Toxovax shortage
Ewes are infected with Toxoplasmosis through exposure to the faeces of infected cats.
We are aware that practices have been notified of a shortage of Toxovax (MSD Animal Health), which may remain unavailable until after the pre-tupping window for most farmers. In the absence of alternative vaccines, farmers may be concerned about the impact of not vaccinating on their flock productivity.
Toxoplasmosis is not passed from ewe to ewe, so mixing replacements with older ewes prior to tupping will not result in the development of immunity. Ewes are infected through exposure to feed and bedding contaminated with faeces from infected cats. Infection is shed for a short period by young or immunocompromised cats, so keeping a healthy adult population of neutered cats on farm may help to keep away risky young or pregnant feral cats. All concentrate, forage and bedding stores should be cat proof.
Early embryo loss, abortion, stillbirth, mummification and birth of weakly lambs can occur when naïve animals are exposed to Toxoplasma gondii during pregnancy. Following initial infection, the formation of bradyzoite tissues cysts leads to a long-lived humoral immunity which protects against the effects of future exposure to the parasite.
In a 2011 serological survey of 125 Scottish flocks, 100% of flocks had seropositive animals, with seroprevalence within flocks varying from 3.7% to 100%. Overall 56.6% of individuals were seropositive, however this reduced to 37.3% in one-year-old sheep. The percentage of seropositive animals within flocks increased from the south (42.3%) to the north (69.2%) of Scotland.
Since the level of humoral immunity in a group of ewe replacements is difficult to estimate, serological testing of 10 animals pre-tupping will help assess the number of naïve animals within the larger group and therefore the number at risk of abortion if animals are exposed to T. gondii oocysts during pregnancy. If a low number of seropositive animals is detected, and the risk of exposure to T. gondii during pregnancy is high (e.g. hill ground with known wildcat population, fields near households with outdoor cats), then medicated feed is an option to help mitigate the impact of exposure during pregnancy. Alternatively, on some farms it may be possible for the group of ‘at risk’ replacements to be grazed on a lower risk pasture throughout pregnancy.
Decoquinate is licenced as an aid in the prevention of abortions and perinatal losses due to toxoplasmosis when fed at a rate of 2mg/kg continuously for 14 weeks prior to lambing. It is likely to be most cost effective in flocks with high value animals or in high risk flocks where the younger, unvaccinated animals can be separated from the rest of the flock post-tupping for medicated feeding. Involving a nutritionist to help formulate a diet to minimise the risk of foetal oversize as a consequence of the extended period of supplementary feeding is advised. Delaying the provision of medicated concentrate until later pregnancy is likely to provide benefits only if exposure to the parasite occurs after the medicated feed is started.
If you would like to discuss this further, then please don’t hesitate to get in touch.
Posted by SRUC Veterinary Services on 20/09/2022
