QUESTIONS

1. What's the difference between a tachyzoite & a bradyzoite?
    
2. How long might a cat that is shedding oocysts be expected to shed oocysts?
    
3. After a cat is infected with toxo orally, is oocyst shedding inevitable?
    
4. How long does it take an oocyst to become a sporocyst & why do we care?
    
5. If a woman is planning a pregnancy & wants to know if her cat presents a risk to her, what type of testing would be appropriate for the cat?
    
6. If you have a cat that has clinical signs that could be compatible with toxoplasmosis (virtually any tissue can be infected so there are no typcial signs), what test is most appropriate?
   
   
   
   
    

ANSWERS

1. Sounds like there ought to  be a punch line!  Both are tissue forms. The tachyzoite is the rapidly dividing tissue form which precedes development of the encysted bradyzoites.  Eating either form will infected you but the prepatency period is different (like 3 weeks if a tachyzoite is eaten & like 3 days if a bradyzoite is eaten) & we do not know why this is.
   
   
   
   
    
2. (Key point:  only cats shed oocysts).  A cat infected after eating a tissue phase will shed for 7-10 days. A cat infected from eating a sporocyst will shed for several weeks. We do not know why this difference exists.  FeLV or FIV infection does not prolong shedding.
   
   
   
   
    
3. If the cat eats a tissue form of toxo, you can pretty much count on shedding.  If the cat eats a sporocyst, there is only a 50% chance that oocysts will be shed.
   
   
   
   
    
4. It takes a day or two for the oocyst to become a sporocyst & we care because the sporocyst is the infective stage.  Because it takes a couple of days for the oocyst to become a problem, daily litter or poop removal can prevent human infection.
   
   
   
   
    
5. First, do a fecal to make sure the cat isn't currently shedding oocysts. Second, run an IGG (yes, that's IGG) titer on the cat. IGG appears within 3 weeks (& sometimes up to 6 weeks) in infected cats & stays positive for about a year.  A cat (unless immune suppressed) will not shed oocysts twice; they only shed shortly after infection.  If the fecal is negative & the IGG is positive, this means the cat was infected already & that they need not worry about the cat shedding again. If the fecal is negative & the IGG is negative, the cat is at risk for infection & they should take steps to see that the cat does not become infected during the woman's pregnancy (no hunting, no undercooked tidbits to eat).
   
   
   
   
    
6. The IgM titer would be more appropriate if one is looking for an active infection.  IgM is positive within 2-4 weeks & is negative by 16 weeks post infection.  A titer of 1:256 or more is considered positive.