Did you know that more than half of all infectious diseases in humans come from animals? Scientists estimate that animals are responsible for spreading over six out of every ten known infectious diseases! Many of these diseases are deadly and can lead to widespread outbreaks, just like the recent COVID-19 pandemic. When harmful germs jump from animals to humans, it’s called zoonosis, or a zoonotic disease. So, while animals can be cute, friendly, or even tasty, it’s important to be cautious if they’re infected! Read on to discover ten horrible zoonotic diseases and how you might catch them.
Toxoplasmosis
Toxoplasmosis is a parasitic infection caused by a microorganism called Toxoplasma gondii, or T. gondii. This parasite is found almost everywhere—in the air, on land, in the sea, and in most warm-blooded animals. However, its main host is the house cat. The microorganism reproduces sexually inside the cat’s intestines. It’s also known as the “mind control” disease because rodents infected with the parasite behave abnormally, making them easier prey for cats. T. gondii can even increase risky behaviors in humans.
People can get Toxoplasmosis by coming into contact with infected cat feces or by eating contaminated raw vegetables or undercooked meat. T. gondii usually enters our bodies as an egg and hides in the brain, heart, or skeletal muscle tissue. When it grows, it becomes an active parasite called a tachyzoite. A tachyzoite can multiply quickly and attack body tissues with limited immune defenses.
Toxoplasmosis affects up to 2 billion people worldwide. Most people don’t notice any symptoms and don’t need treatment. However, up to 20% will experience mild symptoms like swollen lymph nodes or flu-like aches. In rare cases, severe infections can lead to:
- Vision loss
- Brain damage
- Paralysis
- Schizophrenia
- Inflamed heart tissues
If brain damage continues without treatment, especially in people with weak immune systems, Toxoplasmosis can be fatal.
Salmonellosis
Salmonellosis is a bacterial infection caused by Salmonella bacteria. You can find Salmonella in domestic and wild animals, including pigs, cattle, cats, dogs, poultry, birds, reptiles, and amphibians. Infected animals often don’t show signs of the disease, so make sure to wash your hands after handling them. For example, if you have a pet turtle, it can have Salmonella on its body and anywhere in its tank.
Salmonella on your hands can lead to infection or spread to other people and surfaces. Salmonella is pretty tough and can survive throughout the entire food chain. Humans commonly get the bacteria from eating contaminated foods like eggs, meat, and milk. Cook your meals thoroughly!
Although Salmonella isn’t as deadly as the other diseases on this list, it’s a major global cause of diarrheal diseases. It can also cause serious symptoms for people with weakened immune systems, including children and the elderly. For these people, the resulting dehydration can be life-threatening. Other symptoms of Salmonella include:
- Abdominal pain
- Vomiting
- Diarrhea
- Turning bones into fluid
Anthrax (Bacillus Anthracis)
Anthrax is caused by a spore-forming bacteria commonly found in herbivore mammals like pigs, cattle, sheep, camels, antelopes, and goats. The bacteria spores can survive for long periods and spread easily in the air. In our bodies, they become “activated”. Anthrax symptoms include fevers, blisters, swollen lymph nodes, and fluid buildup in the lungs.
The mortality rate of anthrax depends on how you’re exposed. You can get anthrax by:
- Inhaling spores
- Eating undercooked meat from infected animals
- Handling products like wool and hides from infected animals
The deadliest form is through inhalation, with a mortality rate of 80% or higher, even with treatment. Gastrointestinal anthrax from undercooked meat has a mortality rate of 25% to 75%. Cutaneous infection from handling products is about 20%. Cutaneous anthrax is the most common and easiest to cure with antibiotics.
Anthrax isn’t contagious from human to human contact. However, it can be used as a bioterrorism weapon. For example, in 2001, an anthrax attack occurred when terrorists mailed letters containing the bacteria spores, infecting seventeen people and killing five.
Rabies
Rabies is a terrible viral disease that affects all warm-blooded mammals, including humans. An infected animal carries the rabies virus in its saliva and brain tissue. They usually spread the virus to humans through bites or by contact with open cuts or wounds. Bat and dog bites are common sources of humans catching rabies, but skunk, fox, and raccoon bite cases also exist.
While a series of shots, including vaccines, can cure rabies, it’s often fatal if treatment isn’t given soon after exposure. According to the CDC, approximately 59,000 deaths are caused by rabies worldwide each year. Early symptoms include fevers and headaches, but the disease quickly progresses to a neurological illness, leading to paralysis, confusion, difficulty swallowing, and agitation before death. Rabies can also cause unexpected symptoms like hydrophobia, the fear of water.
Protect your pets from catching rabies by getting them their proper shots from the vet!
Zoonotic Influenza
The most prevalent zoonotic influenza is the avian influenza viruses, particularly the A(H5) and A(H7N9) viruses, also known as bird flu. You can contract avian influenza viruses from poultry like chickens, turkeys, and ducks. This virus can transfer to humans directly or indirectly from exposure to infected live or dead poultry and contaminated environments such as bird markets. Eating raw dishes with infected poultry blood can also lead to infection. The swine influenza viruses are another type that affects humans, contracted by being close to infected pigs or their living pens.
While the type of virus determines the incubation period and severity of symptoms, most influenza cases start with respiratory symptoms like sore throats, fevers, and coughs. For the swine influenza viruses, symptoms are usually mild and rarely reported. The most aggressive symptoms come from the avian influenza viruses, which also have the highest fatality rate among similar viruses.
