Significant characteristics of a disease
Virulence is a term that describes “the capacity of a microbe to cause disease after infection”. It is a multifaceted phenomenon is dependent on the “context and nature of the interaction between the host and the microbe.” The number of fatalities that occur as a result of infection by a given pathogen is used as a measure of the virulence. Based on this parameter, the Ebola virus would therefore be considered to be highly virulent as compared to small pox. Give that research has provided better understanding of pathogens thus led to the development of potent forms of therapy; some of the pathogens that were once highly virulent are no longer considered so. Streptococcal and pneumococcal infections were once considered highly virulent but current forms of treatment have led to a reduction of the virulence. Virulence has several attributes which include: replication and transmission, adherence/attachment and aggressiveness. Aggressiveness contributes to the capacity of the disease causing agent to multiply within the tissues of the host.
The aggressiveness of the microbe enables the disease agent to evolve mechanisms of avoiding destruction by the immune system For instance, Streptococcus pneumoniae is considered to be a highly aggressive microbe which therefore contributes to its virulence. The capacity of a disease causing organism to replicate and be transmitted within the organism contributes to the virulence. Once an organism replicates and is transmitted within the organism, it alters the normal function and behaviour of the host. For instance, rats that are infected with Toxoplasma gondii are less fearful of cats which can lead to an increase in predation thus further putting the rats at risk of being exposed to Toxoplasma gondii and contracting toxoplasmosis. A pathogen that possesses a high rate of transmission and low recovery rate of the host has enhanced persistence among the hosts and between the hosts. For transmission to take place effectively, the load of the pathogen within the host must be high. The transmission of a pathogen is dependent on the pathogens’ ability to remain specific to a given range of hosts. High specificity to hosts is therefore linked to high virulence while low host specificity is linked to low virulence.
Toxigenicity refers to the capacity of a pathogen to produce toxins. The toxins that are produced by a pathogen can trigger adverse reactions such as allergic reactions, impaired nutrition of the cells and eventually death of the cells as a result of the metabolic products that have been produced by the toxins. The type of toxin produced and the nature of the toxin can therefore exacerbate the symptoms as a result. Pathogens that produce lethal toxins are said to have the capacity to kill the hosts at a faster rate. This is because after the infection of the host, these pathogens could circumvent the immune system while still producing the toxins that are lethal to the cells. Toxins can be classified into different categories which are: exotoxins or true toxin refers to toxins that are produced by activities of the bacteria which can trigger anti-toxin responses by the host.
Exotoxins are proteinous in nature and are heat labile. They can cause disease through different mechanisms: ingestion of the preformed exotoxins can result in an infection such as staphylococcal infection. Gangrene is as a result of the production of exotoxins at the site of a wound. Production of exotoxin after the colonization of the host can result in disease as in the case of cholera. Ptomains are molecules which are small such as methylamines, cadavarin and putrecine which may not necessarily trigger a humoral response. Endotoxins are toxins that are produced after the death of the pathogen. Endotoxins are capable of producing a variety of symptoms in the host such as fever, diarrhea, inflammation, haemorrhaging of the intenstines, inflammation and fibrinolysis.
Resistance refers to the ability of a disease causing agent to survive under adverse conditions. Pathogens have evolved various mechanisms to survive the adverse conditions of the host cells. For instance, the stomach is highly acidic thus pathogens that invade the stomach can easily be destroyed by the acidic environment. Listeria is known to be capable of surviving in pH conditions ranging between 5 and 9.5 thus the acidic nature of an environment would not necessarily be a challenge. Some pathogens have a polysaccharide capsule that is resistant to destruction by the digestive enzymes present in the stomach. Others form an endospore; a highly resistant capsule that lays dormant for a long period. An endospore is resistant to desiccation, high temperatures, ultra violet radiation, disinfection and extreme freezing. It can also resist anti-bacterial agents hence can thrive on soil or in water prior to infecting a suitable host. Others such as Salmonella have the capacity to survive under low oxygen conditions.
Antigenicity refers to the ability of the pathogen to trigger the production of antibodies within the host. The production of the antibodies as a result of infection by organisms such as measles and rubella could prove to be an advantage to the host later on. The antibodies provide protection in future from an oncoming attack or progression of the disease. In addition to that, antigens that trigger the production of antibodies that offer life long immunity from a disease are used in the production of vaccines. This has proven to be beneficial to society since some diseases which were considered highly virulent in the past such as measles have been effectively eradicated through the use of vaccines. Pathogenicity refers to the ability of an agent to trigger a disease. However, the capacity of some pathogens to trigger disease is dependent on the route of entry. For instance, one can only get typhoid based on the route of exposure and not just because they were exposed to Salmonella typhii which is pathogenic. Infectivity refers to the ability of a pathogen to enter the host and multiply to infectious doses. However, immune responses may hinder replication of the pathogen.
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