Reference: Villar, M., Fernández de Mera, I.G., Artigas-Jerónimo, S. et al. Coronavirus in cat flea: findings and questions regarding COVID-19. Parasites Vectors 13, 409 (2020). https://doi.org/10.1186/s13071-020-04292-y
Cats and COVID
Do you remember reading news stories back in April that reported confirmed COVID infection in two domestic house cats, and even a Malayan tiger at the Bronx Zoo in New York City, all resulting from contact with infected humans? While the accounts were a bit jarring and left us concerned about our pets, thankfully house cats and captive big cats becoming infected with the novel coronavirus are incredibly rare. But, infections are obviously possible and people are left to wonder about the reverse — can these animals infect us? Currently, there is no evidence to suggest that our pet cats can infect us with the virus that causes COVID. Phew!
But what about parasites that can live on our pets — like fleas? Many diseases can be transmitted from animals to humans through the bites of these vectors (which are specifically called zoonotic diseases). For example, certain tick species can spread the bacteria that cause Lyme disease when they feed on an infected rodent (called the disease reservoir host), and then bite a human (who is considered the dead-end or incidental host, meaning humans get infected but can’t spread the disease further) during their next meal. Likewise, mosquitoes can transmit numerous viruses by feeding on an infected reservoir host and then biting a human or other animal. Fleas are no different, and they are certainly no strangers to pandemics.
Where have I seen this before?
You may recall that fleas are the infamous culprit behind the bubonic plague. During the “Black Death” pandemic of the mid-1300s, fleas fed on and transmitted Yersinia pestis bacteria from infected rats to humans, and over the course of three years, killed between 75 and 200 million people across Europe and Northern Africa (DeLeo and Hinnebusch 2005). The specific flea responsible for this disease outbreak was the rat flea (Xenopsylla cheopis), but because all fleas bloodfeed, they’re all capable of acting as disease vectors.
But can fleas spread THIS disease?
In recognition of this new evidence that our feline companions are capable of developing COVID as a result of infection with the causative virus SARS-COV-2, paper authors Villar and others asked the question: can the common cat flea (Ctenocephalides felis, which also parasitizes dogs and humans) 1) harbor coronaviruses at all, and 2) can it carry the novel coronavirus (SARS-COV-2) that causes COVID? Recall that viruses are not considered to be “living” organisms because they cannot replicate themselves on their own – they require a host, and hijack that host’s cell machinery to make copies of themselves. To find evidence of the virus, researchers look for the proteins that are associated with the virus’ structure, as well as its genetic material (the instructions to tell the cell how and what structures to make more of).
To investigate whether fleas were carrying any of the many coronaviruses that exist, and whether they harbored the species causing the COVID pandemic, Villar and colleagues examined lab-reared, unfed cat fleas, as well as ones collected from feral cats living in Ciudad Real, Spain between May and June 2020. The authors used two main molecular-level analyses to identify whether there were coronaviruses present in those fleas: proteomics to look for coronavirus-specific proteins, and polymerase chain reaction (more commonly called PCR) to identify genetic material and then make lots and lots of copies of it to make it detectable.
Their results revealed that the both unfed and fed fleas contained evidence of coronavirus species in their midguts (i.e. their stomachs), which was puzzling. The feral cat-fed fleas could have picked up a coronavirus from the cats, but the lab-reared fleas had not been recently exposed to a blood meal that could’ve contained a coronavirus. Instead, they could be showing evidence that the virus could be maintained as fleas develop from egg to larvae, to pupa, to adult. Importantly, the authors did not find the specific novel coronavirus SARS-COV-2, the virus behind the current pandemic. However, further analysis found that, while the fleas were not infected with the virus causing COVID, they do in fact have the correct cell receptor, or gateway (angiotensin converting enzyme 2, or ACE2) necessary for SARS-COV-2 to infect them. This means that while these particular fleas were not infected at this time with SARS-COV-2, it is possible for them to become infected.
While this is a concerning finding, there are some major limitations to this study. First, the authors had an incredibly small sample size: wild fleas were only collected from two feral cats that were not determined to have COVID, so there could be no direct assessment of whether the fleas could become infected with the virus. Additionally, there was no study to determine the ability of fleas to actually transmit a coronavirus, let alone SARS-COV-2 to a domestic animal or a human. The authors stressed the need for further study into the role that vectors like fleas could play in spreading coronaviruses (McNamara et al. 2020), as well as maintaining an effective flea and tick preventative regimen throughout the year to avoid contact with parasites that could transmit disease between you and your pet.
McNamara T, Richt JA, Glickman L. 2020. A critical needs assessment for research in companion animals and livestock following the pandemic of COVID-19 in humans. Vector Borne Zoonotic Dis. 20: 6.https://doi.org/10.1089/vbz.2020.2650.
DeLeo, F., Hinnebusch, B. 2005. A plague upon the phagocytes. Nat Med 11, 927–928. https://doi.org/10.1038/nm0905-927
Information about SARS-COV-2:
Shereen et al. 2020. COVID-19 infection: Origin, transmission, and characteristics of human coronaviruses. Biomedical Journal. In press. https://doi.org/10.1016/j.jare.2020.03.005
Flea control and prevention: