Ticks: the vampire of the parasites

Spring has sprung and unlocked the tick’s season. Learn more about this blood eater parasite and which diseases it can bring to you, your family and animals.

By Ricardo Ribas

Image from 272447, pixabay.com
Image from 272447, pixabay.com


Ticks are small external parasites that together with the spiders, mites, harvestmen and scorpions form the group of the arachnids. Arachnids are a type of invertebrated animals with their bodies divided into two main compartments: the cephalothorax, corresponding to the front part of the body (containing the head and thorax) and the abdomen. Adults ticks are usually 3-5mm long, ovoid or pear-shaped with eight legs that often become engorged as they feed on blood. Blood is essential for the ticks life-cycle and survival, hence their dependence on vertebrate animals, such as mammals, birds, reptiles, amphibians, as well as humans. But, as blood-eaters, ticks can cause considerable harm to their host either directly or indirectly by transmitting infectious agents. There are more than 800 species of ticks worldwide subdivided into two major groups, according to the presence or absence of a dorsal hard shield: the Ixodidae or hard ticks and the Argasidae also called soft ticks. Hard ticks are the most abundant in nature, accounting for more than 650 of the known species, whilst the soft ticks account for about 155 species. Fossil records suggest that ticks are very old parasites, having been around for over 90 million years.


Ticks are widely distributed around the world, but prefer humid and warm conditions. Spring is their favourite season coinciding with the time when people and animals spend longer periods outside. To help them locate blood, ticks possess senses that attract them to the host smells and body temperatures. Additionally, they often position themselves strategically, such as in tall grasses, which provides them in an ideal spot to crawl and ride into passing hosts. Once attached to the host, ticks can appear anywhere in the body but they often opt for warm and moist areas, such as the armpits, groin and scalp in humans and the head, ears, anus and between the digits in animals.


Ticks life-cycle is divided into four main stages: eggs, larva, nymph and adult. Both males and females attack together, but it is easy to distinguish because the females tend to increase their volume, as they feed in blood. Each adult female can produce between 2,000 and 20,000 eggs at one time, often laying them in areas of moist vegetation, where they have higher chances of meeting the hosts. Depending on the species, ticks may require one, two or three hosts.


Ticks can seriously harm and even kill their hosts, raising concerns for public health. As blood-eaters, they are often responsible for anaemias, skin inflammations and allergies, conditions particularly serious in livestock and accountable for serious repercussions in the wool and animal-source food industries. Some types of ticks can also produce neurotoxins that can cause paralysis or even lead to death of their host. Luckily, toxicity can be reversed by removing the tick. However this should be done carefully to avoid leaving traces of the tick’s mouth in the host’s skin.

Recent studies have also shown that some ticks are responsible for a type of food allergy to red meat, called Alpha-gal syndrome. When feeding on animal’s blood, particularly cattle and sheep, ticks ingest a sugar molecule called alpha-gal. This sugar is stored in the tick’s saliva and subsequently injected in the human’s bloodstream as they feed. In some people, this triggers a strong immune response that culminates with the development of allergies after consumption of red meat often accompanied by hives, itchy skin, swelling of the lips, tongue, throat and face, abdominal pain, diarrhoea and vomiting, wheezing and shortness of breath. Interestingly, alpha-gal syndrome sufferers do not require to become fully vegetarians, as fish and poultry meat consumption does not trigger the reaction. Unlike most food allergies, alpha-gal allergy can disappear over time.

However, ticks are also be involved in the transmission of infection diseases caused by bacteria, viruses and parasites. Tick-Borne Relapsing Fever and Lyme diseases are examples of bacterial infections transmitted by ticks. Tick-Borne Relapsing Fever is predominantly found in North America, Africa, Asia, Middle East and Spain causing fever and flu-like symptoms in humans, dogs, horses, cattle, deer, mice, chipmunks and racoons. Lyme disease is present mainly in North America and Euroasia. The majority of the animals infected by this disease rarely show symptoms, but dogs often develop fever, lameness and joint pain (arthritis), that in rare cases can develop to heart and kidneys complications and death. Human experience flu-like symptoms accompanied by a red bump in site of the bite that can spread into a larger circular red rash. In some cases, the disease can progress and spread to other organs causing joint pain as well as heart and brain complications. Antibiotics are effective, particularly if administered in the earlier stages.

