The Spread of Infections from Animals to Humans

by John May, MD (published 4/1/2002 in Country Folks)

This month's column is written by Dr. John May, MD, Pulmonary Physician at Bassett Healthcare and Director of NYCAMH and NIOSH's (National Institute for Occupational Safety and Health) Northeast Center for Agricultural Health in Cooperstown, New York.

Anthrax... plague... diseases that we have heard much about in recent months are just a couple of examples of "zoonoses"—diseases transmitted to humans through animals. Influenza, a disease which causes far more deaths in our modern world, is another example of an infection which can spread to humans from animals. Of the approximately 150 such diseases, about 25 are of particular interest to American farmers. Fortunately only a few of these are likely to affect the farming community of the Northeast.

There are nearly 150 different diseases which can be spread to humans through contact with animals. About one third of these diseases—or "zoonoses" have been associated with farming activities somewhere in the world. Of these, roughly half can be encountered by farmers in the United States, however for most farmers in the northeastern United States there are only a handful of diseases of real significance.

Some of these diseases can be spread directly from farm animals. Prominent examples of this include human contact with infected urine or manure. In some cases the infection may enter the human body through sores or abrasions on the skin. Examples of this type of spread include leptospirosis and brucellosis. Both of these can cause a significant illness with high fevers and serious flu-like symptoms of exhaustion, muscle aching and nausea. Fortunately, these diseases can be diagnosed with laboratory tests and both usually respond well to antibiotics. More frightening is rabies which has been particularly troublesome in many parts of the northeast in recent years. This potentially fatal disease can be spread by contact of infected saliva or other fluids with open sores or abrasions on the skin. More commonly, a bite—usually from an infected bat—results in the skin break and the infection. Although careful scrubbing of the skin site with strong soap and water may reduce the chance of infection considerably, a chance of this infection must not be ignored. It may take weeks to months before the disease becomes evident, but once it does, it is always fatal. Prompt treatment with immune globulin followed by immunization will prevent the disease.

In other types of zoonoses infected material from the animals can spread from the farmer's hands to his mouth while working or eating. Toxoplasmosis can be transmitted to veterinarians and slaughterhouse workers. It can also be transmitted by the family cat—often through exposures while cleaning the litter box. This disease causes fever, sore throat, muscle aching and markedly swollen glands. Of particular concern in recent years are infections caused by bacteria such as salmonella. There have been several reported instances of farmers becoming infected with precisely the same strain of salmonella as that infecting their herd. Because the infection tends to cause pronounced diarrhea in the animals, it is often fairly easy for the farmer to be exposed. Additionally in infected dairy herds, some of these bacteria can be passed into the milk. Drinking of raw milk which is infected with just a few salmonella organisms can result in a significant human infection with fever and prolonged diarrhea. The recent appearance of salmonella in animals which is highly resistant to nearly all antibiotics makes this a particular area of concern. Campylobacter and Giardia are two other organisms which can cause diarrhea in humans following the type of hand-to-mouth spread described above.

Some skin infections can be readily spread from the skin of animals to that of the farmer. Common examples include the fungus which causes "ringworm," scabies and "milker's nodule". This last is a viral infection of the fingers of dairy farmers which results in a large, swollen, sore nodule on one of the fingers, usually the forefinger.

With other diseases, infections may be picked up from animals and carried to humans by some insect carrier. In this latter group are a number of infections which have received a good deal of publicity in recent years. Viral encephalitis, including last summer's outbreak of "West Nile encephalitis" in the New York metropolitan area, results from virus being carried by mosquitoes from infected birds, horses and rodents to human victims. These people suffer fever, headache, confusion and sometimes coma and death when the virus spreads from their bloodstream to the brain. Lyme disease is known for its skin rash, arthritis, heart and nervous system injury. This occurs following spread of the Lyme organism from rodents to humans by way of tiny ticks.

Although most of the above infections will respond to medication, some are not currently treatable. Thus prevention, education and in some circumstances, preventive immunization are the most important approaches to controlling zoonotic diseases. Prompt cleaning and care of skin wounds, careful hand washing and reconsideration of the use of raw milk are relatively simple methods of disease avoidance. Certainly the use of gloves, goggles or safety glasses and sometimes, respirator masks when dealing with potentially infected animals can prevent most of the forms of disease spread mentioned above. Insect repellents, inspection of the skin with early removal of ticks and wearing proper types of clothing can reduce the risk of insect-borne diseases.

For more information on this subject, or any other agricultural health or safety issue, call NYCAMH at 800.343.7527 or 607.547.6023.