Your dog or cat might be the cause of Blindness!!

Yesterday when I was studying for my MRCP exam, I came across a case scenario. I am going to write down the medical case here. My today's post is regarding the diagnosis of the case.

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The case scenario

A 13 years old male was admitted to hospital with complete loss of vision in his right eye. He had been placed in a juvenile offenders' residential home six months ago for stealing on several occasions. His parents had separated a few months previously, and before that, he had been living with his father, who was an alcohol abuser and had three dogs. Two weeks before admission he had complained of reduced visual acuity in his right eye, which had progressively become worse. [1] (Physical examination and investigation results were included in the scenario which is excluded intentionally as those are not necessary for my post)

image source

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The diagnosis is Toxocariasis or Toxocara infection or more specifically ocular toxocariasis.

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Toxocariasis is a parasitic infection caused by the genus Toxocara. Two species commonly encounter human being and carried by two of our favourite pets. One is Toxocara Canis (dog roundworm), carried by a dog and other is Toxocara Catis, carried by a cat. This is quite common in the western world and it is estimated that around 2-3% of the population carry the antibodies against this parasite (it means they were infected at some stage of their life).

image source A picture of a dog roundworm

T. Catis and T. Canis are natural parasites in cats and dog's intestine, respectively. These parasites can complete their lifecycle within cats and dogs. Accidentally human can acquire this infection. Usually, dog or cat excrete embryonated eggs of Toxocara in this faeces. When humans ingest the eggs the disease process starts. Children who are in close contact with dogs and cats are at higher risk of contracting Toxocariasis by ingesting contaminated fur or soil.

image source Lifecycle of the parasite and how infecting humans

When embryonated egg reaches human intestine they hatch and young larva takes birth. The larva can penetrate the intestinal wall, enter the blood-stream and migrate to distal organs, hence they are called larva migrans { Larva migrans describes a parasitic disease involving migration of immature (larval) worms in various parts of the body}[2]. The larva in human body can't mature into the adult form, rather the disease process starts due to the immune process of the human body against the larval antigen. The manifestation varies according to the affected organs by the migrated larva.

image source A picture of typical cutaneous larva migrans

The larva may penetrate any tissue where they can induce bleeding, the formation of eosinophilic granuloma and necrosis(death of living tissue). Toxocariasis present with two distinct types of clinical features. One is the classical visceral larva migrans (VLM) in where liver, lungs, brain, kidney. heart and skeletal muscles are affected. The severity of the symptoms depends upon (1) number of larva (2) location of the lesions and (3) sensitivity of the affected human body to the larval antigen. A majority of the children or affected person will remain asymptomatic. A small number of the patients may develop muscle pain, cough, sneezing, fever, anorexia, urticarial rash, seizure, lymphadenopathy and enlargement of liver & spleen.

Another type of Toxocariasis is Ocular larva migrans (OLM) where the larva reaches the eyes and form granuloma. If the macular area is involved, blindness may occur.

image source Larva migrans on the macula of the eye

Now, Scroll up and read the case scenario again. You will find the hint (..............and had three dogs.........).

So, Hello parents! Be careful! A cute puppy might be the cause of great sorrow!!

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References:
[1] Rapid review of clinical medicine for MRCP part 2 (second edition) by Sanjay Sharma; Question no: 337
[2] http://www.cfsph.iastate.edu/FastFacts/pdfs/larvamigrans_F.pdf

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