from 1992 to 1996, and suggested a clustering of infection in time and space. The prevalence of Tb in possums declined with increasing possum population size and well being, and decreasing age. The prevalence of Tb in possums was highest in 1980 and 1992, and appeared to roughly coincide with similar upsurges in Tb in nearby cattle. CONCLUSIONS Our results suggest that the exceptional levels of Tb recorded in possums in 1992 had a direct effect on possum numbers, and gave rise to very low trap catches in 1993-94. They also suggest that Tb can persist for several years in possum populations existing well below the 5% trap catch targeted by regional councils for Tb possum control.