A total of 87 horses have been affected by vesicular stomatitis since the outbreak in Texas and Colorado began in May, latest figures show.
The latest update from the Animal and Plant Health Inspection Service (APHIS), released on Thursday, said 77 horses were currently positive for the disease, which has symptoms similar to foot and mouth disease. Four cattle are also affected.
All cases have been identified as the New Jersey serotype.
APHIS says that are now 35 positive premises under quarantine – 12 horse properties and two cattle properties in Texas, and 21 horse properties in Colorado.
In Texas, Bastrop County had the most number of affected animals – 15 horses and two cows across 12 premises. Travis County has one premises under quarantine, where two horses have tested positive. The first case has been confirmed this week in Val Verde, with one horse affected.
In Colorado, Boulder County has 10 affected horses across eight premises, while Weld County has 14 infected horses across 11 premises. Two counties had their first confirmed cases this week, with El Paso and Larimer counties each having an infected horse.
APHIS said 63 positive premises have so far been identified across the two states.
There have been four counties affected in Colorado (Boulder, El Paso, Larimer, and Weld counties) and nine counties in Texas (Bastrop, Guadalupe, Hidalgo, Jim Wells, Kinney, Nueces, San Patricio, Travis, and Val Verde counties).
Six premises in Texas (one in Kinney County, two in Nueces County, two in San Patricio County, and one in Hidalgo County) have been released from quarantine and there are currently four other premises in Texas on a 21-day countdown to quarantine release.
The 81 animals currently positive for the disease comprise 77 horses and four cattle.
APHIS lists the number of susceptible animals, including the positive cases, at 609 horses and 1879 cattle. A small number of other species, including pigs, have also been listed as susceptible.
Since the outbreak began, 87 horses and four cattle have tested positive.
The first case in the current outbreak, in Texas, was confirmed through testing on May 23.
Vesicular stomatitis can be painful for animals and costly to their owners, state veterinarian Dr Keith Roehr said.
The virus behind the disease typically causes oral blisters and sores that can be painful to affected animals, causing difficulty in eating and drinking.
Veterinarians and livestock owners who suspect an animal may have the disease have been told to contact state or federal animal health authorities.
Livestock with clinical signs of the disease are isolated until they are healed and determined to be of no further threat for disease spread. There are no federally approved vaccines for the disease.
While rare, human cases of vesicular stomatitis can occur, usually among those who handle infected animals. VS in humans can cause flu-like symptoms and only rarely includes lesions or blisters.
Susceptible species include horses, mules, cattle, bison, sheep, goats, pigs, and camelids.
Clinical signs include vesicles, erosions and sloughing of the skin on the muzzle, tongue, teats and above the hooves of susceptible livestock. Vesicles are usually only seen early in the course of the disease.
The transmission of vesicular stomatitis is not fully understood but components include insect vectors, mechanical transmission, and livestock movement.