Isn’t cholera a Class A infectious disease? Why are the cases at Wuhan University not dangerous?

For the same cholera, some strains may be just "laxative", while some strains can "attract millions of fans". In the hot summer, with high temperatures, we should pay attention to hydration and prevent diarrhea, which are what we should really pay attention to.

Written by Li Qingchao (Shandong Normal University)

Earlier this week, it was revealed online that a student at Wuhan University had infectious diarrhea and was diagnosed with cholera. Cholera, known as the "king of diarrhea," is the leader in the diarrhea industry and is one of the only two Class A infectious diseases in my country's legal infectious diseases (the other is plague). This acute diarrheal disease can kill people within a few hours if not treated.

Since spreading from the Ganges Delta in India to other parts of the world in the 19th century, cholera has killed millions of people. Currently, cholera infects an estimated 1.3 to 4 million people worldwide each year, killing 21,000 to 143,000 people[1].

Figure 1: Plague is also known as the Black Death, while cholera is also known as the Blue Death. Cholera patients suffer from severe dehydration, which causes their skin to turn gray-blue. [2]

Cholera is so terrible, so why did the government respond so calmly? It lifted the dormitory lockdown on July 12[3]. Weren’t they afraid of an epidemic? This starts with the Vibrio cholerae itself.

Searching for the source of the chaos: the discovery of Vibrio cholerae

Cholera was recorded in Sanskrit as early as the fifth century BC. Throughout history, people were often shrouded in fear of this plague, but little was known about the cause of cholera. Until the mid-19th century, during the third global cholera pandemic, people believed that cholera was caused by "miasma" - bad air.

Now it seems that this theory is certainly incorrect, but it is much better than the explanation of "evil spirit possession". The first historic breakthrough in the understanding of cholera was proposed by British doctor John Snow (1813-1858). After a detailed investigation of the cholera epidemic in London, England, Snow locked the source of cholera transmission on the water source.

Figure 2: John Snow and the map he used for his 1854 survey. The thick black lines on the right represent cholera cases, which are concentrated near a water pump. 丨Source: Wikipedia

Snow's research confirmed that the cholera epidemic was caused by polluted water sources; subsequent research found that cholera is an oral-fecal disease, which provided a theoretical basis for how to prevent cholera. Snow's research can be said to be a major event in the history of public health, and it also pioneered epidemiological research. (For details, see "He used mathematics to predict the origin of epidemics, and was finally recognized after 30 years of injustice")

What is the pathogen of cholera? At almost the same time as Snow conducted epidemiological investigations, Italian doctor Filippo Pacini (1812-1883) performed autopsies on cholera victims and found many comma-shaped bacteria in feces and intestinal mucosa. However, Pacini failed to successfully isolate the bacteria, nor did he prove the relationship between the new bacteria and cholera.

Thirty years later, in the 1880s, German biologist Robert Koch (1843-1901) also discovered this comma-like bacterium while studying cholera, and obtained a pure culture of this bacterium (a culture medium containing only one microorganism). Koch believed that this was the pathogen that caused cholera. However, using this comma-shaped bacterium to infect animals did not cause a disease similar to cholera. His efforts to prove the relationship between bacteria and disease through the "Koch's postulates" he created failed!

It was later speculated that this might be because the experimental animals used at the time had already developed immunity to the comma-like bacteria.

Figure 3: Filippo Pasini (left) and Koch (middle), the discoverers of Vibrio cholerae. The former was more than 30 years earlier than the latter, and the name "Vibrio cholerae" was eventually determined based on the former's naming. Vibrio cholerae (right) looks like a comma with a long tail. 丨Source: Wikipedia

Vibrio cholerae is a highly motile comma-shaped Gram-negative bacterium that is 1.3 microns long and 0.3 microns in diameter. Its average swimming speed reaches 75.4 microns per second, which is equivalent to a 1.8-meter-tall athlete running more than 100 meters per second (calculated in proportion to body length).

Cholera is caused by Vibrio cholerae. But in fact, most people infected with Vibrio cholerae are asymptomatic or have only mild symptoms. Only the O1 and O139 serotypes of Vibrio cholerae can cause large-scale epidemics. Therefore, there is no need to panic when you see the word "cholera", but to see how toxic the bacteria is.

Report on the handling of a cholera case in Wuhan University Cholera toxin - the terrorist's killer weapon

Virulence refers to the ability of a pathogen to cause harm to the host, or pathogenicity. Bacteria can cause disease through five aspects: adhesion (binding to the host or cell surface), colonization (reproduction in a specific location), invasion (entering the body or cells), immunosuppression (inhibiting the host immune system from clearing itself), and toxins (toxic substances).

Figure 4: The basic process by which bacteria cause disease in their hosts. Any factor that helps bacteria complete the above process can be called a virulence factor. [4]

Severe, lethal cholera cases produce highly liquid "rice water" stools during diarrhea. Untreated cholera patients may produce 10-20 liters of rice water stools per day. At this rate, an 80 kg man will be drained in four or five days. What makes highly pathogenic cholera so toxic?

