Our role in the veterinary profession is to uphold the welfare of our patients. Unfortunately, our patients take no such oath! They can sometimes have a negative impact on their carers... us. This can be stress-related, overt aggression and through the transmission of infectious diseases. This latter group are the zoonotic infections. Did you know that the majority of human infections have their origins in animals? Some are regularly transmitted from animals. Others got into the human population some time ago and are now transmitted human-to-human (e.g., human immunodeficiency virus (HIV)). Clearly it is our duty to understand these diseases and to control them, not just for the welfare of our patients, but also for the safety of their carers, whether they are owners or veterinary staff.

Zoonotic infections range from common to rare, and cover all the important pathogen groups. Many are mild and have only temporary effects. However, others can be fatal. Most are more severe in high-risk populations like the old, the young and the immunocompromised.

In this lecture we will briefly review the more important and interesting zoonotic infections. Time will prevent an exhaustive account, but we will hear about some important and interesting diseases--and hey, this is one of the few lectures you attend that might save your life!

Viral Zoonoses

The classic small animal zoonosis is probably rabies. Although this disease is thankfully rare in the UK it remains a common and terrifying disease in other areas of the world (40,000-70,000 deaths annually; WHO). In the UK, related viruses infect bats. If you regularly handle bats you should be vaccinated and you should always be careful when you handle them particularly if they are showing neurological signs. You are more likely to come across rabies in cats and dogs on holiday abroad than in cats and dogs in the UK. However, infected animals could be brought into the UK, particularly illegally. Rarely, cats or dogs could be infected by contacting infected bats.

A new viral zoonosis is avian influenza H5N1 which as of 18 December 2007 is known to have killed 209 people (WHO). Avian influenza generally causes death in birds, but cats and to a lesser extent dogs can be infected by eating infected birds. Affected cats show severe respiratory and neurological disease. Should H5N1 become established in British bird populations, this will place you on the frontline for the control of this potentially devastating disease. It must be emphasised that although cats can be infected and infect other cats, there is no evidence that they can pass the virus on to humans.

Less serious viral zoonoses include cowpox-virus, which has its reservoir in wild rodents but can cause skin lesions in people and cats. Although generally mild, infection of immunocompromised individuals can be extremely severe to the point of death.

Bacterial Zoonoses

Campylobacter is the commonest reported bacterial cause of infectious intestinal disease in people in England and Wales. Undercooked meat (especially poultry) is often associated with illness, as are unpasteurised milk and untreated water. The majority of infections, however, remain unexplained by recognised risk factors for disease. Although dogs frequently carry Campylobacter, it tends to be different strains to those most commonly seen in humans, and the significance of dogs in the transmission of this zoonosis is unclear. However, clearly it pays to be hygienic when handling dog faeces!

Leptospirosis is rare in humans. In England and Wales there were 44 laboratory-confirmed reports of human leptospirosis in 2006, of which 34 were indigenously acquired, two proving fatal (HPA). In both Northern Ireland and Scotland, there were three laboratory-confirmed reports of human leptospirosis in 2006. People most commonly acquire leptospirosis from farms, through contact with livestock and/or rats, and through activities involving freshwater, including canoeing, windsurfing and swimming. Although infection from dogs appears extremely rare, care should be taken when handling suspect dogs.

Cat scratch disease is caused by Bartonella henselae. Cats become infected when bitten by infected fleas and pass on infection as the name suggests by scratching and biting. Affected people show rash and lymphadenopathy. About 30 cases are diagnosed annually in England and Wales, but many more cases are probably not diagnosed.

We have all heard of meticillin-resistant Staphylococcus aureus (MRSA). Many of us carry MRSA without symptoms (1% of the general population, ~10% of healthcare workers including veterinary surgeons). Clinical disease in people is much less frequent and is generally associated with some exposure to healthcare institutions, although rarer sporadic cases do occur. The strains found in cats and dogs are generally the same as those found in the human population (epidemic strains 15 and 16). Although it has been shown that people can become carriers when in contact with clinically affected cats and dogs, this is usually not associated with disease in people. It is also likely that people can infect cats and dogs. Control is associated with rigorous hygiene--for further information see the BSAVA web site.

