Chapter 2 Disease and disease transmission

1 7 Disease AND Disease TRANSMISSIONC hapter 2 Disease and Disease transmissionAn enormous variety of organisms exist, including some which can survive andeven develop in the body of people or animals. If the organism can causeinfection, it is an infectious agent. In this manual infectious agents which causeinfection and illness are called pathogens. Diseases caused by pathogens, or thetoxins they produce, are communicable or infectious diseases (45). In this manualthese will be called Disease and Chapter presents the transmission cycle of Disease with its different elements,and categorises the different infections related to Introduction to the transmission cycle of diseaseTo be able to persist or live on, pathogens must be able to leave an infected host,survive transmission in the environment, enter a susceptible person or animal, anddevelop and/or multiply in the newly infected transmission of pathogens from current to future host follows a repeatingcycle.

2 This cycle can be simple, with a direct transmission from current to futurehost, or complex, where transmission occurs through (multiple) intermediatehosts or cycle is called the transmission cycle of Disease , or transmission cycle. Thetransmission cycle has different elements: The pathogen: the organism causing the infection The host: the infected person or animal carrying the pathogen The exit: the method the pathogen uses to leave the body of the host transmission : how the pathogen is transferred from host to susceptible personor animal, which can include developmental stages in the environment, inintermediate hosts, or in vectors8 CONTROLLING AND PREVENTING Disease The environment: the environment in which transmission of the pathogentakes place.

3 The entry: the method the pathogen uses to enter the body of the susceptibleperson or animal The susceptible person or animal: the potential future host who is receptive tothe pathogenTo understand why infections occur in a particular situation, and to know how toprevent them, the transmission cycles of these infections must be understood. Therest of this Chapter looks at the elements of the transmission cycle in more The different elements of the transmission cycle of diseaseThe environmentTransmissionThe hostThe pathogenThe pathogen leavesthe hostEntry of the pathogenThe susceptible personor animal9 Disease AND Disease The pathogenThe pathogen is the organism that causes the infection.

4 * Specific pathogens causespecific infections . Cholera is caused by the bacterium Vibrio cholerae, forexample, and Leishmaniasis is caused by different species (spp.) of the infections also have specific transmission cycles. To be able to reactappropriately to health problems in a population, the specific infection causing theproblems must be known. Identification of the infection will usually be done bymedical categories of pathogens can infect humans. The pathogens causing thediseases covered in this manual include viruses, bacteria, rickettsiae, fungi, proto-zoa, and helminths (worms). All pathogens go through a lifecycle, which takes theorganism from reproducing adult to reproducing adult.

5 This cycle includes phasesof growth, consolidation, change of structure, multiplication/reproduction, spread,and infection of a new host. The combination of these phases is called thedevelopment of the terms are commonly used to describe pathogens leaving the host throughfaeces or urine: latency and excretion, a latent pathogen must develop in the environment or intermedi-ate host before a susceptible person or animal can be infected. During the latentperiod the pathogen is not infectious. A non-latent pathogen does not need to gothrough a development, and can cause infection directly after being describes how long a pathogen can survive in the environment.

6 Apersistent pathogen remains viable for a long period outside the host (perhapsmonths), while a non-persistent pathogen remains viable for only a limited period(6) (days, or weeks).Active immunity is the resistance the person or animal develops against thepathogen after overcoming infection or through immunisation (vaccination).(45).Depending on the pathogen, the effectiveness of active immunity often decreasesover time.*It is important to realise that not all infections will result in Disease . While a pathogen may cause illness in oneperson, it may be killed or cause asymptomatic infection in AND PREVENTING DISEASEU sually immunity only develops against the specific pathogen that caused theinfection.

7 If there are different types (serotypes or strains) of the same pathogen( in dengue fever and scrub typhus), immunity will often only develop againstthe particular type which caused the infection. The person or animal can stilldevelop the illness when infected with another serotype or strain of the patho-gen(3).Table presents the different categories of pathogenic organisms with some oftheir characteristics, including latency, persistence, and immunity. The informa-tion is general, and exceptions can The hostThe host is the person or animal infected by the pathogen. The importance of thehost in the transmission cycle is its roles as both reservoir and source of are two types of host: definitive and intermediate host.

8 The definitive host isthe person or animal infected with the adult, or sexual, form of the pathogen. Inthe infections covered here, people are usually the definitive host. To keep thingssimple the definitive host is called just the host .The intermediate host is an animal or person infected by a larval, or asexual, formof the pathogen(3). Cysticercosis and hydatid Disease are the only infectionscovered here for which people are the intermediate host. Where intermediate hostis meant, this term is used. Of the infections covered here, only helminths haveboth definitive and intermediate hosts. All other pathogens only have definitivehosts, although vectors function technically as intermediate hosts for : transmission from animal to personSome pathogens are specific to humans, others to animals.

9 Many pathogens areless specific and can infect both people and animals. infections that can naturallybe transmitted from animal to person are called zoonoses (3). Zoonoses are verycommon; over half of the infections covered in this manual are zoonoses. Many ofthese infections normally occur in an animal cycle, with people being infected problem with zoonoses is that a continuous reservoir of pathogens existsoutside humans. Even if all human infections were cured and transmission topeople stopped, the presence of an animal reservoir would remain a continuousrisk to AND Disease TRANSMISSIONT able Categories of pathogenic organisms and their characteristicsPathogenVirusRickettsiaeB acteriaFungiProtozoaHelminths(worms)Desc riptionParticles invade livingcells.

10 The pathogenneeds structures inthese cells toreproduce. (45)Organisms resemblebacteria. (45).However, similar toviruses, the pathogenneeds to developinside the cells of thehost.(2)Bacteria are single cellorganisms. They areconsidered moreprimitive than animalor plant cells. (45)A group of organismswhich include yeast,moulds, andmushrooms. (45)Protozoa area singlecell organisms. (45)Helminths are worms(roundworms, flukes ortapeworms) (45). Oftenmale and female mustmeet in host toreproduce, andsometimes theymultiply inintermediate pathogensare pathogensare pathogensare pathogen islatent. It oftenhas a complexlifecycle with adevelopment inthe environmentor intermediatehosts.