Medical Fun Facts Podcast

MFF0030: Ocular Larva Migrans

It’s Thursday 12 January 2017. A couple of weeks ago a listener asked that I do more parasite facts. There are a few parasitic diseases beginning with O and the one I’ve chosen for this show is Ocular Larva Migrans. Don’t you love the descriptive title? I hope you’re not too squeamish. How do you feel about little worms moving in your eyeball?

The usual cause of ocular larva migrans are parasites in the genus Toxocara. Usually from a dog, T. canis and occasionally from a cat, T. cati. Gee, that’s an original and exciting name, isn’t it? Some days I imagine being an expert in cat bite infections, my aim in life would be to get something, preferably a bacterium designated with a species name of pusi. Given a common genus in the mouths of cats and dogs is Pasteurella, and given my love of alliteration, I’d love to see Pasturella pusi in the Approved Lists of Bacterial Names. I’d spell it p u s i. Despite having attended a grammar school, I didn’t study Latin, so I don’t know what the correct spelling and form would be. Yes, apparently, a Grammar school is a school that teaches Latin. I can’t remember where I learnt that I think it may have been from listening to a book written and narrated by Sir David Attenborough. I attended Brisbane Grammar School as a day student (aka dago because boarders were better) and I remember our school motto was in Latin. It was and I hope still is, nil sine labore which as kids we translated as no sign of work but what our masters (yes, they were referred to as masters) explained meant nothing of substance can be achieved without hard work.

Toxocariasis is a zoönosis caused by these aforementioned roundworms.

People who own dogs and cats are more at risk of infestation.

The toxocaridæ live in the intestines of the host animal and can be found in the faeces of the animal. If you play or work in the dirt where there may be dog or cat faeces, your chance of being infested increases. Occasionally, you can get infected if you eat undercooked meat containing Toxocara larvæ. Now in some countries, eating dogs and cats is not frowned on. Here in Australia, it’s generally not the done thing. I really wouldn’t recommend it. You can buy regular beef mince in Coles for about $5 for 500 grams (that’s 1 pound in mass if you live in the USA, Burma and Liberia). Okay, so that was probably going a bit too far in trying to get a laugh. What mostly happens is that sheep and rabbits, which eat grass that may be contaminated with dog faeces, may get infected and if you eat undercooked lamb or rabbit you may get infected.

There are two main manifestations of toxocariasis.

Visceral toxocariasis involves larvæ moving to various organs and tissues in your body like your liver or brain. You might experience fever, fatigue, abdominal discomfort and pain, along with wheezing and coughing.

Ocular toxocariasis means you have larvæ in your eyeball, it’s usually only one. I don’t know if anyone has ever looked at if there is a preference for the left or right eyeball. With ocular toxocariasis you can lose your vision, you can get inflammation in your eyeball and retinal damage.

The good news is that most people who are infected only have a mild infestation and do not know they have these larvæ moving around inside them. The other bit of good news is that the larvæ in human accidental hosts do not develop into adult worms. Can you imagine having adult worms in you! The pathological manifestations are usually due to reactions to the larval proteins.

Diagnosis can be made by serology or by histopathological examination of affected tissue.

Treatment includes antihelminthic drugs and for ocular larva migrans, the aim is to minimise further damage to the eye. An ophthalmic surgeon may be required to assess and participate in management.

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