toxoplasmosis

Introduction

Toxoplasma gondii was discovered in 1908 and is one of the most successful global parasites which appears to be primarily spread by oocytes shed in cat faeces
cats appear to be the definitive host and the only host capable of shedding oocytes
congenital infection was discovered in 1938
there is only one species but there appears to be genomic variability
it infects approx. one third of humans but infection over the age of 5 yrs in immunocompetent humans is thought to be asymptomatic although there are recent concerns it may have a role in various neurologic conditions such as:
- dementia
- gliomas - possibly a 2-3x increased risk
- schizophrenia - may double the risk
- lowered cognition
- behavioural changes such as more risk taking and aggression
the main concerns in humans are:
- congenital infection
- infection in immunocompromised patients such as those with AIDS

cats ingest a bradyzoite which then undergo sexual phase in the small intestine and then shed millions of oocytes in feaces as a product of schizogony and gametogony
unlike other forms of the parasites, the oocytes can survive for long periods in water
the oocytes are ingested by other mammalian hosts (including marine) which then excyst and invade the host as tachyzoites which may become tissue cysts in the form of bradyzoites which are very resistant to antimicrobial medications
oocytes also excyst in tissues after parenteral innoculation
tachyzoites and bradyzoites are killed by water
cattle and horses are resistant to clinical T. gondii, although a similar organism, Neospora caninum is a common cause of cattle abortions, while a similar organism, Sarcocystis neurona may cause fatal encephalomyelitis in horses

mammalian hosts can acquire infection by either:
- ingestion of oocytes from cats feaces (only 1 oocyte may be required for humans, pigs and mice!)
- ingestion of under-cooked or raw meat - especially lamb, but also beef or horse meat (although unlike cats, mice and presumably humans may need to ingest > 100 bradyzoites to become infected)
  - it is easily killed by prior freezing or cooking meat to inner temp of 66degC (or medium cooked meat)
  - a study in 2020 showed that Australian lamb mince meat had over 40% chance of being infected¹⁾
- congenital vertical transmission during pregnancy
  - only if newly acquired maternal infection, and highest risk of transmission is in 1st and 2nd TM
cats are relatively immune to infection via ingestion of oocytes, but very sensitive to infection after eating raw meat with only one bradyzoite which then can result in the shedding of millions of oocytes in faeces

cervical lymphadenopathy - usually nontender and < 3cm
may have sore throat, fevers, myalgias
may have retroperitoneal and mesenteric lymphadenopathy with abdominal pain
splenomegaly
may infect heart tissue, eyes
- ocular infection may cause recurrent, usually unilateral (3/4 are unilateral) retinal inflammation (toxoplasmic retinochoroiditis) whether as a primary or reactivation of infection, resulting in characteristic retinal scars, and may complain of seeing floaters
  - prevalence in an Australian study was 1 in 149 people ²⁾
  - 60% may develop visual impairment and 25% become permanently blind
  - on resolution of any attack of inflammation, the individual is left with a pigmented retinal scar that has a highly typical clinical appearance, these may occur at any location across the retina although there is a tendency for the lesions (perhaps around 24% of lesions) to occur in the central retina, i.e., in the region of the macula and optic disc.
  - Pregnancy and cataract surgery have both been associated with an increased risk of reactivation
neurologic symptomatic disease is rare in immunompetent adults and became evident in those with AIDS

50% develop CNS involvement which may cause seizure, vertigo, cranial nerve deficits, headache, focal neurology and altered mental state due to either encephalitis, meningoencephalitis or mass lesions
may develop:
- myocarditis
- pneumonitis similar to P. carinii / jiroveci - dry cough, SOB, fever
- organ specific reactivation toxoplasmosis

usually subacute onset with focal neurology
up to 25% have an acute onset with seizures or cerebral haemorrhage
less common presentations include parkinsonian symptoms, neuropsychiatric, spinal cord
untreated, late stage AIDS may develop pneumonitis +/- extrapulmonary toxoplasmosis

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