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Topic 2: Diseases & Public Health

CUMMINS. Would the veterinarians present like to comment on the health and hygiene aspects of handling possums, or marsupials in general? What are the hazards and how do we get around them?

MORIARTY. The two main diseases that spring to mind are leptospirosis and tuberculosis. As a generalisation, in our area (Palmerston North) Tb is not a great health hazard, for I have not seen it on any possums we have handled. However, because of risks from an infected animal, at Massey University we have reduced the use of possums in student practicals.

HAMILTON. We had an extraordinary situation where we were actually stopped from using possums because of the health hazard. I did not think it was all that hazardous. However on examining a urine sample we found it loaded with Leptospira, so that convinced me that the urine could be quite a hazard to the human operator.

BLACKMORE. I agree the major diseases in possums are leptospirosis and tuberculosis but I think we have got to put it into perspective in relation to other animals we are handling. Apparently viable Leptospira occur commonly in most of our wild mammal species but we do not yet know the particular significance of various serotypes that have occurred in the possum. In the meantime we must consider it hazardous. Anyone carrying out postmortems on possums must therefore wear gloves, and if you know you are working in a Tb area particular precautions must be taken. However, it needs to be put into perspective - one should take sensible precautions with any wild animal for any could be transmitting disease which can be contracted by man.

CUMMINS. Yes it is worth noting that the possum is not the only villain on the scene. Do you take any prophylactic measures with your captive animals? For example at Victoria University we routinely dose them with combiotic (following Wallaceville Animal Research Centre's recommendation) before they are placed in the colony.

BLACKMORE. I just think we treat every possum as a potential disease source. We do not yet have a possum prophylactic in terms of a vaccine either for Leptospira balcanioa or tuberculosis.

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BROCKIE. As I recall there are very few substantiated cases of people being infected by possums. I think records of hospital patients on the West Coast were checked and about 10–12 trappers had got skin tuberculosis, presumably from handling possums. It was restricted to their hands and wrists. Another case was a technician who contracted leptospirosis from a rat at Massey.

BLACKMORE. We have to be careful with statistics. We have 550-odd Tb cases in humans a year but few of the mycobacteria are typed. So we don't really know how many cases were M. bovis (which is pathogenic to man as well as possums and cattle) or M. tuberculosis. Again, we need to treat every animal as suspect regarding disease risks.

CUMMINS. I think it's worthwhile that the points raised at this meeting are brough to the attention of the schools. I know possums are quite routinely trapped around Wellington for various class projects. I think it should be noted by all dealing with them that they are a potential health hazard.

BLACKMORE. I have in fact written a Health Department Bulletin pointing out the dangers of using possums in the school situation.

ANONYMOUS. Mr Chairman, I think that the Bulletin has been sent around to the teachers concerned.

MEADOWS. I would be interested to know if anybody has come across ringworm in possums. It is reported in Australia that some species around cities in particular have it, and it is one of the diseases which can be a health hazard.

BROCKIE. I've submitted 40–50 samples of possum fur to the National Health Institute. One or two with ringworm have shown up but they have not been pathogenic to humans. Nearly every other wild animal does carry ringworm that can be transmitted to humans so in a sense it was something of a disappointment. Often possums look very mangy but the cause of this is ectoparasitic mites rather than ringworm.

GREEN. If there is a potential Tb risk from handling possums in New Zealand, why have the appropriate tests for typing the Mycobacterium not been done?

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BLACKMORE. This has been done on occasion. If a doctor sees a case of tuberculosis and feels there is something odd he will submit a culture for typing. However the doctor is usually concerned with whether it is a Tb case or not, and not so concerned as to which type of tuberculosis it might be.

WARD. Where you are handling animals and are at risk of being urinated on, what antiseptic would anyone like to recommend?

COOK. Both formalin and Savlon are contraindicated in the presence of tuberculosis. I would suggest that you use Dettol if you are going to use a disinfectant at all; merely soap and water is probably quite good.

HATHAWAY. I would react strongly regarding the use of Dettol against Mycobacterium, for I don't think it's very effective. Phenol derivatives are most commonly used at Massey University.

MARKHAM. I find Dettol is not so good. Medol or synthetic phenols are better for Tb. As far as leptospirosis is concerned, the organisms appear very susceptible to any of the common disinfectants; water and soap is also very good. However, leptospirosis can survive in damp conditions, so if one is urinated on use a paper-towel - get the urine off and dry oneself.

HAMILTON. Can I suggest that if anybody is worried about picking up tuberculosis from possums it's likely to be through skin infection. It would be wise to test your tubercular sensitivity first of all, and see if you are tubercular negative. The BCG vaccination would almost certainly prevent you having the skin reaction with bovine tuberculosis and it would certainly raise your resistance and diminish your chances of getting pulmonary tuberculosis.

