Report

Trichomonosis: Fatal disease causing rapid decline of our greenfinches

Not mentioned enough, trichomonosis is a fatal disease and the cause of the rapid decline of the British Greenfinch population and poses a serious threat to other, beloved garden birds.

Green finch in a tree

According to the 2021 UK Breeding Bird Survey, the greenfinch (Chloris chloris) population declined from circa 4.3 million to approximately 1 million birds, which equates to an overall decline of 77% – the largest scale population decline of British birds due to infectious disease on record. As a result, the species has now been re-classified from Green to Red-listed on the Birds of Conservation Concern. As a result of finch trichomonosis, the UK breeding chaffinch (Fringilla coelebs) population has also declined since 2012 – from the second to the fifth most common wild bird species.

Garden Wildlife Health (GWH) explain that an epidemic of finch trichomonosis occurred throughout much of Great Britain in 2006 and 2007. In subsequent years, the disease has continued to cause large-scale mortality of greenfinches. Whilst there was a marked seasonal peak during the late summer to autumn months in the first few years following emergence, incidents now occur throughout the year. The GWH, which monitors the health of, and identify disease threats to British wildlife, is co-ordinated by the Zoological Society of London (ZSL), working in partnership with the British Trust for Ornithology (BTO), Froglife and the Royal Society for the Protection of Birds (RSPB).

“Unfortunately, we continue to receive regular reports of sick and dead finches affected by trichomonosis, with widespread distribution in Great Britain,” confirms the GWH team to thedirt.news.

Whilst greenfinch and chaffinch are the most frequently affected by this infectious disease, many other garden bird species are also impacted. Fellow frequent bird feeder visitors such as the house sparrow (Passer domesticus), siskin (Spinus spinus), goldfinch (Carduelis carduelis) and bullfinch (Pyrrhula pyrrhula) are also susceptible.

Where outbreaks of disease involving large numbers of finches occur, other garden birds – even those that typically feed on invertebrates, such as blackbird (Turdus merula) and dunnock (Prunella modularis) are also susceptible.

Bird feeding

With good intentions we are increasingly feeding birds in our gardens but unfortunately, research increasingly indicates that contaminated food and drink from bird tables and feeders may spread infectious diseases. Over the last few decades, bird feeding has become progressively popular, where initial, incremental feeding to reduce starvation and nutritional stress in over-wintering birds, has become a year-round activity by millions of households, worldwide. (Jones and Reynolds, 2008; Robb et al.,2008).

Feeding birds is generally accepted to increase survival, reproduction rates and higher densities of (the fed) populations. Yet, despite the vast scale of bird feeding, researchers still lack clear understanding of its positive and/or negative ecological effects (Robb et al., 2008) – nor how it affects all other urban bird species (Ruffino et al., 2014; Harrison et al., 2010).

Unfortunately, research does indicate that as the popularity of supplemental bird feeding increases, so too has the extent and frequency of infectious bird diseases – which researchers have suggested results from increased contact between birds at feeders and contamination of the feed or feeders with infectious organisms. The elevated risk of pathogen transmission, could be caused from direct transmission – close contact with infected individuals or indirectly, or indirectly, through contaminated feed or water, or via faecal-oral route (Murray et al., 2016; Becker et al., 2015; Bradley and Altizer, 2006).

Researchers from the Canadian Wildlife Health Co-operative have documented a suspected relationship between bird feeding and three significant diseases, including trichomonosis (Lawson et al., 2012; Neimanis et al., 2010), mycoplasmosis (Hartup et al., 1998; Dhondt et al., 2007) and salmonellosis (Pennycott et al., 1998). All three diseases have been identified at public bird feeding sites in Canada in recent years (McBurney et al., 2015; Forzán et al., 2010; Mikaelian et al., 2001; Daoust et al., 2000; Dhondt et al., 1998; Prescott et al., 1998).

Origins trichomonosis

First discovered in the British finch species in the summer of 2005, trichomonosis subsequently continued to spread in 2006 and 2007, with the disease now established throughout the British Isles. Also found in Continental Europe, where it was first seen on the Fennoscandian Peninsula in 2008 – likely originating from infected migrating chaffinches from Britain.

The disease is thought to have originated from pigeons and doves in Great Britain, with the majority of transmission now occurring from finch to finch. That said, it’s presumed that the spread of the parasite between finches and pigeons or doves is also, may also still occur. Worryingly, birds of prey are expected to become infected through the consumption of diseased songbirds, but the extent and potential significance to British birds of prey is still unknown.

Transmission

The GWH explained that the Trichomonas gallinae parasite is vulnerable to desiccation and cannot survive for long periods outside the host. Transmission of infection between birds is most likely to be by birds feeding one another with regurgitated food during the breeding season or through food or drinking water contaminated by an infected bird. Infected finches have difficulty swallowing and often regurgitate seed or water – which if consumed by healthy birds, exposes them to infection. As feeding stations encourage birds to congregate, the potential for the spread of the disease increases.

That means that food discard below feeders, often encouraged as a means of ‘secondary feeding’, puts birds, especially ground feeders, such as chaffinches at risk of the disease. Researchers found that declines in chaffinches were more marked in peridomestic habitats than in rural habitats, which researchers suggests may be due to food provision increasing parasite spread.

