A perilous plague & menacing risk

The gruesome pandemic known as ‘The Black Death’ was one of the worst plagues in history. The disease originated in Asia during the 14th Century and spread throughout Europe by being transmitted via parasitic fleas on rats that lived on trade ships. By June 1348 it had arrived in Dorset and by October of that year it had reached London where it infected and killed King Edward III’s daughter Princess Joan. By 1351 it had caused European-wide deaths of between 25-50 million people and the disease only came under control once effective quarantine procedures had been put in place. 

The term ‘plague’ can refer to affliction, torment, or infestation, and the last of these three terms may be aptly employed to reflect the presence of an unusually large number of rats able to cause extensive human disease and physical damage to the built environment. Infected rodents have caused chaos and mayhem to public health over many centuries, often serving as a reservoir for numerous harmful diseases.  Many of these are spread by parasitic fleas and ticks that live on rats, and so that it may feed a flea or tick will puncture the skin of the rat to inject saliva that will numb the area of skin surrounding the point of entry. This will prevent the animal’s blood from clotting and permit the insect to feed off the rat’s blood for several days. However, these fleas and ticks transmit disease through their bites. 

Rats are considered to be one of the world’s most invasive species, second only to humans in the ecological damage that they inflict. Hence they have far-reaching consequences for the environment, for ecosystems, wildlife, and for human health. Consequently, efforts to control and help eradicate these vermin are crucial for environmental protection and conservation. Albert Szent-Gyotgi, the Nobel Prize winning Hungarian biochemist who first isolated Vitamin C in 1937, once stated that ‘a discovery is said to be an accident meeting a prepared mind.’

Whenever rats live in close proximity to the human population there is a need to understand the contribution they make to the transmission of various pathogenic bacteria. This is of particular importance as rats pose specific hazards due to their ability to amplify pathogen concentrations within the local environment, thereby forming reservoirs of disease. Rat-borne diseases can be spread via two distinct pathways, namely ‘direct’ and ‘indirect’. Direct routes include bites, the consumption of contaminated food and water, physical contact with floodwaters, and the inhalation of germs. In these routes the source of contamination is either rat faeces or rat urine. Indirect routes comprise the spreading of rat-borne pathogens via infected parasitic ticks, mites or fleas. All of these combined effects often account for huge economic losses with people suffering a rat bite needing to be hospitalised for an average period exceeding 11 days.

Rats have spread disease throughout the Ages. The 14th Century ‘Black Death’ that killed 75 million people was followed in the mid-17th Century by a Bubonic Plague epidemic in the UK that caused 100,000 deaths and was designated the ‘Great Plague of London’. Both of these historic outbreaks were caused and spread by fleas living on rats and recent changes in global climatic conditions have triggered the re-emergence of infectious diseases and the increased incidence of rat-borne pathogens. Today the UK is home to 68 million people and to 150-200 million rats, each of which is able to breed easily as soon as it becomes five weeks old. These omnivorous scavengers eat almost anything including waste products and refuse, and their parasitic fleas seek out other prey when their rodent host dies. As the saying goes, ‘to smell a rat’ is to suspect something is wrong and their increasing numbers spell trouble.

Rats and their parasites collectively infect people with bacteria and mycobacteria, giving rise to a number of serious pathogenic diseases that are injurious to human health.  Rat excrement can contain numerous pathological disease causing bacteria that include Listeria, Pasteurella, Leptospira, and Escherichia coli (E.coli), the last of which can persist within the environment in soil and dust that can be inhaled. 

Rats are susceptible to tuberculosis, though transmission via the rat-to-human route is rare.  In this regard though they are quite useful, or at least have the potential to be very useful, since laboratory tests show rats are better at detecting TB in human sputum samples than the traditional sputum smear performed in hospital laboratories. How best to exploit this ability of rats though is still being researched so it remains something of a novelty. Nevertheless, in the future this could change the course of healthcare in the third world where TB is still endemic; the vast majority of TB cases in the UK are imported from the Far East and sub-Saharan Africa.

Rats play host to parasitic ticks and fleas that can host their own parasites termed hyper-parasites, i.e. a parasite of a parasite.  The most important of these is Yersinia Pestis, responsible for Bubonic Plague, Rickettsia Typhus and Borrelia Burgdorfori, the causal agent of Lyme disease. Historically Bubonic Plague was a huge burden on the health of the population but can now be readily treated with antibiotics if diagnosed in a timely manner.

