The effect of fertility control on the transmission of bovine tuberculosis in wild brushtail possums

AIM: To determine the effect of fertility control on the rate of transmission of bovine tuberculosis (Tb), caused by Mycobacterium bovis, in brushtail possums (Trichosurus vulpecula).

METHODS: At two study sites with a history of Tb infection in the resident possum population, a sample of adult male and female possums (n=50), determined by palpation to be Tb-free, was surgically sterilised by gonadectomy. A sample of untreated Tb-free male and female possums (n=54) served as controls. Each study site was trapped every 2 months over a 3-year period, and the Tb status of the individuals in the trial assessed. At the conclusion of the trial, all remaining experimental individuals were killed, necropsied and examined for characteristic lesions typical of Tb. The rate of transmission of Tb infection was estimated using the incidence of tuberculous cases in the experimental animals and comparing it between sites, sex and sterilisation treatment.

RESULTS: Sterilisation by gonadectomy resulted in a reduction in the rate of transmission of Tb in male possums by 53%, but a corresponding increase in sterilised females for reasons that are still unclear. However, this interaction was statistically weak (p=0.10). When the sexes were combined, there was no overall effect of sterilisation on the rate of transmission of Tb (p=0.43). Sterility treatment notwithstanding, there was a highly significant difference in the rate of transmission between the sexes (p=0.01), being almost one order of magnitude higher in untreated males compared with untreated females.

CONCLUSIONS: Although lacking strong statistical support, these results suggest that fertility control that targets endocrine control of fertility may provide some additional benefits for disease control over that achieved by reductions in population density, by reducing the rate of transmission of M. bovis in male possums. However, additional studies are needed to confirm the validity of these results. The large difference in the rate of transmission of M. bovis in untreated males compared with untreated females suggests that transmission of Tb among males may be an important driver of the dynamics of Tb infection in possums.     

The effect of fertility control on the transmission of bovine tuberculosis in wild brushtail possums

AIM: To determine the effect of fertility control on the rate of transmission of bovine tuberculosis (Tb), caused by Mycobacterium bovis, in brushtail possums (Trichosurus vulpecula). METHODS: At two study sites with a history of Tb infection in the resident possum population, a sample of adult male and female possums (n=50), determined by palpation to be Tb-free, was surgically sterilised by gonadectomy. A sample of untreated Tb-free male and female possums (n=54) served as controls. Each study site was trapped every 2 months over a 3-year period, and the Tb status of the individuals in the trial assessed. At the conclusion of the trial, all remaining experimental individuals were killed, necropsied and examined for characteristic lesions typical of Tb. The rate of transmission of Tb infection was estimated using the incidence of tuberculous cases in the experimental animals and comparing it between sites, sex and sterilisation treatment. RESULTS: Sterilisation by gonadectomy resulted in a reduction in the rate of transmission of Tb in male possums by 53%, but a corresponding increase in sterilised females for reasons that are still unclear. However, this interaction was statistically weak (p=0.10). When the sexes were combined, there was no overall effect of sterilisation on the rate of transmission of Tb (p=0.43). Sterility treatment notwithstanding, there was a highly significant difference in the rate of transmission between the sexes (p=0.01), being almost one order of magnitude higher in untreated males compared with untreated females. CONCLUSIONS: Although lacking strong statistical support, these results suggest that fertility control that targets endocrine control of fertility may provide some additional benefits for disease control over that achieved by reductions in population density, by reducing the rate of transmission of M. bovis in male possums. However, additional studies are needed to confirm the validity of these results. The large difference in the rate of transmission of M. bovis in untreated males compared with untreated females suggests that transmission of Tb among males may be an important driver of the dynamics of Tb infection in possums.     

Dominance hierarchies in cattle and red deer (Cervus elaphus): their possible relationship to the transmission of bovine tuberculosis

A behavioural study was conducted to assess the dominance structure of cattle and deer herds and to assess the possible relationship of dominance to the risk of becoming infected with bovine tuberculosis. Five groups of cattle containing newly identified intradermal tuberculin test reactors were evaluated to determine the dominance hierarchy, and then exposed to sedated possums to assess the response of reactors and non-reactors. Eighty-six percent of the tuberculin test-positive cattle were among the 20% most dominant animals in their herds. In four of the five herds, the dominant animals investigated the sedated possum most actively, and in three of these four the reactors were in the investigating group. Six deer were exposed to a naturally tuberculosis-infected possum population, and the four highest animals in the dominance hierarchy (which also showed strong investigative behaviour when exposed to simulated terminally ill tuberculous possums) all subsequently became infected with tuberculosis. The fifth animal in the hierarchy became test-positive for tuberculosis later than the first four, but was subsequently also shown to be culture-positive for M. bovis. The lowest animal in the hierarchy, which showed no active interest in simulated tuberculous possums, did not become infected. This study strongly suggests a central role for terminally ill tuberculous possums in the transmission of tuberculosis to cattle and farmed deer. Management techniques designed to reduce contact between these few possums and farmed livestock may be expected to reduce the incidence of tuberculosis.     

