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Did you know that more than half of all infectious diseases affecting humans are transmitted by animals? Experts estimate that animals are behind the spread of over 60% of these diseases! Many of these diseases are deadly and have led to global outbreaks, such as the COVID-19 pandemic. When harmful germs in animals spread to humans, it's called zoonosis or a zoonotic disease. While animals can be cute, friendly, or even delicious, be careful if they're carrying an infection! Read on to discover ten dreadful zoonotic diseases and how they can be transmitted.
10. Toxoplasmosis
Toxoplasmosis is a parasitic infection caused by the organism Toxoplasma gondii, commonly referred to as T. gondii. This parasite is found everywhere—from the air and land to the sea, and in nearly all warm-blooded animals. However, its primary host is the domestic cat. The parasite reproduces sexually within the cat’s intestines, which isn’t exactly a romantic setting. Often dubbed the ‘mind control’ disease, T. gondii alters the behavior of infected rodents, making them more likely to fall prey to cats. It can also increase risky behaviors in humans.
Humans can become infected with Toxoplasmosis through contact with infected cat feces or by consuming raw or undercooked meat and contaminated vegetables. The parasite usually enters the body in the form of an egg, hiding in the brain, heart, or skeletal muscle. Once inside, it develops into an active parasite called a tachyzoite. These tachyzoites multiply rapidly and can spread, targeting tissues that have weak immune defenses.
Toxoplasmosis affects nearly 2 billion people worldwide. For most individuals, there are no obvious symptoms, and treatment isn't necessary. However, up to 20% of those infected may experience mild symptoms like swollen lymph nodes or flu-like discomfort. In rare instances, severe infections can cause:
If brain damage from Toxoplasmosis goes untreated, especially in those with weakened immune systems, it can be fatal.
9. Salmonellosis
Salmonellosis is a bacterial infection caused by Salmonella bacteria. Salmonella can be found in both domestic and wild animals, such as pigs, cattle, cats, dogs, poultry, birds, reptiles, and amphibians. Infected animals often show no symptoms, so it’s important to wash thoroughly after handling them. For example, a pet turtle may carry Salmonella on its skin and throughout its habitat.
Salmonella on your hands can lead to infection or spread to other people and surfaces. This bacteria is also remarkably tough and can survive throughout the food chain. People most often contract Salmonella by consuming contaminated food, like eggs, meat, and milk. Always ensure your meals are properly cooked!
While Salmonella may not be as deadly as some of the other diseases listed, it is a significant global contributor to diarrheal diseases. It can also lead to severe symptoms in individuals with weakened immune systems, including children and the elderly. In these cases, dehydration can become life-threatening. Other symptoms associated with Salmonella include:
8. Anthrax (Bacillus Anthracis)
Anthrax is caused by a spore-forming bacterium. It is commonly found in herbivorous mammals such as pigs, cattle, sheep, camels, antelopes, and goats. The bacteria's spores can survive for extended periods and are easily dispersed through the air. Once inside the body, these spores become activated. Symptoms of anthrax include fever, blisters, swollen lymph nodes, and fluid accumulation in the lungs.
The mortality rate for anthrax depends on how the infection is contracted. You can contract anthrax by:
The most deadly form is inhalation, with a survival rate of only 20% or less, even with treatment. Gastrointestinal anthrax caused by consuming undercooked meat has a mortality rate ranging from 25% to 75%. Cutaneous anthrax, resulting from contact with infected products, has a fatality rate of around 20%. This form is the most common and can be effectively treated with antibiotics.
Anthrax cannot be transmitted from person to person. However, it can be used as a bioterrorism weapon. A notable example occurred in 2001, when terrorists sent letters containing anthrax spores, infecting seventeen individuals and causing five deaths.
7. Rabies
Rabies is a deadly viral disease that impacts all warm-blooded animals, including humans. Infected animals carry the virus in their saliva and brain tissue, transmitting it to humans through bites. The virus can also be spread through contact with open wounds or cuts. Bites from bats and dogs are the most common sources of human rabies infection, though cases from skunks, foxes, and raccoons are also known.
Rabies can be deadly if not treated promptly, although a series of shots, including vaccines, can effectively cure it. The CDC reports that approximately 59,000 deaths worldwide occur due to rabies each year. Initial symptoms include fever and headache, and the disease quickly escalates to a neurological condition, causing paralysis, confusion, difficulty swallowing, and agitation before death. In some cases, rabies can also lead to rare symptoms, such as Hydrophobia, which is the fear of water.
Make sure your pets are protected from rabies by having them vaccinated by a veterinarian!
6. Zoonotic Influenza
Avian influenza viruses, notably the A(H5) and A(H7N9) strains, also known as bird flu, are the most common zoonotic influenza viruses. You can contract these viruses from poultry such as chickens, turkeys, and ducks. The virus spreads to humans either directly or indirectly through exposure to infected live or dead poultry, as well as contaminated environments like bird markets. Consuming undercooked dishes containing infected poultry blood can also lead to infection. Another form of zoonotic influenza, swine influenza, can be contracted by being in close proximity to infected pigs or their pens.
