Infections by parasites and pathogens can affect not only a host’s physical and physiological wellness, but also its cognitive functioning. There are numerous examples, across many different animal taxa, which illustrate changes to cognitive performance during times of infection or illness. We have to think no further than the “brain fog” experienced by some COVID-19 sufferers, especially those with long-haul symptoms.
As climate change alters the distribution of pathogens and wild animals, it is likely that animals will come into contact with a range of new pathogens and will experience infections from these sources. If these infections lead to cognitive impairment then this may influence the animals’ ability to adapt and succeed in the changing world. In particular, if pathogens affect an animal’s ability to learn, remember, or make decisions, this may compromise the fitness of populations in the wild at a time when they need to call on all these skills to adapt to new conditions.
A review of the effects of infection on cognitive function, published in the journal Trends in Ecology & Evolution finds that, although scientists are well aware that disease impairs cognitive function in many different species, the mechanisms and consequences for fitness are not well understood. One of the obstacles is that different cognitive abilities are tested in infected animals, using different forms of tests, different types of infections and different host species.
This review, written by four US researchers, aims to bring together ideas from the fields of cognitive ecology and disease ecology in order to examine evidence for infection-linked cognitive impairment. In addition, the authors considered why certain pathogens bring about change in cognition and what the ecological and evolutionary implications of this may be, particularly as populations of wild animals face changes to their habitats.
The study authors, including Dana Hawley and Kendra Sewall from Virginia Tech and Anne Leonard from the University of Nevada, said that cognitive impairment could contribute to population declines, particularly for species that rely heavily on learning and memory for foraging or other important functions. “Impaired cognitive performance could compromise the ability of some animals to exploit urbanized and other rapidly changing habitats, where problem solving may be particularly important.”
The experts found a lot of evidence in the scientific literature for infection-linked cognitive impairment, albeit that this evidence was very variable, across host and pathogen taxa, and across cognitive abilities. For example, bumblebees infected with the trypanosome Critidia bombin showed impaired performance on learning tasks, and American crows naturally infected with the bacteria Campylobacter jejuni showed a reduced ability to solve problems. This effect was also recorded for female blue tits infected with the blood protozoan Plasmodium.
The researchers determined that pathogens could impair cognitive performance either directly or indirectly, and either in the short term or over longer periods. Direct effects involved neurological damage caused by pathogens that infect the central nervous system. For example, West Nile virus, feline immunodeficiency virus, Ebola virus and coronaviruses may manifest as meningitis, encephalitis and myelitis, all of which can have long-term cognitive impacts.
Indirect impairment of cognitive function in diseased animals can arise from such consequences as malnutrition (when a sick animal cannot forage effectively or loses motivation to forage), missing out on social contacts and missing out on learning opportunities through being absent and ill. In addition, infections can change the gut microbiome of a host, which can lead to impaired digestion or absorption of nutrients. Furthermore, immune responses, such as inflammation, can also impede a foraging animal’s success.
The timing of infection in an animal’s life may also have significant consequences. For example, young canaries that are infected with Plasmodium when they are learning songs might have a reduced song repertoire and complexity as adults.
The authors state that cognitive abilities are crucial for animals to be able to collect, retain and act on information relating to their survival. Therefore, impairment of cognitive performance due to disease could have widespread ecological consequences for populations of wild animals. As climate changes lead to habitat alterations, wild animals will need all their cognitive skills to adapt and survive, and this against a backdrop of increased pathogen proliferation in warmer conditions.
“In a century characterized by global changes that accelerate pathogen emergence, while simultaneously favoring cognitive flexibility, there is an urgent need to better understand the interactions between animal cognition and infection, and their potential consequences for wild populations,” said the researchers.
The experts call for increased attention to be paid to this topic, saying: “One challenge in all studies of cognitive ecology is the scarcity of comparable data across taxa. Understanding how infection affects specific cognitive abilities will require research across a wider breadth of hosts and pathogens, targeting analogous aspects of cognition (e.g., spatial learning; problem solving; general versus specialized cognitive abilities), and standardizing experimental paradigms to the extent possible.”