Reptile related diseases in humans
At the request of Animal Aid, I am herein reviewing scientific and other material regarding reptile-related disease in humans, as well as providing an interpretation of this subject. Clifford Warwick, EurBiol CBiol MIBiol MInstBl, Consultant Biologist.
In their natural context, reptiles present almost no health threat to humans as a result of the micro-organisms they carry, and even the perceived ‘risks’ from such ‘threats’ as poisonous snakes and crocodilians are greatly exaggerated.
While diverse micro-organisms form part of reptilian (and all animal) microfauna and microflora (microscopic creatures present in and on the body), these organisms occupy established ecological niches and are often very much part of the host animal’s life. In nature, this relationship in general is characteristically balanced and ‘holistic’.
However, in an artificial context, the host-microorganism-parasite relationship changes and gross as well as subtle controls and influences greatly alter the overall picture. Human capture of wildlife and its relocation to captivity imposes such substantial stressors on animals at every stage that it becomes difficult to conceive a more destabilising process on the former biological harmonics. What was balanced and harmless to the host frequently becomes unbalanced and harmful.
Captivity stress and its effects on reptiles as carriers of pathogens
It is routinely recognised that reptiles and other animals become susceptible and fall to disease as a direct consequence of their lives in captivity, and such morbidities and high mortality rates are commonly due to the destabilisation of the host-microorganism-parasite relationship. This issue is further negatively compounded by the placement of animals in environments with different microbiological contents and different controls and influences. These conditions rapidly and chronically promulgate disease.
An additional serious consequence of confining reptiles in captivity is that both the disturbance of natural biological harmonics, as well as the poorly controlled artificial environments, enhance and promote opportunities for reptilian-linked microorganisms to become a real threat to human health and life. This has already occurred in abundance, and is primarily the result of humans placing exotic animals in artificial and ecologically unsound environments.
Diverse and severe medical (Frye, 1991), psychological and behavioural (Warwick, 1990, 1995; Gillingham, 1995), and management (Frye, 1991; Warwick, Frye and Murphy, 1995) problems are continuous features of the lives of captive reptiles. Captivity is also known to undermine reptile immunocompetence – in other words, their ability to fight off disease (Frye, 1991; Warwick 1995; Maas, 2000). In captivity, reptiles and other animals show increased vulnerability to both the normal internal fauna and flora of their bodies as well as the opportunistic and potentially invasive bacteria and viruses that occupy their everyday environments. Having found a host, such opportunistic organisms increase dramatically in numbers and may become pathogenic to the host, or simply become present in such large numbers that the chances of them migrating to the broader environment and other hosts (e.g. humans) are that much greater.
Most commonly, disease in reptiles is associated with captivity-stress-linked opportunistic infections and maladaptation-related degenerative conditions. A German-Austrian study (Klrmair, 1994) considered reptile survival/longevity in the domestic home and found that captive terrapins and tortoises survived an average of 3.9 years; captive lizards survived an average of 2.5 years; and captive snakes lived an average of 3.6 years. Broader analysis largely supports the informed views (e.g. Kaplan 2000) that the majority of reptiles do not even survive their first year in captivity.
Although specific longevities under natural conditions are difficult to assess and infrequently recorded, at least two highly significant factors differentiate longevities, mortality/survival rates in nature with the same considerations in captivity. First, the ecological context in which animals live and die in nature is clearly of holistic harmony, and therefore even if wild animals experienced shorter lives than those of captive animals, the quality of life is in accordance with their evolved life histories. Second, it is very probable that a far greater number of free-living animals attain longevities closer to their natural potential than their captive counter-parts, who rarely do so. One simple yet important fact here is that many animals are captured from the wild as healthy mature adults and then die soon after their placement in captivity, which demonstrates that these individuals were clearly doing fine until made captives.
In short, captive reptiles are far more likely to manifest disease and/or release potentially virulent micro-organisms into the environment than are reptiles in the wild.
Reptile-related disease in humans
Reptiles have been established as a significant source of disease in humans for several decades. In the U.S. in the 1970s, it became evident that the introduction of millions of hatchling terrapins into private homes was responsible for an estimated 280,000 cases of turtle-related salmonellosis (T.R.S.) in humans. This figure represented approximately 14 per cent of all cases of human salmonellosis from all sources that led to the condition. This clearly established a link between the keeping of reptiles and disease in humans, and led to a ban in the U.S. and Canada on the trading of hatchling terrapins.
Today, numerous studies have reinforced the established view that reptiles are a major cause of disease in humans. One important finding is that reptiles other than terrapins are prolific carriers of salmonellae and other microorganisms. It appears that lizards and snakes may be even more likely to harbour known pathogenic micro-organisms than terrapins. Analyses show that around 93 per cent of reptiles generally carry an assortment of pathogenic microorganisms.