Bordetella bronchiseptica causes kennel cough in dogs and cats. In general we do not think of this as being zoonotic. However, there are now many cases reported of severe infection in immunocompromised people (e.g., having chemotherapy or HIV treatment). As with cowpox virus infection, this highlights the susceptibility of some of our owners to whom we also have a duty of care.

Other bacterial zoonoses include Escherichia coli, Salmonella and Chlamydophila psittaci.

Fungal Infection--Ringworm (Dermatophytosis)

Ringworm is a fungal infection and probably represents one of the commonest zoonotic infections. There are many causes of dermatophytosis in people, only one of which is Microsporum canis. Despite what the name suggests, M. canis is more frequently found in cats than dogs. Infection is more common in young children because adults have some level of immunity. A common scenario would be a child picking up infection from a rescued cat, which may show few clinical signs. Lesions in people tend to occur at the site of microtrauma around the hands, arms and face. It is important to remember that about half of M. canis are Woods lamp negative so suspect cases should be cultured.

Protozoal Infections--Toxoplasmosis

Studies have estimated that between 7 and 34% of people in the UK have been infected with the protozoan parasite Toxoplasma gondii. However, the vast majority of these people will not have noticed any symptoms. Disease due to toxoplasmosis is reported relatively rarely. In 2004 a total of 79 confirmed cases were reported in England and Wales (HPA). Infection is particularly severe for the foetus of pregnant women and in immunocompromised people, where it can be fatal.

T. gondii can be found in the faeces of infected cats and in the meat of infected animals. Cats are the only species in which T. gondii can undergo the sexual part of its life cycle. The parasite multiplies in the cat's gut and the eggs (oocysts) are shed in faeces for several weeks. Cats do not usually show symptoms, so people may not know if their cat has been infected.

Humans generally become infected by swallowing water, food or soil contaminated with the faeces of infected cats, or by the consumption of contaminated vegetables. The main way to prevent Toxoplasma infection is to take general hygiene precautions to avoid ingesting the parasite. These include limiting exposure to cats, their litter and soil contaminated with cat faeces, particularly when pregnant. If possible ask someone else to clean cat litter boxes, alternatively wear gloves and wash hands thoroughly afterwards. Litter boxes should be cleaned daily as the parasite in cat faeces needs one or more days after being passed to become infectious. In addition, gloves should be worn when gardening or handling soil and hands should be washed thoroughly afterwards.

Nematodes

Another important zoonosis we can help control is Toxocara canis infection. Humans may acquire infection from soil contaminated with roundworm eggs, by direct ingestion, from unwashed hands or by consumption of contaminated vegetables. Symptoms in humans are due to the migration of larval forms of the parasite into various tissues and include pneumonitis, chronic abdominal pain and skin rash. Larvae may also enter the eye and cause loss of vision. One laboratory report of human Toxocara infection in England and Wales was received during 2006 and one case was also identified in Scotland.

Conclusion

Zoonotic infections are extremely common in the human population. Although many cause mild or inapparent infections in humans and/or animals, some can be life threatening, particularly to high-risk human groups such as the immunocompromised and the developing foetus. Some zoonotic diseases are notifiable. Prevention of human infection requires an understanding of the biology of these pathogens as well as common sense measures such as hand washing, proper disposal of animal waste and ensuring that infected animals are diagnosed and treated. Special precautions are indicated for high-risk persons. The veterinary profession has a crucial role in educating the human population about these diseases.

References

1.  http://www.bsava.com/resources/mrsa/

2.  http://www.defra.gov.uk/animalh/diseases/notifiable/disease/ai/otheranimals/index.htm

3.  http://www.defra.gov.uk/animalh/diseases/zoonoses/reports.htm

4.  http://www.hpa.org.uk/infections/topics_az/zoonoses/default.htm

5.  http://www.who.int/csr/disease/avian_influenza/en/