Routes of transmission of wobbly possum disease

AIMS: To determine the routes of transmission of wobbly possum disease (WPD) virus and whether or not these would favour further examination of its potential for biological control of possums. METHODS: A standard inoculum, prepared as a tissue suspension from possums which had been infected with WPD, was titrated in vivo. Possums were challenged with this inoculum by the intra-gastric, intra-tracheal and intra-dermal routes. Further possums were challenged with blood by the intra-dermal and intra-peritoneal routes, with urine by the intraperitoneal route and with homogenised mites (Trichosurolaelaps crassipes) by the intra-dermal route. Transmission was investigated between possums in closely-adjacent, individual cages and between possums in a group enclosure. RESULTS: Possums developed WPD following administration of the standard inoculum by all of the above routes, following administration of blood by the intra-peritoneal and intra-dermal routes, following administration of urine by the intraperitoneal route and following administration of homogenised mites by the intra-dermal route. Individually caged control possums did not contract WPD. All non-inoculated adult possums in the group enclosure and many joeys in direct contact with infected possums contracted WPD. CONCLUSION: WPD was efficiently transmitted by close contact. Without such contact transmission did not occur. Infectivity was demonstrated in tissue suspensions, blood, urine and mites. Given the routes by which possums are susceptible to these substances and the need for direct contact, infection may be spread in the wild by several mechanisms, including aggressive encounters in which blood is exchanged, contamination of wounds with urine, ingestion of contaminated food, transfer of mites during den-sharing, and other social encounters. WPD has potential as a biological control agent for possums on the basis that it is readily transmitted between individuals in close contact.     

Development of an indirect ELISA for detection of antibody to wobbly possum disease virus in archival sera of Australian brushtail possums (Trichosurus vulpecula) in New Zealand

AIMS: To develop an indirect ELISA based on recombinant nucleocapsid (rN) protein of wobbly possum disease (WPD) virus for investigation of the presence of WPD virus in Australian brushtail possums (Trichosurus vulpecula) in New Zealand.

METHODS: Pre- and post-infection sera (n=15 and 16, respectively) obtained from a previous experimental challenge study were used for ELISA development. Sera were characterised as positive or negative for antibody to WPD virus based on western-blot using WPD virus rN protein as antigen. An additional 215 archival serum samples, collected between 2000–2016 from five different regions of New Zealand, were also tested using the ELISA. Bayesian modelling of corrected optical density at 450?nm (OD₄₅₀) results from the ELISA was used to obtain estimates of receiver operating characteristic (ROC) curves to establish cut-off values for the ELISA, and to estimate the prevalence of antibody to WPD virus.

RESULTS: Western blot analysis showed 5/14 (36%) pre-infection sera and 11/11 (100%) post-infection sera from experimentally infected possums were positive for antibodies to WPD virus. Bayesian estimates of the ROC curves established cut-off values of OD₄₅₀≥0.41 for samples positive, and OD₄₅₀<0.28 for samples negative for antibody to WPD virus, for sera diluted 1:100 for the ELISA. Based on the model, the estimated proportion of samples with antibodies to WPD virus was 0.30 (95% probability interval=0.196–0.418). Of the 230 archival serum samples tested using the ELISA, 48 (20.9%) were positive for antibody to WPD virus, 155 (67.4%) were negative and 27 (11.7%) equivocal, using the established cut-off values. The proportion of samples positive for WPD virus antibody differed between geographical regions (p<0.001).

CONCLUSION: The results suggested that WPD virus or a related virus has circulated among possums in New Zealand with differences in the proportion of antibody-positive samples from different geographical regions. Antibodies to WPD virus did not seem to protect possums from disease following experimental infection, as one third of possums from the previous challenge study showed evidence of pre-existing antibody at the time of challenge. These results provide further support for existence of different pathotypes of WPD virus, but the exact determinants of protection against WPD and epidemiology of infection in various regions of New Zealand remain to be established.

CLINICAL RELEVANCE: Availability of the indirect ELISA for detection of WPD virus antibody will facilitate prospective epidemiological investigation of WPD virus circulation in wild possum populations in New Zealand.     