Even though reptile keeping in the U.S. may be half as common now as it was when the terrapin trade was in full swing (8 million today versus up to 15 million in the ’70s), the proportionate risks to humans remain high: maybe 10 per cent now versus 14 per cent in the ’70s. It is estimated that between 76,000 and 140,000 people in the U.S. contract salmonellosis each year from direct and indirect contact with reptiles. Ill effects can be both acute and chronic, resulting in long-term health damage or even death. The death in February 2000 of a three-week-old British baby from salmonellosis, caught from the family’s pet reptile, caused widespread alarm. As a result, Prof. Liam Donaldson, Chief Medical Officer (CMO) for the UK Department of Health, advised that children under five, pregnant women, and the elderly should avoid all contact with reptiles. Considering normal government language, this is a strong public health warning. (The CMO also states that he is considering other measures.) A second tragic infant death in May 2000, further highlights the severity and reality of reptile-related disease threats.
While children, pregnant women, the elderly and specific immunocompromised individuals are at the greatest risk, no one is immune to the condition that caused the deaths of the infants, or from many other diseases associated with exotic pets. The DoH cited 190 cases of salmonellosis in Britain in 1998; 258 in 1999 and six to February 2000 (five children and one adult). However, these figures almost certainly reflect under-reporting. The May 2000 infant death from reptile-related salmonellosis is a disturbing addition to these figures.
Disease transmission sources and routes
There are many ways in which a pet keeper can contract pathogenic organisms and eventually disease from their exotic pets. Faeces, urine, skin debris as well as saliva and oral detritus can carry such organisms. Standard precautions such as not touching droppings or soiled substrata are not good enough by a long way. Direct contact with the animal or its home environment (e.g. its cage, provisions, furnishings, water supply, etc.) are obvious potential hazards. Consequently, the broader environment to which the reptile may have access is a potential harbour of possibly hazardous microorganisms and parasites. It has been shown that relevant microorganisms may survive for long periods in the general environment, in tap water for three months, and in faecal material for up to two and a half years.
The routes that contagions take may be subtle and highly indirect, meaning that a visitor to someone’s home where a reptile is kept can unwittingly become a carrier of pathogens to his or her own home and others’ homes as well. The speed at which such micro-organisms find their way into humans can be rapid. One doesn’t even need to ingest them. They can enter the body via the eye area or other sites. The mouths of reptiles (many are capable of tissue-breaking bites) routinely harbour large and diverse colonies of micro-organisms, many of which can be harmful to humans. In fact, the bacterial content of the mouths of some reptiles effectively forms a poison for dispatching prey.
Health issues other than salmonella
Reptiles additionally carry worms and ticks which also cause disease in humans. The following extract is from Frye, 1991, in his section entitled: Final note on endoparasitism: the zoonotic potential for human infections and infestations with reptilian parasites
“I would be remiss if I did not comment upon the importance of protecting personnel from possible infestation or infection with those reptilian parasites which can cross species lines and infect humans. At the time this manuscript is being prepared, the status of Cryptosporidium that infect reptiles is unclear. It may be possible for one or more of these protozoan parasites to infect humans.
“Among the metazoan parasites that merit special attention with respect to their potential for inducing zoonotic disease, are the pentastomids, acanthacephalans, and cestodes. One cestode, Spirametra europeae, like its close relative S. mansonoides, employs a multihost life-cycle that can include humans via contaminated finger-to-eye or other mucous membrane contact. Similarly, many other cestodes whose multihost life-cycles permit mammalian intermediate and accidental hosts, can infect humans. It is important to WEAR GLOVES when handling stools or other suspected materials from these animals. Hand-to-mouth contact must be strictly avoided until the hands are thoroughly washed. Eating, drinking, and smoking must be conscientiously avoided by laboratory personnel while they are in the presence of potentially infective material in the laboratory. Other possibly zoonotic infestations and infections include other flatworms, acanthacephalans, and pentastomids which can be transferred via the oral or conjunctival routes by contaminated fingers and inanimate objects.”
The following statement by Davidson & Nettles (1993) offers an example of some general concerns regarding the zoology of individual animals and how this may have far-reaching considerations.
“It is now recognised by wildlife veterinarians that every wild creature that is the subject of a translocation or rehabilitation release must not be regarded as just a single animal but rather as a package containing an assortment of potentially dangerous viruses, bacteria, protozoa, helminths, and arthropods, any of which may become pathogenic in a new situation involving stressed individuals in a changed environment. In addition, the effects of stress on the immune system of animals while held in captivity pending translocations and release may increase this risk, unless well managed.”