Other examples of bacterial infections carried by ticks are Ehrlichiosis, Anaplasmosis and Rocky Mountain Spotted Fever, all caused by a Rickettsial-like bacteria, an obligate intracellular microorganism transmitted by arthropods. Ehrlichiosis and Anaplasmosis are globally distributed and responsible for attacking and destroying the host’s white and red blood cells, respectively. Rocky Mountain Spotted Fever is mainly present in the USA and once inside the host it attacks the blood vessel cells. All conditions affect humans and dogs causing fever, flu-like symptoms, diarrhoea, weight loss, anaemia and bleeding disorders such as presence of blood in the urine as well as the presence of erythema and small red rashes in the skin and the conjunctiva caused by broken blood vessels. Antibiotics are efficient, but if delayed, can lead to brain damage, respiratory failure and, in the case of the Rocky Mountain Spotted Fever, to hearing complications and the loss of parts of the limbs. All conditions could lead to death if not treated.

Finally, Tularemia or Rabbit fever is another example of a rare and highly contagious bacterial disease, particularly found in North America and Euroasia and characterised by the appearance of skin ulcers in humans, dogs, cats, rabbits and rodents. Antibiotics are efficient to prevent complications and death.

Nevertheless, ticks are not only vectors of bacteria. They can also transmit parasites such as the case of the Babesia, a single cell organism belonging to the group of protozoa. Babesiosis or Cattle Tick-Fever is a global distributed disease, affecting both humans as well as wild and domestic animals. Once inside the host, the protozoa attack and destroys the red blood cells and leading to flu-like symptoms, dark-colour urine and jaundice, a yellowing of the skin and mucous membranes due to a bile disorder. In severe cases, the parasite can travel to other parts of the body such as the kidneys, lungs and brain and cause incoordination and potential death. Despite the fact that a wide range of animals can be affected, babesiosis is particularly concerning in cattle and buffalo due to the economic impact imposed in cattle industry. Unfortunately, the disease appears also to blame for many wild life causalities over the years, such as the outbreak in Tanzania in early 2000s that killed many rhinos, lions, buffalos, wildebeest and zebras. Fortunately, antibiotics, quinine and vaccination are effective to treat and prevent the disease.

Finally, ticks can also transmit viruses such as the ones responsible for the Powassan disease in North America and the Tick-borne encephalitis in Euroasia, both affecting the brain and causing a variety of neurological symptoms in humans including mental confusion, loss of memory, seizures, difficulties in speech and lack of coordination. Tick-borne encephalitis can also affects carnivores, ruminants, horses, birds and rodents. Other examples of viral diseases include Colorado Tick Fever in the western parts of North America and the Crimean-Congo Haemorrhagic Fever, a disease endemic in Africa, Middle East and Asia. Whilst often asymptomatic in animals, CCHF can cause serious problems in humans leading to bleeding of the skin and mucosa and responsible for death in 40% of the cases, constituting a public health threat. Unfortunately, antibiotics are not efficient in the treatment of these virus diseases, so prevention is the preferable strategy.


The best way to avoid and control tick’s infestations is to implement prevention measures. Here are some tips:

• Use insect repellents both in humans and animals. A wide variety of sprays, powders, topical products, shampoos and collars for animals are available in the market.

• Try to cover up with clothes when going to grassy and bushy areas

• Whenever find a tick, remove it with gloves or tweezers by grasping it near the mouth to avoid leaving traces of the tick’s mouth on the host’s skin. After removal, burn the tick, disinfect the area and wash your hands in soap and water.

• If notice ticks in your garden, apply mite sprays.

• Inspect frequently for the presence of ticks on yourself, children and on the animals, especially in areas where ticks regularly appear.

• Vaccinate humans and animals against some of the tick-borne diseases, such as Lyme disease and babesiosis, which have an efficiency of 70 to 100 percent.

• People affected by alpha-gal syndrome should refrain from eating red meat.