A neurological syndrome in a free-living population of possums (Trichosurus vulpecula)

AIMS: To determine the clinical and pathological features of a neurological disease syndrome in a free-living possum population in New Zealand and to compare this syndrome with wobbly possum disease. METHODS: An outbreak of a neurological disease in possums in the Rotorua district was investigated in 1994. A variety of tissues was collected and investigated microbiologically and histopathologically. Tissues stored from clinically affected possums were homogenised, clarified and inoculated into healthy possums by the intra-peritoneal route. The clinical signs and histopathological lesions in naturally-infected and in experimentally-inoculated possums were assessed and compared with those of possums affected with wobbly possum disease. RESULTS: Histopathological investigation of three of the naturally-affected possums revealed non-suppurative encephalitis with perivascular cuffing, diffuse non-suppurative meningitis and focal non-suppurative myocarditis. These lesions were suggestive of a viral infection. No pathogenic bacteria were recovered and no viruses were isolated in tissue culture. A neurological disease, indistinguishable from wobbly possum disease, was reproduced in five out of the eight experimentally inoculated possums. In two experimental cases the clinical signs were very mild and, in most cases of the natural and experimental disease, histopathological lesions in the central nervous system were mild in comparison with wobbly possum disease. Possums which did not develop clinical signs of neurological disease or have lesions in the central nervous system did have infiltrations of mononuclear inflammatory cells in the liver and kidney. CONCLUSIONS: This neurological disease, reported for the first time in a free-living population, closely resembles and may be the same as wobbly possum disease. The milder nature of this disease could suggest there may be more than one strain of the aetiological agent.     

Effects of sustained control of brushtail possums on levels of Mycobacterium bovis infection in cattle and brushtail possum populations from Hohotaka, New Zealand

AIMS: To examine the effect of reducing the abundance of brushtail possums (Trichosurus vulpecula) on the distribution and prevalence of bovine tuberculosis (Mycobacterium bovis, Tb) in possums and the incidence of Tb in domestic cattle on a group of farms in the central North Island, New Zealand. METHODS: The cumulative yearly incidence of Tb infection from 12 cattle herds was estimated from annual tuberculin testing and abattoir inspection data over the period 1983-98. Intensive control of possum populations began for six of the herds in 1988, five herds in 1994 and the remaining herd in 1996. The prevalence and distribution of macroscopic M. bovis infection in possums and an index of possum abundance was estimated during yearly cross-sectional surveys from 1988 to 1998. This enabled formal testing of the link between the abundance of tuberculous possums and the incidence of Tb in cattle. RESULTS: Before possum control, infected possums were clustered in foci on or adjacent to the farms with the highest annual incidence of tuberculosis in cattle, and had an overall prevalence of macroscopic M. bovis infection of 2.3%. The prevalence of disease declined to zero with ongoing possum control, although infected possums continued to be found during the first 5 years of control. Maintaining the possum population at an average of 22.1% of its pre-control density significantly reduced the odds of the cumulative yearly incidence of Tb in cattle by 77% during the first 5 years of possum control and a further 65% in the second 5-year period. Nine of 11 tuberculous possums identified since the start of possum control were found within the areas where infected possums were clustered during the pre-control survey, suggesting that the persistence of infection within these clusters rather than infected immigrants was the source of ongoing disease. Annual estimates of the prevalence of tuberculous possums broadly followed the predictions of Barlow's possum-Tb model for a controlled possum population. CONCLUSION: The results support the hypothesis that tuberculous possums transmit bovine tuberculosis to domestic cattle, and therefore that reducing the abundance of tuberculous possums reduces the incidence of Tb in cattle. If the level of possum culling is sufficient, it appears that M. bovis infection may be eradicated from possum populations. Better information on population density, rate of increase and annual culling rates would have been needed for a truly independent examination of the Barlow possum-Tb model.     

