The term “invasive species” carries a bad ring with it because of these species’ highly problematic nature. They are often accidentally introduced into an ecosystem for which they are not naturally a part of. While many non-native species fit in well with their surroundings and even demonstrate benefits, invasive species are over-competitive and unwanted. Most of these animals tend to disrupt the ecosystem both directly and indirectly; they often don’t have predators in their non-native habitat and make the best use of it by growing wildly and sucking out resources from others. In this cascade of chaos, scientists have connected evidence suggesting that certain invasive species could lead to an increased incidence of vector-borne diseases.
Vector-borne diseases include some of the deadliest illnesses we know; according to the World Health Organization, malaria is the cause of over 1.2 million deaths annually, and dengue fever is the fastest growing vector-borne disease. The vectors, largely mosquitoes, ticks, and fleas, carry a deadly pathogen from animal to animal; this leads to a high rate of transmission between animals and humans. In fact, the bubonic plague, also known as the black death, took the lives of 50 million people in the 14th century… the source of transmission? Infected fleas.
A study conducted in the Florida Everglades by Hoyer et al. (2017) discovered interesting correlations between the introduction of the Burmese python, an invasive species, and changes in blood-feeding patterns of a dangerous vector, Culex cedecei. Cu. cedecei is a mosquito species known as the sole vector for the Everglades virus (EVEV) which is related to encephalitis viruses; this usually jumps around within rodents but contact with humans can cause feverish symptoms and sometimes lead to neurological issues. The reservoirs or carriers of this virus, small rodents, are a part of the varying diet of Cu. cedecei. The Burmese python, on the other hand, is an invasive species that reigns from South-East Asia and feeds on medium and large mammals. The study compares historical data from years before the settlement of the invasive Burmese python and more recent data after the introduction of the python to determine the possible role of this invasive species on vector feeding and, in turn, vector disease prevalence.
Historical data revealed that Culex cedecei fed on about 90% of mammals – half of which were rodents – in the 1970s. Many mammals suffered a decline in populations at the hands of the competitive, hungry, and invasive Burmese python; the python led to the collapse of medium- and large-sized mammals but did not touch the smaller ones. Small rodents did not suffer and might have even increased in numbers; with the python feeding on bigger mammals and getting them out of the way, the rodents get more access to resources.
“With the python feeding on bigger mammals and getting them out of the way, the rodents get more access to resources.”
Cu. cedecei doesn’t have a food preference, it blood-feeds on any mammal that’s in their path. With more rodents, Cu. cedecei naturally begins to feed on rodents more. Specifically, blood-feedings on hispid cotton rats, one of the most important EVEV reservoirs, increased from 14.7% to 76.4% of overall blood meals! In shocking contrast, feedings upon deer, raccoon, and opossum decreased from 45.1% to just 0.8% of total blood hosts (Figure 3). With more mosquitoes – EVEV virus vectors – feeding on rodents – EVEV virus carriers – the chance of disease transmission is significantly increased.
Piecing this together, we can see that the invasive Burmese python toppled the balance of the ecosystem by destroying some species and letting others unnaturally flourish. These species that benefited from the python’s chaos turned out to be small rodents – the EVEV virus’s dream. This unequal exposure of animals to the virus’ vector, Cu. cedecei, leads to increased transmission, thereby, increased occurrence of the disease and likelihood of spilling over to humans.
This novel research avenue has raised many questions and concerns; scientists are beginning to view invasive species in yet another bad light and the importance of tackling these species has never been bigger. What are your thoughts? Share in the comments below!
Hoyer IJ, Blosser EM, Acevedo C, Thompson AC, Reeves LE, Burkett- Cadena ND. 2017 Mammal decline, linked to invasive Burmese python, shifts host use of vector mosquito towards reservoir hosts of a zoonotic disease. Biol. Lett. 13: 20170353.