Iguana Answers




An excerpt from the Federation of British Herpetologists 1st Annual Conference
25th October 2003.

Zoonotic Diseases - Are Reptiles the Real Threat to Human Health?.

Dr. Mike Allen.   2003 Claire May-MillerThe speaker was Dr Mike Allen. Mike informed the meeting that for the past 25 years he has been actively involved in clinical research in the fields of critical care and infectious diseases. His comprehensive talk included a review of the scientific literature regarding zoonotic diseases, with particular reference to the risks to human health posed by salmonella carriage in reptiles. Zoonotic diseases are defined as "those infections which are naturally transmitted between vertebrate animals and humans" and as Mike was able to illustrate during his talk, reptiles are probably the least of our worries as far as zoonotic diseases are concerned.

What follows is only an outline of his talk, as it was not possible for the present author to take notes of all the facts and figures presented by Dr. Allen in the form of diagrams and graphs, although he has kindly assisted with this manuscript to ensure its accuracy and has made available some interesting tables and figures from his talk, which are included here.

Antibiotic Resistance - the major threat today
Mike's research interests include the in-depth study of antibiotic resistance, and in particular the rise of the so-called "super bugs". For the past 3 years Mike has been a member of a national working party of infectious disease specialists responsible for bacteraemia (infections caused by bacteria in the blood) antibiotic resistance surveillance. This working party publishes its results on a regular basis (peer-reviewed medical journals and at international and national medical congresses) and reports to Government agencies.

There are an estimated 100,000 cases of "Hospital-Acquired Infection" (HAI) per annum in the UK; HAI causes 5,000 deaths a year, and costs the NHS an estimated 1 billion. The most problematic bacteria are methicillin-resistant Staphylococcus aureus (MRSA), glycopeptide resistant Enterococci (GRE) and Clostridium difficile. The Department of Health, the Health Protection Agency and the National Audit Office are some of the bodies collecting data about the incidence of these infections in the UK. 

Probably the most clinically significant and life threatening infections are bacteraemias. However, Mike presented the bacteraemia data from SCIEH (Scotland) for Quarter 3, 2002. Only 9 of the 2525 (0.36%) bacteraemias were caused by Salmonella (from any source). These figures mirror those reported by other national agencies. These data clearly illustrate that the threat to human health posed by Salmonella is not supportive of argument peddled by those opposed to the keeping of reptiles. Furthermore, salmonellosis is only one of many zoonotic diseases that threaten human health. Of the commonly kept pets, mammalian species harbour many more diseases harmful to humans than do reptiles. To illustrate this point, Mike presented data taken directly from the Centres for Disease Control (CDC) website. It can be seen clearly from Table 1, that Salmonella is the only disease the CDC attributes to reptiles. However, a multitude of serious diseases (including Salmonella) can be contracted by humans from cats, dogs and rodents.

TABLE 1: Diseases Acquired From Pet Species (Ref: www.cdc.gov/healthypets/ )





Campylobacter Brucella canis Infection BSE (mad cow disease) Salmonella Infection
Cat Scatch Disease Campylobacter Infection Brucellosis  
Cryptosporidium Cryptosporidium Infection Campylobacter Infection  
Hookworm Giardia Infection Cryptosporidium Infection  
Leptospira Infection Hookworm E. coli 0157  
Plague Leptospirosis Rabies  
Q Fever Lyme Disease Ringworm  
Rabies Q Fever Salmonellosis  
Ringworm Rabies Yersinia enterocolitica  
Roundworm Rocky Mountain Spotted Fever    
Salmonellosis Roundworm    
Tapeworm Salmonellosis    
Toxocara Infection Tapeworm    
Toxoplasmosis Toxocara Infection    

Chris Newman with Mike Allen.   2003 Steve Woodward Unlike Salmonella, infections caused by GRE and MRSA are major causes of serious infection and significant mortalities in humans. Mike explained that GRE are an international health problem. These bacteria are readily recovered from farm and domestic animals (e.g. cats and dogs), sewage, and some meat products especially uncooked chicken. It has been estimated that up to 20% of humans are now GRE carriers. The reason for its prevalence is the indiscriminate use of growth promoters (substances very similar to antibiotics) such as bacitracin and virginiamycin in animal feeds. Bacteria in the animals' gut become resistant to these substances, and this also confers resistance to medicinal antibiotics. Whilst we may be able to control the use of these feeds in the UK and Europe, this may have only a limited effect upon human acquisition of GRE as an estimated 30% of chicken in processed food consumed in the UK, comes from countries such as Thailand, where controls are less stringent.
It is also noteworthy that antibiotic resistant Campylobacters have been traced back to the use of antibiotics in animal feeds. Campylobacter not Salmonella is the commonest cause of gastro-intestinal infections in the UK.

