Neospora caninum (hereinafter Neospora) is a unicellular parasite (protozoan). It is one of the leading agents of infectious abortions in cattle.
Neospora has a complex cycle with several species of canids as definitive hosts and a wide variety of species, including cattle, as intermediate hosts.
In cattle, Neospora is transmitted mainly transplacentally (vertically) from the mother to the fetus during gestation.
The rate of transplacental transmission is high but variable and probably depends on the degree of immunity of the mothers.
Horizontal transmission (postnatal infection) from a definitive host is also possible but rather uncommon in our rearing conditions.
When the transplacental infection does not end in an abortion, a normal-looking calf may be born and perpetuate the cycle.
Indeed, vertically infected female calves that are bred will in turn be able to produce infected calves.
To break the cycle of infection, it is recommended not to breed heifers from infected mothers and gradually eliminate positive lines.
For animals of high genetic value, it is possible to preserve the lines by collecting embryos and transferring them to naïve recipients.
How to identify heifers that have been infected vertically?
Most mothers infected with Neospora have antibodies detectable by standard serological tests (ELISA).
However, antibody levels can fluctuate considerably over time to the point where they sometimes become undetectable.
A mother’s seronegative status is therefore not an absolute guarantee that she is not infected.
Conversely, seropositive status also does not predict with certainty that her calves will be born infected.
This is because infected mothers do not systematically give birth to infected calves.
The rate of transmission is higher during the first gestations than during subsequent pregnancies.
In infected mothers, Neospora is present in certain organs (heart, brain) in the form of bradyzoites.
Bradyzoites can remain in a quiescent state for months.
Thanks to a transient immunosuppression as is the case during a gestation, bradyzoites can resume their development.
They then turn into tachyzoites that travel through the bloodstream, reach the placenta and can pass through it to infect the fetus.
From about 3 months of gestation, the fetus is able to respond to infection by producing anti-tachyzoites antibodies.
The usual serological tests use tachyzoite extracts as an antigen and can detect antibodies to this form of the parasite.
The presence of Neospora antibodies in a calf born before its first intake of colostrum is evidence of prenatal infection.
In vertically infected calves, the parasite is present in the form of bradyzoites, as in adults.
Bradyzoites provide litle stimulus for the immune system and, as a result, animals produce few antibodies against them.
In any case, the usual serological tests essentially detect antibodies directed against tachyzoites, not bradyzoites.
In very young animals (less than 3 months) that received colostrum from their mother, the presence of antibodies is evidence of the mother’s infection.
The antibodies detected at this age are mostly of maternal origin (colostral).
Some of them can also come from the calf (fetus) itself.
After these antibodies disappear, a vertically infected calf will not always produce enough anti-tachyzoite antibodies for them to be detectable.
A negative serological test in a heifer aged 6 to 12 months is therefore not an absolute guarantee of the absence of infection.
Determining the serological status of heifers before they are bred is strongly recommended to control vertical transmission of Neospora.
A positive test makes it possible to conclude with virtual certainty that they are infected and they will be at risk of abortion or of giving birth to infected calves.
Conversely, a negative test unfortunately is not an absolute guarantee that they are not infected.
Serological control in the middle of first gestation can detect infections that previously went unnoticed.
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André Broes, D.V.M., Ph.D., Technical Services Manager