Infectious and parasitic diseases

Vomiting and diarrhoea in dogs and cats are two of the most common maladies facing the small animal practitioner today, and bacterial, viral and parasitic entero-pathogens play an important role in many of these cases. Dogs and cats harbouring intestinal parasites or even bacterial pathogens can remain asymptomatic, but may develop clinical signs ranging in severity from a mild, self-limiting diarrhoea, to a potentiallyfatal acute haemorrhagic diarrhoeal syndrome. Evaluation of the animal’s history and physical examination are integral to formulating an appropriate list of differential diagnoses ranking the most likely infectious agents involved. Parvovirus enteritis is an important consideration in a young, unvaccinated puppy presenting with a history of lethargy, anorexia and vomiting, whereas salmonellosis is an important consideration in a middle-aged or older immunocompromised animal that is febrile and presents with a history of haemorrhagic diarrhoea.

Faecal examination

Afaecal examination is integral to the diagnostic investigation of dogs and cats with vomiting, diarrhoea and weight loss (). The techniques used most commonly include direct saline smear (wet preparation), stained smear and faecal flotation. A Baermann technique is indicated when parasitic larval stages are being evaluated.

Direct smear

Fresh faeces (ideally <2 hours old) should always be used to preserve the motility of trophozoites, such as Giardia spp. and Tritrichomonas foetus (). A small amount of faeces is placed on a warm slide and a drop of 0.9% saline is mixed with the faeces. It is importantthatthe smear is nottoo thick, as trophozoites may be missed. A simple rule of thumb is that one should be able to read the fine print of a newspaper through the smear. After application of a coverslip, the smear is evaluated for motile organisms by examining at x10 magnification, with confirmation at x40 magnification.

Stained smear

Stained smears have traditionary been examined to evaluate for: the presence of endospores associated with Clostridium perfringens; the presence of spiral-shaped, Gram-negative bacteria consistent with Campylobacter spp.; orthe presence of increased white blood cells. Unfortunately, the diagnostic utility of increased faecal endospores is very limited as some healthy, non-diarrhoeic dogs can also have increased faecal endospores. In addition, there is no correlation between the finding of increased faecal endospores and the presence of enterotoxin. The presence of spiral-shaped bacterial organisms should be interpreted with caution as there are a number of Campylobacter spp. in dogs and cats, some of which may be non-pathogenic. Perhaps the best use of stained faecal smears is to make a diagnosis of Histoplasrna or Prototheca. The diagnostic yield of stained faecal smears can be increased by using a cotton swab introduced into the rectum and gently rotated several times. The cotton swab is rolled on to a glass slide, which is then stained after air-drying. However, this sample is not ideal for culture, where a sample of faeces is preferred.

Faecal flotation

Faecal flotations are indicated forthe recovery of cysts, oocysts and ova in faeces. Fresh faeces should be examined whenever possible, or a fresh specimen can be refrigerated for up to 72 hours for detection of cysts, oocysts or ova via a concentration technique. Fresh faeces can also be placed in 10% buffered formalin if evaluation will be delayed more than 72 hours. Specimens fixed in formalin are suitable for concentration techniques, acid-fast stains and immunoassays. Although standing (gravitational) flotation methods are easier and quicker to perform than centrifugation flotation, there is abundant evidence supporting the superior sensitivity {up to eight-fold) of centrifugation flotation (). Animals with low parasite burdens could have a false-negative result if the gravitational method is utilized. It is important to appreciate that faecal flotations have limitations and should not be used to detect heavy ova that do not float (e.g. Paragonimus) or larvae (e.g. Aelurostrongylus).

The type and specific gravity of the flotation medium used are important considerations. The author recommends zinc sulphate with a specific gravity of 1.18 to 1.2 for flotations. This solution and specific gravity are optimal for flotation of ova and Giardia cysts, while maintaining the structural detail of the Giardia cyst.

Procedure for centrifugal flotation:

  1. 1. Prepare a faecal emulsion using 2-5 g of faeces and 30 ml of saturated salt or zinc sulphate solution.
  2. 2. Strain the emulsion through a tea strainer or cheesecloth into a 15 ml conical centrifuge tube. Suspending a funnel over the tube facilitates filling the tube.
  3. 3. Fill the tube with flotation medium to create a positive meniscus.
  4. 4. Place a coverslip on top of the tube.
  5. 5. Balance the tube in the centrifuge.
  6. 6. Centrifuge the tubes for 10 minutes at 400-600 G (approximately 1500 r.p.m).
  7. 7. Carefully remove the coverslips from the tubes by lifting straight up and place them on a slide.
  8. 8. Examine the slide within 10 minutes. Examine entire coverslip at x10. Use x40 to confirm identification by visualizing internal structures and measuring the organism.

Modification: If using a fixed-angle centrifuge that does not have free swinging buckets, the above technique is used but the centrifuge tube is filled to within an inch or so from the top, and a coverslip is not added for the final spin. When the final centrifugation step is complete, the tube is carefully set upright in a test tube rack. A pipette is then used to gently run additional flotation solution down the side of the tube while disturbing the contents as little possible. A positive meniscus is created and a coverslip set on top. The tube is allowed to stand for 5 minutes only. The coverslip is removed and placed onto a slide and examined as described in step 8.

Faecal immunoassays

The morphological detection of Giardia cysts () or Cryptosporidium oocysts () via faecal flotation can be technically demanding and time-consuming. Implementation of immunoassays avoids the necessityformorphological identification altogether, while relying upon the observation of a colorimetric reaction in a well. In addition, immunoassays can be performed on formalinized or frozen faeces.

Intestinal parasites

Enteropathogenic bacteria

Viral infections

Intestinal fungal infections


Histoplasma capsulatum may affect the GI tract, respiratory tract, bones and eyes; infection has not been reported in the UK. Intestinal involvement in dogs is characterized by diarrhoea (with or without melaena or haematochezia) and severe weight loss. Cats are more commonly presented with pulmonary involvement. In dogs, the colon is usually more severely affected, resulting in clinical signs of tenesmus, mucoid stools and haematochezia. Severe disease can be associated with panhypoproteinaemia.


Identification of the organism via cytology or histology is required. Rectal scrapings should be performed in dogs suspected of having histoplasmosis. A comprehensive diagnostic investigation, consisting of a fundoscopic examination to lookforevidence of chorioretinitis, thoracic radiography to evaluate for pulmonary interstitial infiltrates or hilar lymphadenopathy, and abdominal ultrasonography to evaluate the spleen, liver and mesenteric lymph nodes is usually indicated.


Itraconazole (5 mg/kg p.o. q12h for 4 days, then q24h thereafter) or amphotericin B (0.25 mg/kg i.v. as test dose, then 0.5 mg/kg i.v. up to three times weekly) are usually effective. The prognosis is extremely variable depending at what stage the disease is diagnosed.

Infectious and parasitic diseases: Conclusion

Comprehensive faecal exams are pivotal in the diagnostic investigation of dogs and cats with diarrhoea or vomiting. The diagnostic yield will be markedly increased with the examination of fresh faecal specimens, the utilization of a centrifugation technique with zinc sulphate solution, and the timely incorporation of immunoassays for diagnosing Giardia and Cryptosporidium spp. The clinical documentation of entero-pathogenic bacteria causing diarrhoea in dogs is clouded by the presence of many of these organisms existing as normal constituents of the indigenous intestinal flora. The diagnosis of a putative bacterial enteropathogen(s) in dogs and cats should be made based on a combination of parameters, including clinical details and predisposing factors, clinical signs, serological assays for toxins, faecal culture and PCR. Relying on results of faecal culture alone is problematic because Clostridium perfringens, Clostridium difficile, Campylobacter spp., and pathogenic and non-pathogenic E. coli are commonly isolated from apparently healthy dogs and cats. Faecal cultures may be useful in procuring isolates for the application of molecular techniques, such as PCR, fordetection of specific toxin genes or molecular typing of isolated strains to establish clonality in suspected outbreaks. The over simplistic attempt to characterize acute bacterial-associated diarrhoea by anatomical localization of clinical signs should be discouraged, as most of the above mentioned bacteria have been associated with both small and large intestinal diarrhoea. Accurate diagnosis of infections may require diagnostic laboratories to incorporate PCR-based assays using genus- and species-specific primers to facilitate detection of toxin genes and diffe rentiation of species that appear phe n otypical ly and biochemically similar.