Neospora
Neospora is a devastating cause of abortion often underestimated by many dairy producers and can have catastrophic consequences, as David Black and Miriam Hedley of Paragon Vet Group explain
N
eosporosis is the name given to the disease caused by Neospora, an important infectious disease of cattle worldwide that causes abortion and stillbirth; in fact in the UK, laboratory data shows it to be the most commonly detected cause of bovine abortion. The economic signifi cance of this disease is not in doubt, with signifi cant losses not only due to abortion, but also from reduced yield and premature culling. Neospora is a protozoan parasite similar to Toxoplasma, an organism which causes abortion in sheep. Their lifecycles are similar in that they both involve an asexual cycle within the cow or sheep and a sexual lifecycle within a defi nitive host outside of the cow.
So for Neospora, the two stages of the asexual lifecycle involve a fast growing state - the tachyzoite, and a slow growing state - the bradyzoite. The tachyzoite multiplies rapidly within a cell, causing it to burst and re- lease more Neospora to attack more cells.
The slow growing state, the bradyzoite, comes into play once the cow’s immune system starts to kick in. The immune system attempts to kill the tachyzoite, but it cleverly switches to become a bradyzoite. This multiplies slowly and causes less damage, but stays “hidden” from the immune system by forming tissue cysts in the brain and spinal cord.
The sexual lifecycle involves another host. In the UK, this other host is usually the dog. The dog becomes infected by ingesting material containing Neospora tissue cysts, such as infected placentas or parts of aborted calves.
The dog is then able to produce Neospora oocysts (eggs) which are excreted in its faeces. Shedding of these eggs only lasts for about three weeks after initial infection and after this the dog is immune, so as a result, shedding is usually seen in puppies or young dogs after their fi rst exposure to the parasite. So how do cows actually become infected? Most foetal infections occur in cattle which were already infected before they became pregnant. This is called vertical transmission. In these cows, Neospora exists as tissue cysts in the brain.
During pregnancy, the mother’s immune system is weakened and, therefore, less able to suppress the parasite, which escapes from the cyst, circulates in the blood stream and crosses over into the placenta and foetus. This tends to be seen as a relatively low number of sporadic abortions throughout the year. However, some infections can be transmitted horizontally – this is when a naive (non-immune) pregnant cow
100 THE JOURNAL FEBRUARY 2015
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