Zoonotic diseases

Definition

Domestic and wild animals and their ecosystems play an important role in human health and well-being. Animals provide protein-rich nutrients, transportation, fuel, recreation, and companionship. However, along with the many benefits people derive from animals come health risks. The human-animal-ecosystem interface can be described as a continuum of direct or indirect human exposure to animals, their products, and their ecosystems.

Zoonotic diseases, also known as zoonoses, are diseases caused by infectious agents (viruses, bacteria, and parasites such as worms and protozoa) transmitted or shared by animals and humans. These diseases are caused by a diverse group of pathogenic microorganisms that ordinarily live among animals. Some zoonotic diseases are transmitted directly from animals to humans, some result from contamination of the environment by animals, and some require a vector, such as a tick or mosquito.

Viral Infections

Arthropod-borne viruses, or arboviruses, include any of a large group of ribonucleic acid (RNA) viruses that are transmitted primarily by arthropods. There are more than four hundred species of arboviruses. Zoonotic diseases caused by these viruses include the following:

Encephalitis. Encephalitis is inflammation of the brain caused by infection. All arboviral encephalitides are zoonotic. They are maintained in complex life cycles involving a nonhuman primary vertebrate host and a primary arthropod vector. Many arboviruses that cause encephalitis have a variety of vertebrate hosts, and some are transmitted by more than one vector.

Four main flavivirus agents of encephalitis exist in the United States—eastern equine encephalitis, western equine encephalitis, Saint Louis encephalitis, and La Crosse encephalitis—all of which are transmitted by mosquitoes. Most human infections are asymptomatic or result in nonspecific flu-like symptoms. In some infected persons, infection may progress to full-blown encephalitis, with permanent neurologic damage or even death. Because the arboviral encephalitides are viral diseases, antibiotics are not effective for treatment, and no effective antiviral drugs have been developed. There are no commercially available human vaccines for these diseases, so treatment is supportive.

West Nile virus. West Nile virus (WNV) is a flavivirus commonly found in Africa, West Asia, the Middle East, and the United States. The virus can infect humans, birds, mosquitoes, horses, and other mammals. WNV was first diagnosed in the United States in 1999.

Most human infections are asymptomatic or result in a nonspecific flu-like syndrome. Approximately 80 percent of people who are infected with WNV will not show any symptoms. Up to 20 percent of infected people develop swollen lymph glands or a skin rash on the chest, stomach, and back. About 1 in 150 infected persons will develop severe illness, with symptoms that can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, and muscle weakness.

There is no specific treatment for WNV infection. In milder cases, symptoms resolve on their own. In more severe cases, infected persons should seek supportive treatment in a hospital.

Hantavirus pulmonary syndrome. Hantavirus pulmonary syndrome (HPS) is contracted from rodents and has been identified throughout the United States. Rodent infestation in and around the home remains the primary risk factor for hantavirus exposure. In the United States, deer mice, cotton rats, rice rats, and white-footed mice carry hantaviruses that cause HPS.

Although rare, HPS is potentially deadly. Humans can contract the disease when they come into contact with infected rodents or their urine and droppings or when they breathe in the hantavirus from the air. HPS cannot be transmitted from one person to another.

Early symptoms include fatigue, fever, muscle aches, headaches, dizziness, chills, and abdominal problems. Four to ten days later, additional symptoms appear. These symptoms include coughing and shortness of breath, as the lungs fill with fluid. There is no specific treatment or vaccine for hantavirus infection. Supportive care is the basis for therapy.

Lymphocytic choriomeningitis. Lymphocytic choriomeningitis (LCM) is a rodent-borne viral disease that appears as aseptic meningitis (inflammation of the membrane surrounding the brain and spinal cord), encephalitis, or meningoencephalitis (inflammation of the brain and meninges). LCM is caused by the lymphocytic choriomeningitis virus (LCMV). The common house mouse, Mus musculus, is the primary host. The virus is found in the saliva, urine, and feces of infected mice, and people become infected when exposed to these substances. Other types of rodents, such as hamsters, can become infected with LCMV in pet stores.

Initial LCM symptoms include fever, malaise, lack of appetite, muscle aches, headache, nausea, and vomiting. In the second phase of the infection, persons have symptoms of meningitis (fever, headache, and stiff neck) or characteristics of encephalitis (drowsiness, confusion, and sensory disturbances), or have symptoms such as motor abnormalities (for example, paralysis). LCM is usually not fatal. Aseptic meningitis, encephalitis, or meningoencephalitis require hospitalization. Anti-inflammatory drugs, such as corticosteroids, may be helpful in treating the disease.

