Public Health - Recognising zoonoses

Animals can pass on infections harmful to humans. Barry Hill explains the routes of transmission.

Infection can occur if the hands are not washed after contact with animals
Infection can occur if the hands are not washed after contact with animals

Last year, an outbreak of Escherichia coli 0157 among visitors to a Surrey petting farm highlighted the problems posed by zoonoses - infections acquired from animals.

All animals, particularly farm animals, such as sheep, cattle and goats, can carry a range of micro-organisms and diseases that can be transmitted to humans and cause ill health.

Who is at risk and how?
Agricultural workers and other professions that come into regular contact with animals are particularly vulnerable to zoonoses but, as was seen with the E coli 0157 outbreak, the general public is also at risk.

Also, the trend towards owning exotic pets, such as reptiles, large birds and even small primates, has opened up more potential routes for contracting zoonoses. Therefore, it is important that primary care practitioners are aware of the more common symptoms affected patients may present with.

There are many possible contamination routes, the most obvious being accidental ingestion through food or drink following poor hand hygiene after coming into contact with animals or their faeces.

Zoonoses can also enter the body through broken skin due to bites, pecks or scratches, or through the eyes, mucous membranes or lungs. Additionally, the development of allergic conditions due to animal hair, fur, urine, faeces or bedding, and the possibility of vector-borne disease from insects associated with animals, such as fleas, must also be considered.

The E coli outbreak
In the case of the petting farm outbreak, where small children were particularly affected, the likely route of transmission was oral. E coli 0157 is a bacterium that lives in the gut of some farmed animals, causing them little or no harm.

The organism is unusual in that very few individual bacteria are needed to cause infection in humans; classically, this occurs by coming into contact with the animals or their faeces and then putting fingers into the mouth, or consuming food and drink shortly afterwards without washing hands - habits small children are prone to.

The typical incubation period is one to six days after initial contact. Once infected, the toxins produced by the bacteria in humans can cause symptoms ranging from diarrhoea to a haemorrhagic colitis characterised by a combination of severe abdominal cramps and blood in the stools.

Kidney failure may potentially follow, which in severe cases can prove fatal, children being particularly vulnerable.

Some notable zoonoses
The table below lists examples of zoonoses, with likely sources and routes of transmission.

Salmonella is found in most types of farm animal, and its transmission and symptoms are similar to those of E coli 0157.

Cryptosporidiosis is caused by a protozoan and can occur in farm animals and pets. Symptoms in humans include diarrhoea, fever and abdominal pains, young children being particularly susceptible.

Ringworm is due to a dermatophyte fungus that infects cattle, pigs, sheep, horses and domestic pets such as dogs. In infected humans, inflamed crusty lesions form on the hands, forearms, head and neck as a result of fungal spores entering the skin via cuts and abrasions.

Q fever is caused by Coxiella burnetii bacteria, carried mainly by sheep and cattle but also deer. It usually leads to acute illness with flu-like symptoms, although pneumonia and other complications sometimes follow. A more serious, chronic form of the disease can also develop, leading to liver and heart valve damage or death.

The Q fever agent can survive for many months in a spore-like form under suitable conditions. It is transmitted by inhaling dust contaminated with animal faeces or urine; however, infection can also occur from drinking unpasteurised milk, via tick bites or through skin abrasions.

Psittacosis, sometimes known as ornithosis, is caused by an organism often carried by ducks and other poultry, as well as caged, wild and exotic birds. In humans it can lead to flu-like illness that may lead to pneumonia, endocarditis, hepatitis and even death. Transmission is usually by inhaling dust or aerosol from faeces or via a nasal discharge from infected birds.

Some zoonoses are notifiable - further information on this and all other zoonoses can be found on the Health Protection Agency website.

  • Barry Hill is chief biomedical scientist at the blood transfusion department, Wigan Royal Infirmary.

Resource
Health Protection Agency information on zoonoses: www.hpa.org.uk/HPA/

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