Figure 5: For the same cholera, some strains may be just a laxative, while others can attract millions of fans. Source: The cover of the French magazine Le Petti Journal in 1912.

This problem was not solved until the middle of the twentieth century by Indian scientist Sambhu Nath De (1915-1985): he discovered cholera toxin (CTX). Sambhu Nath De used ileal loop ligation on rabbits to establish an animal model for studying diarrhea. In short, a long ileum was found in rabbits that were fasting and emptying their intestines, and then several ileal loops were used to ligate the ileum, dividing the rabbit ileum into several unconnected segments, and injecting bacteria that could cause diarrhea into each separated ileal segment. If this bacteria can cause diarrhea, the intestinal wall will release a large amount of fluid into the ileal cavity; these fluids will accumulate in the corresponding ileal segments due to the obstruction of the ileal loop. In this way, the ability of bacteria to cause diarrhea can be detected by comparing the amount of accumulated fluid. In addition, he also injected the supernatant of Vibrio cholerae culture (fluid after removing bacteria) into the rabbit ileal loop ligation model, and found that fluid without Vibrio cholerae could also cause severe diarrhea.

This experiment proved that Vibrio cholerae can release a substance that causes severe diarrhea. This substance is cholera toxin (CTX). Later studies have shown that cholera toxin is a protein. People use cholera toxin as an antigen to make vaccines to prevent cholera. The use of cholera toxin as a vaccine also proves that the main virulence factor of cholera is cholera toxin. As long as cholera toxin can be neutralized, Vibrio cholerae will find it difficult to cause serious disease.

Figure 6: The structure of cholera toxin analyzed by Indian scientist Sambhana and later

Cholera toxin is a hexamer composed of one enzyme subunit A and five receptor binding subunits B, which can be written as AB5. The A part is the "biggest source of chaos", while B plays the role of "leading the wolf into the house" for A to enter the cell. The receptor binding subunit B can pry open the door to enter the cell by binding to the surface components of human intestinal cells (mainly gangliosides, other polysaccharides are also possible). Once cholera toxin enters the cell, the enzyme subunit A can make trouble. In layman's terms, enzyme subunit A can cause a sharp increase in the level of steady-state signal molecules in the cell (by binding to Arf6, adenylate cyclase continues to work and produces a large amount of cAMP), so that the chloride ion channel on the cell surface "opens the gate to release chlorine" without restraint (under the stimulation of the cAMP signaling pathway, the chloride ion channel CFTR is abnormally activated). The normal ion level difference inside and outside the cell membrane is broken. While excessive chloride ions are released outside the cell, water, potassium and sodium ions are also forced to be secreted into the intestine, causing a large loss of water and inorganic salt ions. In a nutshell, cholera toxin can slip into intestinal cells, open the valve, and cause people to have diarrhea and lose their souls.

Ironically, Vibrio cholerae was blessed with cholera toxin not because of God’s favor, but because it was “sick.” People found that the gene encoding cholera toxin CTX does not belong to cholera, but to a bacteriophage CTXφ. You know, bacteriophages are the natural enemies of bacteria.

The CTXφ phage was first discovered in 1996. It is a lysogenic filamentous single-stranded DNA phage that can infect Vibrio cholerae. The so-called lysogenicity means that after the phage infects the bacteria, it does not cause the bacteria to lyse, but enters a state of carrying the phage. In this way, Vibrio cholerae gets the coding gene for cholera toxin. This is really a complete "we have no guns and no cannons, but the enemy makes them for us." What's more, Vibrio cholerae carrying phages can also release phages, allowing other Vibrio cholerae to be infected with phages and gain the ability to produce cholera toxins. Vibrio cholerae infected with CTXφ phages are like terrorists who have obtained a big killer weapon. Their combat effectiveness explodes instantly, and people infected with Vibrio cholerae carrying CTXφ phages are in trouble.

Figure 7: CTXφ phage infects Vibrio cholerae and enables it to secrete cholera toxin. Vibrio cholerae, which is not highly toxic (upper left), acquires the CTX gene after being infected with CTXφ phage (red line in the upper left circle), thereby being able to express cholera toxin (lower left) and having higher infectious and pathogenic abilities. [5]

At this point, everyone should understand that the cholera cases discovered by Wuhan University this time, although the Vibrio cholerae infected by them are serotypes that can cause large-scale epidemics, do not carry cholera toxins, so its danger and harm are limited. What really deserves our attention is the symptom of diarrhea itself.

A hero is no match for three puffs of urine

What does a brave man fear? A brave man fears a penny, two fists, three diarrheas, and four hands. The third “diarrhea”, or diarrhea, is a very common disease, with an estimated 170 million to 5 billion cases of diarrhea each year worldwide[6], especially in developing countries. Diarrhea is no small matter, with 1.6 million people dying each year from severe diarrhea (2017 data)[7]. If a brave man can’t handle it, then a child certainly can’t. Diarrhea is the biggest enemy of infant health and the second leading cause of death among children worldwide, killing more than 500,000 children under the age of five each year[8].