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.     

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.     

Age-dependent patterns of bovine tuberculosis in cattle

Bovine tuberculosis (BTB) is an important livestock disease, seriously impacting cattle industries in both industrialised and pre-industrialised countries. Like TB in other mammals, infection is life long and, if undiagnosed, may progress to disease years after exposure. The risk of disease in humans is highly age-dependent, however in cattle, age-dependent risks have yet to be quantified, largely due to insufficient data and limited diagnostics. Here, we estimate age-specific reactor rates in Great Britain by combining herd-level testing data with spatial movement data from the Cattle Tracing System (CTS). Using a catalytic model, we find strong age dependencies in infection risk and that the probability of detecting infection increases with age. Between 2004 and 2009, infection incidence in cattle fluctuated around 1%. Age-specific incidence increased monotonically until 24–36 months, with cattle aged between 12 and 36 months experiencing the highest rates of infection. Beef and dairy cattle under 24 months experienced similar infection risks, however major differences occurred in older ages. The average reproductive number in cattle was greater than 1 for the years 2004–2009. These methods reveal a consistent pattern of BTB rates with age, across different population structures and testing patterns. The results provide practical insights into BTB epidemiology and control, suggesting that targeting a mass control programme at cattle between 12 and 36 months could be beneficial.     

Interactions between cattle and badgers at pasture with reference to bovine tuberculosis transmission

The normal behaviour of badgers, as described here, would not result in direct transmission of tuberculosis from badgers to cattle via air expired by badgers or via bodily contact. All activities of wild badgers in cattle fields at three different sites were observed at night for a total of 359 h. In addition, all activities of pairs of wild caught badgers were observed in a 0.5 ha enclosure with an artificial sett on 20 nights when cattle were present. Badger foraging in cattle fields was infrequent during dry conditions and variable at other times. Clover fields were preferred to grass pastures in two autumnal studies and under dry conditions the badgers did not prefer to forage on short pasture. Badgers consistently avoided close contact with cattle by changing routes from sett to foraging site and by foraging much less in areas of fields occupied by cattle. When foraging they preferred to remain at least 10-15 m from cattle and they avoided compact groups of cattle more than individuals. Some cattle would move towards badgers, especially if they were carrying out unusual behaviour, but badgers fled rapidly from every approach. In all potential encounters badgers were able to keep at least 2-3 m from approaching cattle. Badgers in the artificial sett delayed entering the enclosure if cattle were within 15 m of the entrance. When cattle were managed on a strip-grazing (rotational) system the whole area which they occupied was avoided by the badgers. However, badgers came closer to individual cattle and foraged in areas grazed by the cattle more if the cattle were set-stocked. If cattle are managed so that they are concentrated in a small area (rotational) system the risk of disease transmission is minimized because they are less likely to encounter badgers or their fresh products than are cattle managed on a large area (set-stocked) system.     

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.     

Risk factors for bovine tuberculosis in New Zealand cattle farms and their relationship with possum control strategies

This paper reports the investigation of farm-level risk factors for confirmed bovine tuberculosis (TB), based on a retrospective cohort study of a population of cattle in the lower North Island of New Zealand. Data were obtained from the TB testing surveillance programme operational in this area since the mid-1970s and comprised 190,665 cattle-years at risk from July 1980 to June 2004 (inclusive). A mixed-effects Poisson regression model was used to investigate the influence of farm-level covariates on the number of cattle confirmed with TB throughout the study period. This model was interpreted in context of depopulation strategies for the wildlife reservoir for TB, the brushtail possum Trichosurus vulpecula, that were applied in this area. The model showed that, despite intensification of possum control strategies over time, proximity to forest parks (a principal possum habitat in this area) remained a significant predictor of the number of confirmed cases of TB detected per farm per year. Our analyses showed a significant, three-fold increase in TB risk in dairy cattle relative to beef conditional on the size of local possum habitat, and confirmed the positive influence of cattle population size and the presence of previous infection status as a determinant of the number of confirmed TB cases per farm per year.     

