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Trichomonosis (fat finch disease) has decimated the greenfinches that visited my garden, from 15 to nil. This really concerns me. It was participants in the Garden Bird Watch scheme (BTO-British Trust for Ornithology ) that brought this plight to the attention of BTO scientists, who were concerned because the organism that causes the death of greenfinches has ‘jumped’ species from wood pigeons to greenfinches. I decided to contact the BTO with a number of questions. Mike Toms, Head of Garden Ecology kindly found the time and responded. The questions and responses ( in italics ) are printed below in full. Don’t forget to keep your feeders clean and give the birds fresh water, daily. I hope you find it interesting. If you do, please consider joining the Garden Birdwatch scheme…….

www.bto.org/gbw

Dear George

Many thanks for your email and queries about urban trichomonosis. I have responded to each query in turn in the hope that it makes for a more sensible response.

1. I appreciate that targeted medical dosing of wild populations living in the fields and open countryside would not be a practical option. But taking no action is, in my opinion, not an option either. However, many of these birds may well visit garden birds feeders, including on some farms.  If we attract birds to our gardens for our pleasure, do we not owe them some responsibility? Simply administering general hygiene measures for garden bird feeding, feeders, tables, positioning,  water, etc.,  does not appear to be working. Could it be possible to approach bird seed companies to administer some form of medication to seed mixes to kill this organism or give immunity to birds visiting bird feeders?

You are quite right. If we attract birds to our gardens for our pleasure then we do have a responsibility for their welfare. We are working to understand the risk factors associated with disease transmission so that we can provide advice that will help to reduce transmission rates. This advice might, for example, suggest changes to feeder design that could help reduce transmission rates. The question of hygiene measures and their success has not been resolved and I don’t think it is the case that administering general hygiene measures has failed. It is clear from studies that have been carried out that only a minority of those providing food for wild birds actually have any hygiene measures in place. This suggests that efforts should be made to increase awareness of the need for good hygiene at feeding stations. Not only should we promote this message through the media but we should also be pushing bird food suppliers to increase the profile that hygiene gets on their packaging. The administration of medication through seed mixes does not appear to be a practical solution as it is impossible to control the dose that individual birds receive.

2. As the organism may live for a few days in moist seeds/bird foods, does the fact that such food is offered in tubular feeders, as opposed to a bird table, allow the organism some protection from the elements aiding its longevity and survival ?

We don’t know but work looking at the ‘residue’ that collects in the base of tube feeders is on the cards and we have been looking at the role of different types of feeder in the wider issue of disease transmission. More targeted work – most probably lab based – is needed here and I know that such work has been discussed.

3. Parasites usually do not kill their hosts as this then kills the parasite and therefore stops it infecting new hosts and spreading. Why does the parasite kill its hosts, i.e. greenfinches?

Mortality appears to result from secondary problems (secondary infection or the inability to feed that follows the development of lesions) so it is not really the case that the parasite itself is killing the host. This is really a question for the vets but my understanding is that the Trichomonad can complete its lifecycle effectively within the period over which the bird is infected and before its death, so that death of the ‘host’ is less relevant than would be the case in something like your more typical parasite.

4. Is it possible that the visitation to feeders by wood pigeons, by using such means as wrap around pole hooks, aids the transmission of this disease to greenfinches?

We don’t know but transmission from pigeons to finches is most likely to have been through contaminated salival contact – e.g. spilt food. Again, this is something that is being looked at and will be looked at in more detail if some of the suggested areas for further study are followed up.

5. Would it be possible that squirrels also act as a vector for this parasite?

We don’t know. It is unlikely, as Trichomonas gallinae is a parasite of birds and not mammals. Disease transmission involving Grey Squirrels has been examined in relation to the squirrel pox virus and there may be a mechanical pathway here, so we could look at this.

6. Is the disease trichomonosis or trichomoniasis?

When we first started the work the term ‘trichomoniasis’ was in wide use. However, the term ‘trichomonosis’ is now favoured by vets.

7. What caused the organism to jump species?

It is not host specific and had been reported from a range of species for a number of years. We don’t know why it suddenly took off in finches, nor do we know definitively that it came from Wood Pigeons but the dna work is highly suggestive that it did. My guess (and it is only a guess) is that it spilled over from wood pigeons as a consequence of the large increase in Wood Pigeon numbers at garden feeding stations.

8 Why did it do this?

See answer to 7.

9. If this is not solved what could happen to the UK greenfinch population?

We don’t know but are monitoring the situation. It exists at a low level in wood pigeons so we might expect it to rumble along at a lower level in finches once the initial impact has subsided.

10. As 8 but the European population ( Norway, Holland and Sweden have reported deaths)

They have and we have a paper in review on this so I cannot comment at this stage, except to say that we have good evidence of why it has now occurred in these other countries.

11. Has there been an corresponding increase in trichomonosis in the wild bird population?

We don’t distinguish between ‘garden birds’ and ‘wild birds’ as they are all ‘wild birds’ and, as you know, there is a great deal of movement between different habitats. The decline in the GBW reporting rate is almost identical to the index of the breeding population derived from the Breeding Bird Survey.

I hope that this helps.

With best wishes

Mike Toms

Head of Garden Ecology Team

mike.toms@bto.org

__________________________________________________________

British Trust for Ornithology

The Nunnery

Thetford

Norfolk

IP24 2PU

tel. 01842-750050    fax. 01842-750030

www.bto.org/gbw