final core diagnostic tool with which the shelter practitioner should be proficient is the appropriate use of response to treatment to make a presumptive diagnosis. Response to treatment refers to the empirical prescribing of medications or courses of treatment without confirmation of a diagnosis. Rather, a presumptive diagnosis and treatment are based on historical information and physical examination findings, and the diagnosis is presumed to be correct based on the resolution of the clinical problem. Careful utilization of this technique can provide benefits of immediately addressing existing conditions, minimizing opportunities for infectious disease transmission, and, when the presumptive diagnosis is correct, reducing the time and costs associated with obtaining a definitive diagnosis before initiating treatment. However, there are significant downsides to relying on this technique on a routine basis. When used indiscriminately, reliance on response to treatment can result in misdiagnosis; misuse of pharmaceuticals; prolonged time to diagnosis, treatment, and disease resolution; and an overall ineffective use of resources. Utilizing response to treatment as a basic diagnostic tool may be considered in the following circumstances:
An urgent threat to animal health or welfare
Severe medical resource limitations
Diagnostic testing options are limited in accuracy, availability, or practical turnaround time
Inability to obtain veterinary care (e.g. after‐hours or irregular access to veterinary support)
Treatment is likely to provide substantial benefit but unlikely to cause harm.
In each case, a response to treatment protocol should be developed under the direction of a veterinarian familiar with the shelter population, community resources, and limitations. Model protocols include the following components:
Re‐evaluation of patient status after starting treatment and regularly thereafter
The appropriate use of first‐line antimicrobials (e.g. amoxicillin vs. amoxicillin and clavulanic acid) (See Chapter 7 for more information.)
Defined end‐points after which definitive diagnostics or a revised treatment plan will be pursued
Periodic review of SOPs including evaluation of antimicrobial choices based on definitively diagnosed conditions.
Common shelter scenarios in which response to treatment may be beneficial include mild upper respiratory infections (URIs), mild enteritis, first‐time urinary tract infections (UTIs), minor wounds, and superficial pyoderma.
4.3.2 Primary Diagnostic Testing
4.3.2.1 ELISA
The ELISA is perhaps the most readily available and widely utilized diagnostic test in veterinary medicine. In general, ELISAs provide immediate results, are easy to use, cost‐effective, able to be conducted outside of a laboratory setting, highly accurate, and adaptable for the detection of a wide variety of analytes. In the shelter setting, ELISAs are commonly utilized for the detection of both antigen and antibodies to common pathogens (e.g. Dirofilaria immitis, Giardia spp., and feline leukemia virus [FeLV] antigen; Ehrlichia spp., Anaplasma spp., Borrelia burgdorferi, Leptospirosis spp., and feline immunodeficiency virus [FIV] antibody), assessment of antibody titers against common pathogens (e.g. canine distemper virus, canine parvovirus, canine adenovirus‐2, feline panleukopenia, feline herpesvirus‐1, feline calicivirus), and the detection of hormone levels for example, relaxin and luteinizing hormone (see Table 4.3). Test kits for the detection of metabolic biomarkers are also available. Anticoagulated whole blood, serum, or plasma can typically be utilized in point‐of‐care ELISA test kits depending on manufacturer instructions. Particularly as pertains to feline retrovirus (FeLV and FIV) testing, samples of saliva or tears have also been used in such test kits and while historically have resulted in an unacceptably low sensitivity, newer devices may be more accurate (Westman et al. 2016, 2017).
Table 4.3 Common diagnostic tests, indications, and collection and handling techniques for in‐house laboratories (Belford and Lumsden 1998; Bowman 2014; Rosenfeld and Dial 2010a, b; Zajac and Conboy 2012).
| Diagnostic Test | Indications | Collection and Handling Techniques |
|---|---|---|
| Cytology | ||
| Fine needle aspirate | Evaluate mass lesions in the skin, subcutaneous tissue, thorax, and abdomen | (A) Insert a 21–25 g needle attached to a ≥5 ml syringe into the lesion; apply 3–5 ml of vacuum; move the needle forward and back while altering direction; release the vacuum and withdraw needle; (B) Insert a 21–25 g needle directly into the lesion; move the needle forward and back while altering direction; withdraw the needle.Remove needle (A) and aspirate 3–5 ml of air into the syringe; (A, B) attach the needle to the syringe and depress the plunger to deposit a drop of aspirated material onto the slide. |
| Impression smears, imprints, or swabs | Superficial lesions, otic cytology, surgical biopsy specimens | Ensure the sample site is clean and dry.Gently press clean glass slide on or rub cotton swab across the surface of the tissue.Roll cotton swab across a clean microscope slide. |
| Scrapings | Diagnosis of external parasites (e.g., Demodex spp.), pyoderma, cutaneous lesions, surgical biopsy specimens | Hold a sterile scalpel blade at a 90° angle to the surface of the tissue; scrape vigorously across the surfaceThe edge of a glass slide can also be utilized to obtain superficial surface samplesFor diagnosis of demodicosis, utilize a scalpel blade and scrape until capillary bleeding occursFor cutaneous lesions and surgical biopsy specimens, ensure the tissue is clean and dry prior to scraping |
| Immunoenzyme assays | ||
| ELISA | Antigen or antibody detection: infection or exposure (e.g., Anaplasma spp., Borrelia burgdorferi, canine parvovirus, Dirofilaria immitis, Ehrlichia spp., feline panleukopenia, feline leukemia, Giardia), antibody titer assessment (canine parvovirus, canine distemper, feline panleukopenia) Hormonal assays: luteinizing hormone, relaxin | Vary by assay; see individual techniques below and refer to test manufacturer guidelines |
| Multi‐well test kits (e.g. ASSURE®/FeLV, DiroCHEK®, TiterCHEK® CDV/CPV [Zoetis, Parsippany‐Troy Hills, NJ]) | Anticoagulated whole blood, serum or plasma samplesaSamples can be fresh, refrigerated for up to 7 days, or frozen and thawedaStore test kits under refrigeration until the expiration dateAllow samples and test kits to reach room temperature before testing | |
| Single‐well test devices (e.g. SNAP tests [IDEXX Laboratories, Westbrook, ME]) |
Anticoagulated whole blood, serum, plasma, or fecal samplesaSamples can be fresh, refrigerated up to 1 week, or frozen and thawedaAllow samples and test kits to reach room temperature before testingStore test kits under refrigeration until expiration date OR at room temperature for 90 days (or until expired)Test kits removed from refrigeration
| |
