HOST PARASITE RELATIONSHIPS
Virulent strains of Newcastle disease virus (NDV) cause Newcastle disease (ND) . To ensure advanced biosecurity, all the tests were carried out in the SEPRL . close phylogenetic relationships between the historical and recent virulent . These facts preclude the accumulation of infectivity in natural habitats of birds (53 ). The purpose of Introduction to Infectious Diseases and Immunology is to provide students with a sound quizzes. 24% (6% per module). ~20min each Practical and. Workshop material. Weeks 4- the terms used in describing host-parasite relationships. • the four details of the key microbial virulence factors. Reference . Virulence = degree or intensity of pathogenicity. Disease Infection, when parasitic microorganisms increase in number either within or on the body of the host.
This type of antigenic variation occurs during the course of a single infection. Another type of antigenic variation occurs in certain agents, such as the foot-and-mouth disease virus, that are highly infectious in nature and that depend for their survival on a continuous cycling through host populations of relatively long-lived animals. The ability to reinfect the same host at a later date is obviously desirable for the agent's survival, and this is dependent on the generation of a relatively short-lived immunity combined with the ability of the agent to undergo antigenic variation during its passage through the host population.
In such circumstances there is a strong selection pressure for antigenic variants.
The two main types of variation are: Antigenic shifts are of particular significance when the control of a disease is being attempted by vaccination, since in effect they represent the introduction of a new agent against which the existing vaccine is likely to confer little or no immunity. The capacity of parasites to evolve rapidly has important implications in other areas of disease control.
The very act of introducing a control measure or disease treatment may, in itself, create conditions whereby a strong pressure is exerted on the agent population to select strains which are resistant to the measures or treatments imposed. The evolution of such resistant strains will, in turn, jeopardise the effectiveness of the control measure or treatment.
Resistant strains of agents are most likely to develop when the measures or treatments are carried out on a wide scale but improperly - as, for example, in the case of antibiotic resistance arising through the widespread, unsupervised use of antibiotics by livestock producers. Methods of transmitting infectious agents Ascertaining the means by which disease agents are transmitted is a major objective in epidemiological studies, since once the mechanisms by which a particular disease is transmitted are understood, it may become possible to introduce measures to prevent transmission from taking place.
There are three main ways by which disease agents are transmitted from infected to susceptible hosts. An agent may be transmitted through contact between infected and susceptible individuals, or it may be conveyed between these individuals by means of an inanimate object or via another animal serving as a vector or intermediate host.
These methods of transmission are not mutually exclusive; the same disease agent may be transmitted by more than one of the following ways. In contact transmissions the agent is conveyed between hosts through direct physical contact, as in the case of venereally transmitted diseases such as vibriosis or trichomoniasis, or through indirect contact.
In cases of indirect contact the agent is normally contained in the excretions, secretions or exhalations of the infected host i. Susceptible hosts contract the infection either by direct exposure to these or through exposure to substances contaminated by them. Diseases spread in this fashion include rinderpest, foot-and-mouth disease, Newcastle disease, and contagious bovine pleuropneumonia.
Contact transmissions can be further distinguished according to whether they occur horizontally between individuals of the same generation or vertically between individuals of different generations.
In vertical transmissions the infectious agent is usually passed from dam to offspring either in the uterus or through the colostrum. The main factors determining whether or not transmission takes place in contact-transmitted diseases are: Rinderpest virus, for example, is easily destroyed in the environment, so contact between infected and susceptible individuals must be close and immediate for transmission to take place, whereas, under certain circumstances, foot-and-mouth disease can spread between widely separated stock.
The control of livestock movements is, therefore, a vital factor in the control of contact-transmitted diseases which, in Africa, normally occur more frequently during the dry season when livestock movements are at their highest.
In vehicular transmission the agent is transferred between infected and susceptible hosts by means of an inanimate substance or object sometimes called fomitesuch as water, foodstuffs, bedding materials, veterinary equipment and pharmaceuticals, or on the skin, hair or mouthparts of animals.
In contrast to indirect transmission, the survival time of the agent in or on the vehicle is usually prolonged. This means, in effect, that vehicular transmission can take place over greater distances and over longer time periods. Hygiene, disinfection and control over the distribution of likely vehicles of transmission are important factors in the control of vehically transmitted diseases.
Certain agents may take the opportunity to reproduce themselves during vehicular transmission. This occurs in the transmission of food-borne bacteria, such as salmonella and coliforms, and underlines the importance of strict hygiene in the handling of foodstuffs and livestock feeds, since a small initial contamination may eventually result in the gross contamination of a whole batch of food or feed. Vectors and intermediate hosts. Confusion frequently arises between the terms "vector", "intermediate host" and "definitive host".
The latter two terms are essentially parasitological terms and describe the different types of hosts that are biologically necessary in the lives of agents with relatively complicated life cycles. The definitive host is usually a vertebrate, while intermediate hosts can be either vertebrates or invertebrates. Essentially, vectors can transmit infectious agents in two ways. They can serve as a vehicle whereby the infectious agent is conveyed from one host to another without undergoing a stage of development or multiplication.
This is known as mechanical transmission. Alternatively, the infectious agent can undergo some stage of development or multiplication in the vector - this is known as biological transmission - and in this case the vector is serving either as an intermediate or definitive host, depending on which stage of the development cycle of the agent takes place within it.
Principles of Epidemiology: Lesson 1, Section 9|Self-Study Course SS|CDC
Vertebrate intermediate hosts play the same role in the transmission of their disease agents as biological vectors. In mechanical transmission the agent is carried on the skin or mouthparts of the vector from an infected to a susceptible host.
The survival time of the agent in or on the vector is usually short, and as a result the transmission of the agent has to be accomplished rapidly. The carriers are normally winged haematophagous insects, and transmission usually takes place when susceptible and infected hosts are in close proximity and when large numbers of vectors are present. In biological transmission, since the agent develops in the vector, a period of time elapses between the acquisition of the infectious agent by the vector and its becoming infective.
Multiplication of an infectious agent within the body. Multiplication of the bacteria that are part of the normal flora of the gastrointestinal tract, skin, and so on is generally not considered an infection; on the other hand, multiplication of pathogenic bacteria eg, Salmonella species —even if the person is asymptomatic—is deemed an infection.
The process whereby bacteria, animal parasites, fungi, and viruses enter host cells or tissues and spread in the body. Microbial flora harbored by normal, healthy individuals. A microorganism that does not cause disease; may be part of the normal microbiota.
An agent capable of causing disease only when the host's resistance is impaired ie, when the patient is "immunocompromised".
Lesson 1: Introduction to Epidemiology
A microorganism capable of causing disease. The ability of an infectious agent to cause disease. This term reflects the spread of the disease. Total of cases of disease in a population at any given time. This term reflects how sick the population is.
Occurrence of a disease in a population over a defined period of time.
When bacteria are actually growing and dividing in the blood. Commonly felt by the patient. Commonly observed by a physician. Nosocomial infections -- Hospital acquired infections are especially fearsome because: The incubation period - initial infection and the first appearance of signs or symptoms.
The prodromal period -short duration.
Period of initial mild sign or symptoms.