Horses are often moved back to stable conditions as the grazing season comes to an end. As a result, horses are in closer proximity to each other, and available space per horse decreases. Especially in autumn, it is not uncommon that horses infect each other, and diseases spread. Usually, infectious diseases affect the respiratory system of horses, but parasites such as worms also pose a threat to their health.
Pathogens
Pathogens are organisms that are harmful to the body (1). Most pathogens are single-celled microorganisms, such as bacteria, fungi, and parasites (2). Microbes themselves are not harmful; they are an essential part of a healthy body. Most microbes do not cause diseases and exit the body naturally (3).
The term virulence refers to the pathogen’s disease-causing ability. In addition to virulence, the body’s resistance and the number of invading microbes influence whether an inflammation occurs (3). Virulence can also measure the body’s reduced vitality, especially when the body has inflammation caused by pathogens (1).
Viruses
Viruses are pathogens that cause diseases. However, viruses are not single-celled, and they do not belong to the group of microbes. While viruses have genetic material, they lack cellular structure or metabolism. As a result, viruses can only replicate within host cells (2). Replication does not occur through binary fission (diving) but can generate thousands of new viruses rapidly (4). In horses, various herpes and influenza viruses are concerning. Diseases caused by viruses cannot be cured with mediaction, and treatment plans mainly consist of rest and symptom management (5).
Bacteria
Bacteria are among the earliest inhabitants of the Earth and are part of the living world. Like viruses, bacteria have genetic material. However, they are larger than viruses and have a clear cell wall. Unlike viruses, bacteria can generally reproduce independently in nature, with a few exceptions. Tens of thousands of different bacterial species are known, but many remain unidentified (2).
Today, bacteria are categorized into different species based on their genes. Known bacteria include Salmonella, E. coli, and Staphylococcus (MRSA). Bacterial diseases can typically be treated with antibiotics. There are several dozen different antibiotics available because certain bacterial species are naturally resistant to specific antibiotics. Additionally, the excessive and often unnecessary use of antibiotics has created bacterial strains resistant to antibiotics. This is known as antibiotic resistance (2).
Parasites
Parasites exploit the body of another organism and depend on it. Parasites include various worms and parasitic insects (2). In the case of horses in Finland, recognized internal parasites include roundworms, tapeworms, and pinworms. Like bacteria, parasites have developed resistance to certain parasitic drugs. It is advisable to check the effectiveness of a specific drug before administering it to a horse (8).
What Is Immunity and How Does It Work?
Immunity is one of the body’s most vital functions. Its purpose is to prevent harmful pathogens, or disease-causing agents, from entering the body. The impact of pathogens varies depending on whether they are viruses or bacteria, and different pathogens affect different cells in the body. Some pathogens can go unnoticed for a long time, while others aggressively invade healthy cells, causing rapid destruction in the body. Some pathogens can be life-threatening. A well-functioning immune system is therefore crucial (9).
The operation of immunity is based on learning and constant development. Viruses or cancer cells that it cannot detect or act against, pose challenges to immunity. However, immunity has a good memory, allowing it to react more quickly to pathogens to which it has been exposed previously (9).
Factors Weakening Immunity
Age: Age is known to weaken immunity, making older horses more susceptible to various diseases (10, 11).
Weather: Different weather conditions can affect morbidity. In autumn, the quickly changing cool and damp weather can strain a horse’s body, weakening immunity and allowing pathogens to affect the body more easily. Horses are also kept in closer proximity to each other during the winter season, making it easier for diseases to spread (12).
Some diseases, however, spread more aggressively in the summer. Especially diseases affecting foals are exacerbated by the foaling season (13). Due to climate change and the rising temperatures, various pathogens can now survive in areas where it would have been impossible in the past (5).
Stress: Stress always upsets the body’s balance. Physical exercise especially, is known to be stressful for a horse’s body. This may also weaken immune function. The intensity of training and a horse’s physical condition influence how much exercise affects immunity. Research indicates that excessive training for a horse with weak basic fitness, also weakens immune function and may expose the horse to diseases caused by influenza viruses (14).
Common Infectious Diseases (in Finland)
The most common virus-caused diseases in horses are Equine Viral Arteritis (EVA), Equine Herpesvirus-1 (EHV-1), and equine influenza caused by the A virus (5). The best-known bacterial respiratory disease in horses is strangles, but tetanus, caused by a soil-dwelling bacteria, is also significant (15).
Symptoms of strangles include fever and anorexia. Later the lymph nodes in the head area swell, and form abscesses. Strangles is caused by a contagious streptococcal bacterium, and infection spreads through droplet transmission or through people and equipment (15). The disease rarely leads to death, but in some cases, abscesses can also develop in the abdominal cavity, which is then often fatal to the horse, especially if the abscesses burst. Some horses may become asymptomatic carriers and excrete the bacterium (15).
Equine influenza is caused by A viruses, and its symptoms include high fever, a dry cough, and nasal discharge. The disease spreads through droplet transmission or through people and equipment (15). There is no cure, but vaccination is used to prevent horses from getting sick. However, influenza viruses are highly adaptable, affecting the horse’s ability to fight the virus. Vaccinations do not provide complete immunity, but vaccinating all horses in a stable provides the best protection against the disease (15).
