horse_quiz_2This poor horse developed this lesion under its tail.  On examination there were also two other lesions on the horse’s flank. You were asked to come up with a few “differential diagnoses” and after receiving several excellent entries, we selected Meredith as the winner, for her suggestion of Habronema, or “summer sores”.







meredithAt left is a picture of the WINNER – Meredith, looking very sharp in her new  VetweRx jacket.

So where does Habronema fit in our chart of organisms?  It is a nematode, or multicellular eukaryote, that lives in the horse’s stomach. The adults lay eggs that pass in the horse’s feces, where they hatch into larvae.  Now here’s where it gets interesting. Insects – house flies (Musca) and stable flies (Stromoxys) lay their eggs in horse manure, and these eggs hatch into fly larva (called Maggots).  The maggots ingest the nematode larva in the manure, and become infected.  When the infected maggots mature into the adult fly, the nematode larva (which are still there) migrate to the mouth parts of the adult fly. These infected flies hang around moist areas – like horses lips. The nematode larva are thus deposited on the horse’s lips and ingested, where they enter the stomach and complete their life cycle.

OK, what does this have to do with this horrible looking mass under a horse’s tail?  Well, flies are attracted to moist wounds as well, and these same nematode larva can be deposited in wounds, were they don’t mature into adults (abortive life cycle), but do survive, causing a lot of problems and preventing a wound from healing.  This is an excellent example of the complexity of biological ecosystems, and the interdependence of multiple organisms in the transmission of disease.  Unfortunately, in this case, it has nothing to do with the mass under the horse’s tail.  It is not Habronema.

So back to the drawing board.  What else is on the list?  Is it trauma?  Trauma from fighting with another horse could certainly produce raised masses where they have been bitten or kicked. Being hidden under the tail base in this case and being ulcerated as it is makes this less likely.

Is it infectious?  This could be a result of an infection causing an abscess in this location. Given that the horse has multiple masses in multiple areas this diagnosis becomes less likely. Also, the horse is not sick and has normal bloodwork. Sometimes abscesses however can be well walled off and not produce any changes in the blood work. In any event, you could try to ultrasound these masses to look for a fluid pocket filled with puss. You would then have the option to obtain a sample for culture in order to identify the type of bacteria present.  So far other horses on the property are unaffected.  Are we missing something?  What else should be on the list, and how can we definitively determine this horse’s problem?


Now let’s check back in on our first patient.