Ever wondered why you felt as sick as a dog?

Zoonosis is the medical term for any disease that can be transmitted from animals to humans. The term is derived from zoion, the Greek word for animal.

Many zoonoses are associated with the poor cooking and preparation of food. The Food Safety Promotion Board and the Food Safety Authority of Ireland have mounted vigorous campaigns to highlight gastro-enteritis, a largely avoidable illness.

Salmonella in humans is linked to the consumption of poorly cooked poultry. Campylobacter is another bacterium that causes bloody diarrhoea and crampy abdominal pain in humans. And the dreaded 0157 version of E. Coli can be fatal in young children and the elderly; eating undercooked beefburgers and manure-contaminated vegetables has caused serious outbreaks of the disease in the last number of years.

While such bacteria are transmitted from animals via the food chain, people probably think of them as gastro-enteritis rather than as zoonoses.

Some skin infections are zoonoses. Ringworm is a fungal skin infection that can be picked up from cats, dogs and cattle. It causes a dry, itchy, ring-like rash.

Orf is an interesting infection caused by a poxvirus. It primarily affects sheep farmers and is picked up from infected lesions on lambs and from contaminated fences, hedges and barn doors. In humans, the rash becomes a painful blister that opens into a sore.

Lyme disease is spread by tick bites. First described in 1977, following an outbreak of juvenile arthritis in Lyme, Connecticut, it is caused by a bug called Borrelia burgdorferi. The tick lives in deer, sheep, horses and field mice. Many people who become infected get no symptoms; those who do may notice a red rash, known as erythema migrans, spreading out from the site of the bite. Profound fatigue, muscle ache and joint pain are other early symptoms. Treatment is by a 10- to 21-day course of antibiotics.

A "flu-like" illness, especially one that occurs outside the winter months, could be symptomatic of a zoonotic infection such as leptospirosis, toxoplasmosis or psittacosis.

Leptospirosis is caused by different types of Leptospira, a finely coiled, thread-like spirochaete (or flexible, spirally twisted bacterium). Humans become infected incidentally; animal hosts excrete leptospirae in their urine for a long time without showing evidence of disease. Skin contact with urine or stagnant water is enough to infect a human.

Leptospirosis icterohaemorhagiae is spread by rat urine and has been known to infect golfers who have retrieved balls from drains and ditches.

Leptospirosis hardjo is found in cattle. It produces a less aggressive form of disease than the rat-borne infection, but is more common. A Northern Ireland study demonstrated an 8 per cent infection rate among farmers there. Recent research carried out by the faculty of veterinary medicine at University College Dublin and Teagasc shows that almost 80 per cent of unvaccinated dairy herds in the State are infected by leptospirae.

Following an initial flu-like illness of fever, sweating and muscle aches, a person with leptospirosis will characteristically develop severe headaches, abdominal pain, cough and breathlessness. Early antibiotic treatment can shorten the duration of symptoms.

Of the more common zoonoses, toxoplasmosis is the most likely to affect the urban dweller. Spread by cat faeces, the toxoplasma bug is a particular hazard for pregnant women. It can enter a baby's bloodstream by crossing the placenta. Infection early in pregnancy may cause miscarriage or congenital abnormalities.

Psittacosis is a condition caused by infection with Chlamydia psittaci. The bug can be inhaled with the dust from bird droppings and feathers, and infection also results from contact with aborting ewes and their discharges. Sore throat, dry cough and a flu-like illness are the typical symptoms.

By their nature, zoonoses are a particular hazard for farmers and their families. Schering-Plough Animal Health Ireland has just produced a medical information card for farmers. As well as giving details of the various illnesses, it contains good advice to help prevent infection from animals. Copies are available by calling 01-2050900.

You can e-mail Dr Muiris Houston, Medical Correspondent, at mhouston@irish-times.ie or leave a message on 01-6707711, ext 8511. He regrets he cannot reply to individual medical problems