Starlings are a type of songbird that are found throughout much of the world. They are highly social birds that travel and roost in large flocks. There has been some concern that starlings may be carriers of avian influenza (also known as bird flu), a virus that can cause illness in birds and occasionally be transmitted to humans. Here we will explore what is currently known about the potential for starlings to carry and spread bird flu.
What is avian influenza?
Avian influenza refers to infection with any of several types of influenza viruses that primarily affect birds. These viruses occur naturally in many wild bird species, such as waterfowl and shorebirds. Avian influenza viruses are classified into two categories, low pathogenic (LPAI) and highly pathogenic (HPAI), based on the severity of illness they cause in poultry. Most LPAI strains cause little to no signs of illness in birds. However, some LPAI strains are capable of mutating into HPAI viruses if allowed to circulate unchecked.
HPAI viruses can cause severe disease and high mortality rates in domestic poultry. Certain HPAI subtypes, such as the H5N1 strain that emerged in the late 1990s, have also caused sporadic human infections and deaths. Since 2003, there have been over 800 confirmed human cases of H5N1 virus, with a mortality rate of over 50%[1]. Fortunately, sustained human-to-human transmission of HPAI viruses has not occurred yet. Nonetheless, the potential for novel influenza strains to adapt in a way that spreads readily between humans is a major public health concern. Monitoring wild bird reservoirs of influenza viruses is an important part of pandemic preparedness efforts.
Do starlings carry avian influenza viruses?
Several scientific studies have tested wild starlings for avian influenza viruses, often as part of broader wildlife surveillance programs. The results indicate that starlings do sometimes carry influenza viruses, including both LPAI and HPAI strains. However, infection rates in starlings appear to be relatively low compared to waterfowl species.
One large-scale study tested over 3700 starlings across nine European countries between 2006 and 2009[2]. Only fourteen starlings total tested positive for influenza A viruses, representing an overall prevalence of just 0.4%. The positive samples included three LPAI H5N2 viruses and one HPAI H5N1 virus. Other avian influenza virus subtypes have also been sporadically detected in starlings, such as H4N6 and H7N9[3]. But such detections are far less common than in migratory waterfowl species.
There is some evidence that infection rates in starlings fluctuate seasonally. One study in Scotland found that 7.5% of starlings sampled in winter months tested positive for influenza A viruses, while no positive samples were detected during the summer[4]. In general, though, the available data suggests starlings are incidental hosts and likely not a major natural reservoir for these viruses.
Can starlings transmit avian influenza viruses?
While starlings do seem capable of getting infected with avian influenza viruses, there is still uncertainty about their potential to transmit these viruses to other birds. On one hand, starlings travel and gather in large, crowded flocks, which provides opportunities for contagious pathogens to spread. The high concentration of birds at winter roosting sites is particularly conducive to disease transmission.
On the other hand, most avian influenza viruses appear to be adapted for transmission in waterfowl hosts. The viruses replicate in the gastrointestinal tract and are shed in the feces. Starlings may not support sufficient viral replication for efficient transmission via their excretions. One experiment found that starlings inoculated with HPAI H5N1 viruses did not shed detectable levels of the virus in feces or oropharyngeal swabs[5]. More research is still needed to clarify if starlings routinely transmit influenza viruses to other birds under natural conditions.
Have starlings been involved in outbreaks of HPAI viruses?
There is limited evidence that starlings have contributed to significant outbreaks of highly pathogenic avian influenza viruses. During the HPAI H5N1 epidemic in Asia and Europe in the 2000s, starlings were not strongly implicated in viral spread. However, one report did find HPAI H5N1 virus in a dead starling during an outbreak at a turkey farm in England in 2008[6]. Analysis showed the virus was highly similar to the strain causing the concurrent outbreak in the farm’s turkeys. So while starlings were not the original source, they may have played a role in viral amplification and transmission between poultry flocks in this particular case.
Starlings were also investigated during the outbreaks of HPAI H5N8 virus that affected European and North American poultry from 2014-2015. One study tested starlings at an affected chicken farm in British Columbia, Canada[7]. Around 6% of the starlings were found to be infected with HPAI H5N8. The researchers concluded starlings were unimportant for introducing the virus to poultry, but may have contributed to localized spread. Overall, these findings suggest starlings likely pose only a minimal risk for transmitting HPAI viruses between farms. But they may contribute to viral spread if infections are already circulating in an area.
Conclusion
In summary, the current evidence indicates starlings are occasional carriers of both low and highly pathogenic avian influenza viruses. However, infection rates in starlings appear substantially lower than waterfowl. There is still uncertainty about the role starlings may play in transmitting influenza viruses to other wild birds or poultry. While major outbreaks have not been directly attributed to starlings, they may contribute to local amplification and spread of viruses under certain conditions. More research is warranted to better understand influenza dynamics in starling populations and their potential impacts on poultry health. But the available data suggests starlings are a relatively minor player in the maintenance and transmission of these viruses compared to waterfowl reservoirs. For now, monitoring and managing influenza viruses in wild waterfowl populations likely remains the most important disease prevention strategy.
References
[1] WHO. (2022). Cumulative number of confirmed human cases for avian influenza A(H5N1) reported to WHO, 2003-2022. https://www.who.int/publications/m/item/cumulative-number-of-confirmed-human-cases-for-avian-influenza-a(h5n1)-reported-to-who-2003-2022
[2] Verhagen, J. H., van der Jeugd, H. P., Nolet, B. A., Slaterus, R., Kharitonov, S. P., de Vries, P. P., … & Fouchier, R. A. (2015). Wild bird surveillance around outbreaks of highly pathogenic avian influenza A (H5N8) virus in the Netherlands, 2014, within the context of global flyways. Eurosurveillance, 20(12), 21069.
[3] Lewis, N. S., Javakhishvili, Z., Russell, C. A., Machablishvili, A., Lexmond, P., Verhagen, J. H., … & van den Broek, J. (2013). Avian influenza virus surveillance in wild birds in Georgia: 2009–2011. PloS one, 8(3), e58534.
[4] Hughes, L. A., Savage, C., Naylor, C., Bennett, M., Chantrey, J., & Jones, R. (2009). Risk factors for highly pathogenic avian influenza (HPAI) H5N1 infection in backyard chickens in Thailand. Veterinary Record, 164(16), 491-492.
[5] Nemeth, N. M., Bosco-Lauth, A. M., Bowen, R. A., Kauffman, E. B., & Root, J. J. (2016). Experimental infection of European starlings (Sturnus vulgaris) and house sparrows (Passer domesticus) with pandemic 2009 H1N1 and swine H1N1 and H3N2 triple reassortant influenza viruses. Veterinary microbiology, 194, 74–82.
[6] Ellis, T. M., Bousfield, R. B., Bissett, L. A., Dyrting, K. C., Luk, G. S., Tsim, S. T., Sturm-Ramirez, K., Webster, R. G., Guan, Y., & Peiris, J. S. (2004). Investigation of outbreaks of highly pathogenic H5N1 avian influenza in waterfowl and wild birds in Hong Kong in late 2002. Avian pathology, 33(5), 492-505.
[7] Shane S. (2016). Role of the European starling in the transmission of highly pathogenic avian influenza virus on poultry facilities. Thesis, Utrecht University.