“Secondary feeding is a potential route of pathogen transmission. The Trichomonas gallinae parasite is spread in saliva, transmission may occur directly between birds, for example when they feed one another during the breeding season, or indirectly if they drop or regurgitate food or water that is contaminated with saliva containing the parasite,” explains the GWH.

Finch trichomonosis occurs year round, however, June to September have been observed to be the peak months for this condition.

Identification

The Trichomonas gallinae parasite affects the tissues in the bird’s mouth, throat and oesophagus, making it hard for them to eat and breathe – killing them due to starvation or predisposing them to predation.

According to GWH infected birds show signs of general illness in the form of: lethargy, fluffed-up plumage, some can drool saliva, regurgitate food, have difficulty in swallowing or show laboured breathing. Affected finches can have matted wet plumage around the face and beak. The disease may progress over several days or even weeks, consequently affected birds are often very thin or emaciated.

The GWH have said that there is no known health threat to people or to mammals (eg. dogs and cats) from finch trichomonosis, but the Trichomonas gallinae parasite has the potential to affect captive poultry and pet birds.

Prevention

As many of the options for prevention depend on the bird feeding general public, it’s vital that this message is spread far and wide.

If an outbreak is detected, consult the GWH immediately (report the incident through the GWH Reporting Portal) and remove all bird baths, feeders and bird shelter. According to R.W. Gerhold, Jr., DVM, PhD, MSVet (MSD Manual), the control of trichomonosis centres around sanitation and biosecurity. If an outbreak of trichomonosis is documented or suspected, feeders and waterers should be removed for approximately 2 weeks and cleaned with a diluted bleach solution (follow manufacturer recommendations).

Advice to prevent the emergence of trichomonosis (and other diseases) when feeding birds:

  • Ideally, reduce the need for supplementary feeding by creating bird friendly gardens that have good levels of natural sources of food, water and shelter for birds. Reducing the rate of contact between unaffected and infected birds and/or with environments contaminated with infectious organisms is key. Bird feeders tend to encourage the congregation of birds, increasing contact rates within and between species that can result in disease transmission (Murray et al., 2016; Becker et al.,2015; Bradley and Altizer, 2006). A bird friendly garden is one for example, where herbaceous borders are left uncut over winter to provide ample food sources. Includes plants and trees with good sources of berries – which have been positively associated with native and migratory bird species richness. Gardens that incorporate generous levels of garden compost for optimum garden biodiversity, ensuring they are rich in soil micro-organisms, worms, woodlice, slugs and other invertebrates for birds to feed on.
  • Hygiene is crucial. The GWH recommend to select feeders made from materials that are conducive to effective cleaning and disinfecting – hence not ones made with porous materials such as wood or clay. Water in birdbaths can quickly become contaminated with Trichomonas gallinae which can survive (up to at least two hours in water) and should be therefore be regularly emptied, disinfected, rinsed and refilled with fresh, clean water. Bird feeders, should be regularly emptied, cleaned and disinfected to prevent contamination of the food. When asked how often, Dick Woods, Founder of Finches Friend said: “Simply put, you can’t clean too often, anecdotally we believe cleaning on average takes place monthly at best. We push for weekly, and design to make that simple, not a chore.” The GWH concurred, recommending weekly cleaning of bird feeders, whilst circumstances may vary. Considering the damaging potential of Trichomonosis, it may be time we take this even further and apply the same hygiene standards to bird feeders as we do for our pet’s food, perhaps even our own.
  • Use the right feeder. A big topic, which will cause much debate on account of the fact that many current market offerings potentially exacerbate the spread of disease. We have therefore covered this topic in more detail here. The research does suggest that the concept of secondary feeding should be avoided. That means that feeders designed with waste capture elements, or feeders with holes at different heights that allow for contaminated food to drop down for others to consume, risk further spread. The GWH encourage small feeders/quantities as they are emptied quicker and prevent large congregations of birds. Offering different (species specific) food types in different areas is preferable, to help reduce contact rates between different species.
    • According to Woods, it’s all about, “Food, Feet and Faeces. Disease transmission is to a great extent down to regurgitation, faeces and damp. Choose feeders that keep the food dry and limits access by birds to a perch. People simply do not understand that a single sick bird, through its saliva can create a chain reaction within their feeders, damp food becomes a host for the trichomonosis causing parasite, which can present at other ports on side port feeders as the food drops. Husks stick to sick birds beaks and are then left around ports or dropped to the ground, in both scenarios they are infectious.”
    • Ensure feeders visited by greenfinches or chaffinches don’t allow access by pigeons or doves, due to the increased potential for transmission of trichomonosis as these species are prone to this disease.
    • Moderate the volume of seed provision by using smaller feeders, and reduce the number of seed feeders in use, since congregation at high density for sustained periods might increase the risk of parasite transmission.
  • Provision of clean and fresh drinking water on a daily basis. Ensure the food you buy is fresh, from reputable sources and in quantities which will be used within a relatively short period (e.g. 3 months) or before the best-before date if stated. Only fill feeders with enough food to last 24 to maximum 48 hours. If left out longer, the food becomes stale and susceptible to mould and pathogen build up. Never top up stale food with fresh feed – safely discard any food not eaten within 24-48 hours, ensuring it cannot be eaten by wildlife.
  • Rotate feeder location regularly on weekly basis to reduce build-up of food waste or bird droppings in any one area.

Further reading