For example, the H5N1 virus has a mortality rate of about 60% in humans and can cause symptoms like diarrhea, vomiting, internal bleeding, chest pain, and complications like:
- Multi-organ dysfunction
- Pneumonia
- Hypoxemic respiratory failure
- Septic shock
Arbovirus
Arbovirus refers to arthropod-borne viruses, such as dengue fever, Zika virus, and West Nile virus. Mosquito bites are a common transmitter of arboviruses, with different arthropods carrying different arboviruses. For example, dengue arises from the bite of an infected Aedes aegypti mosquito. The West Nile virus spreads from mosquitoes that caught the virus after feeding on infected birds or horses.
Every arbovirus presents different symptoms and mortality rates. The Centers for Disease Control and Prevention (CDC) reports around 400 million people contract dengue fever annually, and 22,000 die as a result. Severe cases of dengue fever will have symptoms including:
- Vomiting blood
- Continuous vomiting
- Blood in your bowel movements
- Bleeding from the gums or nose
- Feeling tired or restless
For West Nile virus, most infected people don’t get sick or have only mild symptoms. Severe reactions, however, affect the nervous system and can lead to paralysis, meningitis, encephalitis, or death. Meningitis and encephalitis are both types of inflammations in the brain that can lead to death.
Malaria
Although also transmitted by mosquitoes, Malaria is not a virus, but a disease caused by the Plasmodium parasite carried by the Anopheles mosquito. Mosquitos use saliva to prevent blood clots when they bite humans, and the parasite is transmitted into our bloodstream through this saliva. Once in our bodies, the parasites multiply in the liver, then infect and destroy red blood cells.
According to the World Health Organization (WHO), about 229 million people globally contracted Malaria in 2019, and approximately 409,000 died, with children under five years old accounting for 67% of deaths.
Depending on the strain of the parasite, Malaria symptoms can be mild or severe. Mild symptoms are more flu-like and include fevers, headaches, muscle aches, vomiting, diarrhea, and tiredness. More severe cases include:
- Impaired consciousness
- Convulsions
- Respiratory distress
- Abnormal bleeding
With severe loss of red blood cells, Malaria can also cause anemia, vital organ dysfunction, and jaundice, which is the yellow coloring of the eyes and skin.
Ebola Virus Disease (EVD)
Ebola virus disease (EVD), formerly known as Ebola hemorrhagic fever, can be transmitted to humans by wild animals. You can get infected if you contact an infected animal’s bodily fluids, especially those found ill or dead. Examples of animals that can infect humans include:
- Porcupines
- Antelopes
- Fruit bats of the Pteropodidae family
- Monkeys
- Chimpanzees
- Gorillas
Once in the human population, EVD becomes even more contagious. Human mortality rates have varied between 25% and 90% with each outbreak. It’s common for healthcare workers to become infected after treating ill patients because direct contact with the following can lead to infection:
- Blood or body fluids of someone who is sick or has died from EVD
- Contaminated objects with EVD-infected body fluids
The symptoms can be sudden, including headaches, fatigue, fevers, muscle pain, and sore throats. Then it can lead to rashes, vomiting, diarrhea, or more severe symptoms such as:
- Impaired kidney and liver function
- Internal and external bleeding
- Low white blood cells
HIV and AIDS
The human immunodeficiency virus (HIV) has been a global health issue since scientists first identified it in the 1980s. At the end of 2019, the WHO estimated that 38 million people globally were living with HIV. The virus targets the immune system and weakens our body’s defenses against infections. Acquired immunodeficiency syndrome (AIDS) is the final stage of HIV infection and involves developing certain cancers.
HIV is a type of lentivirus similar to the Simian Immunodeficiency Virus (SIV) that attacks monkeys’ and apes’ immune systems. Scientists have found that HIV strains are closely related to a strain found in chimpanzees and a strain found in sooty mangabeys. The most common theory of how the SIV strain became HIV in humans is that hunters contracted the virus from eating chimpanzees or monkeys as “bushmeat” or getting infected blood into wounds. The SIV, once in the human body, then adapted to become HIV.
Initial symptoms of HIV in humans include weight loss, fever, diarrhea, and swollen lymph nodes. As the virus weakens the immune system, it can also develop into severe illnesses such as:
- Bacterial infections
- Cryptococcal meningitis
- Tuberculosis (TB)
- Cancers such as Kaposi’s sarcoma and lymphomas
There are treatment regimens that can suppress HIV and allow infected people’s immune systems to recover towards fighting off infections. Currently, lifelong antiretroviral therapy (ART) helps prevent transmission of the virus as the search for an HIV cure continues.
Novel Coronavirus (COVID-19)
The novel coronavirus called SARS-CoV-2 caused the COVID-19 disease that turned pandemic in 2020. While genetic analysis of the virus suggests it originated in bats, officials in China suspect the initial outbreak in Wuhan links to a seafood market. Two theories speculate that the pangolins, or snakes, the market sold could be the intermediate animal that transferred the virus from bats to humans.
The symptoms vary depending on the strain and who is infected. It can range from mild symptoms of sore throat, headaches, diarrhea, and skin rashes to more severe indicators:
- Loss of taste or smell
- Persistent chest pain
- Sleep disorders
- Neurological complications
Since research on disease origins takes time, we don’t know exactly how the virus started to infect humans. We do know it has had deadly effects and is extremely contagious. At the beginning of April 2021, there had been over 132.4 million cases of people infected globally and over 2.87 million deaths from COVID-19.
From toxoplasmosis carried by cats to the recent COVID-19 pandemic, zoonotic diseases pose a significant threat to human health. These diseases remind us of the interconnectedness between humans and animals and the importance of understanding and preventing the spread of these illnesses. As we continue to interact with animals, staying informed about these risks and taking preventive measures are crucial for protecting ourselves and future generations.
What steps do you take to protect yourself from zoonotic diseases? Share your thoughts and experiences in the comments below!
