Microorganisms that Cause Chronic Fatigue in Animals

Chronic fatigue syndrome is a disease that occurs in both people and animals. Below we’ll discuss the role of microorganisms in this disease further.
Microorganisms that Cause Chronic Fatigue in Animals
María Muñoz Navarro

Written and verified by the biologist María Muñoz Navarro.

Last update: 21 December, 2022

There are several microorganisms capable of affecting living beings. Because of this, s ometimes both humans and animals can become ill. In this sense, it’s interesting to know which microorganisms cause chronic fatigue.  

To better understand this, we must first clarify two fundamental issues:  

  • A microorganism is a single-cell organism so small that we can only look at it under a microscope or an electron microscope as with viruses, which are ultra-microscopic.Pathogenic microorganisms are capable of causing diseases in animals, plants, and also people. 
  • We associate it as a symptom of various disorders such as physical exhaustion, decreased cognitive ability, or impaired sleep. This shouldn’t be confused with the fatigue we feel after physical activity. 

Below, we’ll show you some examples of animals suffering from this illness and the scientific evidence known to date , which will then help us understand the importance of these tiny beings , that play a crucial role in animal health.  

Chronic fatigue syndrome

Chronic Fatigue Syndrome (CFS), also known as ‘myalgic encephalomyelitis’ (ME), is a disease that starts with something similar to the flu.  

It’s a disease with symptoms of intense fatigue, both physical and mental, and is prolonged over time (at least 6 months).   It can also lead to concentration, memory, and muscle pain problems. Some professionals even link it to mental health.  

Only recently did we believe that this disease was unique in humans , however, several studies have revealed cases of CFS in animals, such as horses and pets, indicating a possible case of zoonosis

A sad dog.

Its cause, diagnosis, and treatment aren’t very clear nowadays , but all revolve around a viral or bacterial infection in the central nervous system (CNS), consequently causing an immune system disorder.  

Research has also revealed the presence of a variety of diseases  capable of causing life-long infections , by evading the immune system:  

  • Canine Parvovirus (CPV) 
  • Human herpesvirus 
  • Mycoplasma pneumonia 
  • Common herpes 
  • Epstein-Barr variants 
  • And even fungi or other biotoxins 

Microorganisms that cause chronic fatigue

Bacteria, parasites, and viruses are microorganisms that can cause fatigue, or are involved in chronic fatigue syndrome.  


A study in England suggested that horses could also suffer from CFS. In fact, they confirmed that  there is a link between humans and pets.  

Four horses were used to carry out an experiment, two of them had chronic fatigue and weakness that made it difficult for them to make any effort.   They appeared depressed, with their head down and drooping lips, as well as sleepy and apathetic. On the other hand, t he other two horses had symptoms of weakness, asthenia, and reduced weight.  

A pair of horses grazing.

Blood smear and blood culture tests found that all of them were carrying micrococcus-like bacteria  that  healthy horses  didn’t have .  

The author of the same study previously tested cats and dogs carrying micrococci in the blood,  and tested positive for s taphylococcus intermedius and s taphylococcus xilosus . Therefore, bacteria may be microorganisms that cause chronic fatigue.  


In another essay, published in the  Progress in Neurobiology  journal, scientists used mice as a fatigue study model.   As a result, they found that both Cryptosporidium parvum and Toxoplasma gondii infections affected the activity of mice for several weeks. 

The parasite Trypanosoma  brucei (causing sleep disease) caused daytime sleepiness and insomnia in infected rats.   Once in the blood, it can invade other tissues, such as the nervous system.  

In all cases, it was shown that these unicellular parasites can also cause chronic fatigue.  


Other microorganisms that cause chronic fatigue are viruses.   Numerous studies have confirmed that   CFS is due to a set of brain syndromes attributed to viral encephalopathies. In fact, it has been found that the following viruses could be associated with this disease:  

  • Parvovirus 
  • Enterovirus 
  • Epstein-Barr 
  • Common herpes virus 

Some of them are found in the cerebrospinal fluid itself   and experts suggest that this is where it causes the infection. However, others are found in other parts of the body and indirectly become infected with the CNS.  

Another article points out that viruses are more likely to cause this disease than bacteria, based on the following ideas:  

  • This disease is associated with altering brain functions and we know that viruses cause behavioral changes. 
  • Viruses of the circovirus and Polyomavirus genus can penetrate the brain and replicate there.Circovirus doesn’t cause pathogenicity in humans, but does cause it in animals, such as the post-weaning multisystemic wasting syndrome in pigs, which has symptoms similar to CFS. 
  • CFS usually occurs because of systemic disease, infection, or severe stress. These conditions can cause the virus to enter the blood and pass to the CNS, suppressing the immune system and increasing its viral load, as seen with the circovirus. 

An underweight pig.

The viruses from the Picornaviridae  family, which includes enterovirus, are present in the healthy population.   However, many enteroviruses have caused CFS  and several subtypes are known to occasionally affect the brain.   The herpes family and parvovirus also have a similar profile.  

There are several microorganisms that can cause chronic fatigue and are still being studied.  

Microorganisms always give clues

To conclude, the study of bacteria, parasites, and viruses allows us to understand infectious diseases and also develop effective treatments against them.  

Microbiology, however, goes far beyond that.   This scientific field tries to help us  understand the importance of these ‘invisible’ organisms, since life wouldn’t exist without them. Its role in nature is important to numerous biological and vital processes.

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All cited sources were thoroughly reviewed by our team to ensure their quality, reliability, currency, and validity. The bibliography of this article was considered reliable and of academic or scientific accuracy.

  • Bjorn, Grinde. Is chronic fatigue syndrome caused by a rare brain infection of a common, normally benign virus? Medical Hypothesis (2008) 71, 270-274.
  • Walter, Tarello. Chronic fatigue syndrome in horses: diagnosis and treatment of 4 cases. Comparaative Immunology, Microbiology and Infectious Diseases (2001) 24, 57-70.
  • Harrington, M.E. Neurobiological studies of fatige. Progress in Neurobiology (2012) 99, 93-105
  • Garg, V., Gupta, S., Kulkarni, K. and Singh, S. Role of antioxidants in chronic fatigue syndrome in mice. Indian Journal of Experimental Biology (2002) 40, 1240-1244

The contents of My Animals are written for informational purposes. They can't replace the diagnosis, advice, or treatment from a professional. In the case of any doubt, it's best to consult a trusted specialist.