Veterinary Parasite

Well, as this is a vet student’s blog, and also a self-proclaimed parasite’s blog, I thought it was about time to write something about veterinary parasites! I have a “special affinity” with parasites, after a year resitti…I mean studying them in more depth, and to my shame and horror found myself enjoying the subject. Anyway, here are my four favourites!
First on our list is Dictyocaulus viviparus, the bovine lungworm, which has a quite fantastic means of getting about. To set the scene, the momma worm is living in the cow’s lungs, lays some eggs, which the cow kindly coughs up, swallows, and the craps out. So the poor little parasite is now embedded in a big pile of cow turd on the ground, where it has the pleasure of hatching from its egg. Now this baby parasite would quite like to get to another cow, so it can have babies of its own, but what self-respecting cow is going to eat its own crap. Perhaps it would be best off getting off this steaming pile that it calls home, and finding some grass, but how? This is the fantastic part, as it turns out that a species of fungus (Pilobolus spp) grows on the cow dung, and when it’s ready to send out its spores, it does so in an explosive fashion. So our little baby worm climbs onto the fungus, waits until it sporulates, then rides the explosion out onto the grass! Truly fantastic I’m sure you’ll agree.

Our second fantastic parasite is called Onchocerca, and our more knowledgeable readers may question this designation, as it causes River Blindness in Africa, a far from fantastic disease. However, what is fantastic is how this worm avoids being killed by the immune system. To grasp the beauty of its evasiveness, we must first learn a small amount about how the human body kills invaders. Parasites such as Onchocerca are attacked using a type of cell known as an eosinophil, whereas bacteria are generally destroyed by another type, called neutrophils. These two types of cell are pretty specific in their killing, so neutrophils are useless at killing parasites. Now brace yourself for the fantasticness! Onchocerca worms allow bacteria to live beneath their cuticle (skin), so the body finds these bacteria, and despatches neutrophils to kill them. It also notices the worm, and sends out eosinophils to kill them. However, the response to the bacteria surrounds the worm with neutrophils, which block the eosinophils from reaching their target, protecting the worm from the immune attack. Further, the worm’s cuticle protects the bacteria from being killed, so the worm can sit happily in a protective cocoon of neutrophils, kindly provided by the human’s own immune system.

Next up we have the first of our mind-controlling parasites, Toxoplasma gondii, a protozoon parasite, that can cause disease in immuno-compromised humans, such as those with AIDS, but is in fact generally transmitted between cats and mice. Now for this parasite to complete its life cycle, it requires the infected mouse to be caught by a cat, which represents a small problem, as mice are usually repelled by the smell of cat urine, keeping them away from cat hunting grounds. This is but a small problem for Toxoplasma though, as it “simply“ alters the mouse’s behaviour, making it attracted to cat urine. Thus, mouse is more likely to be caught by a cat, completing the parasite’s life cycle.

Our second mind-controlling parasite is a liver fluke of cattle, which goes by the name of Dicrocoelium dendriticum. Its life cycle is complex, but involves first a snail (Galba truncatula), and then an ant (Formica spp.), with the ant being eaten by a cow to complete the cycle. Another small parasitic problem…ants tend to live either underground, or on the surface of the ground, but cows tend to eat tall grass. How then can we get the cow to eat the ant. Well we could do this by getting the ant to stop being repelled by light (negatively phototrophic), and instead be attracted to it. So the stage of the fluke found in the ant (the cercariae), send some emissaries to the ant brain, where they encyst to become metacercariae. This causes a lesion in the ant brain, changing its behaviour, so it’s attracted to light, and climbs tall grass blades to get as close to the sun as possible. A perfect target for a cow to eat.