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Laboratory Diagnosis of Infectious Diseases

1
Laia Solano‐Gallego, DVM, PhD, DACVP and Gad Baneth, DVM, PhD, DACVP 2
1 Dep. Medicina i Cirurgia Animals, Facultat de Veterinària, Universitat Autònoma de Barcelona, Barcelona, Spain
2 School of Veterinary Medicine, Hebrew University, Rehovot, Israel

Veterinarians use diagnostic testing for infectious clinical signs, clinicopathologic abnormalities, treat-
diseases for two main reasons: ment, prognosis, and prevention will be described in
detail in other chapters in this section.
● to confirm acute or chronic infection in a dog or cat The most commonly employed techniques for the
with clinical signs or clinicopathologic abnormalities diagnosis of infectious diseases in small animals include
compatible with infectious diseases microscopic examination of microorganisms in cyto-
● to detect subclinical infection or certify that animals logic preparations or histopathologic specimens, serol-
are free of infection. ogy, polymerase chain reaction (PCR), and culture of the
Detection of infection may be pursued to screen clinically organism in appropriate medium, as described in the fol-
healthy animals living in endemic regions, to prevent lowing subsections of this chapter. The advantages and
transmission by blood transfusion, to avoid importation of disadvantages of these diagnostic techniques are listed in
infected dogs and cats to nonendemic countries, to avoid Table 79.1. Sample collection, transport, and preserva-
transmission of disease to people in contact with the animal, tion of specimens for the different diagnostic tests and
including immune‐suppressed owners, to monitor response pathogens are described in Table 79.2.
to treatment, and for research. Different diagnostic proce-
dures can be used depending on the purpose of the diag- Protozoal and Arthropod‐Borne Infections
nostic investigation and the pathogen most likely affecting
the dog or cat based on the differential diagnosis list. Test Microscopic Examination
results might also be interpreted according to the aim of the
diagnosis, type of pathogen, and clinical status of animals. Diagnosis can be based on cytologic or histologic detec-
Accurate diagnosis of infectious diseases requires an tion of pathogens, either contained inside cells or free in
integrated routine approach consisting of thorough routinely stained smears. Detection by light microscopy
clinical history, physical examination, pertinent routine of pathogens in cytologic preparations or histopatho-
laboratory tests such as complete blood count (CBC), logic specimens might be difficult, depending on the
complete biochemical profile and urinalysis, diagnostic type of microorganisms causing infection. However,
imaging and additional diagnostic testing, depending on cytology is a more rapid and simple technique for the
the differential diagnoses list for each individual patient. detection of some microorganisms compared with histo-
In addition, specific pathogen diagnostic assays should pathology. Histopathology commonly requires special
be carried out when infectious diseases are at the top of staining to detect bacterial, mycobacterial, fungal or pro-
the differential diagnoses list. tozoal infections. Standard Romanowsky stains such as
This chapter describes general concepts of diagnosis of Giemsa are used for cytologic detection of the majority
viral, bacterial, protozoal, and fungal infectious diseases of organisms (Figure 79.1). Special stains can also be
with an overview of the most common diagnostic meth- used in cytology for enhancing the visualization of
ods employed for these infections. Specific diagnostic microorganisms but are less frequently needed.
features of common infectious diseases in dogs and cats, Although the specificity of these methods is high, it is
geographic distribution, main transmission modes, chief important to highlight that identification of species

Clinical Small Animal Internal Medicine Volume II, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical