July 18 - Strongyloides westeri

Poor babies - baby horses, that is. Strongyloides westeri is a nematode known as a threadworm that can make foals very sick by giving them very bad diarrhea, resulting in weakness, weight loss and poor growth. The adult worms reside in the small intestine and lay eggs that pass out in the feces. Horses can be infected by ingesting the larvae or if the larvae penetrate through their skin, however, one of the most common routes of infection is via the mother's milk, which is somewhat unusual among parasites. For this reason, veterinarians commonly recommend that mares be dewormed on the day that they give birth and that foals be dewormed several times in their first few weeks.

July 13 - Dioctophyma renale

The aptly named Dioctophyma renale reaches maturity in the host’s kidneys. Also known as giant kidney worms, they are one of the largest nematodes capable of infecting humans-adult females grow to be over 100 cm long (they are also bright red and about 10 mm wide). Although they don’t feed on kidney tissue, they cause pressure necrosis (imagine a meter-long, centimeter wide worm living in your roughly computer mouse-sized kidney), turning the kidney into a non-functioning hollow shell. It has a cosmopolitan distribution and affects humans, several species of canids, horses and pigs, and uses annelids as intermediate hosts (fish can be paratenic hosts). Vertebrate hosts become infected by eating an infected intermediate or paratenic host or by drinking water in which they are swimming. Luckily, human infections are uncommon. On rare occasions, they become encysted in other organs (e.g. ovaries, uterus, urethra, mammary gland or subcutaneous tissue) or migrate through the digestive tract or abdominal cavity, making for an unpleasant finding during veterinary surgical procedures.

Contributed by Andrés Gómez.
Photo from this site.

June 29 - Gasterophilus intestinalis

Growing up, I was an avid horseback rider and had both a horse and a pony. Today's parasite was always one of the things I dreaded about summer. Gasterophilus intestinalis, the horse botfly, is an annoying creature to horses and horse owners alike. The females are large and aggressive and pester the horses until they eventually alight and deposit small eggs that stick to the horse's hair like glue, most often on their legs. The eggs itch and so the horse licks and bites at it, eventually swallowing the eggs. The larvae then live out the fall and winter months in the stomach of the horse, which can make the horse lethargic and induce colic. In the spring, they are passed out in the feces, where they pupate and then emerge as new - annoying - flies. We would always try to prevent infection by scraping the eggs of the horses' legs using specially made rough sponge-like things - I spent hours doing that each summer!

The photo shows a very heavy infestation of botfly larvae in the stomach of a horse and comes from this site.

May 19 - Pythium insidiosum

Oh man - this is a nasty one. I had a really hard time finding a photo that wasn't completely disgusting and opted for just a nice image of the simple fungi themselves. The parasite is Pythium insidiosum, a fungal parasite that causes a disease known as pythiosis in dogs and horses. It's a common parasite in warmer parts of the world including the American South, eastern Australia, and south America and primarily exists in swampy, standing water. Dogs can become infected in their GI tract from drinking tainted water and it can cause thickening of the tissues or granulated lumps. Horses more often have subcutaneous lesions of these fungi, which they get from standing in the swampy water. The damage to their legs, bellies, and chests can be quite gruesome and often the lesions will have bits of dead tissue in them with the somewhat amusing name of "klunkers."

The disease is apparently on the rise in the U.S. and is troublesome because many vets are not familiar with it. I found this website with some rather sad tales of beloved pets who have been lost to the disease.

The image is from this site.