Spillover infection, also known as pathogen spillover and spillover event, occurs when a reservoir population with a high pathogen prevalence comes into contact with a novel host population. The pathogen is transmitted from the reservoir population and may or may not be transmitted within the host population. Due to climate change and land use expansion, the risk of viral spillover is predicted to significantly increase.
Spillover is a common event; in fact, more than two-thirds of human viruses are zoonotic. Most spillover events result in self-limited cases with no further human to human transmission, as occurs, for example, with rabies, anthrax, histoplasmosis or hidatidosis. Other zoonotic pathogens are able to be transmitted by humans to produce secondary cases and even to establish limited chains of transmission. Some examples are the Ebola and Marburg filoviruses, the MERS and SARS coronaviruses or some avian flu viruses. Finally, some few spillover events can result in the final adaptation of the microbe to the humans, who became a new stable reservoir, as occurred with the HIV virus resulting in the AIDS epidemic.
Most of the pathogens which are presently exclusive of humans were probably transmitted by other animals sometime in the past. If the history of mutual adaptation is long enough, permanent host-microbe associations can be established resulting in co-evolution, and even on permanent integration of the microbe genome in the human genome, as it is the case of endogenous viruses. The closer the two species are in phylogenetic terms, the easier it is for microbes to overcome the biological barrier to produce successful spillovers. For this reason, other mammals are the main source of zoonotic agents for humans.
During the late 20th century zoonotic spillover increased as the environmental impact of agriculture promoted increased land use and deforestation, changing wildlife habitat. As species shift their geographic range in response to climate change, the risk of zoonotic spillover is predicted to substantially increase, particularly in tropical regions that are experiencing rapid warming.
Commercially bred bumblebees used to pollinate greenhouses can be reservoirs for several pollinator parasites including the protozoans Crithidia bombi, and Apicystis bombi, the microsporidians Nosema bombi and Nosema ceranae, plus viruses such as Deformed wing virus and the tracheal mites Locustacarus buchneri. Commercial bees that escape the glasshouse environment may then infect wild bee populations. Infection may be via direct interactions between managed and wild bees or via shared flower use and contamination. One study found that half of all wild bees found near greenhouses were infected with C. bombi. Rates and incidence of infection decline dramatically the further away from the greenhouses the wild bees are located. Instances of spillover between bumblebees are well documented across the world but particularly in Japan, North America and the United Kingdom.
- Carlson, Colin J.; Albery, Gregory F.; Merow, Cory; Trisos, Christopher H.; Zipfel, Casey M.; Eskew, Evan A.; Olival, Kevin J.; Ross, Noam; Bansal, Shweta (28 April 2022). "Climate change increases cross-species viral transmission risk". Nature. 607 (7919): 555–562. doi:10.1038/s41586-022-04788-w. ISSN 1476-4687. PMID 35483403. S2CID 248430532. Retrieved 6 May 2022.
- Power, AG; Mitchell, CE (Nov 2004). "Pathogen spillover in disease epidemics". Am Nat. 164 (Suppl 5): S79–89. doi:10.1086/424610. PMID 15540144. S2CID 16762851.
- Woolhouse M, Scott F, Hudson Z, Howey R, Chase-Topping M. Human viruses: discovery and emergence. Phil. Trans. R. Soc. B (2012) 367, 2864–2871
- Wolfe, Nathan D.; Dunavan, Claire Panosian; Diamond, Jared (May 2007). "Origins of major human infectious diseases". Nature. 447 (7142): 279–283. doi:10.1038/nature05775. ISSN 1476-4687. PMC 7095142. PMID 17507975.
- Graystock, P; Yates, K; Evison, SEF; Darvill, B; Goulson, D; Hughes, WOH (2013). "The Trojan hives: pollinator pathogens, imported and distributed in bumblebee colonies". Journal of Applied Ecology. 50 (5): 1207–15. doi:10.1111/1365-2664.12134. S2CID 3937352.
- Sachman-Ruiz, Bernardo; Narváez-Padilla, Verónica; Reynaud, Enrique (2015-03-10). "Commercial Bombus impatiens as reservoirs of emerging infectious diseases in central México". Biological Invasions. 17 (7): 2043–53. doi:10.1007/s10530-015-0859-6. ISSN 1387-3547.
- Durrer, Stephan; Schmid-Hempel, Paul (1994-12-22). "Shared Use of Flowers Leads to Horizontal Pathogen Transmission". Proceedings of the Royal Society of London B: Biological Sciences. 258 (1353): 299–302. Bibcode:1994RSPSB.258..299D. doi:10.1098/rspb.1994.0176. ISSN 0962-8452. S2CID 84926310.
- Graystock, Peter; Goulson, Dave; Hughes, William O. H. (2015-08-22). "Parasites in bloom: flowers aid dispersal and transmission of pollinator parasites within and between bee species". Proc. R. Soc. B. 282 (1813): 20151371. doi:10.1098/rspb.2015.1371. ISSN 0962-8452. PMC 4632632. PMID 26246556.
- Otterstatter, MC; Thomson, JD (2008). "Does Pathogen Spillover from Commercially Reared Bumble Bees Threaten Wild Pollinators?". PLOS ONE. 3 (7): e2771. Bibcode:2008PLoSO...3.2771O. doi:10.1371/journal.pone.0002771. PMC 2464710. PMID 18648661.
- Graystock, Peter; Goulson, Dave; Hughes, William O.H. (2014). "The relationship between managed bees and the prevalence of parasites in bumblebees". PeerJ. 2: e522. doi:10.7717/peerj.522. PMC 4137657. PMID 25165632.
- Graystock, Peter; Blane, Edward J.; McFrederick, Quinn S.; Goulson, Dave; Hughes, William O. H. (2016). "Do managed bees drive parasite spread and emergence in wild bees?". International Journal for Parasitology: Parasites and Wildlife. 5 (1): 64–75. doi:10.1016/j.ijppaw.2015.10.001. PMC 5439461. PMID 28560161.
- Imported bumblebees pose risk to UK's wild and honeybee population. Damian Carrington. theguardian.com, Thursday 18 July 2013