Mares do not harbour the virus once they have gone through the clinical stages of the disease (unlike stallions who may harbour and shed the pathogen permanently in their semen). This means that once a mare has been infected, and gone through the actual infectious stages (about 3 weeks) her immune system will - as you note - kick in, and she will have antibodies against the virus, but not be infectious. Consequently she will not become clinically affected even in the event that she is re-exposed to the virus (for a few years - antibody levels may decrease over time). There is no concern therefore about the mare becoming a "habitual aborter" - and in fact would be a lesser risk from EVA in subsequent years.
One of the few causes of fatality in EVA involved foals born to mares that contract the disease late in pregnancy, and the foal may be delivered full-term but then succumb shortly after. In other words, yes, the foal may be positive for the virus, and negatively impacted by it.
If a mare is exposed to the virus and becomes infected earlier in pregnancy, then there is a concern about the risk of abortion (which is quite high). This usually will occur within the three weeks after contracting the disease. Note that an aborted fetus and placental tissues are likely to be high in viral content, so care to prevent contaminating other animals must me taken.
As foals are born without immunity, if the foal does happen to survive a viral insult to the mare while pregnant, it is unlikely that there would be antibody presence at birth, although colostral transfer may be possible.
Thanks for that. Another question I have is does youngstock harbour the infection?
For instance if you brought in a youngster and then after a while the mares abort, would the contributing factor be due to the youngster spreading the disease via the respiratory route, and if so, would it have to be showing signs of a snotty nose etc?
The virus is testosterone-dependent for harbouring in the animal beyond the clinical infection period of about 3 weeks. That is why a stallion has the potential to harbour it, but not a mare (i.e. once the mare has gone though the clinical infectious stage, she will not shed the virus). Consequently young stock - specifically colts - will not retain the virus unless they have reached sexual maturity. One of the vaccination protocols for farms involves vaccination of all young stock prior to 270 days of age, as even if the foal has encountered the virus environmentally prior to vaccination, the colts will not retain the virus in their system as they do not yet have elevated testosterone levels. Similarly, if a shedding stallion is castrated, he will cease to harbour and/or shed the virus.
Note that an animal with the disease can be in the active stages (clinical) of the disease but be asymptomatic. In other words, that animal can be infectious, but show no outward signs of illness whatsoever - so no snotty nose. It is therefore possible that one could bring an apparently healthy animal onto the farm - youngster or not - but if it was in the infectious stages, other horses on the farm could become infected. In the case of young stock (colts <270 days) or mares, after about three weeks, the animals would no longer be infectious. Only sexually mature entire colts/stallions would have to potential to remain infectious. 3 weeks quarantine for incoming horses is therefore a good idea. That does not solve the problem of being stabled next to a strange horse at a show or other function that could be clinically infectious but asymptomatic though. Vaccination is therefore a far more practical idea. There are however some countries that have restrictions on use of vaccines and importation of titre-positive horses or semen from them, so one would have to check out that aspect before vaccination. Those countries are in the minority, but it is worth checking before vaccination.
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