Experimental infection of brushtail possums (Trichosurus vulpecula) with Mycobacterium bovis by conjunctival instillation

In New Zealand, the brushtail possum (Trichosurus vulpecula) is the major wildlife reservoir of Mycobacterium bovis. Procedures for experimentally infecting possums are required to study the pathogenesis of the disease and to challenge possums in vaccine efficacy studies. Conjunctival instillation of a suspension of M. bovis was effective in producing bovine tuberculosis in captive possums. The experimental disease progressed slowly with the development of palpable lesions in superficial lymph node lesions, both characteristics of the disease in wild, naturally infected possums. At necropsy there was widespread distribution of macroscopic and microscopic lesions. The proportion of possums that became diseased, the rate of development and severity of lesions, the severity of clinical signs, all increased when the dose of M. bovis was increased. Of the three doses used, the medium dose (1000-2000 colony forming units) produced the disease with the most desired characteristics. As a procedure for exposing possums to infection with M. bovis the conjunctival route has advantages in that it is simple and safe to perform, and possums need only to be sedated for infection.     

Contact patterns as a risk factor for bovine tuberculosis infection in a free-living adult brushtail possum Trichosurus vulpecula population

The aim of this study was to identify risk factors for bovine tuberculosis (TB) in a free-roaming, capture-mark-recapture monitored possum Trichosurus vulpecula population in a 22-ha study site at Castlepoint, New Zealand from 1 April 1989 to 31 March 1994. A matched case-control design was used to evaluate the influence of sex, habitat and contact opportunities on TB risk. Cases comprised possums identified as TB-positive throughout the study period. Controls were selected from the group of possums that were captured and showed no clinical signs of TB throughout the study period. Measures derived from a social network analysis of possum capture locations such as degree, clustering coefficient (CC) and betweenness were used to represent potential contact opportunities among possums. Network analysis measures recorded for individual possums in the 12-month period before a diagnosis of TB were evaluated in a conditional logistic regression model. We found no evidence of an association between case status and the total number of possums with which there was potential contact (degree) (P=0.5). The odds of cases being exposed to unit increases in the number of TB-positive contacts was 2.50 (95% CI 1.24-5.05; P<0.01) times that of controls. This effect was conditional on the total number of potential contacts made, with a negative interaction with increasing degree. These findings indicate that potential contact with TB-positive possums increases the odds of disease whereas potential contact with large numbers of possums does not. This suggests that multiple contacts with TB-positive possum(s) are necessary for transmission of TB and this is more likely to occur in networks that are smaller. We challenge the hypothesis that contact with large numbers of individuals increases the probability of becoming TB infected and argue that individual contact behaviour is a determinant of the creation of TB foci within free-living possum populations.     

Toxoplasmosis as a cause of ovine perinatal mortality

Aims. To determine temporal and spatial patterns of bovine tuberculosis (Tb) in a population of brushtail possums (Trichosurus vulpeculu) 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 Th 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 thatTb can persist for several years in possum populations existing well below the 5% trap catch targeted by regipnal councils forTb possum control.     

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.     

Mycobacterium bovis infection in the brush-tailed possum (Trichosurus vulpecula): II. Comparison of experimental infections with an Australian cattle strain and a New Zealand possum strain

Experimentally induced M. bovis infection in brush-tailed possums (Trichosurus vulpecula) with a possum-adapted strain from New Zealand, produced a more rapidly progressive, severe disease than did that induced by an Australian cattle strain. Possums infected with either strain excreted large numbers of mycobacteria in urine, faeces and discharges from abscesses. Aerosol transmission of infection occurred only with the New Zealand possum strain. The infectious aerosol most probably was of respiratory origin but urine may have been the source.All possums had focal interstitial nephritis consistent with Leptospira infection. Histopathological evidence of severe stress was observed in only one possum; the remainder had either mild or no stress-associated lesions.     

Behavioural studies on the potential for direct transmission of tuberculosis from feral ferrets (Mustela furo) and possums (Trichosurus vulpecula) to farmed livestock