The severity and incubation period of influenza depend on the specific virus. Most influenza cases begin with respiratory symptoms like sore throat, fever, and cough. Symptoms of swine influenza are generally mild and rarely reported, while the more aggressive avian influenza viruses present with severe symptoms and have the highest fatality rate among similar viruses.
The H5N1 virus, for instance, has a human mortality rate of approximately 60%. It leads to symptoms such as diarrhea, vomiting, internal bleeding, chest pain, and other complications, including:
5. Arboviruses
Arboviruses are viruses transmitted by arthropods, including those responsible for diseases like dengue fever, Zika virus, and West Nile virus. These viruses are commonly spread through mosquito bites, although different arthropods carry different types of arboviruses. For example, the Aedes aegypti mosquito transmits dengue, while mosquitoes carrying the West Nile virus often become infected after feeding on birds or horses.
Each arbovirus causes its own set of symptoms and has varying mortality rates. According to the Centers for Disease Control and Prevention (CDC), approximately 400 million people worldwide contract dengue fever every year, with 22,000 deaths resulting from the disease. Severe cases of dengue can lead to symptoms like:
While most people infected with West Nile virus remain asymptomatic or experience only mild symptoms, severe cases can impact the nervous system, potentially resulting in paralysis, meningitis, encephalitis, or even death. Both meningitis and encephalitis are brain inflammations that can be fatal.
4. Malaria
Malaria, although transmitted by mosquitoes, is not caused by a virus. Instead, it is the result of an infection by the Plasmodium parasite. The Anopheles mosquito can carry the parasite responsible for Malaria. When these mosquitoes bite humans, they inject saliva into the bloodstream, which contains the parasite. The parasite then travels to the liver where it multiplies and eventually attacks red blood cells.
The World Health Organization (WHO) reported that in 2019, approximately 229 million people worldwide contracted Malaria. That year also saw around 409,000 fatalities, with 67% of these deaths occurring among children under five years of age.
Malaria symptoms can range from mild to severe, depending on the parasite strain. Mild symptoms are similar to those of the flu and include fever, headaches, muscle pain, nausea, diarrhea, and fatigue. In more severe cases, the symptoms can worsen and include the following:
Severe loss of red blood cells due to Malaria can lead to anemia, dysfunction of essential organs, and jaundice. Jaundice is characterized by a yellow tint to the skin and eyes.
3. Novel Coronavirus (COVID-19)
The novel coronavirus SARS-CoV-2, which caused the COVID-19 pandemic in 2020, is believed to have originated in bats, according to genetic studies. However, Chinese authorities suspect that the initial outbreak in Wuhan may have been linked to a seafood market. Two leading theories propose that either pangolins or snakes, sold at the market, could have acted as intermediate hosts, transferring the virus from bats to humans.
The symptoms vary depending on the type of strain and the individual affected. They can range from mild symptoms such as sore throat, headaches, diarrhea, and skin rashes to more severe manifestations, including:
Since researching the origins of diseases is a lengthy process, we still do not know the exact way the virus began infecting humans. What we do know is that it has been highly deadly and spreads very easily. By early April 2021, over 132.4 million people worldwide had been infected, and more than 2.87 million deaths had been attributed to COVID-19.
2. HIV and AIDS
The human immunodeficiency virus (HIV) has posed a significant global health threat since it was first identified in the 1980s. By the end of 2019, the World Health Organization (WHO) estimated that 38 million people worldwide were living with HIV. The virus attacks the immune system, weakening the body's ability to fight off infections. AIDS (Acquired Immunodeficiency Syndrome) is the final stage of HIV infection and is marked by the development of certain types of cancer.
HIV belongs to a group of viruses known as lentiviruses, which are similar to the Simian Immunodeficiency Virus (SIV) that affects monkeys and apes. Studies show that HIV strains are closely related to those found in chimpanzees and sooty mangabeys. The most widely accepted theory is that humans contracted the virus by consuming monkey or chimpanzee meat, known as bushmeat, or by coming into contact with infected blood through cuts or wounds. Once inside the human body, SIV adapted and evolved into HIV.
The early signs of HIV in humans often include weight loss, fever, diarrhea, and swollen lymph nodes. As the virus weakens the immune system, it can lead to more serious conditions, such as:
There are treatment protocols available that can suppress HIV and enable the immune systems of affected individuals to recover, helping them combat other infections. Currently, ongoing antiretroviral therapy (ART) continues to be used as a lifelong treatment to prevent the virus from spreading, while the search for a definitive cure for HIV persists.
1. Ebola Virus Disease (EVD)
Ebola Virus Disease (EVD), previously known as Ebola hemorrhagic fever, is a deadly illness transmitted to humans by wild animals. Infection occurs when humans come into contact with the bodily fluids of infected animals, particularly those that are sick or deceased. Some animals that are capable of transmitting the virus to humans include:
Once Ebola Virus Disease (EVD) reaches the human population, it becomes significantly more contagious. Human mortality rates have ranged from 25% to 90% in various outbreaks. Healthcare workers are often at high risk of infection after treating sick individuals. Direct contact with the following can result in infection:
Symptoms of EVD can appear suddenly and may include headaches, fatigue, fever, muscle pain, and sore throat. As the disease progresses, additional symptoms such as rashes, vomiting, diarrhea, and more severe signs may develop, including:
