While PCR is an excellent way to rule out KNOWN causes of disease, most diagnostic PCR assays are not designed to detect newly emergent disease. This is another reason to include histologic analysis of tissues in the diagnostic plan: by ruling out common diseases, a newly emergent cause for disease can be more quickly identified. The most difficult emergent diseases to recognize and identify are those that mimic known diseases. For retrospective analyses, DNA can also be extracted from tissue embedded in paraffin, however, this is offered by a more limited number of laboratories.
1 Toxicology:It is best to contact a toxicology laboratory, state laboratory, and/or a poison control center such as the ASPCA (http://www.aspca.org/apcc) for guidance. The appropriate sample for analysis is dependent on the type of toxin, among other variables. In the case of GI illness, source (food) and stomach contents should be saved. For heavy metal analysis, samples of liver and kidney should be collected, placed in separate plastic bags, and frozen until submitted. If a toxin is suspected but unknown, a necropsy should be performed and, in addition to histologic samples, liver, kidney, fat, stomach contents, and muscle samples should be frozen until submitted.
2 Serology:Serodiagnostic tests are tests performed on serum or plasma to detect either the presence of antibodies to a particular pathogen or the presence of circulating antigens from the pathogen itself. Both of these types of tests can be performed postmortem on serum obtained from a pooled blood source (e.g. heart, major veins). The significance of the result should be considered before this particular technique is used; few tests are validated for postmortem serum. Nonetheless, a positive titer is generally considered significant. See Chapter 4 for more details on serologic testing.
5.5.2.2 Parvovirus (Canine Parvovirus (CPV), Feline Panleukopenia Virus (FPV))
In the shelter, the most common causes of intestinal disease associated with mortality are the canine and feline parvoviruses. Suspicion and caution for this disease, therefore, is high; however, no clinical or gross finding is specific to parvoviral enteritis. This is a good reason to perform a necropsy on a dog or cat that is either suspicious for or known to be infected with parvovirus.
1 Establishing cause:Tissues for histology: Necropsy with histology can confirm the presence of parvovirus and would be important in ruling out parvovirus during an investigation of an unusual outbreak of GI disease. Acute cases of parvovirus are nearly pathognomonic by histologic analysis and chronic (historic) cases can also be detected by a pathologist; the architecture of the small intestine is not restored to normal for two to three weeks post‐infection.Other tests: In dogs, although the parvovirus antigen tests on feces are highly sensitive during viral shedding in the early stages of infection, these peak viral titers are brief and occur at the same time as, or prior to, the onset of clinical signs (Greene 2011). Subsequent viral shedding is known to fluctuate, and if the fecal antigen test is performed late after infection, virus in feces may be undetected. Testing of nearby, recently exposed animals is warranted, and in an animal that has died, feces should be retested at the time of necropsy with fecal material collected from pooled segments of the lower intestine (duodenum, jejunum, and colon). Numerous cases of dogs or cats have been seen whose feces were negative one to two days prior to death and submitted for evaluation of “non‐parvoviral diarrhea.” These same animals were often positive by fecal antigen tests at the time of necropsy and in these cases, there was concurrent histologic confirmation of parvoviral disease to establish etiology.
Tissue and fecal samples from dogs or cats collected at the time of necropsy are also useful for PCR amplification of virus. There is a higher sensitivity by using the PCR on infected tissues as compared with fecal antigen retrieval (Decaro and Elia 2005). In addition, many laboratories offer additional diagnostic methods on tissue samples such as immunofluorescence or immunohistochemistry.
1 Unusual presentation:
The progression of any disease can vary greatly among affected animals. Among the factors that can alter the “normal” course of disease are co‐infections, viral dose and virulence, and/or the animal's age, breed, and clinical presentation.
1 Concurrent disease(s):
Parvoviral disease, even when suspected or confirmed clinically, may be exacerbated by concurrent infections with bacteria, Giardia, hookworms, or other enteric viruses such as coronavirus. Samples should be gathered that can potentially rule concurrent disease either in or out.
5.5.2.2.1 Gross Findings of Parvoviral Disease
The gross findings of parvoviral disease, although caused by a similar virus, manifest somewhat differently in the dog and cat. It would be unlikely that a puppy or adult dog would die suddenly of parvovirus in the absence of dehydration, diarrhea, and other well‐documented clinical signs. On the other hand, kittens can die per‐acutely of panleukopenia, with no preceding signs noted. On necropsy, dogs commonly have segmental to diffuse sub‐serosal hemorrhage (reddening) that predominantly affects the small intestine (See Figure 5.8).
The small intestine can be flaccid and/or dilated. There will be scant ingesta within the intestinal system and no formed feces within the colon. On section of the small intestine, the mucosa is segmentally to multifocally discolored tan to dark red (necrosis, congestion, hemorrhage). Peyer's patches, which are more concentrated in the distal small intestine and ileum, can be dark red (lympholysis).
In cats, the findings can be similar but are usually subtle. The small intestine is flaccid or dilated, but not always reddened, and the GI contents, although typically watery or scarce, do not always contain blood. In both dogs and cats, the mesenteric lymph nodes are enlarged, congested and wet (edematous). Because the effect of panleukopenia on bone marrow and primary lymphoid tissues is quite predictable in the feline form of the disease, it is a good idea to include these tissues when performing a necropsy on either a dog or a cat.
Figure 5.8 Canine parvovirus (CPV). The intestines are segmentally thick, edematous, and hemorrhagic, and the mucosal surface (pictured here) is dull and felt‐like.
5.5.3 Respiratory Disease, General
The following collection would be a good starting point for sampling any respiratory disease of unknown origin in dogs or cats. While causes for respiratory distress can be remote from the respiratory system, most cases of infectious pneumonia or upper respiratory infections (URI) result from a direct attack on the respiratory tissues. Gross lesions of the lung are difficult to interpret. This is, in no small part, because there are often peri‐mortem lung changes, and such variability makes a baseline interpretation of “normal” very difficult. Histologic samples are of paramount importance when trying to discern factors contributing to lung disease. In cats, in particular, analyzing the upper respiratory tract as well as the lung is important; many common infections of the upper respiratory tract can contribute to fulminant respiratory disease, and severe URI is often interpreted as pneumonia (infection of the lower respiratory system).
1 Microbiology:In respiratory disease, depending on the lesions and clinical course, bacterial cultures can be taken from the nasal cavity, frontal sinus (swab immediately after opening) and/or lung. In general, because URI is so common in kittens and cats in a shelter, cultures should be taken from both the nasal