Many rat-borne infections give rise to symptoms that vary from the distressing and transient phenomena that include fevers, rashes and fatigue, to the very dangerous and permanently disabling. Chronic fatigue is often associated with Lyme disease, whilst Typhus or Bubonic Plague in individuals with a weakened immune system can lead to kidney failure, liver failure, and overwhelming sepsis. 

Worthy of its own paragraph is Leptospirosis, which in its most acute and severest form (accounting for a tenth of cases) is named Weil’s disease after the 19th Century German Physician and Professor of Special Pathology & Therapy, Adolf Weil. This disease is principally transmitted via the urine of infected rodents with the Leptospira bacterium responsible for a blood infection that causes liver failure, leading to jaundice and haemorrhaging by compromising the liver’s ability to make blood clotting factors. It may cause congestive heart failure, though most sufferers of Leptospirosis suffer a self-limiting course of deeply unpleasant illness for a fortnight or so. Around 50 percent of patients suffer a sterile Meningitis causing confusion and irritation, and sadly approximately 15 percent will succumb to permanently-debilitating disease. 

Infectious diseases spread by rats can be spread through direct contact, through food and water, or from latent organisms left in the environment from rat saliva or excrement. Listeria and pasteurella bacteria are sometimes spread by rat bites. Listeriosis causes stillbirth in pregnant women, though it is rarely fatal to the expectant mother. Symptoms of Pasteurellosis can include swelling of the skin around the bite area, often forming abscesses, and if it reaches the circulation it can cause a septic arthritis comprising infection of joints; this results in hugely debilitating secondary arthritis from the joint destruction wrought by the infection. Salmonella is usually self-limiting and tends not to require treatment but it does carry a mortality rate of around 1 percent.  

Contrary to the popular perception of rats being dirty and offensive in appearance, these rodents fastidiously clean themselves to maintain their own well-being and are remarkably sociable animals. As they are omnivorous and able to eat pretty much anything they can if necessary survive on only 3 grams of food per day and without water for up to two weeks, whilst at the same time living in the most inhospitable conditions. Rats have a very keen sense of smell to detect food sources and to detect their nesting place to which they can readily retreat using their excellent navigational skills. Their adaptability has led them to being perhaps the most successful mammal in terms of geographical spread, present in all continents bar Antarctica.  Black rats, rattus rattus and Brown rats, rattus norvegicus are the dominant rats in the UK and together can be a problem in urban, suburban and rural areas, threatening the food supply as well as being directly hazardous to human health.  Being omnivorous they happily raid fruit trees, nut trees, and even sections of ornamental shrubs and flowers, and are thought to be responsible for destroying approximately one fifth of global agricultural produce per annum. 

Within the urban environment, rats often infest older buildings within crowded and unsanitary areas but they can also create problems where newer homes and sanitary conditions exist. Damage caused to the built environment is often the result of rats chewing and gnawing at plastics, metals and wiring. Softer metals including aluminium and lead are readily damaged, as is electric cable insulation comprising rubber, polyethylene and PVC, and such damage may present a fire hazard. Further damage to infrastructure and the built environment may result from rats burrowing under buildings causing subsidence and long term problems with settlement. Their burrowing over long distances from nest to food sources in order to reduce their exposure to potential predators can result in excavated tunnels stretching to vertical depths of up to 1.5 metres into the earth. 

Having established an excavated nesting site, female rats collect a variety of waste materials such as rags, paper and straw to thermally insulate the nest, and these materials may be supplemented with additional material created by the rodents gnawing at books, wood or upholstery products. Once the nest is completed it provides a warm and safe environment that affords protection from cold winter weather and a haven where the rodent is able to breed profusely. However, in terms of environmental impacts these sites can be extremely difficult to eradicate.

Rats are the most prolific breed of rodents. In the wild a female rat’s life spans on average around one a year with her becoming sexually mature at the age of 6 weeks and reaching social maturity after approximately 5-6 months. A rat will breed throughout the year with a peak in summer and less often in autumn. Rats have a gestation period of between 21 and 26 days and the young are born without fur, sight or hearing. Each litter usually comprises anything between five and twelve in number with the mother typically bearing six litters per year. This means that one female rat can produce many offspring in just one year who themselves will rapidly begin to reproduce. This spiralling growth rate potentially leads to an exponential expansion in the rat population, and if left unchecked this astounding phenomenon means that a pair of rats could theoretically produce nearly half a billion descendants in just three years.

To conclude, it is essential to take proactive measures to address rat infestations promptly and effectively in order to minimize any associated risks to human health. Edward Topsell, the 17th Century English cleric who authored books on zoology, religion and moral themes, once described rats as ‘evil, apt to steal, insidious, and deceitful.’ A menacing risk if ever there was one.