Though on the face of it a kill rate of 90% is highly improbable and I can see no realistic way that such a high death toll can be reached, let me give a boost to this thread by drawing attention (via cut&paste from wikipedia) to this OTL epidemic:
1872 American outbreak
An epizootic outbreak of equine influenza during
1872 in North America became known as "The Great Epizootic of 1872." The outbreak is known as the "most destructive recorded episode of equine influenza in history."
[3] The impact of the outbreak is marked as one of the major contributors to the
Panic of 1873 in the United States.
The first cases of disease in pasture horses were in the townships of
Scarborough,
York, and
Markham in
Ontario,
Canada. By October 1, 1872, the first case occurred in
Toronto. It took only three days before all the
street car horses and major
livery-stables were affected. By the middle of the month,
Montreal,
Detroit, and most of the
Dominion of Canada and New England reported cases.
[2]
By the start of November
Ohio,
Massachusetts, and
South Carolina were reporting cases. So was
Chicago,
Illinois. The contagion reached
Florida and
Louisiana by the end of November and Cuba on December 7. The height of the plague was December 14, when the Mexican government had to supply disease-free horses to the stricken United States.
[2] One major factor was that cities were not clean back in those days, which meant that germs spread all that much more quickly (especially through contaminated food and water).
The rate of infected horses approached 100%, and mortality rates ranged between 1% and 10%. Many horses were unable to stand in their stalls. Those that could stand coughed violently and were too weak to pull any loads or support riders.
[4]
The
street railway industry ground to a halt in late
1872. Every aspect of American transportation was affected. Locomotives came to a halt as coal could not be delivered to power them, while fires in many major cities raged unchecked. One
fire in Boston destroyed over 700 buildings (November 9-10 of that year). Indeed, many a fireman just stood there helpless and horror-stricken, for lack of any equipment to work with. Even the
United States Army Cavalry was reduced to fighting on foot against the
Apaches (as the plague had swept not only south to Mexico and Cuba, but also west to the Pacific Ocean within two months!), who likewise found their mounts too sick to do battle. The outbreak forced men to pull wagons by hand; while trains and ships full of cargo sat unloaded (perishables, such as milk, often became spoiled), tram cars stood idle and deliveries of basic community essentials (including food and clothing) were no longer being made. The Long Riders' Guild Academic Foundation founder CuChullaine O'Reilly said, "The Great Epizootic was the worst equestrian catastrophe in the history of the United States - and perhaps the world."
[4]
The Great Epizootic of 1872 was also a contributor to the
Panic of 1873, which lasted six years; hence, it would be about seven years total before things were restored to normal operation.