Once organisms in the environment, and in our bodies, are resistant to antibiotics this resistance can be transferred between different species of bacteria, particularly those which cause serious infections in man. MRSA is probably the bacteria causing greatest concern amongst health professionals. MRSA has been isolated from cats, dogs, horses, cattle, meat and milk. Staphylococcus aureus (non MRSA) has been shown to be responsible for Toxic Shock Syndrome (TSS) in humans. Studies have identified Staphylococcus aureus contracted from dogs, cats, chickens, pigs, horses, sheep and cattle as the cause of TSS; but there are no reports of a link with reptiles.

The incidence of antibiotic resistance is likely to continue to rise, because antibiotics are still used heavily in our society in veterinary medicine, particularly to treat farm animals. Slurry containing antibiotics and bacteria are regularly released into the environment (e.g. soil, drainage water) and their presence may have serious consequences for bacterial resistance to commonly used antibiotics and disinfectants. Some of the antibiotics which have been detected in the environment include penicillins, tetracyclines, aminogycosides, macrolides and fluoroquinolones. The veterinary and agricultural use of antibiotics poses a major potential threat to our health.

Salmonellosis in humans
The incidence of salmonellosis in humans is steadily decreasing. The common symptoms of Salmonella infection are diarrhoea, fever and abdominal cramps. In most cases the illness lasts for 4-7 days and resolves without the need for treatment. A small proportion of sufferers may become debilitated by severe diarrhoea, or the organism may move from the gut into the body or bloodstream (bacteraemia), causing more serious illness. The young, the elderly, and those with an impaired immune system are most at risk. Recent figures from the CDC in the USA include an estimate of 1.4 million cases of salmonellosis annually, of which about 30,000 are culture-confirmed and reported to the CDC. There are an estimated 500 deaths from salmonellosis a year in the United States. This is a very small number when one considers that the population of the USA is about 277 million and the number of high risk patients who contract Salmonella.

There are over 2,000 serotypes of Salmonella. Over half of salmonellosis cases are caused by just two serotypes, S. enteritidis and S. typhimurium. These are not serotypes normally found in reptiles, though they are commonly found in chickens. Increasingly, salmonella bacteria are becoming resistant to antibiotics. As described previously, this is largely due to the use of antibiotic growth promoters in farm animal feeds. Salmonella bacteria are common commensals in animals, causing them no illness, and excreted in their faeces. Making animal feeds from waste products from slaughterhouses increases the carriage rate in animals. Salmonella organisms may be isolated from almost all domestic animals, farm animals and pets; up to 30% of dogs, for example, are carriers. Reptiles also commonly carry salmonella species, predominantly unusual serotypes. 30-70% of reptiles may carry salmonella.

The primary source of human infection is oral ingestion from contaminated food, usually animal products. The bacteria can also be accidentally ingested following contact with contaminated soil, animal faeces, fish and bone meal, etc. A report published by the CDC in 2001 reveals that by far the commonest sources of clinical infections were farm animals and their products. Reptiles were the least common source, accounting for only 2.9% of cases (Table 2).

TABLE 2: Clinical Salmonella Isolates from 7,243 Nonhuman Sources (Ref: CDC Salmonella Annual Summary 2001)

SOURCE (Species) NUMBER (%)
Bovine (cattle) 2,670 (36.9)
Porcine (pig) 1,486 (20.5)
Turkey 914 (12.6)
Equine 908 (12.5)
Other Domestic Animals 364 (5.0)
Other Birds / Wild Animals 303 (4.2)
Chicken 298 (4.1)
Reptiles 207 (2.9)
Other 93 (1.3)
TOTAL 7,243

One of the reasons for the widespread public concern regarding the role of reptiles in transmission of salmonellosis is historical. In the 1970s, following the "Ninja Turtle" craze, millions of baby red-eared slider turtles were suddenly kept as pets. At the height of the craze, an estimated 4% of households in the USA kept a turtle. Within a short time, the proportion of cases of salmonellosis from reptile-related salmonella serotypes increased to 14% of all cases reported to the CDC. This was largely due to the naivety of the public at the time. The turtles were being bred in vast numbers for rapid sale. The public were given little or no education regarding basic husbandry or hygiene. The turtles were being handled by children 2-3 years old and younger, some of whom, according to the case histories, were putting the turtles in their mouths and drinking the water from the turtles' tanks. No wonder an epidemic of salmonellosis occurred at this time. Restrictions upon the sale of these turtles soon followed.

The risk of transmission of salmonella from reptiles to humans has now been clearly defined. Various public health agencies such as the CDC in the USA, and the Health Protection Agency in the UK, have now designed clear, simple and common sense guidelines that should pose no problem at all for responsible herpetologists. Nowhere do they recommend the banning of reptile keeping. On the contrary, the CDC, for example publish helpful posters and website information outlining sensible precautions to be taken when reptiles are kept as pets. They state that simple hand-washing routines and practice of good hygienic measures, as would be expected with all pets, are sufficient to control the spread of salmonella organisms in most cases. They do recognise that the very young, the elderly, and immunocompromised persons are at increased risk (from any source of salmonella infection) and recommend that these persons avoid contact with reptiles. If these guidelines are followed, the risks of reptile-associated salmonellosis are minimal.