Studies were conducted to evaluate the response of cattle and deer to ferrets which were sedated so they behaved like terminally tuberculous animals, and to compare this with the response of cattle, deer and sheep to sedated possums. Six groups of deer and two groups of cattle were exposed to a sedated ferret and to a sedated possum. Both livestock species showed interest in the possum by sniffing and licking it, but they only briefly touched the ferret and no licking or extended investigation was observed. The proportion of available time spent in physical contact with the possum by cattle was 7.7 times as high as for the ferret, and for the deer was 5.7 times as high. The behavioural response of three groups of sheep to a sedated possum was investigated, and sheep showed limited interest beyond viewing the possum from a distance. The amount of time spent by sheep investigating the possum was very low and the intensity of exploration was also low. For possums, at least one deer was within 1.5 m (an estimate of the distance that tuberculosis can be transmitted by aerosol) for 50.9% of observation time, and in physical contact with the possum for 9.5% of time. The figures for cattle were 69.3% and 17.3%, while those for sheep were 6.9% and 0.3%. In interactions with ferrets, the equivalent figures were 29.8% within 1.5 m and 2.2% in physical contact for cattle, and 20.8% and 1.1% for deer. Tuberculous possums commonly and tuberculous ferrets less commonly have lung lesions and/or discharging sinuses, and may excrete Mycobacterium bovis intermittently or continuously in aerosols or discharges. The exploratory behaviour of deer and cattle in this study would provide opportunities for them to become infected with M. bovis if they had contact with infectious possums, and less probably with ferrets. The response of sheep to possums suggests that they would be much less likely to contract the disease.     

Trends in the incidence of tuberculosis in possums and livestock, associated with differing control intensities applied to possum populations

AIM: To determine the trap-catch index (an estimate of abundance) of brushtail possum (Trichosurus vulpecula) populations infected with bovine tuberculosis (Tb; Mycobacterium bovis) that must be achieved, and the length of time such an index must be maintained, for Tb to be eliminated from possum populations and adjacent livestock. METHODS: Between 1997-1998 and 2000-2001, trap-catch surveys of possum populations naturally infected with Tb and subjected to population-control measures were undertaken at four forest sites and two farmland sites. At the same time, possum carcasses were collected at these sites and their Tb status determined, and all contiguous cattle and deer herds were Tb tested and abattoir slaughter data for these herds were interrogated. RESULTS: Trap-catch surveys indicated that numbers of possums on the farmland sites surveyed were usually very low and well below the control targets set (i.e. a 5% trap catch or approximately 0.5-1 possum/ha) for the study. In contrast, trap-catch surveys undertaken in forest sites indicated possum numbers were more variable, and often recovered rapidly from control operations to exceed control targets within 1-3 years. The annual rate of recovery of possum populations in half of the forest population surveys undertaken exceeded published intrinsic rates of increase for possums. The overall prevalence of Tb in possum populations was < or =1.9% at 5/6 sites, and was 6.5% at the sixth site. Juvenile possums infected with Tb were trapped within but near the edge of control zones and appeared to represent an immigrant source of infection. Mature infected possums survived control operations apparently by having home ranges in uncontrolled patches within control areas. Infection in possums appeared to be eliminated from one study site by the intensive control undertaken, but elimination at other sites appeared less likely. Levels of Tb in livestock on or adjacent to the study sites fell by at least 50% during the study, and cattle in one area tested clear for the first time in 20 years. CONCLUSIONS: Initial control of possums in forest appeared to achieve national control targets set by the Animal Health Board (AHB), despite trap-catch data often providing misleading population estimates. Such targets were often exceeded within 1-3 years. By comparison, possum control on farmland appeared to maintain populations at very low levels, while control on forest margins maintained populations at intermediate levels. Control was least effective in deep forest where human access was most difficult. Intensive population control measures appeared to have led to a reduced incidence of Tb in livestock at 3/4 sites, and elimination of Tb in livestock at one site. This result supports modelling studies that predict the eradication of Tb from possums through ongoing intensive control and may explain the lower success achieved with earlier less-intensive possum control.     

Resolution of Mycobacterium bovis infection in wild brushtail possums (Trichosurus vulpecula)

The brushtail possum (Trichosurus vulpecula) is the principal wildlife reservoir of Mycobacterium bovis in New Zealand (Morris and Pfeiffer 1995). Tuberculosis in possums is a progressive, fatal infection. The complete resolution of infection in possums has not been reported. The purpose of this communication is to present two cases of complete and spontaneous resolution of tuberculosis in wild, naturally infected possums. During an 11-year (1989-2000) longitudinal study of a possum population at Castlepoint in the Wairarapa, 90 bacteriologically-confirmed cases of M. bovis infection were observed, and 88 of the tuberculous possums were either found dead or were presumed to have died when they ceased to be trapped. As the home range of adult possums is stable (Efford et al 2000) it was assumed that if an adult possum could no longer be trapped, it had died. In many cases, the declining physical condition of a tuberculous possum prior to its disappearance supported the assumption. However, two cases survived and were euthanised, one 15 months and the other 30 months after they first showed clinical signs of infection. When examined post mortem, neither showed evidence of M. bovis infection. The case histories are described and the significance of the observations discussed.     