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.     

Longevity of Mycobacterium bovis in brushtail possum (Trichosurus vulpecula) carcasses,and contact rates between possums and carcasses

Abstract

AIM: To determine, for a variety of environmental conditions, how long Mycobacterium bovis might remain viable inside the carcass of a brushtail possum (Trichosurus vulpecula) that died of bovine tuberculosis (Tb), and to measure the rate of contact between free-ranging possums and possum carcasses.

METHODS: Lesions of M. bovis were simulated by inoculating excised spleens weighing 0.5–1 g with 0.2 mL liquid culture containing approximately 5 x 10⁷ cfu M. bovis/mL. Simulated lesions were inserted into possum carcasses (n=48) at the peripheral lymph nodes. Carcasses were placed in the field at two sites (a tussock grassland and a podocarp-broadleaved forest site) and in two seasons (summer and winter) for up to 62 days. Survival rates of M. bovis were estimated by sampling the simulated lesions over time, and culturing the recovered lesion to determine if any viable M. bovis bacteria were present.

The time taken for a free-ranging possum to first encounter a dead possum in its home range was estimated by live-trapping possums and fitting them with proximity loggers (n=13). A ‘contact’ was recorded if these possums came within 40–50 cm of proximity loggers fitted to possum carcasses.

RESULTS: There were strong seasonal and site effects in the survival rate of M. bovis in possum carcasses. In the grassland habitat, no viable bacilli were cultured from any carcass after 3 days in summer, whereas in winter all samples were culture-positive for the first 20 days, and some were still positive after 27 days. The survival rates for forest habitat were intermediate between the results for grassland, and there were no culture-positive carcasses after 9 days in summer or 27 days in winter.

In summer, infected carcasses (n=6) were first encountered by possums a mean 1.9 (range 0.4–6.7) days after placement.

CONCLUSIONS: Possum carcasses were contacted by free-ranging possums within the period that viable M. bovis were shown to survive in a carcass. The risk of such infection is likely to be most significant in winter or in areas with microhabitats where the survival of M. bovis is high. However, the generally low survival rate of M. bovis in possum carcasses and the low frequency of possum-to-carcass contacts indicate this route of transmission alone could not maintain Tb in a possum population.     

Effects of farm management practices on cattle grazing behaviour and the potential for transmission of bovine tuberculosis from badgers to cattle

Despite the large body of circumstantial evidence to suggest a link, the means by which bovine tuberculosis is passed from badgers to cattle remains unclear; pasture contamination with the urine, faeces and/or sputum of infectious badgers is believed to be the main route of transmission. Therefore the behaviour of grazing cattle was studied to determine whether they avoided investigating and/or grazing pasture contaminated with badger excreta, and whether different farm management practices enhanced the potential for disease transmission. Active latrines were avoided by most cattle until the sward length in the rest of the field was reduced, after which there was an increasing likelihood that active badger latrines would be grazed. Most of the cattle grazed active badger latrines, but cattle of low rank within the herd grazed latrines more heavily. Farm management practices that reduced the availability of long swards shortened the period of investigative behaviour and greatly enhanced the risk that cattle would graze active badger latrines. Cattle were more likely to graze pasture away from latrines that was contaminated either with badger urine or single faeces. Because bacilli remain viable in the soil for up to 2 years, there is the potential for bacilli to accumulate at active badger latrines, and these could pose a significant risk to cattle, even when the latrine is no longer being used by badgers. Cattle readily grazed the lush sward at disused latrines, during which they could ingest contaminated soil; the amount of soil ingested increases as sward length decreases.     

Transdisciplinary habitat models for elk and cattle as a proxy for bovine tuberculosis transmission risk

