16. Protozoa (Toxo) Flashcards
Is Toxo an intracellular parasite?
yes, obligate intracellular
does Toxo have high or low host specificity?
low - possibly infects any mammal
what tissues does Toxo infect?
almost all tissues
what is the definitive host of Toxo?
a cat, domestic or wild
how do cats become infected with Toxo?
by eating the oocysts or infected rodents or birds
what is the intermediate/reservoir host of Toxo?
most species of warm blooded animals
what are some ways that humans can become infected with toxo?
by ingesting a cyst, eating raw meat, transplacentally, or organ transplants
how does Toxo reproduce in the IH
asexually
how does Toxo reproduce asexually in the IH?
- an ingested oocyst goes to the digestive tract, they are engulfed by macrophages
- in the mac, the tachyzoites develop and travel to various parts of the body via the blood
- once the immune system is triggered, the tachyzoites encyst into pseudocysts that contain bradyzoites
what is the infectious stage of Toxo?
all parasitic stages are infectious and can cause toxoplasmosis
how long following infection do cats shed oocysts?
for only 1-2 weeks following infection
when do oocysts become infective?
24-48 hours after remaining in feces
how long can oocysts remain viable in warm, moist soil?
years
what are the three forms of Toxo?
- oocysts
- Tachyzoites
- Bradyzoites (tissue cysts)
what does a sporulated oocyst contain?
2 sporocysts, each of which contain 4 sporozoites
what shape are the tachyzoites of Toxo?
crescent shaped
what is the parasite membrane called?
parasitophorous vacuolar membrane
what phase of infection is marked by the tachyzoites?
the acute phase
are tachyzoites the sexual or asexual form?
asexual
what kind of cells do tachyzoites invade?
all nucleated cells
tachyzoites invade nucleated cells and occur in _________.
in groups
at what stage in infection are tachyzoites rapidly growing?
in the early stage
what do tissue cysts contain?
slowly growing trophozoites known as bradyzoites
what is the appearance of a bradyzoite in body cells?
a circular mass
what do bradyzoites contain?
thousands of parasites
how long do brandyzoites remain in tissue?
for the life of the host
what stage of infection is marked by bradyzoites?
the chronic phase
when can reactivation of infection occur?
in immunocomprimized hosts
what are bradyzoites resistant to?
low pH and digestive enzymes (for stomach passage)
what do bradyzoites transform into?
tachyzoites
what has to happen before bradyzoites transform into tachyzoites?
the protective cyst wall has to be dissolved
where are bradyzoites released?
in the intestine
what is needed for tachyzoite to bradyzoite conversion?
- high pH
- low pH
- heat shock
- mitochondrial inhibition
- presence of nitric oxide
what is needed for bradyzoite to tachyzoite conversion?
- lack of nitric oxide
- lack of IFN-γ
- lack of TNF-α
- lack of T-cells
- lack of IL-12
what are the two phases of the T. gondii life cycle?
- the intestinal (or enteroepithelial)
- the extra-intestinal
where does the intestinal phase occur?
only in cats
what does the intestinal phase produce?
oocysts
where does the extra-intestinal phase occur?
in all infected animals including cats
what does the extra-intestinal phase produce?
tachyzoites and eventually bradyzoites
what can occur if a kitten is infected?
rarely, acute infections can result in numerous intestinal lesions that kill kittens
what are the clinical features of toxoplasmosis in humans, if there are symptoms?
- mild fever
- sore muscles
- swollen glands and lymph nodes
how are the clinical symptoms resolved in humans
clinical course is benign and self-limiting, usually resolves within a few months to a year
who is most at risk for toxoplasmosis?
individuals who are immuno-compromised - patients suffering from HIV, organ transplant receivers, people on chemotherapy
what is the most common cause of intra-cerebral mass lesions in patients with AIDS?
toxoplasmic encephalitis
what do immunodeficient patients often have?
CNS disease, but many have retinochoroditis, or pneumonitis
how many cases of congenital toxoplasmosis is estimated to occur per year in the US?
400-4000
how many human deaths are associated with the clinical disease of Toxoplasmosis?
750, making it the 3rd most common lethal food poisoning
what can congenital toxoplasmosis result in?
spontaneous abortion, a stillborn, or a child that is severely handicapped mentally or physically
is the fetus at risk if it is not the mother’s first exposure to Toxo?
no, only if it is the first exposure
are most cats actively shedding oocysts?
no
about how many women that are infected actually pass the infection to the unborn baby?
30-40%
what is the risk of passing the infection in the third trimester?
70%
what is the risk of passing the infection in early pregnancy?
15%
what are some common defects of congenital toxoplasmosis?
- chorioretinitis (leading to blindness)
- intre-cerebral calcification
- hydrocephaly/microcephaly
which is the least common defect of congenital toxo?
hydrocephalus, but it is the most dramatic
what is the most common defect of toxo?
occular disease
how is diagnosis of toxo done?
- ELISA test for IgG and IgM - different titer results indicate different stages of infection. For exaxmple, acute vs. chronic
- amniotic/spinal fluid isolation and culture of parasite
- direct detection by microscopy
what is the most reliable serological method for toxo diagnosis? how does it work?
Sabin-Feldman dye test - live tachyzoites are the antigen and are exposed to dilutions of the serum
which medication are most commonly prescribed for toxoplasmosis infection?
pyrimethamine
sulfadiazine
clindamycin
spiramycin
what is the use of pyrimethamine?
antimalarial medication
what is the use of sulfadiazine?
antibiotic used in combination with pyramethamine to treat toxoplasmosis
what is the use of clindamycin?
antibiotic used most often for people with HIV/AIDS
what is the use of spiramycin?
antibiotic used most often for pregnant women to prevent the infection of their child
what is the host manipulation that has been observed in rats infected with Toxoplasma gondii?
rats will no longer be prude about areas that have cat urine, instead they possibly even seek out those areas