By Ricardo Ribas, Veterinary Doctor, doctorate in veterinary sciences and researcher in the area of oncology in London

Reference Sources

1. Simon LV, West B, McKinney WP. Tick Paralysis. [Updated 2020 Apr 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan. 2020 Apr 28. PMID: 29262244

2 Mabelane T, Ogunbanjo GA. Ingestion of mammalian meat and alpha-gal allergy: Clinical relevance in primary care. Afr J Prm Health Care Fam Med. 2019;11(1), a1901. https://doi.org/10.4102/phcfm.v11i1.1901

3. Elelu, N. Tick-borne Relapsing Fever as a Potential Veterinary Medical Problem. Vet Med Sci. 2018 Nov;4(4):271-279. doi: 10.1002/vms3.108.

4. Chomel, B. Lyme Disease. Rev Sci Tech 2015 Aug;34(2):569-76.doi: 10.20506/rst.34.2.2380.

5. Ismail, N; Bloch, KC; McBride, JW. Human Ehrlichiosis and Anaplasmosis. Clin Lab Med. 2010 Mar;30(1):261-92. doi: 10.1016/j.cll.2009.10.004.

6. Nijhof, AM; Penzhorn, BL et al. Babesia bicornis sp. nov. and Theileria bicornis sp. nov.: Tick-Borne Parasites Associated with Mortality in the Black Rhinoceros (Diceros bicornis). J Clin Microbiol. 2003 May; 41(5): 2249–2254. doi: 10.1128/JCM.41.5.2249-2254.2003

7. Shi, J; Hu, Z et al. Tick-Borne Viruses. Virol Sin 2018 Feb;33(1):21-43. doi: 10.1007/s12250-018-0019-0.

8. Tick, Wikipedia, Wikipedia Foundation, Last update: 30 April 2020. https://en.m.wikipedia.org/wiki/Tick, Accessed May 2020

9. Tick-borne disease, Wikipedia, Wikipedia Foundation, Last update: 25 April 2020. https://en.m.wikipedia.org/wiki/Tick-borne_disease. Accessed May 2020

10. Tularemia. Wikipedia, Wikipedia Foundation, https://en.m.wikipedia.org/wiki/Tularemia. Accessed May 2020

11. Ticks, Centers for disease Control and Prevention, last reviewed 28 May 2020, https://www.cdc.gov/ticks/index.html, Accessed May 2020

12. Carter, PD & Rolls, P. Babesiosis. MSD Manual, Veterinary Manual. Last update: Feb 2015. https://www.msdvetmanual.com/circulatory-system/blood-parasites/babesiosis. Accessed: May 2020


The material, ideas and prevention measures discussed on this article are for informational purposes only. For more information consult a vet or a professional in the area. Whilst every effort is made to make sure the article is accurate at the time of publication, I take no liability for any new developments on the subject as well as any errors or omissions.

Insects that transmit deadly parasite…

Leishmaniasis is an infectious disease endemic in nearly 100 countries around East Africa, the Americas, Southeast Asia and the Mediterranean. It belongs to the list of neglected tropical diseases together with rabies and dengue, despite the fact that the World Health Organization estimates that over 1 million humans are infected each year, making it one of the most dangerous and deadly tropical parasitic diseases, after malaria. The agent is a single-cell parasite named Leishmania and is spread via the bite of infected female sandflies.

Learn more about one of the oldest and more mysterious diseases of mankind, responsible for thousands of deaths around the world yearly.

By Ricardo Ribas

Image from  272447, pixabay.com
Image from 272447, pixabay.com

Leishmaniasis is an endemic disease that affect humans and animals in tropical and subtropical regions around the world, particularly in countries affected by malnutrition, poor housing conditions and a lack of financial resources. Not every human infected develops symptoms, but children, the elderly and people with a weakened immune system, are the more vulnerable groups.


Leishmaniasis is caused by a microscopic single-cell parasite named Leishmania and is spread by the bite of infected female sandflies. There are over 90 species of sandflies around the world known to transmit the disease to animals and humans. Adult sandflies are small measuring around 3mm, yellow, with black eyes and hairy all over their body, wings and legs.

In nature, Leishmania can be found in two different forms according to the stage of their life-cycle and the host they infect. The promastigote form corresponds to the phase of the cycle when the parasite is living inside the sandfly, whilst the amastigote form is present in the vertebrate (humans or animals). In order to complete the life cycle, the leishmania needs to pass through both hosts, hence the transmission between vertebrates is only possible via the sandfly’s bite.


