What are zoonoses?

Causes and Symptoms

There are many types of contact between humans and animals. Some produce pleasure, such as stroking a kitten’s fur; some have strictly utilitarian considerations, such as farming or meat processing; and some are to help animals themselves, such as the veterinary sciences. Unfortunately, some also result in the transmission of infectious diseases to humans, which are called zoonoses. The most common symptoms of zoonoses are headache, fevers, general malaise, diarrhea or bloody stool, and sometimes skin rashes, eruptions, or inflammation (in the event of a bite or sting).

Approximately 150 types of zoonoses can be transmitted either directly or indirectly to human beings. Direct exposure results from coming in contact with an infected animal or its excrement, blood, or saliva. Indirect exposure results from being bitten by an insect carrying an infected animal’s blood. In either case, a disease may or may not develop.

Although many diseases transmissible to humans from animals now can be cured, it is important to avoid the methods of transmittal, especially the handling of infected animals. Humans must wash their hands after handling animals, especially after cleaning cages or litter boxes. Small children should be discouraged from kissing and cuddling pets. In rural or mountainous areas, humans should be discouraged from handling wild animals. In some communities, especially rural areas, keeping wild animals such as wolves or raccoons as pets is popular. Unfortunately, wild animals can carry many serious diseases that can be passed to humans, notably rabies.

Incubation periods for zoonoses can range from a few days to several years. If it is suspected that a human has contracted a disease from an animal, it is important to seek medical attention. Most zoonoses can be diagnosed and treated. In most cases, treatment will clear up the disease with no lasting aftereffects. If the infected person waits too long for treatment, however, therapy may take longer, and problems may persist. In rare cases, surgery is necessary, and in even rarer cases, death can occur.

Zoonoses vary greatly in their sources and symptoms. Among the most important of these diseases are anthrax, brucellosis, cat-scratch fever, encephalitis, Lyme disease, malaria, cattle tuberculosis, plague, rabies, ringworm, Rocky Mountain spotted fever, roundworm, salmonella poisoning, sporotrichosis, and toxoplasmosis.

Anthrax is an infectious disease of warm-blooded animals such as cattle or sheep, caused by the bacterium Bacillus anthracis. The disease can be transmitted to humans by the handling of infected products, such as the animals’ hair. The disease is characterized by lesions in the lungs and by external ulcerating nodules.

Brucellosis is characterized by repeated fevers accompanied by weakness and joint pain. It is contracted from the amniotic and fetal membranes of pregnant and newborn animals. More typical in farm animals, it can also be present in dogs that are bred. While it is not fatal, it does cause severe flulike symptoms. It can be difficult to treat and cure completely.

Not as serious as toxoplasmosis, which can also be transmitted through a bite or scratch from a cat, cat-scratch fever is a bacterial infection that can result when a human is bitten, nipped, or scratched by a feline. Symptoms include a blistery inflammation at the site of the infection, fever, malaise, and sometimes swelling of the lymph nodes. Such symptoms appear two to ten days after the skin is broken and generally last about a month. The infection will generally clear up on its own, but, after washing the affected area with soap and water, a consultation with a physician is recommended. Approximately twenty thousand people are infected annually with cat-scratch fever, mostly children. It is important for parents to instruct children not to play roughly with cats.

The bite of mosquitoes infected with
encephalitis can cause this disease in humans. The illness is an infection of the lining of the brain; symptoms include a high fever, general malaise, and usually a very strong headache. Another disease transmitted by the bite of mosquitoes is malaria, which is caused by sporozoan parasites. Symptoms include intermittent chills and fever. Malaria may persist for years, and once the disease has been contracted, those affected are discouraged from donating blood, to avoid passing it on to others.

Lyme disease is an insect-related disease that is usually caused by the bite of a deerfly or tick. The disease was named for an area of Connecticut where it was first discovered. A doctor treating patients with flulike symptoms and skin inflammations realized that patients complaining of the symptoms had all been walking in woody areas and had been in contact with brush, weeds, and flowers where the tiny ticks and deerflies could have been harbored. The insects preyed on deer and other animals in the area, then probably jumped off or were brushed off the host animal into the grass. (Bitten animals may become infected with the disease as well.) Cases of Lyme disease have been found across the United States, with the highest percentage being reported in the Midwest and on the East Coast. Although humans can develop the disease from a tick or deerfly bite, there is no evidence yet that a bite or scratch from an infected animal can transmit the disease.

The bacterium Borrelia burgdorferi is responsible for Lyme disease. Its symptoms include inflammation, skin lesions and redness, joint inflammation, fever, fatigue, general malaise, and headaches or a stiff neck. These symptoms may last for weeks after the bite. Nerve-related disorders and heart ailments may follow the preliminary symptoms. Early treatment is a fourteen-day regimen of antibiotics, which can then be followed by treatment with antimicrobial agents until symptoms cease. Later stages of Lyme disease, such as arthritis and heart disorders, can be treated with penicillin-type antibiotics. Nevertheless, joint and muscle pain may persist for several weeks.

Rocky Mountain spotted fever is another disease caused by tick bites. The disease is found in all areas of the United States, not only in the Rocky Mountains. Symptoms include headache, fever, and skin rash. Early diagnosis and antibiotic treatment are very important in order to prevent more serious complications.