Case Histories of Reptile-Related Salmonellosis
Mike presented five case histories from the USA of serious salmonellosis infections acquired from reptiles, representative of the reports seen in the media in the UK and USA. All cases were associated with lack of adherence to the simple advice given by the CDC and HPA. Namely, a clear breakdown in the basic hygienic measures; the keeping of reptiles in food preparation areas; and the exposure of high-risk patients. One baby was infected from faeces from a savannah monitor kept in the family kitchen. A six-week-old baby was infected in a home where the reptile's water bowls were washed in the kitchen sink. A three week-old baby was infected from their pet iguana, handled by all the family. Another 5-month-old girl contracted the illness from an iguana kept by her babysitter, who handled both iguana and child frequently.

Mike then told us of a disturbing case of salmonellosis in the UK. A consultant microbiologist who works with Mike on an MRSA study in North East England, was called to a house where a young child had contracted salmonella. What the microbiologist discovered was a free roaming iguana shedding faecal matter in the living areas of the house. The young child unaware of the dangers was putting the faecal matter into her mouth! As Mike pointed out these cases of salmonellosis are newsworthy for both their human interest and their rarity. While catastrophic for the patient and their families, sensational headlines are no substitute for well designed clinical and epidemiological studies and audit (as performed by the CDC, SCIEH and the HPA), but are peddled as solid evidence by the opponents of responsible reptile keeping.

Animal faeces: the greatest risk
The greatest risk of salmonella infection comes from animal faeces. However, as Mike pointed out, what has to be taken into account is the access to the faecal matter. Having established that cats, dogs, rodents and farm animals are major carriers of Salmonella as well as reptiles, Mike clearly illustrated that the risk of faecal contamination is far greater from the non-reptile species. If basic husbandry criteria are followed, the faecal matter from reptiles is contained within a clearly defined area with restricted access, such as within a vivarium. These areas can be easily and effectively decontaminated. The converse is true of the mammalian species, where faecal matter is readily accessible in the environment and in the homes of the vulnerable groups (e.g. children). Mike illustrated this with recent examples where cats had defecated in his children's sand pit, and where he photographed dog faeces adjacent to his children's school. The latter occurred in spite of the presence of numerous signs targeting the fouling by dogs and the provision of a specific bin for dog mess. Farm animal slurry presents another significant risk of salmonella.

Mike concluded his talk by making the following points

  1. Of all the potential bacteria causing infectious diseases in humans, Salmonella does not represent the greatest threat.
  2. Salmonella is not a major cause of bacteraemia. 
  3. Of the commonly kept pet species, reptiles carry the least number of known zoonotic diseases (only Salmonella is identified by the CDC.)
  4. Salmonella is carried by many commonly kept mammalian pet species as well as reptiles.
  5. If the simple guidelines on reptile ownership provided by the CDC and HPA are adhered to, the risks of salmonella infections are minimal. 
  6. When non-human sources of clinical cases of salmonellosis were reviewed by the CDC, reptiles were implicated in less than 3% (207) of cases vs. 36.9% for cattle, 20.5% for pigs and 12.6% for turkeys.
  7. It is estimated that 7.3 million reptiles are kept in households in the USA. The 207 cases above are hardly indicative of an unacceptable risk of reptile associated salmonellosis.
  8. With the use of antibiotics and growth promoters in agriculture, it is somewhat perverse that reptiles are probably more at risk of acquiring infectious diseases from us (from what we feed them) than we are from them.
  9. All the evidence presented in the talk can be accessed either from the websites of the CDC, HPA and SCIEH etc. or from peer reviewed medical journals. Unless one is incapable or unwilling to see what is blindingly obvious from these respected sources, the case against reptiles on the basis of the excess risk to human health is comprehensively discredited. 
  10. If the risk to human health is the primary argument against the keeping of reptiles, then taking this argument to its logical (perverse) conclusion based on proven risks to health should see the banning of the keeping of cats, dogs, birds, fish, rabbits etc. Furthermore, we would need to ban common hobbies such as gardening , since most dangerous bacteria are found in soil, ponds, etc. 

At the end of Mike's talk, we all felt that the risks of reptile-related Salmonellosis had well and truly been put into perspective, especially compared to the huge and growing risk of antibiotic-resistant bacteria spread from mammals, primarily farm animals fed antibiotic "growth promoters". Responsible pet ownership involves gaining full knowledge of any risks associated with the keeping of that animal, and actively working to minimise those risks to oneself and to others. From what we heard, it is perfectly feasible to achieve this goal, and it is pure scaremongering, misleading and irresponsible to suggest otherwise.


The contents of this report may be freely reproduced provided the source, "The Rephiberary, Newsletter of ASRA" is acknowledged.