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Principles of infectious disease and transmission

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Abstract

Infectious diseases can be a major problem in the shelter environment. This chapter provides a very brief overview of the infectious agents that can cause disease, how the pathogens of importance are transmitted, and how they might be managed and treated.

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Figures

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8.1 Canine coronavirus. (Unstained; original magnification X200,000) (Courtesy of B Getty)
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8.2 Gram-positive bacilli. (Reproduced from the )
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8.3 (a) Cocci seen in this smear are stained blue. (Diff-Quik® stain; original magnification X63) (b) Bacilli can be seen in this smear taken from a dog with otitis caused by . Some of the bacteria have been taken up by neutrophils (arrowed). (Diff-Quik® stain; original magnification X63) (a, Courtesy of L Grau Roma; b, © Jenny Stavisky)
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8.4 Stained faecal smear showing characteristic appearance of with its long undulating membrane. (Reproduced from the )
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8.5 in a smear taken from the ear of a dog with otitis. (Diff-Quik® stain; original magnification X63) (© Jenny Stavisky)
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8.6 Microscopic appearance of ectoparasites. (a) The canine biting louse , suspended in liquid paraffin. (Original magnification X100) (b) The canine sucking louse , suspended in liquid paraffin. (Original magnification X100) (Reproduced from the )
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8.7 Feline immunodeficiency virus (FIV) is considered a relatively host-specific virus. (a) FIV is most commonly identified in feral or stray male cats and can be transmitted by deep bites. (b) Rapid diagnostic tests for FIV and feline leukaemia virus (FeLV), showing control (C) and test (T) bands in the window where the results are read. These samples are negative for FeLV and positive for FIV. (Courtesy of R Elmore)
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8.8 Severely ill or malnourished animals may be at increased risk of infectious disease. (Courtesy of R Elmore)
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8.9 (a) Limited ventilation, cracked surfaces and poor drainage can contribute to disease persistence and transmission. If organic debris is not cleaned properly, disinfection will not be adequate. (b) This scoop was used for cleaning faeces from dog kennels, then left to soak in disinfectant without having been cleaned. (© Jenny Stavisky)
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8.10 (a) Barren environments are relatively easy to clean and disinfect. However, they can be highly stressful for residents. (b) Enriched environments are beneficial for reducing stress. However, in the event of an outbreak of disease, decontamination may be difficult. (© Jenny Stavisky)
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8.11 Immunofluorescent microscopy of FCV-F9 infected feline embryo A cells immunostained for FCV (green). FCV = feline calicivirus (© A Radford and M Afonso)
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8.13 Polymerase chain reaction (PCR) searches for a target molecule of DNA (or RNA). If this target is present, multiple copies are made and the results are visualized on a gel. (a) Pre-stage: If the target is RNA (e.g. for many RNA viruses), reverse transcription is used to make an identical DNA copy. (b) Stage 1: Sample containing DNA target (or a DNA copy of the original RNA target if the pre-stage has been required) is heated to separate the strands and then mixed with ingredients for DNA synthesis. Primers are added that will bind only to the target sequence the assay is designed to detect. (c) Stage 2: The mixture undergoes repeated cycles of heating and cooling. At each cycle, the primers move along the strands of DNA making copies of it. (d) Stage 3: As there is a large excess of primer in the mix, each new strand is duplicated with every cycle. Therefore, with each cycle of heating and cooling, the number of copies doubles. After around 40 cycles, there is a large quantity of identical copies of short strands of DNA. This will only occur if the target is present. (e) When the reaction has finished, the products are introduced into a gel and an electrical charge is applied to separate the contents by size. By knowing how far apart the primers are placed on the target DNA, the size of the DNA fragment can be predicted. If a large amount of fragments of DNA of the correct size are seen on the gel, the sample is considered positive. In this example, dog 1 is negative and dogs 2–6 are all positive.
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8.14 (a) In real-time, quantitative or ‘fluorogenic’ PCR, during each cycle fluorescence is emitted when the target sequence is duplicated. The amount of fluorescence generated in each cycle is proportional to the number of copies of the target present, and therefore increases with each cycle. (b) This fluorescence is measured by using a machine, rather than the naked eye, making real-time PCR 10–100 times as sensitive as conventional PCR. It is also possible to estimate the quantity of target that was present in the initial sample. This is because if more target is initially present, it will take fewer cycles of replication for the reaction to reach the threshold at which it can be detected. The number of cycles required to reach this is termed the cycle threshold (CT). Samples which have a larger amount of the target molecule will reach the threshold sooner and therefore have a lower CT number. Rn is a measure of the amount of fluorescence generated.
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8.15 Bacterial culture and determination of antibacterial resistance.
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8.16 Virus growing in cell culture causes cell death, known as a cytopathic effect (CPE). (a) Healthy cell culture. (b) CPE caused by infection with feline herpesvirus. (© A Radford and M Afonso)

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