A behaviour study on the potential for direct transmission of tuberculosis from possums (Trichosurus vulpecula) to alpacas (Lama pacos), and the converse from alpacas to possums

AIMS: To evaluate the potential for the direct transmission of tuberculosis from possums to alpacas, and vice versa. METHODS: A field study was conducted on an alpaca farm in Northland, New Zealand on 7 January 1999. Observations were recorded on the interaction of one group of male alpacas with a simulated dead possum, one male and one female alpaca group with a simulated terminally tuberculous possum, and one group of male alpacas with a normal possum in an enclosure from which the animals could not escape. The possum was sedated with ketamine as hydrochloride to simulate death (inactive; no movement), and terminal illness (active, inco-ordinated movement around the paddock). The observations were based on the focal animal sampling technique, they were un-replicated, and recorded visually and manually with intermittent still photography. RESULTS: Both male and female alpacas showed strongly inquisitive interaction with the possum. They clustered around the possum (focal animal) very soon after it was observed by the first member of the group. The interest of the majority of both sex groups remained high for the observation periods of approximately 30 minutes, and most individuals remained within 5 metres of the possum for that time. Approximately 50% of the alpacas were within possible aerosol transmission distance of 2 metres from the sedated, erratically mobile possum with their heads towards it for approximately 50% of the two observation periods. Aggressive behaviour was recorded for a young male with stamping on the moving possum. Similar, but more vigorous and prolonged stamping behaviour was recorded for a female with a young (<1 week) offspring (cria). The stamping behaviour was accompanied by very close nose to nose contact of the alpaca and possum. At one point the female threw the possum approximately 1.5 metres in the air with her teeth. The group of male alpacas placed in an enclosure with an unsedated normal possum generally moved away from the possum during its rapid active attempts to escape. When it became inactive their approaches were cautious and only once elicited a defence reaction from the possum, from which they recoiled. One male made one attempt to stamp on the active possum. Soon after the possum became inactive in a small loose hay pile, the alpacas lost interest in it. CONCLUSIONS: The alpaca / possum behavioural interactions show there is potential for direct aerosol transmission of tuberculosis from possums to alpacas, but probably not from alpacas to possums.     

Ranging behaviour and duration of survival of wild brushtail possums (Trichosurus vulpecula) infected with Mycobacterium bovis

AIM: To quantify the duration of survival of possums (Trichosurus vulpecula) infected with Mycobacterium bovis, and identify aspects of their behaviour which may influence the likelihood of disease transmission to domestic stock or wildlife. METHODS: Capture and den locations of 14 naturally infected tuberculous possums, eight possums experimentally infected with M. bovis and eight non-infected possums were recorded between May 1998 and February 2000 at a study site near Castlepoint on the Wairarapa coast of the North Island in New Zealand. Denning behaviour was observed weekly using radiotelemetry, and possums were captured, examined and released bi-monthly. Data were used to estimate survival period; create denning, activity, and total ranges; and to identify extended forays by possums as individuals and groups. RESULTS: Seventeen tuberculous possum carcasses were recovered, of which 14 (82%) were close to or within their activity range. Denning ranges were known for 10/17 possums that died. Four tuberculous possums were found dead within their denning range. Three possums made extended forays in the 3 weeks before death. Twelve possums were found dead in dense scrub, three in long grass in open woodland and two on pasture. Mean duration of survival of naturally infected possums following detection of clinical signs was 3.4 months (95% CI=2.1-5.4) and the instantaneous mortality rate was 0.293 per month (95% CI=0.184-0.470). Signs of disease were obvious for about 3 weeks prior to death. Tuberculous possums were commonly trapped on only part of the area where the total non-infected population was trapped. CONCLUSION: Most tuberculous possums died within their activity range and in scrub, representing a risk of transmission of M. bovis to wildlife and livestock that forage in scrub. Smaller proportions dying on pasture represent a less frequent, but highly visible risk. Tuberculous possums were clustered on the study site, and localised possum control operations would be more effective if focussed on such areas.     

