Mad cow disease | what exactly is it?
Mad cow disease, also known as Bovine Spongiform Encephalopathy (BSE), is a disease of high interest worldwide due to its economic and health impact on animal production.
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Mad cow disease, also known as Bovine Spongiform Encephalopathy (BSE), is a disease of high interest worldwide due to its economic and health impact on animal production. In this article we will explain everything about this disease: the causes, how it is transmitted and what measures can be taken to prevent it.
It was called “mad cow disease” because of the neurological signs it generates in cows, altering their behavior. At a technical level, it is also called “Bovine Spongiform Encephalopathy” (BSE) due to the lesions it generates in the bovine brain, which have the appearance of a sponge under the microscope.
Bovine Spongiform Encephalopathy (BSE) was first diagnosed in the UK in 1986 and has spread around the world since then. According to certain sources, its origin was in the feeding of cattle with products of goat origin (flours) contaminated with a prion that was called Scrapie, which was adapted to cattle, thus generating a new type of prion.
In the epidemic of the 1980s in the UK, around 400,000 cows died from the disease. Besides, approximately 4,500,000 cows were slaughtered as a preventive method. The economic impact on the Gross Domestic Product (GDP) of those countries was very high. For this reason, mad cow disease is one of the most monitored bovine diseases in the world.
What is the cause of this disease?
The disease, so far known, is caused by a particle called prion. This prion consists of an abnormal protein that is transmitted between animals, which has “infectious” behavior. The prion is an etiology of disease that is difficult to classify and identify.
In general terms, animal cells produce a cellular prion protein (PrPC) that is digested by enzymes as it occurs Â with proteins. However, the prion that causes Bovine Spongiform Encephalopathy (BSE) has a three-dimensional structure different from normal PrPC. This structural change gives it the pathological capacity to accumulate in the nervous system of animals, in addition to being resistant to high temperatures. For this reason, it has been called Infective Prion Protein (PrPSc) or commonly called a prion.
What do we observe in animals?
The development of the disease has an important component: it has long incubation periods. It can take between 4-5 years after infection to observe the appearance of clinical signs. This imposes a difficult diagnostic challenge as producers do not easily know if they may have the disease in their farms. Once the signs appear, the course of mad cow isease ranges from two to six months, until the death of the animal.
The signs that may appear are: aggressive or nervous behavior; excessive alteration of the senses (hearing, vision, touch); abnormal body positions and incoordination when moving; weight loss, and depression. For this reason, they must know how to differentiate BSE from other diseases of a similar course, such as rabies, electrolyte disturbances, tetanus, botulism, among others.
How to get to the diagnosis?
Regarding the macroscopic pathological study, no evident lesions are observed. On the other hand, at the microscopic level, notorious lesions are found in the brain, which loses its normal texture and begins to have holes caused by prions, thus generating a spongy-looking brain.
Because of this, the diagnosis is done mainly by histopathology. In this, we observe those lesions of brain tissue under the microscope. Besides, immunohistochemistry allows prions to be found around amyloid plaques located in the brain or tonsils.
Additionally, laboratory blood tests provide poor information for the diagnosis, because the immune system of bovines does not generate any type of reaction against this abnormal protein called “prion” (prions, plural).
It is a Notifiable Disease for the World Organization for Animal Health (OIE). There is no treatment, and it has a mortality of 100%.
How is it transmitted?
The transmission of the prions that cause mad cow disease occurs when healthy cattle are fed with products of animal origin, either bovine or ovine, which are contaminated with the prion. For example, transmission can occur when cattle are fed with meat or bone meal.
To avoid transmission by contaminated food, in 2004 the FAO issued a manual on the correct handling of feed, which you can check here: http://www.fao.org/tempref/GI/Reserved/FTP_FaoRlc/old/prior/segalim/animal/eeb/pdf/inocui.pdf
State of the world disease
The first diagnosed case was in Britain in 1986. Subsequently, the first native cases (that is, arising within the same country) were reported in Ireland in 1989, in Japan in 2001, in Canada in 2003, and in the USA in 2005. We observe in this brief chronology, that the disease has spread across the continents reaching America. For this reason, surveillance systems have been strengthened to monitor the disease in countries where cases have emerged and in which they have not.
In Figure 3 we can see the world map updated by the OIE where the health status of all countries concerning BSE is indicated.
It is classified as follows:
- Indeterminate risk: countries without official risk status, that is, the state of the disease in that country is not well known.
- Negligible risk: a country that meets all the surveillance requirements requested by the OIE to declare its risk to BSE as negligible. Most Latin American countries are part of this group.
- Controlled risk: countries that have controlled outbreaks of BSE through surveillance and currently do not present new cases. In Latin America, only Ecuador is part of this group.
How to prevent mad cow disease?
One of the major challenges in preventing the disease is that prions resist high temperatures. So that, apply heat methods when preparing meat or bone meal is not an effective method against the disease.
Therefore, the key points in its prevention have focused on the following:
- Eliminate Specific Risk Material (SRM) such as brain or spinal tissue during the processing of bovine carcasses destined for consumption.
- Prohibit the use of Specific Risk Materials (Brain, spinal cord) as a raw material in the preparation of feed for animal consumption.
- Establish robust surveillance methods in the event of possible cases or outbreaks of the disease, making constant follow up and monitoring nervous-type pictures.
- Implement a rapid and transparent notification system to take timely action.
- Conduct screaming studies on cattle during slaughter (post-mortem analysis of a large number of samples)
- Monitor and regulate the importation of live ruminants or products of bovine origin.
Other Spongiform Encephalopathies
In addition to mad cow disease, other encephalopathies produced by these abnormal proteins known as prions have been described, affecting other animals and humans.
Some of these are:
- Scrapie: it was the first prion encephalopathy described in the world, which mainly affects sheep and goats. The course of the disease is similar to BSE.
- Creutzfeldt-Jakob disease (CJD): it is a prion disease that affects humans. According to certain sources, Creutzfeldt-Jakob disease had its origin in BSE. For this reason, the surveillance systems in veterinary medicine and human health are joining forces to prevent it.
- Chronic Wasting Disease (CWD): it is a disease also caused by prions that affect deer and elk, both domestic and wild; CWD is present in North America. There is a strong vigilance in people who consume meat from these animals or hunt them. When there is an outbreak in cervids, mortalities are high. Besides, the prion remains in the environment for up to 2 years.
Spongiform Encephalopathies are a group of neurological diseases of great importance in veterinary medicine with the so-called mad cow disease. Global surveillance has managed to keep it under control, but an eventual outbreak could be catastrophic at an economic and health level. Therefore, it is necessary to know this disease, its characteristics, and how to prevent it to guarantee the health of animals and humans.
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Jerson AndrĂ©s CuĂ©llar SĂˇenz
Estudiante medicina veterinaria (V aĂ±o); Universidad Nacional de Colombia.