​Trypanosoma cruzi trypomastigotes, observed in the blood of acutely infected dogs for a short time after infection. Morphological identification can be accomplished based on the presence of a terminal/subterminal kinetoplast that is darkly stained at the posterior end, a centrally located nucleus, an undulating membrane, and a flagellum that runs along the length of the trypomastigote body extending anteriorly beyond the margin of the flagellate. Trypomastigotes range in length from 12 to 30 µm and are often C-shaped (arrow) or J-shaped in fixed preparations.
Despite finding trypomastigotes in this case, diagnosing Chagas disease in dogs is usually accomplished by T. cruzi antibody detection, using immunofluorescence antibody (IFA) tests in chronically infected dogs. Trypanosoma cruzi infected dogs are more likely to have ventricular arrhythmias, combinations of ECG abnormalities, and cardiac troponin I (cTNl) >0.129 ng/mL. Surveys of dogs in T. cruzi endemic areas, showed that infected dogs were significantly younger than negative dogs and the highest prevalence of infection were in non-sporting and toy breed dogs. Odds of T. cruzi infection were 13 times greater among dogs where there was an infected housemate or littermate.

​Oxyuris equi, are the pinworm parasites found in the colon of horses. Gravid females measure 4 to 15 cm.  They migrate down the colon to the rectum through the anus to cement the eggs in the surrounding anal skin.  The egg masses are a yellowish fluid with thousands of eggs. The fluid and egg masses dry out causing pruritus and contaminating fomites as well as the environment where the eggs become infective.  Eggs are asymmetrical with a single polar plug (Image 2). Adults have a pointed tail (image 3) and an esophageal bulb (Image 4).

Baylisascaris procyonis, the raccoon roundworm that causes visceral larva migrans in a wide range of hosts including humans. The infective larvae tend to invade the central nervous system of paratenic hosts and it can be deadly to small mammals and birds.  Hay, straw and other bedding materials contaminated with raccoon feces can be a source of infection.  Dogs can be definitive hosts for the adult parasites as well.

​Oestrus ovis, larva third stage (L3).  The nasal botfly, is a small fly ~ 1cm long with vestigial mouthparts. They are active during warmer hours of the day, depositing larvae (L1) in the nostril of sheep.  The L1 crawls onto the mucous membrane of the nasal passage remaining there for about 2 weeks, or in arrested stage during the winter. The larva then moves to the frontal sinuses where they reach the L3 stage.  After full development, the L3 larva crawl down into the nasal passages and is expelled by the sheep’s sneezing.
O. ovis larvae are not pathogenic, but heavy infestation can cause, nasal discharge, sneezing and partial blockage of the nasal passages.  Diagnosis is made by location of the larva on the host and morphology of spiracles. (Image 1)  Although these larvae are commonly called the sheep botfly, larvae have been also found in deer, goats, and cattle. There are also reports of horse, dog and human infestation.