Milk jaundice

Milk jaundice is a disease that affects newborn mammals, but it's rare outside of equines.

The search for a cause of this disease is what lead to the discovery of blood types.

History
Milk jaundice has been identified in equine populations for much of history, although its causes have been unknown. It was often blamed on crossbreeding and related to the taboo of couples of different races and species in many cultures.

The first strides towards the identification of the cause of the disease was made when blood-based vaccines were developed for tick fever in Cattle by mixing infected blood with uninfected blood.

In the vaccinated subjects, the first cases of milk jaundice in Cattle were identified, giving researchers a hint about the cause of the disease being related to blood and the blood contact between different creatures.

Subsequent tests were arranged which consisted of mixing of blood of parents. In cases where milk jaundice had been present in calves, blood had always curdled while in healthy calves this wasn't always the case.

Tests were extended to Ponies and known cases of the disease, yielding the same results.

The theory is that creatures have different types of blood and the disease results. These "blood types" are labelled based in the order in which they are identified: I, II, III, IV...

Causes
The disease occurs when two parents have incompatible blood types.

It's possible to test the risk of the disease by mixing the blood of both parents. If the blood curdles, there's a risk of the disease happening.

This disease is more likely to develop if the parents are of different races or different species. The test for this disease is mandatory in Equestria for parents of different species.

Symptoms
Symptoms appear days after first drinking the colostrum. The main symptoms are hastened heart rate and respiration and yellowing mucous tissue.

The disease shares several symptoms with swamp fever. In areas where the latter is endemic, a differential diagnosis may be necessary in patients to accurately identify the disease.