The strict definition of diarrhea refers to at least three liquid or watery stools a day (it is normal for infants who are exclusively breastfed or formula-fed to have loose, unformed stools but no water). According to different clinical symptoms, it can be divided into three categories: watery diarrhea that lasts for several hours or days is called acute watery diarrhea, such as cholera; acute diarrhea with blood is called acute bloody diarrhea, also known as dysentery; diarrhea that lasts for 14 days or longer is called persistent diarrhea. The main reason why diarrhea can cause harm is electrolyte imbalance and dehydration caused by a large amount of water loss in a short period of time, which is very dangerous; and the problem caused by chronic diarrhea is mainly nutrient loss, which can cause malnutrition and developmental delay.

Tips: Dehydration

Dehydration is a condition in which the human body loses too much water. The human body can tolerate a water loss of 3-5% (mild dehydration), but more than 5% will cause fatigue and dizziness (moderate dehydration), more than 10% will cause severe discomfort (severe dehydration), and more than 15%-25% water loss is life-threatening [9]. There are many reasons for dehydration, such as hot weather, excessive exercise leading to sweating and dehydration, vomiting, diarrhea causing fluid loss and dehydration, diuretics, drug effects and disease effects leading to dehydration. The main signs of dehydration are headache, fatigue, general discomfort, irritability, oliguria, etc., and in severe dehydration, sunken eyes, no tears, oliguria, poor skin elasticity, and even loss of consciousness can be seen.

Figure 8: Symptoms of dehydration in infants. Infants often become dehydrated due to diarrhea. If they experience the above symptoms, they need to be rehydrated or treated promptly. [10]

If you lose a lot of water due to sweating, vomiting or diarrhea, you should drink water immediately. In severe cases, you should take oral rehydration or intravenous infusion containing electrolytes to replenish body fluids. Here, we especially want to remind the elderly that the sense of thirst becomes weaker with age, so they should develop the habit of drinking water even if they don't feel thirsty.

Figure 9: Judging whether the body is dehydrated based on the color of urine. [11]

There are many causes of diarrhea, the main factors are as follows:

Infection. Virus, bacteria or parasite infection can cause diarrhea. Diarrhea caused by infection is called gastroenteritis. Among them, the most common pathogens of viral diarrhea are rotavirus, adenovirus and norovirus, and the common pathogens of bacterial diarrhea are mainly Vibrio cholerae, Shigella, Salmonella, pathogenic Escherichia coli, etc. The risk factors for diarrheal infection mainly include unclean food and water sources, public sanitation conditions and poor personal hygiene habits.

Malnutrition. Adequate nutrition is very important for health and disease prevention, especially for infants and young children whose immune systems are not yet fully developed. Under malnutrition, the human immune system is stunted, the disease resistance is low, and the body is susceptible to infection, which causes diarrhea. Diarrhea in turn causes nutrient loss, thus forming a vicious cycle. Among them, zinc and vitamin A deficiency are closely related to frequent diarrhea.

Economic backwardness is an important source of the above reasons. Deaths caused by diarrhea mainly occur in economically underdeveloped areas such as sub-Saharan Africa, South Asia, and Southeast Asia. The incidence and mortality of diarrhea can be reduced through improved sanitary conditions and nutritional status brought about by social and economic development.

After the founding of the People's Republic of China, the prevention and control of cholera has been very effective. There is no need to panic about the cholera cases this time. We should pay attention to food and water hygiene and drink water (especially in the current hot weather), which is what we should keep in mind.

Summarize:

There are many serotypes of Vibrio cholerae, but only O1 and O139 have the potential to cause a pandemic.

Generally, only Vibrio cholerae carrying cholera toxin can cause fatal diarrhea.

Diarrhea is dangerous because severe diarrhea can cause fatal dehydration.

Identifying dehydration and promptly rehydrating can greatly reduce the life-threatening effects of diarrhea.

References

[1] https://www.who.int/zh/news-room/fact-sheets/detail/cholera

[2] The Sick Rose: Disease and the Art of Medical Illustration, by Richard Barnett

[3] http://news.hnr.cn/shxw/article/1/1546995940000223234

[4] https://en.wikipedia.org/wiki/Dehydration

[5] https://microbewiki.kenyon.edu/index.php/CTX%CF%86_Bacteriophage

[6] https://en.wikipedia.org/wiki/Diarrhea

[7] https://ourworldindata.org/diarrheal-diseases

[8] https://www.who.int/news-room/fact-sheets/detail/diarrhoeal-disease

[9] https://medical-dictionary.thefreedictionary.com/dehydration

[10] Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. 2003

[11] https://en.wikipedia.org/wiki/Dehydration

Produced by: Science Popularization China-Starry Sky Project

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