Leishmaniasis is transmitted to humans and animals through a bite of an infected female sandfly while feeding on blood required for their eggs production. After biting the animal, the sandfly ingests the infected blood containing the amastigote parasites allowing them to lodge in their intestine, multiply and convert into the promastigote form. When the sandfly bites the animal or the human again, it deposits the parasite back into their blood. The parasites penetrate the vertebrate’s defence cells, multiply and are transported to the various parts of the body causing damage to the different organs. Fortunately, not all individuals get sick and hosts with a good immune system are able to destroy the parasite and prevent damage.

Transmission is more likely to occur in the warmer months when there are an increased number of sandflies, and usually during dusk and dawn, when the insects are most likely to be looking for food. Once infected, it may take months or even years for the host to develop symptoms.


Over 70 species of animals can be natural reservoir for the Leishmania parasites. These include cats, goats, wolfs, rabbits, rodents and even birds, but dogs seem to be the main reservoir of the disease. Leishmaniasis can affect any breed of dog, yet middle-aged German Shepherds, Boxers and Dobermanns are more susceptible. Males and females are equally affected, but the disease is rare in animals with less than six months. The disease is slow and progressive, and can lead to a wide variety of symptoms such as: painless skin lesions and hair loss in the biting site; fever; lack of appetite; weight loss; diarrhoea and vomiting; swelling of the lymph nodes; conjunctivitis and blindness due to the presence of the parasite in the eyes; lameness and joint inflammation; enlargement of spleen and liver as well as serious kidney problems that can lead to the animal’s death. Cats can also develop the disease and suffer from symptoms similar to the dogs. Due to the presence of nonspecific symptoms, diagnosis can be challenging, often requiring specific blood tests. At times animals may show no symptoms, making it important to perform periodic screening tests particularly to animals inhabiting endemic regions.


Humans are generally more resistant to leishmaniasis infection than dogs, however immunocompromised individuals, as well as children and elderly people tend to be more susceptible. Direct transmission between animals and humans is not possible unless via the insect vector.

Humans can be affected with three forms of the disease:

1) Cutaneous is the most common form of the disease, characterised by the presence of skin lesions in the insect biting site, as well as the presence of painless ulcers leaving life-long scars and serious disability;

2) Visceral or Kala-azar is the most serious form of the disease affecting internal organs. If left untreated, it can be fatal in over 95% of the cases. Symptoms are similar to the ones observed in animals, including fever, weight loss, anaemia and enlargement of the spleen and liver. Most cases occur in Brazil, East Africa and in India.

3) Mucocutaneous: is a form of the disease that affects predominantly the mucous membranes and skin of the nose, mouth and throat. More than 90% of the cases occur in South America (Bolivia, Brazil, Peru) and Ethiopia.


Many factors affect the likelihood of contracting the disease. Poverty particularly in developing countries, where people live in overcrowded and under bad sanitary conditions, as well as certain human behaviours such as sleeping outside, increases the risk of contracting the disease, creating more opportunities for the insects to breed, to find resting sites and to have more access to human or animal blood. Malnutrition is also a very important risk factor since it compromises the host’s immune system and its capacity to fight the parasite.

Environmental, climate and urbanisational changes are also vital. For instance, human expansion into forested areas and subsequent deforestation affects insect’s numbers, distribution and survival, creating ideal opportunities for the spread of the disease outside the endemic regions.

Treatment of leishmaniasis is possible but involves long duration therapies generally administered into the vein. Its success is also dependent on multiple factors, such as the form of the disease, the type of leishmania as well as the capacity of the host’s immune system to get rid of the parasite. Is important to remember that if left untreated, leishmaniasis can kill.


Although several laboratories are trying to develop vaccines against the disease, these are still not effective and the reason why preventive measures are crucial. Here is some advice:

1. Early identification of the disease in humans and animals is crucial to prevent its spread and allow proper and quick treatments.

2. In endemic regions, avoid leaving pets outside the house, particularly at dawn and dusk, since insects are more likely to be looking for food.

3. Use repellents and insecticides in humans in animals and in the environment to keep kennels and homes free from insects. The use of mosquito nets is equally effective, particularly in endemic areas.

4. Perform periodic screening tests on animals inhabiting endemic regions.

6. Educate the community about the disease, prevention measures and encourage behavioural changes.

It is crucial to increase the focus on this neglected disease by helping implement control and prevention measures as well as promoting research to discover new treatments to avoid many thousands of deaths each year.