PRACY. I think people dealing with possums especially in these informed situations know the risk and take precautions. The problem is the occasional trapper who is unaware of the risks. When skinning it is quite possible to cut through an infected lymph node. I must admit I have done this four times. Also I am in the habit when opening-uppossums of holding them between the knees and this is where I think there is a danger.

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HAMILTON. I would like to present to the symposium a statement on possums in virus research based on our work at the Virus Special Projects unit of the National Health Institute. We were asked by Dr R.E. Brockie if we could utilise possums discarded after his studies of Leptospira, ringworm, and so on. We decided to try and infect possums with human influenza viruses.

It is known that the ferret Mustela putorius could be infected with human influenza viruses, both type A and type B. Andrews had indeed first demonstrated the viral aetiology of human influenza, using these animals in 1933. He had previously tried a number of experimental animals without success, and it is said that his pre-deliction for blood sports determined the choice of ferrets and his access to supplies of this exotic experimental animal! Later work by Burnet demonstrated that the human influenza virus could be grown in fertile eggs utilising the amniotic cavity of 10–15 day old embryos. During adaptation to the egg, the human influenza virus undergoes a mutational change from the original (O) human phase to the derived (D) egg-adapted phase of the virus. This latter phase, D, grows faster than the O phase, and also in the allantoic cavity, which the O phase is unable to do. The important fact is that ferrets could also be infected with the D phase egg-adapted influenza virus which is completely avirulent for man. This later finding is fundamental to the ecology of human influenza viruses, for it precludes the possibility of using egg-adapted virus as attenuated live influenza virus vaccines in humans. This work of Burnet's has never been repeated except in minor ways (e.g. the virus isolated in human kidney tissue cultures seems to remain in the O phase). Thus, based on this type of thinking we attempted to infect possums with first, egg-adapted virus in high doses and later, with human throat secretions found positive for influenza virus in eggs.

Wild caught possums were obtained and bled by cardiac puncture under ether anaesthesia. When the animals recovered they were inoculated intranasally with the virus-containing inocula under light ether anaesthesia. Swabs were obtained after 3 days from the throat also under ether anaesthesia and virus isolation was attempted in eggs. A post-inoculation sample of blood was taken 14 days after inoculation, also by cardiac puncture. A large amount of blood could be obtained from a mature possum - 180 ml with recovery from cardiac puncture.

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Now I shall outline the results. Two possums were infected with Myxovirus influenzae type A, from egg-adapted virus 1974 that had been stored at -90°C. The virus was recovered from the throat of one animal 3 days after inoculation; the serum antibody response was marked, the pre-inoculation titre of 1/10 going to 1/160 and 1/320 respectively in the two animals.

The possums were similarly infected with egg-adapted Myxovirus influenzae type B. In this case, virus recoveries after 3 days were negative; the preinoculation titre to the inoculated virus went from < 1/10 to 1/40 and 1/320 respectively in the two animals.

A young male in contact with the infected animals above did not develop antibody and attempted virus recovery was negative.

Single attempts to infect possums with human throat secretions were negative, but the stored material was not viable when simultaneously tested in eggs. Thus the results were not conclusive. A total of eight possums was used for these experiments.

To summarise, egg-adapted virus will infect possums; the antibody development could be of similar specificity to that developed in ferrets and so could be very useful for strain-specific identification of human influenza viruses.

We also carried out tissue culture studies. Kidney tissue cultures were set up in glass test-tubes as dispersed epithelial cell cultures. Fourteen specimens of influenza viruses from -90°C storage were used to inoculate fully grown tissue cultures from several animals. Cytopathogenic effects and positive haemadsorption were demonstrated with virus of egg and human origin. Thus virus could be isolated directly from human source material in possum kidney cultures.

In possum antibody studies serum from just over 100 wild caught possums was examined for antibody to human A and B influenza viruses by both a complement fixation and haemoglutination technique. No specific antibody was demonstrated. The H.I. tests were bedevilled by much non-specific inhibition, unlike sera obtained under laboratory conditions.

From these studies I conclude that possums could be very useful animals in human myxovirus research for:

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(1)	production of large amounts of post-infection antibody which has the promise of being highly specific;
(2)	use of the kidney epithelial cell cultures for isolation and antibody studies in human infections; these promise to be better than monkey kidney cell cultures which can be infected with SV5 virus contaminant (another myxovirus);
(3)	the investigation of the immunological abnormality which allows the growth of egg-adapted influenza virus could have important results in the field of live attenuated influenza viruses for human influenza immunisation;
(4)	the use of possums in other aspects of human respiratory virus investigation is worthy of exploration, such as organ cultures of the trachea and the attempted isolation of other human respiratory viruses.

I gratefully acknowledge the technical assistance and expertise of Mr Maurice Wilson and Mr Norman Kuttner in this work.