Angiostrongylus Infection:

The diagnosis of Angiostrongyliasis  is complicated due to the difficulty of presenting the angiostrongylus larvae themselves, and  will usually be made based on the presence of eosiniphilic meningitis and history of exposure to snail hosts. Eosiniphilic meningitis is generally characterized as a meningitis with >10 eosiniphils/μL in the CSF (cerebrospinal fluid), or at least 10% eosiniphils in the total CSF leukocyte count. Occasionally worms found in the cerebrospinal fluid or surgically removed from the eye can be identified in order to diagnose Angiostrongyliasis. 

Lumbar puncture 

Lumbar puncture should always be done is cases of suspected meningitis. In cases of eosiniphilc meningitis it  will rarely produce worms even when they are present in the CSF, because they tend  to cling to the end of nerves. Larvae are present in the CSF in only 1.9-10% of cases. However, as a case of eosiniphilic meningitis progresses, intracranial pressure  and eosiniphil counts should rise. Increased levels of eosinophils in the CSF are a trademark of the eosiniphilic meningitis. 

Brain imaging 

Brain lesions, with invasion of both gray  and white matter, can be seen on a CT or MRI. However MRI findings tend  to be inconclusive, and usually include nonspecific lesions and ventricular enlargement. Sometimes a hemorrhagic, probably produced by migrating worms, is present and of diagnostic value. 

Serology 

In patients with elevated eosiniphils, serology can be used  to confirm a diagnosis of Angiostrongylias rather than infection with another parasite. There are a number of immunoassays that can aid in diagnosis, however serologic testing is available in few labs in  the endemic area,  and is frequently too non-specific. Some cross reactivity has been reported between A. cantonensis and trichinosis, making diagnosis less specific.

The most definitive diagnosis is from the identification of larvae found in the CSF or the eye, however due to this rarity, a clinical diagnosis based on the above  tests is most likely.