Bovine tuberculosis in a possum from the Orongorongo Valley, Wellington

The first confirmed case of a tubercular possum in the Orongorongo Valley forest is reported from a population intensively studied for 20 years. The victim was a five-year-old male recaptured 20 times over 3% years. Its annual home range was about 1.8 hectares but this dropped to 0.4 ha during its last year of life. Its range overlapped with at least 15 others. The animal appeared normal and healthy in July 1985 but died of advanced, generalised bovine tuberculosis 12 weeks later. The nearest confirmed cases of tubercular possums were reported 1.3 km away in 1976 and 4 km away in 1979. The Orongorongo possum was unlikely to have contacted any cattle for over three years. The infection was probably transmitted from the Wainuiomata Valley by possums or pigs. The Orongorongo River was no barrier to the spread of the infection. Although most of the neighbouring possums were trapped and examined several times over the following year, no tubercular animals were detected.     

Mortality rate and movements of brushtail possums with clinical tuberculosis (Mycobacterium bovis) infection

AIM: To describe the mortality rate and movements of brushtail possums (Trichosurus vulpecula) with clinical tuberculosis (Mycobacterium bovis) from a site in native podocarp/hardwood forest, in the Orongorongo Valley, Wellington. METHODS: Possums with clinical tuberculosis (Tb) were identified through routine trapping on live capture grids and were fitted with radio-collars, and located at approximately monthly intervals for 2.5 years. The survival times of radio-collared tuberculous and non-diseased possums were used to estimate the instantaneous additional mortality rate due to clinical Tb. The spatial behaviour of clinically tuberculous possums was compared with that of non-diseased possums, using both den and trap locations. The frequency of long-distance movements of both tuberculous and non-diseased possums was also determined. A possum was defined as having undertaken along-distance movement when it moved at least 200 m from the centre of its activity range and then returned to its established activity range or died. RESULTS: The mean survival time of clinically tuberculous possums was 4.7 (SE 0.79) months. The instantaneous additional mortality rate due to clinical Tb was estimated to be between 1.08 year-1 and 2.38 year-1, with the lower figure including an unknown portion of the incubation period in the estimate. Estimates of denning and activity ranges of tuberculous possums were approximately 22-30% larger, on average, than the ranges of non-diseased possums, but the differences were not significant. Both tuberculous and non-diseased possums undertook long-distance movements up to 700 m from the centre of their activity range. However, the majority of tuberculous possums died within or near their established activity range. CONCLUSIONS: This study estimated the mortality rate during the clinical stages of Tb and, as such, did not represent a true estimate of the disease-induced mortality rate (alpha) used in most epidemiological models of possums and Tb. However, the lower estimate of the additional mortality rate due to clinical disease could be considered an approximation to alpha. The movement patterns of clinically tuberculous possums were, overall, not substantially different from non-diseased possums. The occurrence of long-distance movements by tuberculous possums sometimes coincided with the advanced stage of clinical disease. However, the majority of tuberculous possums died within their established activity area. Long-distance movements by resident possums during the terminal stages of disease represent a direct risk for transmission of Tb to livestock on farms within approximately 1 km of infected possum populations.     

A behaviour study on the potential for direct transmission of tuberculosis from possums (Trichosura vulpecula) to alpacas (Lama pacos), and the converse from alpacas to possums

Aims. To evaluate the potential for the direct transmission of tuberculosis from possums to alpacas, and vice versa.

Methods. A field study was conducted on an alpaca farm in Northland, New Zealand on 7 January 1999. Observations were recorded on the interaction of one group of male alpacas with a simulated dead possum, one male and one female alpaca group with a simulated terminally tuberculous possum, and one group of male alpacas with a normal possum in an enclosure from which the animals could not escape. The possum was sedated with ketamine as hydrochloride to simulate death (inactive; no movement), and terminal illness (active, into-ordinated movement around the paddock). The observations were based on the focal animal sampling technique, they were un-replicated, and recorded visually and manually with intermittent still photography.