Herpesvirus infections are also respiratory diseases that affect the horse’s respiratory tract and nervous system, caused by Equine Herpesvirus 1 and 4. Symptoms include respiratory and neurological symptoms and fever. In pregnant mares, the disease can cause abortion. The virus spreads through droplet transmission or through people and equipment. Herpesviruses can also establish latent infections, where the pathogen remains hidden within the horse’s body. Stress can trigger the virus’s release in asymptomatic carriers. Vaccinations aim to control the disease’s manifestation (15).
Prevention
Vaccinations and parasite control programs are essential in preventing infectious diseases. Tetanus vaccination should be a part of every horse’s vaccination program (15). Horses must be vaccinated against equine influenza for local, national, and international competitions under FEI rules (16), as per the regulations found in the Finnish Equestrian Federation’s (SRL) general competition rules. In some countries, herpesvirus vaccines are mandatory for horses used in breeding, and in Finland, broodmares should be vaccinated during pregnancy. The best protection against EHV-1 is obtained through herd immunity, achieved by vaccinating all horses (5).
It is crucial to remember that diseases spread not only from horse to horse but also through people working in the stable and those interacting with the horses. The same equipment should only be used at one stable, or equipment should be washed and disinfected between stable visits. Unnecessary time spent in the stable and unnecessary handling of horses should be avoided. Guest horses should be kept separate from other stable horses. Generally, the spread of diseases can be prevented through good stable hygiene and daily monitoring of horse health. When illness is detected early, the spreading can also be effectively prevented (15).
References
1. Méthot PO, Alizon S. What is a pathogen? Toward a process view of host-parasite interactions. Virulence. 2014;5(8):775–85.
2. Vuento R, Huovinen P, Lumio J. Infektioiden aiheuttajat: loiset, bakteerit, arkit, sienet, alkueläimet, virukset ja prionit. Teoksessa: Lääkärikirja Duodecim [Internet]. 2020 Accessed Aug. 20, 2023.. Available at: https://www.terveyskirjasto.fi/dlk00562
3. Anttila VJ. Infektioiden tartunta, taudin synty ja leviäminen. Teoksessa: Lääkärikirja Duodecim [Internet]. 2022 [ Accessed Aug. 20, 2023. Available at https://www.terveyskirjasto.fi/dlk00569
4. Taylor MW. What Is a Virus? Teoksessa: Viruses and Man: A History of Interactions [Internet]. Springer International Publishing; 2014. s. 23–40. Available at : https://dx.doi.org/10.1007/978-3-319-07758-1_2
5. Steinman A, Steinman A, Erster O. Virus Infection in Equine. Basel: MDPI Books; 2022.
6. Robert Poulin. Evolutionary Ecology of Parasites : Second Edition [Internet]. Vsk. 2nd ed. Princeton, N.J.: Princeton University Press; 2007. Available at: https://search.ebscohost.com/login.aspx?direct=true&db=e000xww&AN=370807&site=ehost-live&scope=site
7. Valibasha H, D’Souza PE, Dhanalakshmi H. Prevalence Study of Gastro-Intestinal Parasites in Horses. Intas Polivet. 2019;20(2):229.
8. Suomen Hevostietokeskus ry. Loislääkkeiden resistenssitilanne [Internet]. 2022 [Accessed Aug. 20, 2023. Available at: https://hevostietokeskus.fi/i/terveys/hevosten-loiskontrolli/loislaakkeiden-resistenssitilanne
9. Nicholson LB. The immune system. Essays in Biochemistry. 26. lokakuuta 2016;60(3):275–301.
10. Hansen S, Baptiste KE, Fjeldborg J, Horohov DW. A review of the equine age-related changes in the immune system: Comparisons between human and equine aging, with focus on lung-specific immune-aging. Ageing research reviews. 2015;20:11–23.
11. Horohov DW, Adams AA, Chambers TM. Immunosenescence of the Equine Immune System. Journal of comparative pathology. 2010;142:S78–84.
12. Gao R, Shi L, Guo W, Xu Y, Jin X, Yan S, ym. Effects of Housing and Management Systems on the Growth, Immunity, Antioxidation, and Related Physiological and Biochemical Indicators of Donkeys in Cold Weather. Animals. 2022;12(18):2405.
13. Dawson TRMY, Horohov DW, Meijer WG, Muscatello G. Current understanding of the equine immune response to Rhodococcus equi. An immunological review of R. equi pneumonia. Veterinary immunology and immunopathology. 2010;135(1–2):1–11.
14. Folsom RW, Littlefield-Chabaud MA, French DD, Pourciau SS, Mistric L, Horohov DW. Exercise alters the immune response to equine influenza virus and increases susceptibility to infection. Equine Veterinary Journal. 2010;33(7):664–9.
15. Hautala K, Nevalainen M. Hevosen tarttuvat hengitystiesairaudet [Internet]. 1. p. Vsk. 2007. Suomen Hippos ry; 30 s. Saatavissa: https://www.hippos.fi/uploads/sites/1/2022/05/10b4bb7d-tarttuvat_taudit.pdf
16. Suomen Ratsastajainliitto ry. Yleiset kilpailusäännöt [Internet]. Accessed Aug. 20, 2023. Available at: https://www.ratsastus.fi/site/assets/files/20477/ksi_2023_puhdas_ja_liitteet.pdf