By Ricardo Ribas, Veterinary Doctor, doctorate in veterinary sciences and researcher in the area of oncology in London

References sources

1. Sumter, J & Gull, K. Shape, Form, Function and Leishmania Pathogenicity: From Textbook Descriptions to Biological Understanding. [published correction appears in Open Biol. 2018 Aug;8(8):]. Open Biol. 2017;7(9):170165. doi:10.1098/rsob.170165

2. Ribeiro, RR; Michalick, MSM et al. Canine Leishmaniasis: An Overview of the Current Status and Strategies for Control. Biomed Res Int. 2018. doi:10.1155/2018/3296893

3. Leishmaniasis. World Health Organisation. 2 March 2020. https://www.who.int/news-room/fact-sheets/detail/leishmaniasis & https://www.who.int/leishmaniasis/disease/en/. Accessed April 2020

4. WHO Report on Global Surveillance of Epidemic-prone Infectious Diseases – Leishmaniasis. World Health Organisation. https://www.who.int/csr/resources/publications/CSR_ISR_2000_1leish/en/. Accessed April 2020

5. Parasites- Leishmaniasis. Centre for Disease Control and Prevention. Last updated February 2020. https://www.cdc.gov/parasites/leishmaniasis/index.html. Accessed April 2020

6. Leishmaniasis. Wikipedia, Wikipedia Foundation. https://en.m.wikipedia.org/wiki/Leishmania. Accessed April 2020

7. Leishmaniasis in other animals. Stop leishmania.org. http://www.stopleishmania.org/leishmaniosis-animals.php. Accessed April 2020


The material, ideas as well as control and prevention measures discussed on this article are for informational purposes only. For more information consult a vet or a professional in the area. Whilst every effort is made to make sure the article is accurate at the time of publication, I take no liability for any new developments on the subject as well as any errors or omissions.

Antibiotic Crisis: Are animals really to blame for it?

By Ricardo Ribas

Image from skeeze, pixabay.com​
Image from skeeze, pixabay.com

Antibiotics and their history?

Antibiotics are a group of medicines used to treat and prevent infections and diseases caused by single cell microorganisms called bacteria. They act either by killing bacteria directly or by preventing them from dividing and reproducing. A limited number of antibiotics can also treat diseases caused by single-cell parasites called protozoa, but none of them are effective against viruses, the agents responsible for cold and flu.

There is evidence that antibiotics have been used for thousands of years to cure infections. Various plant extracts, herbs, honey and even animal faeces were used by earlier civilisations to treat bacterial infections. In ancient Egypt, for example, people used to apply moulded bread to treat infected wounds. Despite the fact that antibiotics have been used throughout history, was only in the 19th century, that scientists linked bacteria as the cause of some infections. Also, up until the 1930s, many people often died with minor bacterial infections and surgeries were very risky. The discovery of penicillin revolutionised medicine, when in 1928 Alexandre Fleming noticed that bacteria present in his culture plates have been killed by a type of mould that accidentally contaminated them. The specie of mould (fungus) that invaded Fleming’s plates was a Penicillium notatum, what subsequently led to the name of penicillin. Recognising the potential of this discovery, Fleming and his colleagues rapidly tried to commercialise it as a medicine, but it took over 10 years for the penicillin to be available in the market. The 1940s and 1950s was a great period for discovery of new antibiotics, but since then, not many new ones have been developed.

Antibiotic Resistance

Despite the fact that antibiotics have changed the face of medicine in the 20th century, very soon after, scientists realised that bacteria could evolve and change overtime to become resistant to them, meaning that they were able to survive and grow in the presence of antibiotics that otherwise would kill them. Currently this is one of the biggest threats for global public health. As resistance occur worldwide, treatments are becoming less effective making it harder or even impossible to treat diseases such as pneumonia, gonorrhoea, blood poisoning, tuberculosis and others. According to the world health organisation, antibiotic resistance is responsible for 700.000 humans’ deaths around the world every year and these numbers are increasing, with some studies predicting that by 2050, it could reach 10 million yearly deaths, becoming the leading cause of mortality worldwide.