Results. Both male and female alpacas showed strongly inquisitive interaction with the possum. They clustered around the possum (focal animal) very soon after it was observed by the first member of the group. The interest of the majority of both sex groups remained high for the observation periods of approximately 30 minutes, and most individuals remained within 5 metres of the possum for that time. Approximately 50% of the alpacas were within possible aerosol transmission distance of 2 metres from the sedated, erratically mobile possum with their heads towards it for approximately 50% of the two observation periods. Aggressive behaviour was recorded for a young male with stamping on the moving possum. Similar, but more vigorous and prolonged stamping behaviour was recorded for a female with a young (<1 week) offspring (cria).The stamping behaviour was accompanied by very close nose to nose contact of the alpaca and possum. At one point the female threw the possum approximately 1.5 metres in the air with her teeth. The group of male alpacas placed in an enclosure with an unsedated normal possum generally moved away from the possum during its rapid active attempts to escape. When it became inactive their approaches were cautious and only once elicited a defence reaction from the possum, from which they recoiled. One male made one attempt to stamp on the active possum. Soon after the possum became inactive in a small loose hay pile, the alpacas lost interest in it.

Conclusions. The alpaca / possum behavioural interactions show there is potential for direct aerosol transmission of tuberculosis from possums to alpacas, but probably not from alpacas to possums.     

Interactions between beef cattle and simulated tuberculous possums on pasture

Brushtail possums (Trichosurus vulpecula) were sedated with ketamine and placed within a fenced observation area with 17 Hereford cross steers. The behaviour of the sedated possums simulated that of terminally ill possums. Behavioural analysis was carried out during eight observation periods in which sedated possums were successfully released and interactions occurred. Cattle were attracted from over 50 m away by the movements of the possums, and for 34% of observation time cattle investigated the possums. Inactive animals engendered far less interest, and cattle investigated them for 3% of observation time. An average of seven to eight cattle came in physical contact with the possum in each of the observation periods. Cattle actively investigated the possum while following it as it moved, commonly close enough to be within aerosol transmission distance, and some sniffed the possum, touched it with their noses, and in many cases licked it extensively. All of these activities would expose the cattle to high risk of receiving an infective dose of Mycobacterium bovis, especially via the respiratory route. The type and duration of contact between individual steers and the possum varied between animals and observation period. The cattle exposed themselves to greater risk, overall, during the first week of observations, and there was evidence of habituation with increased contact. Although all 17 steers came within 5 metres of the possum at some point during the observation periods, some individual cattle behaved in ways which would put them at greater risk of contracting tuberculosis than did others. Cattle showed little interest in possum carcasses placed on pasture. These were unlikely to be a significant source of infection for cattle, although they could be for scavenger species.     

The relationship between prevalence of Mycobacterium bovis infection in feral ferrets and possum abundance

AIM: To determine the relationship between the prevalence of macroscopic Mycobacterium bovis infection in feral ferrets (Mustela furo) and the abundance of brushtail possums (Trichosurus vulpecula). METHODS: The predictive power of a previously reported positive association between the prevalence of macroscopic M. bovis infection in ferrets and possum abundance was examined by undertaking surveys of M. bovis infection in ferrets at sites of low and high possum abundance. The association was then tested by a manipulative experiment that measured changes in the prevalence of macroscopic M. bovis infection in feral ferrets after reducing possum abundance. RESULTS: The positive relationship between the prevalence of macroscopic M. bovis infection in ferrets and possum abundance remained valid for new survey data, although the goodness of fit of the relationship was reduced. Experimentally reducing possum abundance reduced the odds of macroscopic M. bovis infection in ferrets by 80% in the years immediately following possum control (Odds Ratio=0.23, p=0.003). CONCLUSIONS: There is a causal link between possum abundance and the prevalence of macroscopic M. bovis infection in feral ferrets in areas in which M. bovis infection is known to occur in ferret populations. This suggests that possum-to-ferret transmission of disease occurs and accounts for most of the disease evident in ferret populations, though does not determine whether ferrets are spillover or maintenance hosts of M. bovis. Management to reduce the prevalence of M. bovis infection in ferrets should consider reducing possum abundance as a control tactic. KEY WORDS: Mustela furo, ferret, Trichosurus vulpecula, brushtail possum, Mycobacterium bovis, bovine tuberculosis.