An infection that often strikes herded animals such as cows, deer, or elk is a subspecies of
tuberculosis caused by Mycobacterium bovis. Individuals who come in contact with such animals, such as veterinarians, farmers, and slaughterhouse workers, are most susceptible to this disease because they breathe in the tiny droplets of bacteria. In addition to lung infections, other diseases are reported to be associated with the M. bovis bacterium. Treatment for infected individuals is with antibiotics.

Plague is an infectious disease transmitted by the bite of a rodent flea infected with the bacillus Yersinia pestis. Two forms of plague affected millions of humans in Asia and Europe during the Middle Ages and continue to occur today, although not in epidemic form.
Bubonic plague results in the formation of buboes, or swellings of the lymph glands. The Black Death was caused by the same bacterium and was probably pneumonic plague; this form is transmissible between people and is characterized by black patches appearing on the skin of its victims. Both types cause fevers and heavy coughing. Before the discovery of antibiotics, most victims died from this very contagious disease.

Rabies is an acute viral disease of the nervous system of warm-blooded animals, usually transmitted through the bite of the infected animal. Rabies can be found in a small number of animal species across the world. There are two main types: urban rabies, carried mainly by domesticated animals such as dogs; and sylvatic rabies, carried by wild animals such as bats. Symptoms of the disease include fever, nausea, vomiting, shortness of breath, abdominal pain, and a cough. If left untreated, the muscles become paralyzed and breathing and heartbeats stop, causing death. There are three methods of treating the disease: vaccination of people at risk of exposure before any exposure has occurred; animal control and immunization, especially preexposure immunization; and postinfection treatment, usually a series of expensive, painful shots.

Generally, the incubation period for rabies in humans is three weeks to three months. There have been cases, however, in which the victim was bitten by a rabid animal a year before the onset of the disease. Two documented cases showed that the victims were bitten by infected dogs while traveling in Asia six or seven years prior to the onset of the illness. The World Health Organization (WHO) estimates that approximately 55,000 people die of rabies every year. While domesticated animals, such as dogs and cats, can be treated against the development of rabies, it is difficult to inoculate wild animals, mostly because vaccines are not licensed for use on them. One method to attempt to control rabies in wild animals is the use of edible bait laced with vaccine.

Ringworm is a skin disease caused by a fungus, not a worm. The fungus usually occurs on exposed skin, especially the scalp, and looks inflamed and scaly. Diagnosis in animals is made by exposing the hair or fur to an ultraviolet lamp (the infected area will appear greenish in color). In humans, antifungal soaps or drugs will cure the disease.

Salmonella
bacteria in food cause gastroenteritis, inflammation of the mucous membrane of the stomach and intestine. Symptoms of the disease are severe, bloody diarrhea and sometimes dehydration. The illness is very contagious and often spread rapidly in daycare or home settings. Although salmonella bacteria are usually found in uncooked meat, they can also be present in pet turtles raised on farms—as many as 20 percent of turtle eggs may carry the bacteria. Thus parents buying pet turtles for their children may unknowingly bring the disease into their homes. Many of these strains of salmonella are quite resistant to commonly used antibiotics, thus making treatment difficult and allowing the disease to spread.

Most spider bites result in redness and itchiness or soreness at the site. They usually heal by themselves in a few days. The bites of some venomous spiders, however, such as the brown recluse, or violin, spider (Loxosceles reclusa), can cause serious complications in humans. Symptoms of the bite of a brown recluse spider include an eruption that turns black in the center, surrounded with a characteristic bull’s-eye pattern of red, white, and blue circles. The sore is accompanied by flulike symptoms, weight loss, and extreme fatigue. If treatment is not sought, the flesh at the site becomes gangrenous, and surgery is necessary.

Almost all newborn puppies carry
roundworms, and because children love to cuddle puppies, the children often pick up the worms without knowing it. Symptoms in humans are a cough, fever, headache, and poor appetite. Treatment for both humans and puppies is with anthelminthic (worm-destroying) drugs. It is important that all puppies be seen by a veterinarian when very young.

Sporotrichosis is a fungal infection transmitted by cats. The fungus is found as mold on decaying vegetation, soil, and timber, usually found along southern US waterways and in places with similar climates. Left untreated, the fungus can spread throughout the human body. The organism enters the body through a cut or abrasion in the skin or through inhalation of the fungus. Therefore, it is a good idea to wash one’s hands after handling cats.

Protozoan toxoplasmas are found in undercooked meat, on unwashed raw fruits, and in the feces of cats, and cause a condition called toxoplasmosis. The toxoplasmas may enter the human body through the skin or by respiration. Extreme care should be taken, especially by pregnant women, when disposing of used cat litter. In pregnant women, the disease invades fetal tissues, causing damage to the baby’s central nervous system. The antibiotic spiramycin is effective in combating the disease’s effects. Toxoplasmosis also poses a threat to AIDS patients as a major cause of encephalitis (inflammation of the brain or brain’s lining). Experiments have been conducted with the drug clindamycin in treating toxoplasmic encephalitis in AIDS or HIV-positive patients; however, such side effects as diarrhea and rashes have resulted.

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