Zoonotic diseases such as bovine tuberculosis (TB) that infect wildlife and livestock are particularly difficult to eradicate where wild animals make extensive use of agricultural landscapes. Transmission of TB between cattle (Bos taurus) and wild elk (Cervus elaphus) in southwestern Manitoba, Canada remains poorly understood but there is a risk when comingling occurs on summer pasture. Elk use of cattle summer pastures was assessed using ecological data (187 VHF and 25 GPS collared elk monitored over four years representing 8% of the elk population). Local knowledge was documented by conducting interviews and participatory mapping exercises with 86 cattle producers (98% of those within the study area). Of the 294 cattle pastures mapped by farmers, 13% were used by radio-collared elk, 38% were reported by farmers as being used by elk, and 42% were identified as used by elk when both when all datasets were combined. Cattle pastures that had been used by elk and those that had no elk were compared using binary logistic regression based on each of the three datasets (i.e. farmer observations, radio-collared elk on pasture, and combined dataset). For all three datasets, distance to protected area and proportion of forest cover on the cattle pasture were identified as the most and second most important predictor variables, respectively. There was strong agreement among the relative probabilities of elk occurrence on each pasture derived from the resource selection function (RSF) models developed using farmer interviews and elk collaring data. The farmer interview and collar datasets were then combined to generate a final integrated RSF map summarizing the probability of elk–cattle comingling and were contrasted over each of four cattle grazing seasons (spring, early summer, late summer, and autumn). These predictive maps indicate that use of cattle pastures by elk is extensive, particularly in spring and early summer. Farmer observations indicate that elk and cattle share water sources and livestock mineral supplements on pasture. Local knowledge and conventional ecological data complement and validate one another and help us better understand the temporospatial aspects of shared space use among wildlife and livestock and more generally the risks of disease transmission in agricultural landscapes.     

Introduction of bovine tuberculosis to north-east England by bought-in cattle

The source of bovine tuberculosis was investigated in 31 herds in north-east England that experienced confirmed breakdowns between January 2002 and June 2004; nine of the herds had been restocked after the UK outbreak of foot-and-mouth disease in 2001. In all but one of the breakdowns the most likely source of infection was identified as one or more purchased animals. In 17 of the breakdowns, reactor animals were traced to herds from which the same combination of spoligotype and variable number tandem repeats was isolated, and in five breakdowns a different spoligotype was isolated. The most likely sources were located in Wales and the west and north of England, and included a Cheshire herd that was the most likely source of nine of the breakdowns. Three breakdowns were traced to Irish imports. Reactors in five of the breakdowns included homebred as well as purchased animals, providing evidence for the likely spread of the disease within the herds. The lack of geographical clustering of molecular types pointed to the overwhelming source of infection being cattle that had been bought-in.     

Endoparasites of brushtail possums (Trichosurus vulpecula) from the South Island,New Zealand

As part of a study to assess whether endoparasites could assist in the biological control of brushtail possums in New Zealand, we investigated the composition and distribution of possum endoparasites in the South Island. Possums were collected near five of the original release areas in the South Island: Banks Peninsula, Hokitika, Nelson, Dunedin and Invercargill.

Among the nematodes, those most frequently encountered were Trichostrongylus spp., which were present in possums from all five study areas. Trichostrongylus species from possums in the Invercargill area comprised 4.5% T. colubriformis, 0.9% T. vitrinus and 11.3% T. retortaeformis. Paraustrostrongylus trichosuri and Parastrongyloides trichosuri were found only in the Invercargill area, where they infected 1.4% and 14% of possums respectively. The cestode Bertiella trichosuri was present in possums from all locations except Dunedin. The protozoan Eimeria spp. occurred in all areas.

These are the first records of Parastrongyloides trichosuri, Paraustrostrongylus trichosuri, T. vitrinus, T. retortaeformis and Eimeria spp. in South Island possums.

The prevalence of endoparasites and the intensity of infection was very low compared to the lower North Island of New Zealand. Endoparasites at the existing levels in the South Island probably have very little effect on possum populations.     

Endoparasites of brushtail possums (Trichosurus vulpecula) from the South Island, New Zealand

As part of a study to assess whether endoparasites could assist in the biological control of brushtail possums in New Zealand, we investigated the composition and distribution of possum endoparasites in the South Island. Possums were collected near five of the original release areas in the South Island : Banks Peninsula, Hokitika, Nelson, Dunedin and Invercargill. Among the nematodes, those most frequently encountered were Trichostrongylus spp., which were present in possums from all five study areas. Trichostrongylus species from possums in the Invercargill area comprised 4.5% T. colubriformis, 0.9% T. vitrinus and 11.3% T. retortaeformis. Paraustrostnmgylus trichosuri and Parastrongyloides trichosuri were found only in the Invercargill area, where they infected 1.4% and 14% of possums respectively. The cestode Bertiella trichosuri was present in possums from all locations except Dunedin. The protozoan Eimeria spp. occurred in all areas. These are the first records of Parastrongyloides trichosuri, Paraustrostrongylus trichosuri, T. vitrinus, T. retortaeformis and Eimeria spp. in South Island possums. The prevalence of endoparasites and the intensity of infection was very low compared to the lower North Island of New Zealand. Endoparasites at the existing levels in the South Island probably have very little effect on 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.