Bacteria can become resistant to antibiotics naturally, but this process is very often fastened by the incorrect or overuse of antibiotics. This is a big reason for concern particularly because many countries do not follow treatment guidelines, allowing antibiotics to be over-prescribed and permitting their retail without medical prescription. Similarly, lack of sanitation and hygiene conditions in some hospitals, as well as the shortage of quick and reliable laboratory tests to identify infections, can further contribute to the increasing number of antibiotic resistance happening worldwide.

This is particularly concerning in veterinary medicine. In recent years, the increased number of pets and the requirement for their welfare has enhanced treatments for sick animals, and with it, the administration of antibiotics. More importantly, the increased demand of meat production as a result of the population growth, means that farmers are more reliable on antibiotics to treat and prevent diseases in their livestock. Furthermore, some countries still allow the routinely use of antibiotics to promote animal’s growth, making it a further reason for concern. It is believed that the meat industry on its own accounts for 73% of the global use of antibiotics. Additionally, the fact that farm animals are constantly under stress and often confined to overpopulated spaces, highly compromises their immune system making them more vulnerable to infections and increasing the risk of transmission. As such, animals often become reservoir of the resistant microorganisms increasing the chances of transmission to humans, either directly by close contact; during consumption of contaminated meat or through spread of the animal’s waste into the environmental surroundings.

Future Prospects

Despite the fact that is important to incentive research to discover and develop new antibiotics, it is crucial to learn how to use them more effectively. Countries around the world should follow international guidelines to change the way they prescribe and use the antibiotics and should create actions to reduce the spread of infections, particularly in an era of easy travel.

It is also critical to restrict the use of unnecessary human antibiotics to promote animal growth and to prevent disease in healthy animals. As alternatives, farmers should consider vaccinate the animals, create better and more hygienic housing conditions and improve husbandry procedures such as better ventilation as well as reduction of animal confinement and stress conditions. It is, therefore, vital to educate farmers and pet-owners to collaborate with the vets to understand how to use these drugs responsibly.

Urgent actions are needed to avoid history repeating itself and to see common and minor infections killing again. Yet gain, humans actions, and not animals themselves, are to blame for the antibiotic crisis. In your opinion, what do you think can be done to revert this process?

By Ricardo Ribas, Veterinary Doctor, doctorate in veterinary sciences and researcher in the area of oncology in London

Reference Sources

1. Aminov RI. A brief history of the antibiotic era: lessons learned and challenges for the future. Front Microbiol. 2010;1:134. Published 2010 Dec 8. doi:10.3389/fmicb.2010.00134

2. Gould K. Antibiotics: from prehistory to the present day. J Antimicrob Chemother. 2016;71(3):572‐575. doi:10.1093/jac/dkv484

3. Ventola CL. The Antibiotic Resistance Crisis. P T. 2015 Apr; 40(4): 277–283. PMID: 25859123

4. Antibiotic Resistance. World Health Organisation. 5 February 2018. https://www.who.int/news-room/fact-sheets/detail/antibiotic-resistance. Accessed February 2020

5. New report calls for urgent action to avert antimicrobial resistance crisis. World Health Organisation. 29 April 2019. https://www.who.int/news-room/detail/29-04-2019-new-report-calls-for-urgent-action-to-avert-antimicrobial-resistance-crisis. Accessed February 2020

6. Antibiotic. Wikipedia. Wikipedia Foundation. https://en.m.wikipedia.org/wiki/Antibiotic. Accessed February 2020

7. Antimicrobial resistance. Wikipedia. Wikipedia Foundation. https://en.m.wikipedia.org/wiki/Antimicrobial_resistance. Accessed February 2020


The subjects and ideas discussed in this article are for informational purposes only. For more information consult a professional in the area. Whilst every effort is made to make sure the article is accurate at the time of publication, I take no liability for any new developments on the subject as well as any errors or omissions.

Smart viruses that jump species: Covid-19, HIV, Ebola and others…

By Ricardo Ribas

Image from syaibatulhamdi, pixabay.com
Image from syaibatulhamdi, pixabay.com

Viruses are microscopic agents that infect living organisms and cause diseases. They are considered the most abundant biological entity on the planet, existing for over 1.5 billion years. Viruses have both life and non-life properties. They do not have cells and cannot turn food into energy, but have genetic material (either DNA or RNA). Without a host cell, viruses are inert containers of chemicals so they need to infect cells in order to reproduce, evolve and survive. Hosts can be humans, animals, plants, fungi and bacteria. Some viruses are harmless whilst others cause diseases, some of which leading to the death of the host. To date there are no drugs capable of killing viruses so the host is dependent on its own immune system to fight the infection. Nowadays, there are some antiviral drugs capable of inhibiting them from reproducing and developing, as well as vaccines able to prevent infections and control the spread of the disease.

