Preventing human infection – an overview

Preventing human infection – an overview

Preventing infection relies on a combination of techniques, and when applied correctly it is possible to reduce the risks to almost zero even in highly-contaminated environments.

Of course the only 100% effective way is to avoid contact with contaminated water and animals, thus avoiding exposure to the bacterium. If you are in a high-risk area, you should always attempt to minimise contact, as there are many hundreds of other bacteria, viruses and parasites you may be dealing with!

Exposure to water

The vast majority of human cases are from contaminated water, and of those the majority are occupational cases from areas of the world where agriculture and rodents mix – rice-farming, cane-growing and so forth. Recreational exposure is next, with cases amongst swimmers being the obvious top group. Lowest of the risk groups is occupational exposure in the developed world – water and sewer engineers, construction, pest control and so on.

Clearly there are problems in preventing exposure in the highest risk activities (rice-farming and such) and in those areas the only option is to be aware of symptoms and seek early treatment. At this time there is no universally-agreed human vaccine, and the preventative use of antibiotics can only be considered for short periods of very high risk due to their side effects.

Swimming is the greatest risk, and many cases are reported each year from swimming in contaminated water in activities such as adventure racing, and simple recreational swimming in local lakes and ponds. There is no practical way to prevent exposure during swimming, as some water will always enter the mouth. For one-off activities such as expeditions there is a potential argument for prophylaxis but we would usually advise education and followup monitoring of participants rather than widespread medication. Education is also critical for preventing children being exposed, when playing on or near contaminated water.

Anglers using potentially contaminated sites (urban canals, small ponds etc.) and bankside/sewer workers should wear splashproof clothing and expecially gloves. Anglers are at higher risk as it is reasonably common to cause minor cuts with hooks, knives and the like, and this greatly increases the ease by which the bacteria can enter the body. Fish caught from suspect areas should of course never be eaten. Whilst cooking does in theory kill any bacteria within a fish, very often the level of cooking is insufficient to guarantee safety. Specific guidance is given for protecting employees in the workplace.

Recreational exposure (swimming, waterskiing, sailing, caving, etc) is clearly done at the person’s own risk and they must weigh up their own balance of risk vs. desire. The same preventative measures apply – minimise the risk of water entering the body by any and all means, be aware of the symptoms and seek treatment immediately should you feel unwell. There are no ‘quick fixes’ to prevent infection, and we currently do not advise the use of antibiotics to prevent infections from recreational exposure. Some swimmers wash their mouths with antibacterial rinse, though this has not been proved to offer any significant benefit other than keeping their teeth clean.

Scuba divers, who are particularly at risk of both swallowing water and contact with broken skin, should opt for drysuits and try as much as possible to avoid swallowing any water when purging or changing regulators. Commercial divers are required by their employment regulations and insurance to comply with strict rules when working in contaminated water, these include the use of hard-hat systems, wash-down stations and regular medical testing. Specialist tasks such as police recovery dives must also follow these protocols. It’s important for divers to remember that water can enter the mouth via the hands even without immersion, so when preparing for a dive, washing masks, wading through shallow water or even washing equipment after use, sensible hygiene should be followed and broken skin covered with waterproof dressings.

Remember that this advice applies to FRESH water – the risks in saltwater are virtually zero.

Animal exposure

One of the major concerns of people contacting us over the years is transmission from animals. In the developed world the direct exposure to water can often be minimised, but domestic pets can either bring in rodents or become infected themselves, and occupational exposure to rodents and other mammals (as part of garbage collection, pest control, construction or agriculture) is often hard to avoid. Distinction must be made between infection and carrier-state. An infected animal has an illness, and will either recover from it or die. A carrier has a colony of bacteria living in their body but shows no illness. They can remain like this for their entire lives and suffer no major adverse effects. Both are potential sources of infection, but clearly an animal that is not a carrier will eventually be safe, one way or another.

The dominant carriers are rats, but many other rodents and small mammals can take the role depending on the location, such as raccoons, squirrels, field mice, etc. – however rats are naturally incontinent and so spread far more urine than other species. It is usually safe to assume that across the developed world, 1 in 5 wild rats are carriers of pathogenic leptospires. Carrier-state rats emit bacteria in their urine, and this is distributed everywhere they go – normally the bacteria die once the urine dries out, but if it enters water the bacteria can remain viable for longer periods. Anyone handling wild rats should be aware that their urine, damp soil, bedding and nearby water are all potential sources of infection – and rat bites themselves can cause infection both by leptospira and other unrelated organisms.

A dead rat (wherever it may be) remains a risk for a short while, but only in terms of direct contact. Simple disinfection of the surface after disposal is all that is required. Once decomposition has advanced, acidity changes in the body will kill leptospires and of course they cannot survive on a dry skeleton. Similarly, domestic pets that are contaminated on their coats but not infected (such as for example after swimming) can be rinsed and dried as usual, and will be no significant risk thereafter. Dogs and farm livestock are usually vaccinated, cats rarely contract any infection.

A domestic or farm animal that is infected and ill will be a potential source of infection during the first few weeks of the disease, but only in terms of bites or contact with body fluids. Airborne and skin-to-skin transfer is almost impossible and has never been reported in the wild. During treatment the animal should be handled with care, and any body fluids cleaned away using disinfectant, but beyond that there are few risks. Animals should be kept apart to minimise the risk of direct infection, though again this is statistically rare.

Endemic exposure

Visitors and residents of countries where the bacterium is widespread (such as the Indian subcontinent) are likely to be exposed to contaminated water as a matter of course. In these cases it is almost impossible to prevent exposure, and the best option is to be aware of the early symptoms and react to them. Leptospiral contamination is akin to malaria in that it will probably always be there, and cannot realistically be avoided in certain countries.

Visitors to these countries are at a higher risk than residents, as in the resident population there is a statistically higher immunity, due to childhood infection or placental antibody transfer. Having said this, there are many pathogenic serovars of leptospira, and immunity to one does not prevent infection from the others. Nobody is immune to the disease as a whole, and the serovars found at a selected location can change over time as host populations change. The highest concentrations of cases will always be in developing countries where wet farming and rodent populations combine. Tourists visiting these areas should be aware of the risks and limit their activities accordingly. Of course, the majority of ‘tourist’ activities are perfectly safe even in areas of high risk, but the growth of ‘adventure vacations’ with swimming, survival and back-country hiking all increase the dangers. As more of the world is opened up to tourism, we must accept that the dangers of those areas are opened up to us.