Rats and Fleas That Spread Disease

We all know rats and mice can be pests and one of the key attributes for falling into this category is their well know ability to harbour and transmit diseases specifically zoonotic diseases. 

So, what is zoonoses?

It is unlikely that you will give your dog your cold, and equally unlikely that your dog will give you its kennel cough.  So why worry about the diseases that other animals, especially rats, carry? We after all, are not rodents, so how can their diseases affect us?  Although the majority of diseases are species specific, there are certain pathogens that are able to move between hosts of different species and when this happens, it is called zoonosis.

Zoonotic pathogens can have unpredictable behaviours and can have impacts in their secondary hosts in excess of their normal behaviours.  When the incidental or secondary host is a human the pressure on public health can be severe.  This is always a challenge especially when we consider diseases from apparently unknown sources, such as a hidden population of rodents, as this can reduce the suspicion of illness when symptoms present themselves, increasing the time to detection and therefore a subsequent cure. 

There are many pathogens carried by rodents which can harm both humans and animals we care for, with an example of this being the relation between rodents, poultry and avian flu.  Diseases can be transmitted in several ways by intermediaries called vectors.  The simplest way this can occur is the direct physical contact between the two host animals.  Such simplicity affords in of itself a modicum of control, but the management of the spread of disease becomes increasingly more difficult as the interactions between hosts become less obvious.

Vectors fall into two major categories; biological and mechanical.

Biological vectors are organisms which will harbour a pathogen usually within their own bodies.  Whilst there that pathogen finds itself in an environment where conditions are sufficiently favourable that it has the capacity to grow.  The growth of leptospirosis within the kidneys and its subsequent shedding in the urine can be considered a biological vector, the malaria parasite within anopheles mosquito is another example.

Mechanical vectors on the other hand are host organisms which will physically transport a pathogen from location to location, with the key distinction that in this time the pathogen does not grow whilst this occurs.  Such mechanical transportation of pathogens can be tremendously dangerous as it requires no incubations nor favourable climates, simply an animal to move over a contaminated surface onto a previously clean surface.

Changing climates.

Climate change is quite real.  Regardless of whether you believe that humans are to blame for it entirely or simply the speed at which the change is occurring, the inescapable truth is that the climate is changing. 

This change in climate is bringing a real and sometimes dramatic shifting of the boundaries that pests observe, sometimes bringing new pests and their diseases across previously untenable borders.

Globalization has also played a pivotal role within this equation too, not only bringing humans closer together but also allowing plants and animals to cover much greater distances than ever before.  A classic example of this is finding invasive species of crab and other crustacean in harbors around the world after they hitchhike their way across the globe in the bilge tanks of deep-water transit vehicles.

With pests moving into new territories, their parasites and diseases also come hand in hand.  The COVID-19 pandemic and its links to its origin in wild animals has been a stark reminder of this.  It also reminds us that we are not that far removed from nature as we in our cities may like to think.  We are incredibly fortunate that, at present, many of these zoonotic diseases are not present within the pest populations of the UK.  Yet as discussed above, we are in a world which is shrinking every day.

Reverse Zoonoses?

So, if zoonosis is the transition from one species to another, what happens when the pathogen in question may be passed from humans to pests and then back again?

This is a process referred to as reverse zoonoses and may well be the reason behind the mystery that is the sudden appearance of the Omicron variant of Covid.

Omicron’s sudden emergence and drastic difference in number and type of mutations has caused the scientific community to take pause to re-evaluate where this strain may have originated.  Under normal circumstances the rate of mutation in a virus can be (on a large enough scale) relatively predicable.

Two of the pieces of evidence that indicate mice as being the potential culprit for this reverse zoonotic event is that firstly, the mutation speed is highly indicative of an animal with a high fecundity which will itself be mutating multiple resistances to SARS-CoV-2.  Secondly, studies of the receptor’s sites on the surface of mouse cells seem to be perfectly shaped to accept these spike proteins of this new variant of virus, or vice versa.  A recent paper in the Journal of Genetics and Genomics went so far as to state that, “that the progenitor of Omicron jumped from humans to mice, rapidly accumulated mutations conducive to infecting that host, then jumped back into humans, indicating an inter-species evolutionary trajectory for the Omicron outbreak”.

As laypeople and observers, we must be mindful when looking at such data to ensure we do not fall foul of the age-old axiom that apparent correlations must equal causation.  To put this statement into context a Harvard Criminologist undertook an analysis which showed data that clearly intimated that the average ice cream sales in the USA rose in direct proportion to the number of shark attacks recorded.

Although it is entertaining to think that a sinister cabal of man hunting sharks is luring innocent people to the beach with ice cream, it does bring to light an important concept in data analysis and that is that there is often an underlying and often unaccounted, causal event which is distinct from either of the observed events, often referred to as the third variable.

The point I am trying to make is that we currently do not possess the scope of knowledge sufficient to corroborate these observations with certainty.  And much like our menacing man-eating makos, the relationship between spike protein and receptor site may well indeed be being driven by a third, yet unidentified cause, such as the exposure of the host to another similar pathogen.

A laboratory study showed that mice could in fact carry human variants of SARS-CoV-2, but in order to achieve this the levels of exposure required were prohibitively high.  The paper concluded that the level of uncertainty around the potential fate of SARS-CoV-2 was high.  However, it cannot be overlooked that despite the brute force tactics employed to artificially transfer human SARS-CoV-2 variants into mice, it was achievable, leading to an uneasy conclusion that this zoonotic event may well be less of a case of ‘if’ and more a case of ‘when’.

So in Close.

Therein lies the real danger of pests and disease. It is not simply a case of their ability to be vectors of many pathogens, but in their ability to create this perfect storm of disease transmission.  A critical factor which makes pests such virulent transporters of disease is their propensity to move.  Some pests can travel considerable distances from their homes to search for new locations to infest. With this migration pests will also be taking pathogens with them from pre-existing infected sites into potentially quarantined sites as well as coming into contact with a multitude of pathogens on the journey in between.

These seemingly invisible mechanisms of transmission can therefore undermine even the most stringent biosecurity measures if they are not fully considered.  Therefore, reactive strategies may well prove to be too little too late, and so a proactive integrated pest management strategy is imperative to maintaining public safety.