There are many types of viruses, each one adapted to its own host, but at times, viruses can jump and infect different hosts through a process called “spillover”. This can happen as a result of increased contact between different species or due to mutations that allows the viruses to evolve and gain the capacity to spread more efficiency in order to survive and to become more dangerous. In recent years, we have heard more and more about viral diseases able to jump from animals to humans. Here are some examples:

Influeza: The Flu Viruses

In the last century, there have been many examples of flu-like diseases caused by viruses that jumped from animals to humans. Some are caused by influenza viruses, a family responsible for attacking the respiratory system and causing flu like symptoms in humans.

In 1918, a deadly flu-like pandemic (Spanish Flu) appeared and rapidly spread around the world, infecting 500 million people and killing an estimated of 50 million. There is still some controversy about where the virus was originated from since Europe, America and Asia were affected simultaneously, but recent studies tend to agree that the outbreak may have originated in the North of China, either from pigs or from domestic and wild birds.

But there are other more recent examples of influenza viruses that jump species, such as the ones responsible for the outbreaks of swine-flu and bird-flu.

Although pig’s influenza viruses are rarely transmitted to humans and are hardly spread between people, they were responsible for the worldwide outbreak of swine-flu in 2009-2010. The disease was firstly identified in 2009 when many cases of human respiratory illness were reported in Mexico. We now know that this virus is the result of a merge between genetic material from pigs, birds and human viruses. In animals, the disease is responsible for fever, coughing, nose and eyes discharge, sneezing, breathing difficulties and lack of appetite, however, some animals show no signs of illness. Humans show flu like symptoms and can be infected either by close contact with pigs through farming and fairs or by person-to-person contact during sneezing, coughing and by contaminated surfaces. Fortunately, the 2009 outbreak was not as serious as previously anticipated and only a relatively small number of cases lead to serious illnesses and deaths. Swine flu is now considered a normal type of seasonal flu and is included in the vaccination program.

Bird-flu is another infectious type of influenza that spreads among wild and domestic birds. The most common form of the disease is caused by H5N1, a deadly virus for birds. H5N1 was firstly identified in humans in Hong-Kong in 1997. Human cases of the disease were linked with the handling of faeces, nasal and mouth secretions of infected poultry, particularly in farms and markets. Bird symptoms include: sudden death, lack of coordination, purple discoloration of the wattles, combs and legs, misshapen eggs, diarrhoea, nasal discharge, coughing and sneezing. Humans generally suffer from fever, cough, sore throat, muscle ache but the disease can lead to pneumonia. The disease has high mortality rate and is responsible for causing the death of about 60% of those infected. The good news is that H5N1 only rarely infect humans and is not capable to spread from person to person. In spite of this, it is a fast mutating virus and if it gains the capacity to be transmissible between humans while retaining its capacity to cause severe disease, the consequences can be very serious.

Human Immunodeficiency Virus (HIV): one century with humans

Another example of a virus that crossed species is the one responsible for human immunodeficiency. Once infected, the virus attacks the immune system of the host, opening the door for other opportunistic diseases. This disease was first observed in 1981 in the USA, but scientists believe that humans got infected in the 1920’s in West-Central Africa, possibly in what is now the Democratic Republic of Congo. Scientists believe this happened when hunters consumed meat from non-human primates carrying a virus closely related to the HIV (Simian Immunodeficiency Virus or SIV), a virus that also attacks the immune system of these animals. It is suspected that the infection has reached the Caribbean islands in 1960s, New York City in the 1970’s and San Francisco a decade later, when it was finally discovered.

Ebola, the terrifying virus

The Ebola virus was first discovered in 1976 near the Ebola River in the Democratic Republic of Congo. Since then, the virus has been responsible for occasional outbreaks in human and primates in many African countries. Between 1976 and 2013, the world health organisation reported 24 human outbreaks of the disease with 1.600 deaths in total, but the largest outbreak occurred between 2013 and 2016 and was responsible for over 11.000 deaths. The disease is rare but very severe, often causing fever, sore throat, body aches, vomit, diarrhoea and sometimes bleeding inside and outside the body, with death occurring in 50% of the human infected. Scientist think that the Ebola virus may be carried by bats, because these animals don’t die of the disease. It is believed that humans get infected by contact with the infected animal’s body fluids, either directly from bats or during consumption of nonhuman primates. The virus spreads through human-to-human contact and it is also possible via blood or the body fluids of the infected person.

