Several infections that are naturally transmissible between vertebrate animal hosts and human beings exist. These infections result from being in direct contact with infected pets, or during daily activities. Ingesting foods or drinks that are contaminated with fecal matter can also lead to infections. Some gram-negative rods that are associated with zoonotic infections include Salmonella species, Brucella species, and Yersinia pestis among others (Cornelissen, Harvey & Fisher, 2012).
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Pathogen and its varieties
The infection Katie is experiencing is due to Salmonella species, which are a group of bacteria that are associated with gastroenteritis, typhoid fever, and paratyphoid fever. Salmonella is gram-negative bacilli that mostly inhabit the intestines of animals and man, and are shed through stool. Salmonella can also be isolated from the environment, including soil, plants, and water. People get salmonella infections usually through the consumption of contaminated foods or water or direct contact with susceptible animals.
These bacteria are non-spore-forming, rod-shaped gram-negative bacteria that are facultative anaerobes belonging to the Enterobacteriaceae family. They show high motility and can move about spontaneously. These bacteria also can grow and multiply outside the cells, and therefore they have enhanced survival capabilities. Many strains exist, but the most important Salmonella enteric serotype are the ones that are frequently involved in human infections. Some of these serotypes are Salmonella typhi, Salmonella typhimurium, and Salmonella enteritidis (Cornelissen, Harvey & Fisher, 2012).
Salmonella bacteria may have a combination of O, H, and Vi antigens. The O antigen is found on the cell wall of the bacterium, the H antigen is flagellar, while the Vi antigen is found in the capsule. The H antigen is heat-labile and is responsible for the motility of the bacterium. The Vi antigen is the one that is responsible for the virulence of the Salmonella bacterium. In addition, Salmonella bacterium particularly Salmonella typhi has an endotoxin that is characteristic of all gram-negative organisms, which enhances its virulence. It also secretes a protein called invasin that enables non-phagocytic cells to ingest it, thereby surviving in those cells. The majority of the Salmonella serotypes are not confined to particular hosts, but exhibit cross infectivity; therefore, they are readily transferred from animal to man or vice versa. Salmonella typhi and Salmonella paratyphi serotypes are the notable ones that show host-specificity to humans, whereas there are serotypes that are specific to avian species, cattle, and pigs (Schlossberg, 2008).
Considering the history given by Katie’s mother, such as visiting a petting zoo and the meals consumed, the most likely infection is a zoonotic infection. The clinical symptoms in Katie’s case, such as fever and severe diarrhea, laboratory findings such as blood in stool and unusually high number of gram-negative rods lead to the conclusion that Katie is having salmonellosis. Mammals, birds, and reptiles act as natural hosts and carriers of most Salmonella strains. Being a zoonotic disease, humans can acquire the infection through fecal contamination of the mucous membranes.
Katie’s history also shows that they were allowed contact with the pets after which they washed their hands on their own before taking lunch. This information helps in ruling out other possible infections like Shigellosis due to Shigella species and Cholera due to Vibrio cholera. Dysentery due to E. coli O157:H7 is also ruled out because it is rare and is usually not associated with fever. Other zoonotic diseases like Brucellosis, Plague, and Tularemia have characteristic etiology and clinical and laboratory findings that are not relevant to Katie’s case. In Brucellosis, there are no pets involved as the hosts are cattle, goats, and sheep, and the bacteria cause infection after they become localized in the reticuloendothelial system. They are also associated with non-specific symptoms (Corbel & WHO, 2006). Tularemia can be acquired following close contact with pets like rabbits or by vector bite, which frequently cause characteristic ulceration on the skin. It is not associated with diarrhea or blood stool, although a patient may present with low-grade fever (Schlossberg, 2008).
Risk factors and transmission routes
Salmonella infections occur worldwide, although the prevalence is higher in regions of intensive animal husbandry. The prevalence of Salmonellosis is also greater during hot weather and, therefore, it occurs more frequently in the tropics. Individuals whose immunity is vulnerable, such as children and old people are also more prone to Salmonella infection. Annually, Salmonella is estimated to cause typhoid fever in 16 million people and gastroenteritis infections in 1.3 billion people. The annual mortality due to salmonellosis globally exceeds 3 million individuals (Bannister, Gillespie & Jones, 2009).
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Risk factors for Salmonellosis include engaging in activities that increase one’s exposure to the bacteria, such as visiting poor sanitation areas, owning pets or reptiles, and living in congested areas. Other risk factors include poor immune status due to health problems or certain medications like corticosteroids. Food and water contaminated with the bacterium such as raw foods are also great risk factors (Bannister, Gillespie & Jones, 2009).
A person acquires Salmonella infection by consuming food or drinking water contaminated with the bacterium. Infections can also result from being in contact with carriers of Salmonella, may it be humans or animals. If Salmonella bacteria can survive the acidic conditions of the stomach, they colonize the intestine and invade intestinal and dendritic cells. This subsequently stimulates the production of inflammatory mediators like cytokines, chemokines, macrophages, and neutrophils which are attributable to the severity of Salmonella infections. The bacteria replicate in macrophages and then colonize other organs like the lungs, kidneys, and gallbladder. The nontyphoid strains, do not have human virulence factors and, therefore, they produce more localized responses (Bannister, Gillespie & Jones, 2009).
In most cases, the Salmonella infections are self-resolving and, therefore, antibiotics are not recommended. When Salmonella infection is associated with diarrheal dehydration or spreads to the intestines, it may become life-threatening and, therefore, it is necessary to carry out fluid and electrolyte replacement. Antibiotics are usually recommended for severe or invasive salmonellosis. The most commonly used antibiotics include, “Ciprofloxacin, ofloxacin, ampicillin, chloramphenicol, quinolone and macrolide” (Bannister, Gillespie & Jones, 2009, p. 93). However, the use of antibiotics is not effective in shortening the course of the infection or symptoms. In addition, antibiotics are associated with adverse events, prolonged carrier state, and a high risk of relapse. Cases of antibiotic resistance have also been reported, particularly about quinolones thereby limiting the usefulness of the drugs.
Salmonella infections when left untreated may often lead to complications such as typhoid fever, endocarditis, and arthritis. Endocarditis is a very rare condition that usually results from bacteremia, and is characterized by inflammation of the inner lining of the heart, mostly the heart valves. Arthritis results from a chronic state called Reiter’s syndrome and are characterized by painful joints and irritation. Typhoid fever is a more serious disease that is very fatal if not treated (Bannister, Gillespie & Jones, 2009).
Salmonella infections are mostly asymptomatic and frequently transmitted through ingestion of the bacteria in contaminated foods or water. The infection may also be acquired directly from carrier animals. Most Salmonella infections get resolved after a few days; although the infections can be life-threatening in some populations such as the elderly, infants, and those people whose immune system is compromised. Proper diagnosis and treatment of Salmonellosis require detailed information regarding the patient’s history, risk factors, and epidemiological data.
Bannister, B., Gillespie, S., & Jones, J. (2009). Infection: Microbiology and management. Boston, MA: John Wiley & Sons.
Corbel, M. J & W.H.O. (2006). Brucellosis in humans and animals. Geneva: W.H.O.
Cornelissen, C. N., Harvey, R. A., & Fisher, B. D. (2012). Microbiology, 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins.
Schlossberg, D. (2008). Clinical infectious disease. New York, NY: Cambridge University Press.