Rabbit hemorrhagic disease’s spread appears to be slowing

Since the latest strain of rabbit hemorrhagic disease virus type 2 (RHDV-2) was first detected more than three years ago, it continues to spread in North American wild rabbit and hare populations, but reports of detections are greatly reduced.

An outbreak of the highly infectious and lethal virus that affects lagomorphs was first confirmed in late March and early April of 2020 in New Mexico in domestic rabbits and wild rabbits and hares. Since then, it’s been identified in domestic and wild species in 28 U.S. states, 19 Mexican states, and four Canadian provinces. In California, RHDV-2 spread widely across several southern counties between 2020 and 2021.

More recently, detections of RHDV-2 in domestic rabbits have become infrequent, according to the U.S. Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS), with the last detection occurring earlier this year. Increased awareness, biosecurity, and vaccination are proving to be effective in limiting the spread of RHDV-2 in domestic rabbits, an APHIS spokesperson said.

Only one case of RHDV-2 in domestic rabbits occurred between April and June in Sacramento County, according to a USDA map as of the end of June. Fifteen cases of RHDV-2 in 12 counties among Colorado, Kansas, Montana, New Mexico, and Texas were recorded during the same timeframe among wild species.

The California Department of Food & Agriculture (CDFA) noted that the virus was also detected in domestic rabbits in Sacramento County on February 28 and in a wild jackrabbit in Santa Clara County on February 8. It was previously detected in a wild jackrabbit in Sacramento County in December 2022.

Dr. Andrea Mikolon, assistant branch chief of the Animal Health Branch at CDFA attributed the reduction in cases in domestic rabbits to both the decrease in the virus spreading in the wild population, “and also some help from the vaccination of the rabbits with the European vaccines previously and now the Medgene vaccine.”

In fall 2021, Medgene Labs received emergency use authorization from the APHIS’s Center for Veterinary Biologics (CVB) for its RHDV-2 vaccine. The vaccine is now available in 45 states and the District of Columbia.

Dr. Mikolon explained that in California, endangered riparian brush rabbits are the only wild rabbits authorized to be vaccinated. While there seems to be a decrease in the virus in wild populations, it’s likely not because of the vaccine as only a small number of wild rabbits were vaccinated and in such a small area of the state.

Dr. Mikolon said veterinarians should still include RHDV-2 in a differential diagnosis for sudden death. “We’re hoping that it’s going away but we can’t be sure,” she said.

The Texas Parks and Wildlife Department (TPWD) has seen RHDV-2 move from previous 2020 endemic areas such as West Texas and the North Texas Panhandle to more eastern areas with two potential cases on the eastern border that are still being investigated.

Dr. Sara Wyckoff, a wildlife veterinarian at TPWD, noted that the confirmed wild rabbit RHDV-2 detections in the central part of the state are currently the most eastward detections of RHDV-2 in wild rabbits in the United States.

“As of right now, we have 16 counties with positive RHDV-2 samples and two pending counties since this current 2022 outbreak started,” Dr. Wyckoff said. 
RHDV-2 is the first disease that veterinarians in the U.S. actively vaccinate pet rabbits against.

The virus can be transmitted through direct or indirect contact. It has a mortality rate ranging from 70% to 100%. Exposed rabbits that survive may become carriers of the virus.

APHIS encourages veterinarians nationwide to watch for RHDV-2, report any suspicious illnesses or deaths to state and federal regulators, and submit samples for testing through the National Veterinary Services Laboratories’ (NVSL) Foreign Animal Disease Diagnostic Laboratory (FADDL).

There are currently no licensed vaccine candidates for RHDV-2 outside of emergency use. Research is ongoing for more vaccines.

Correction: An earlier version of this article misstated the location of the initial outbreak of rabbit hemorrhagic disease virus type 2 (RHDV-2) in the U.S., the number of Canadian provinces affected, and the number of most recent cases.