Coronaviruses repeated stories

Recent history also showed us how coronaviruses can jump from animals to cause a large number of human deaths. Coronaviruses are a large family of viruses known to exist for over 10.000 years. Its name derives from the latin word “corona”, that means crown due to the presence of crown-like spikes in their surface.

Coronaviruses are responsible for respiratory and intestinal diseases in mammals and birds, but the symptoms vary according to the host animal and the type of virus. For instance, in chickens, coronavirus attacks the respiratory and urogenital tracts, whilst in dogs, cats, ferret, rabbits, turkeys, pigs and cows the virus tends to affect the animal’s intestine leading to diarrhoea.

Humans, can only be infected by seven types of coronaviruses. Four of these are very common and responsible for mild respiratory infections with symptoms similar to the ones observed with a cold, but the remaining three viruses can lead to serious and sometimes lethal respiratory infections, such as pneumonia. These include the viruses responsible for the Severe Acute Respiratory Syndrome (SARS), the Middle East Respiratory Syndrome (MERS) and the most recent outbreak in China in 2019-2020 (Covid-19).

SARS was first identified in Southern China during the outbreak of the disease in 2003. Scientists believe that bats are the reservoir of the virus and that the disease was transmitted to humans through consumption of civet cats meat infected by the bats. Once in humans, the disease can spread from person to person. By the end of the outbreak, over 8000 people have been infected and approximately 10% of those died (774 people). Fortunately, no cases of SARS have been reported since 2004.

MERS was reported for the first time in 2012 in Saudi Arabia. It is known that the virus was firstly transmitted to humans by dromedary camels in the earlier 2010s, possibly during consumption of camel meat. This virus has also been found in bats and is believed that it may have infected camels in Africa in the 1990s. Even though the possibility of human to human transmission is very low, the mortality of the disease is quite high standing above 35%. So far, MERS killed 858 out of the 2494 reported cases since it appeared in 2012.

Very recently a new outbreak of coronavirus has emerged in Wuhan, China (Covid-19). Similarly to SARS and MERS, this infection is also responsible for severe acute respiratory infection that can lead to either mild or severe symptoms such as pneumonia, kidney failure and death. Since this is a very recent virus, not much is known about it yet. To date scientists haven’t yet identified which animal the reservoir for the virus is, however, it appears that the contamination to humans may have occurred in a wildlife market, possible during consumption of wild animals. Despite the fact that the rate of transmission between humans is high, the mortality rate seems low. So far, the disease has spread within China and other countries around the world and an international effort is being made to control the spread of the disease.

When human viruses infect animals?

Sometimes viruses can jump the opposite way. There are also examples of human viruses that alter and adapt to infect animals. This is the case of the human respiratory syncytial virus (HRSV) and the human metapneumovirus (HMPV) that have jumped from humans to chimpanzees. It is believed that this has occurred between 1999 and 2006 in West Africa possibly through tourists visiting the area. Whilst these viruses rarely cause severe disease in healthy adult humans, they are responsible for outbreaks of respiratory diseases among African chimpanzees leading to the death of entire populations.

Why are animal-derived outbreaks getting more frequent?

The fast-growing world population and the development of large metropolis are incentives to the expansion of wildlife trade, which favoured diseases to spill over from animals to humans. Additionally, environment and climate changes are shifting animal habitats, altering their life-styles, their geographic locations and their eating habits (who eat whom). Finally, in an era of international travel means that diseases can spread very quickly.

But a question remains. In a world more vulnerable than ever to new animal-borne diseases, what do you think can be done to prevent future outbreaks?

By Ricardo Ribas, Veterinary Doctor, doctorate in veterinary sciences and researcher in the area of oncology in London

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The subjects and ideas discussed on this article are for informational purposes only. For more information consult a vet or a professional in the area. Whilst every effort is made to make sure the article is accurate at the time of publication, I take no liability for any new developments on the subject as well as any errors or omissions.