Mpox. Mpox, formerly known as monkeypox until it was renamed in 2022, is a rare viral disease that usually occurs in central and western Africa. It is caused by the mpox virus, first found in 1958 in laboratory monkeys. In June 2003, several people in the United States contracted mpox after having contact with pet prairie dogs that were sick with the disease. The disease was traced to a shipment of Gambian rats that were imported to the United States and later kept near prairie dogs at an Illinois animal vendor. Again in 2022, cases of mpox were identified in the United States and Singapore, non-endemic countries. At its peak in August of that year, 631 cases in the US were recorded by the Centers for Disease Control and Prevention (CDC). In total in the US, 30,225 cases were reported and 38 people died, and globally, 86,231 cases were recorded by March 2023.

However, by May 2023, the outbreak had been contained enough for the World Health Organization (WHO) to declare that the mpox outbreak was no longer a Public Health Emergency of International Concern (PHEIC). Still, the disease continued to circulate, and by the end of June 2024 the outbreak had resulted in 97,786 confirmed cases and 207 deaths across 116 countries.

People can get mpox if they are bitten by an infected animal or if they touch the animal’s blood or body fluids. The disease may also spread from person to person. In 2023, the CDC released new data showing the ability of the virus to be transmitted between people up to four days before symptoms of the virus appear. After infection, symptoms include fever, headache, muscle aches, backache, and swollen lymph nodes. A few days later, symptoms include a skin rash that develops into raised bumps filled with fluid; these bumps will eventually fall off the skin. The illness usually lasts two to four weeks. There is no specific treatment for mpox, but by March 7, 2023, 1.2 million vaccines had been administered in the US.

Rabies. Rabies is a viral disease of mammals transmitted through the bite of a rabid animal. Transmission is through the virus-containing saliva of an infected host. Most rabies cases occur in wild animals such as raccoons, skunks, bats, and foxes.

The rabies virus infects the central nervous system of humans, ultimately involving the brain and leading to death. Early symptoms include fever, headache, and general weakness. As the disease progresses, more specific symptoms appear, including insomnia, anxiety, confusion, partial paralysis, hallucinations, hypersalivation (increased saliva), difficulty swallowing, and hydrophobia (fear of water). Once clinical signs of rabies appear, the disease is nearly always fatal, and treatment is mainly supportive.

Several tests are required for the diagnosis of rabies. Thorough wound cleansing has been shown to markedly reduce the likelihood of contracting rabies. A tetanus shot should be given if the infected person has not received one within the previous ten years. A doctor will determine if antibiotics should be used. Persons not previously vaccinated should receive a postexposure vaccination against rabies that includes administration of both passive antibody and vaccine.

Bacterial Infections

Zoonotic diseases caused by bacterial infections include the following:

Anthrax. Anthrax is an acute infectious disease caused by the bacterium Bacillus anthracis. It most commonly occurs in wild and domestic mammals such as cattle, sheep, and goats, but it also can occur in humans who are exposed to infected animals or to tissue from infected animals. B. anthracis spores can survive in the soil for many years. Humans can become infected by handling products from infected animals or by inhaling anthrax spores in contaminated animal products. Anthrax can also be contracted by eating undercooked meat from infected animals.

Anthrax infections can be of three types: cutaneous (skin), inhalation, and gastrointestinal. Most cutaneous infections occur when the bacterium enters a cut or abrasion on the skin. About 20 percent of untreated cases of cutaneous anthrax result in death, but death is rare with antimicrobial therapy. The first symptoms of inhalation infection resemble a common cold, but after several days, the symptoms may progress to severe breathing problems. Inhalation anthrax is usually fatal. The gastrointestinal form of anthrax follows the eating of contaminated meat and is followed by an acute inflammation of the intestinal tract. Intestinal anthrax results in death in 25 to 60 percent of cases of infection. Antibiotics are used to treat all three types of anthrax. Early identification and treatment are critical.

Lyme disease. Lyme disease is caused by the bacterium Borrelia burgdorferi and is transmitted to humans by the bite of infected blacklegged ticks. The Lyme disease bacterium lives in deer, mice, squirrels, and other small animals, and ticks become infected by feeding on these animals. In the northeastern and north-central United States, Lyme disease is transmitted by the deer tick Ixodes scapularis. In the Pacific Northwest, the disease is spread by the Western blacklegged tick (I. pacificus).