Education versus legislation
At the height of the terrapin trade in the U.S. when there were 280,000 cases of turtle-related salmonellosis (TRS) in humans, the government tried to reduce the human pathological (disease-causing) effects of the trade through comprehensive voluntary and mandatory precautionary hygiene and health advice to prospective and actual animal keepers. The turtle producing industry devised various cleansing processes in an attempt to sterilise the eggs and baby terrapins before they were introduced to the marketplace. All these measures failed.
The pet store chain Pets-Mart attempted to educate its staff and the public about the dangers of keeping reptiles, but these attempts failed because of high staff turnover and the limited ability of staff to take guidance as well as the refusal of the public to follow advice. There are likely further complicating factors that would have made even strictly-followed advice insufficiently effective.
Clearly, education did not work. So compelling were the data that in 1975 the U.S. Food and Drug Administration (FDA) introduced a nation-wide ban on the industry, specifically the buying and selling of terrapins of four inches (10 cm) or smaller. It was thought that children were less likely to handle larger animals. The size limit was contentious on both sides of the debate. Traders were angered because the ban on baby terrapins all but destroyed the domestic market, while many public health observers believed reptiles of any size should be included in the ban.
Canada followed suit in 1976, banning its domestic market of about three million animals a year. Since then, several other countries have imposed terrapin import bans on human health grounds. In the U.S., studies conducted after the ban revealed a substantial 18 per cent decrease in the number of cases of TRS. This supported both the view that there is a link between reptiles and human disease, and that the FDA had acted prudently in banning the trade.
Kindler and Tanis (2000) of the Humane Society of the United States (HSUS) have recently (March 2000) prepared and submitted a detailed and comprehensive report petitioning for a ban on the sale of all reptiles for use as pets. This document summarises key historical medical and legal findings, and presents salient facts concerning reptiles and health threats to humans. I can confirm that this petition is a conscientious and true investigation into reptile-related disease in humans, and that it offers important observations and warnings concerning threats to human health.
Importing exotic diseases
Medical advice is routinely urged for anyone proposing to venture to the tropics. Very specific advice and cautionary preventive medicine, vaccines, etc. are administered to guard against disease that may result from insufficient protection. Yet at the same time, millions of wild-caught and captive-bred animals (reptiles, amphibians, fish and invertebrates) from exactly the same (as well as unknown) tropical regions and pathogenic hot-spots are shipped to pretty much every corner of the world each year, including nations with no importation-health regulations on these animals whatsoever (which includes Britain). Within as little as 24 hours, an animal from a tropical disease hot spot in Africa can be in the hands of an unsuspecting member of the public in Britain.
On pathological examination, many imported animals carry organisms that are not discovered when the animal is acquired, but which show up after different screening or after a particular incubation period. No amount of screening guarantees that every virus will be identified. Furthermore, most people who deal with exotics, whether wild-caught or captive-bred, have only a vague knowledge of the disease risks associated with such animals. Nevertheless, in my experience, these traders are usually aware that they risk human health through their business. And by paying money for such animals, the public unwittingly runs the risk of bringing the diseases they potentially carry into their homes.
As the above items indicate, reptiles placed in captivity are far more likely than their wild relatives to become ill themselves and to infect their human handlers and others with various forms of microorganism. It is beyond doubt that reptiles are a significant source of disease, in particular salmonellosis, in humans.
Efforts by governments, special interest groups, and the pet industry to curtail the problem through advice have failed. Education doesn’t work. Neither does taking in-store precautions such as not allowing customers to handle reptiles, because the reptiles’ broader environment (cages, sale area, floor etc. as well as the rest of the store) may also be contaminated with microorganisms.
Attempts by Focus Do It All to create a family atmosphere in its Pet World sales areas reinforce an incorrect, even irresponsible, view that taking one’s children to a place where exotic animals are on sale is somehow a sensible and safe fun day out. Furthermore Pet World locations potentially serve to turn what is now a niche market into a mainstream trade.
Perhaps particularly disturbing in the Pet World arrangement is that routine contact between staff and exotic sale animals means that these same people themselves offer simple and clear transmission routes for microorganism, so potentially distributing pathogens throughout the far wider environment of the store itself and inanimate items and products in general. It is, therefore, quite reasonable to expect that some totally unsuspecting member of the public could buy a box of nails or a power drill and carry away a potential for disease, not only to him or herself, but to other members of the family. Thus the only sure way to protect humans from contracting illness from reptiles is not to sell reptiles at all.
The problem of pet-linked disease in humans is not resolvable so long as people keep reptiles. The solution proposed by the HSUS and others is a ban on selling reptiles as pets. In my view this is the only responsible and technically supportable action that will substantially reduce and eventually largely eliminate reptile-related salmonellosis and other pet-linked diseases in humans.
For more information see the reptile campaign index.