Solving the common mystery of the cat flea

You may be inclined to think that we know everything we need to know about the flea but we don’t. They infest our pets and our homes; we treat them with a variety of substances and yet they are near impossible to exterminate. Importantly, they occasionally bite people, causing annoyance and sometimes severe skin reactions. You may also think this is all we really need to know them. In fact, these parasites are often overlooked in terms of their significance to animal health, their competence as disease vectors and the impacts they make on our everyday lives. There is much more to these irritating insects than meets the eye.

This is the first “guest post” on my blog and comes from my PhD student Andrea Lawrence (University of Sydney) ahead of her presentations at the Australian Society for Parasitology conference in Canberra next week (looks like a wonderful program of events this year!). I’m hoping that there will be plenty more guest posts from Andrea and my other students in the near future.

The most common flea encountered in Australia is the cat flea, Ctenocephalides felis. Just because your dog has fleas, it doesn’t mean it has dog fleas (Ctenocephalides canis). This is a common misconception. In fact, it appears as if the dog flea is something of a mythical creature in Australia. Despite historical records and anecdotal reports of dog flea infestations, there is no recent literature confirming their presence. A recent study of over 2,500 pets failed to find a dog flea. As such, if your pet is troubled with fleas, you can likely lay the blame solely on the cat flea.

The cat flea is the top ectoparasite affecting cats and dogs globally for a variety of reasons. They are the cause of up to 50% of all dermatological cases presented to vet clinics world-wide. Pet owners are spending $40 to $70 on flea and tick control products per month and, based on figures from the United States, over $1 billion annually. That is a lot of money to spend only to have the fleas come back time after time.

As well as the nuisance-biting, the cat flea also carries zoonotic pathogens such as Bartonella (bacteria that causes cat scratch disease in hypersensitive or immunocompromised people) and Rickettsia (bacteria that causes murine typhus and flea-borne spotted fever).

There may also be many cases of underdiagnosed febrile illnesses caused by flea-borne pathogens that fly under the radar due to the presentation of generic fever and flu-like symptoms that rarely warrant further pathological investigation. Of course, the most famous and historically significant pathogen spread by fleas is the plague bacteria: Yersinia pestis. Plague is certainly not a thing of the past with recent outbreaks in Madagascar and up to 17 cases reported from North America each year. Considering the highly ubiquitous nature of fleas in human environments, and many species’ tendency to be host generalists – particularly the cat flea – shouldn’t we be more concerned, or at least more aware, of their biology, taxonomy and potential public health risks?

Professional Ratcatchers from Views taken during Cleansing Operations, Quarantine Area, Sydney, 1900

Although the pathogens that cause plague are not endemic to Australia, plague has touched Australia with significant impact. Here are some professional ratcatchers from Sydney, Australia, during the plague outbreak in 1900 (Source: State Library Image Collection)

Given the impact these little parasites have on our lives, it is baffling how little we know about them. The genetic profile of the cat flea is highly understudied and yet within the genetic code lies hidden implications for the evolution of insecticide resistance, disease transmission and the passage of fleas across continents and the global sphere. A study from the Veterinary Parasitology unit at the University of Sydney found that in 2011 across 5 states of Australia cat fleas collected from veterinary practices were 100% genetically identical at the mitochondrial DNA. This was a very unusual result as populations of other flea species are generally very diverse. The result was comforting news at the time for the regulation of veterinary pharmaceuticals as the efficacy of flea control products were able to be compared against flea populations across the entire country.

Taken from “How to get rid of fleas at home” via Appliances online blog.

We know fleas from Australia are genetically similar but what about elsewhere? We broadened the scope of the investigation and compared the fleas from Australia to those collected from Thailand, Fiji and Seychelles: a group of Islands north-east of Madagascar. These results showed that from a global perspective, cat fleas are genetically diverse. The 2013 flea season yielded a novel second Australian haplotype found in north-east Australia which contradicts the unanimous results from the previous study in 2011. This haplotype was shared with most fleas tested from Fiji, suggesting some recent flea transfer between the two countries. With the rapid emergence of this second haplotype since the previous study, it sparks the question of whether there may be a division of fitness between the two haplotypes. Could this division be resulting in a steady ‘invasion’ of Australia by the second haplotype?

To investigate the haplotype diversity in this study we developed a novel genetic marker capable of clearly delineating different flea species, subspecies and haplotypes. Previously, genetic studies primarily used a mitochondrial DNA marker called cox2. However, there is an emerging global standard of genetic taxonomy called DNA barcoding, which uses a similar gene called cox1. This method involves storing massive amount of short DNA sequences in an electronic database, accessible to anyone with internet access. Currently the database called Barcode of Life Database or BOLD holds 3 million ‘barcodes’, 2 million of which are arthropod barcodes. I wanted to align fleas with this emerging global standard by developing a cox1 marker that would work for fleas. It is surprising given the global significance fleas that the marker has not been optimised before. The ‘barcodes’ collected from this study are now available on BOLD and can be searched allowing greater dissemination of and accessibility to flea genetic data.

A change in the genetic makeup of Australia’s flea population as discovered recently has implications for the pharmaceutical companies who can no longer apply a blanket approach to flea control efficacy testing. Research is continuing this year in the Veterinary Parasitology Unit at The University of Sydney to monitor the rate of spread of this second haplotype. In time I hope this may yield greater understanding of the cat flea genetic puzzle that will lead to finding the key to effective control of these tenacious blood-sucking creatures and the diseases they carry.

The abstract for Andrea’s paper is below:

The cat flea, Ctenocephalides felis (Siphonaptera: Pulicidae) (Bouché), is the most common flea species found on cats and dogs worldwide. We investigated the genetic identity of the cosmopolitan subspecies C. felis felis and evaluated diversity of cat fleas from Australia, Fiji, Thailand and Seychelles using mtDNA sequences from cytochrome c oxidase subunit I (cox1) and II (cox2) genes. Both cox1 and cox2 confirmed the high phylogenetic diversity and paraphyletic origin of C. felis felis. The African subspecies C. felis strongylus (Jordan) is nested within the paraphyletic C. felis felis. The south East Asian subspecies C. felis orientis (Jordan) is monophyletic and is supported by morphology. We confirm that Australian cat fleas belong to C. felis felis and show that in Australia they form two distinct phylogenetic clades, one common with fleas from Fiji. Using a barcoding approach, we recognize two putative species within C. felis (C. felis and C. orientis). Nucleotide diversity was higher in cox1 but COX2 outperformed COX1 in amino acid diversity. COX2 amino acid sequences resolve all phylogenetic clades and provide an additional phylogenetic signal. Both cox1 and cox2 resolved identical phylogeny and are suitable for population structure studies of Ctenocephalides species.

The full reference of the paper is:

Lawrence, A. L., Brown, G. K., Peters, B., Spielman, D. S., Morin-Adeline, V. and Šlapeta, J. (2014), High phylogenetic diversity of the cat flea (Ctenocephalides felis) at two mitochondrial DNA markers. Medical and Veterinary Entomology [early view]doi: 10.1111/mve.12051 [Online]

(The image of the cat flea, Ctenocephalides felis, at the top of this blog post is taken from the PaDIL image collection by K Walker)