In approximately 70 to 80 percent of infected persons, the first sign of infection is usually a circular rash that appears three to thirty days after the tick bite. This “bull’s eye” rash gradually expands in several days, reaching up to twelve inches in diameter. Other early symptoms include fever, chills, headache, fatigue, swollen lymph nodes, and joint and muscle aches. If Lyme disease is left untreated, it can spread to other parts of the body. Symptoms of late-stage Lyme disease include painful, swollen joints; severe headaches and neck stiffness from meningitis; and nervous system problems, such as impaired concentration and memory loss.

Several laboratory tests for Lyme disease are available to measure antibodies to the infection. These tests may return false-negative results in persons with early disease, but they are reliable for diagnosing later stages of disease. Most cases of Lyme disease can be treated and cured with antibiotics.

Plague. Plague is an infectious disease of animals and humans caused by the bacterium Yersinia pestis. It is transmitted from animal to animal and from animal to human by the bites of infected fleas. Humans usually contract plague from being bitten by a rodent flea that is carrying the plague bacterium or by handling an infected animal. Plague is also transmitted by inhaling infected droplets expelled by the coughing of an infected person or animal, especially domestic cats, which may become infected by eating infected wild rodents. Fleas become infected by feeding on rodents, such as chipmunks, prairie dogs, ground squirrels, mice, and other mammals that are infected with the bacterium. Fleas transmit the plague bacterium to humans and other mammals during the feeding process.

The characteristic sign of plague is a very painful, swollen lymph node called a bubo. This sign, accompanied with fever, extreme exhaustion, headache, and a history of possible exposure to rodent fleas, should lead to suspicion of plague. The disease progresses rapidly; the bacteria can then invade the bloodstream and produce a severe form of the illness called septicemic plague, as well as lung infection. Once a human is infected, a progressive and potentially fatal illness generally results unless specific antibiotic therapy is given. The plague vaccine is no longer commercially available in the United States.

Rocky Mountain spotted fever. Rocky Mountain spotted fever (RMSF) is the most severe tickborne illness in the United States. It is caused by infection with the bacterial organism Rickettsia rickettsii, which is transmitted by the bite of an infected tick. The American dog tick (Dermacentor variabilis) and Rocky Mountain wood tick (D. andersoni) are the primary arthropod vectors in the United States.

The early symptoms of RMSF are often nonspecific. Initial symptoms may include severe headache, lack of appetite, muscle pain, nausea, and fever. Later signs and symptoms include diarrhea, joint pain, abdominal pain, and rash. RMSF can be a severe illness, and the majority of infected persons are hospitalized. Diagnosis is based on a combination of clinical signs and symptoms and laboratory tests. It is best treated using a tetracycline antibiotic, usually doxycycline.

Salmonellosis. Salmonellosis is an infection with the bacterium Salmonella, which lives in the intestinal tracts of humans, animals, and birds. Salmonella is usually transmitted to humans through foods contaminated with animal feces. Contaminated foods are usually of animal origin and include beef, poultry, milk, and eggs, but any food, including vegetables, may become contaminated. Salmonella is killed by thorough cooking. Salmonella may also be found in the feces of some pets. Reptiles, such as turtles, lizards, and snakes, and chicks and young birds, are particularly likely to carry Salmonella in their feces. People should always wash their hands immediately after handling one of these animals, even if it appears healthy. Most persons infected with Salmonella develop diarrhea, fever, and abdominal cramps twelve to seventy-two hours after infection. The illness usually lasts four to seven days, and most persons recover without treatment.

Parasitic Infections

Zoonotic diseases caused by parasitic infections include the following:

Cryptosporidiosis. Cryptosporidiosis is a disease caused by parasites of the genus Cryptosporidium. Both the disease and the parasite are known as crypto. Many species of crypto infect humans and a wide range of animals. The parasite is protected by an outer shell that allows it to survive outside the body for long periods of time. Crypto is one of the most frequent causes of waterborne disease among humans in the United States and throughout the world. Crypto lives in the intestines of infected humans or animals. An infected person or animal passes the parasites in the stool. Crypto is found in soil, food, water, or surfaces that have been contaminated with the feces from infected humans or animals. Some people with crypto have no symptoms, but the most common symptom is watery diarrhea. Other symptoms include stomach cramps, dehydration, diarrhea, nausea, fever, or weight loss. Diagnosis is made by examination of stool samples. Most people who have healthy immune systems will recover without treatment.

Cysticercosis. Cysticercosis is an infection caused by the pork tapeworm Taenia solium. Infection occurs when the tapeworm larvae enter the body and form cysticerci (cysts). When cysticerci are found in the brain, the condition is called neurocysticercosis. The tapeworm that causes cysticercosis is most often found in rural, developing countries where pigs are allowed to roam freely and eat human feces.

When pigs swallow pork tapeworm eggs, the eggs are passed through the bowel movement. The eggs are subsequently spread by people who ingest contaminated food or water. Once inside the stomach, the tapeworm egg hatches, penetrates the intestine, travels through the bloodstream, and may develop into larvae in the muscles, brain, or eyes. Although rare, larvae may float in the eye and cause swelling or detachment of the retina. Symptoms of neurocysticercosis can include seizures, headaches, confusion, lack of attention, or difficulty with balance. Death can occur suddenly with heavy infections.

Diagnosis is usually made by magnetic resonance imaging (MRI) or computed tomography (CT) brain scans. Infections are generally treated with antiparasitic drugs in combination with anti-inflammatory drugs.

Trichinellosis. Trichinellosis, also called trichinosis, is caused by eating the raw or undercooked meat of animals infected with the larvae of a species of worm called Trichinella. Infection occurs commonly in domestic pigs. When an animal eats meat that contains Trichinella cysts (larvae), their stomach acid dissolves the hard covering of the cyst and releases the worms. The worms migrate into the small intestine and mature in one to two days. After mating, adult females lay eggs that develop into immature worms and travel through the arteries to muscles. Inside the muscles, the worms curl into a ball and become enclosed in a capsule. Infection in humans occurs when these capsules are consumed in undercooked meat.

The first symptoms of trichinellosis include nausea, diarrhea, vomiting, fatigue, fever, and abdominal discomfort. These first symptoms are later followed by headaches, fevers, chills, cough, eye swelling, aching joints, muscle pains, itchy skin, or diarrhea. In severe cases, death can occur. For mild to moderate infections, most symptoms subside within a few months, although fatigue, weakness, and diarrhea may last for months afterward. Several effective prescription drugs are available to treat trichinellosis.

Coronaviruses

Coronaviruses are a type of virus which can cause a wide range of illnesses with a wide range of severity. While there are several types in existence which infect humans and cause illness, three had a significant global impact between 2002 and 2023— SARS-CoV-2, which causes COVID-19; SARS-CoV, which causes severe acute respiratory syndrome (SARS); and MERS-CoV, which causes Middle East respiratory syndrome (MERS).

COVID-19. This virus, caused by the SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) pathogen, is suspected to have been transmitted to humans through bats, causing the global COVID-19 pandemic in 2019. Other animals which may have served as intermediate hosts between bats and humans include pangolins, raccoon dogs, cats, and minks, among others. Identified in late 2019 after an influx of reported cases of severe pneumonia in Wuhan, China, the virus spread to nearly every country in the world within one year, causing 6,866,434 deaths resulting from 759,408,703 cases by March 2023. Immunizations were developed at a revolutionary speed and approved for emergency use beginning in 2021. By 2023, four COVID-19 vaccine options were available in the United States— Moderna, Pfizer-BioNTech, Johnson & Johnson Janssen (J&J/Janssen), and Novavax. Despite these preventative measures and advancements in treatments, this virus was still a major global health concern throughout 2023 and 2024 due to periodic outbreaks and surges.

Severe Acute Respiratory Syndrome (SARS). This airborne virus was first identified in 2003 in China as the first severe and highly transmissible disease of the twenty-first century. Incubation is typically 2 to 7 days and causes hospitalization in approximately 10 to 20 percent of cases. While symptoms are typically respiratory in nature but may also include gastrointestinal issues and a rash.

Middle East Respiratory Syndrome (MERS). In 2012 Saudi Arabia, MERS emerged causing fever, cough and shortness of breath in most infected people, and in some, pneumonia and gastrointestinal issues may also be present. Likely transmitted from camels in the Middle East, 35 percent of the WHO reported cases were fatal.

Impact

Zoonotic diseases have the potential to spread efficiently across international boundaries, thereby affecting not only human health and well-being but also international travel and trade. More than 60 percent of the newly identified infectious agents that have affected people since the mid-twentieth century have been caused by pathogens originating from animals or animal products. Of these zoonotic infections, 70 percent originated in wildlife.

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