BLUETONGUE
DISEASE:
Possible Cause of High Calving
Losses in Alabama

 Gary Mullen, Renee Anderson, James Wright, and John Owen

During the past several years, unusually high calving losses occurred in a brood herd of mixed Angus-Hereford cattle in Tallapoosa County. There is reason to believe that these losses were related to deaths of white-tailed deer in adjacent Lee County during this same period. AAES research suggests a link between two closely related insect-borne viruses that cause bluetongue disease in cattle and hemorrhagic disease in deer.

Problems first were noticed in a 106-cow herd maintained at the AAES’s Piedmont Research Station near Camp Hill. Calf-related problems included prolonged labor (up to 24 hours or more), abortions, stillbirths, dummy-calf syndrome (unresponsive calves that weaken and die after failing to suckle), heart murmurs, and other congenital anomalies in newborn animals. Similar calving losses were experienced at other farms within a 25-mile radius of the Station.

The brood herd was serviced by two bulls during the nine-week period from late October through the end of December 1997. Seventy-six cows were confirmed to be pregnant by palpation at three to four months of gestation. Of these, 69 produced calves during the period from late July to early November. Sixteen of the calves were dead at birth or died shortly thereafter. Six other cows never completed gestation of their calves. This represented a 29% calving loss among the cows known to be pregnant. The cause of these reproductive losses could not be determined, despite extensive testing of the herd for infectious agents and nutritional deficiencies.

Concurrent with this problem in cattle, AAES entomologists investigated a die-off of white-tailed deer in a captive research herd in Lee County located less than 15 miles from the Piedmont Station. Approximately 45% of more than 100 deer at the facility died due to hemorrhagic disease during a three-week period from late September to mid October 1998. This disease is caused by epizootic hemorrhagic disease virus (EHDV) transmitted by biting midges of the genus Culicoides, commonly known as “no-see-ums.” (Epizootic is the term used for disease outbreaks in animal populations.)

Terminally ill, captive white-tailed deer in late stage of hemorrhagic disease at North Auburn, October 1998. (Photo courtesy of Steve Cotney) .

Epizootic hemorrhagic disease virus is closely related to bluetongue virus (BTV), the causative agent of bluetongue disease in cattle, sheep, and other ruminants. Bluetongue virus also is transmitted by biting midges and causes clinical disease in cattle and sheep similar to hemorrhagic disease in deer. Bluetongue disease is an arthropod-borne animal pathogen that poses great danger and financial losses to U.S. agriculture. Bluetongue virus infects cattle throughout most of the southern and western United States.

Previous studies by AAES researchers, in cooperation with the AU College of Veterinary Medicine and C. S. Roberts Veterinary Diagnostic Laboratory located on the Auburn campus, have documented the occurrence in Alabama of the five strains (serotypes) of bluetongue virus known to occur in North America (serotypes 2, 10, 11, 13, and 17). Survey data indicate that more than 50% of the cattle herds in Alabama have tested positive for BTV antibodies, with up to 81% of herds testing positive in some parts of the state.

Monitoring the blood serum of cattle for antibodies to bluetongue virus during the past 15 years in east-central Alabama has documented active transmission of BTV year after year. Infected cattle generally exhibit only mild or inapparent illness. However, under circumstances that are still not clear, adult animals may become acutely ill and die. Cows that become infected with BTV during pregnancies may experience reproductive disorders, including abortions, stillbirths, and congenital anomalies in newborn calves. This raised suspicion that calving losses at the Piedmont Station might be due to bluetongue disease. This was reinforced when BTV was detected in a bone-marrow tissue sample taken from one of the dead calves.

Blood samples were taken from cattle at Piedmont Station for BTV and EHDV assays.

Blood samples were collected from all cattle in the brood herd from December 1998 to April 1999 for laboratory testing to determine exposure to BTV and EHDV. Blood sera from approximately half of the animals in the herd were tested each month using a serum neutralization assay. The results indicated the presence of antibodies to BTV serotypes 2, 10, 11, and 17 (Table 1); blood samples have not yet been tested for BTV serotype 13. A high percentage of the cattle also tested positive for EHDV-2, one of the two serotypes of this virus known to occur in the United States. Virtually all of the cattle tested were positive for antibodies to BTV serotype 11 and EHDV serotype 2 during January, February, and March 1999, indicating concurrent exposure of individual animals to both of these viruses during that time.

Table 1. Results of Serological Monitoring of Cattle at Piedmont Station, 1998-99
Virus serotype	Percent of herd with positive antibody response to bluetongue virus (BTV) and epizootic hemorrhagic disease virus (EHDV) based on serum neutralization
	DEC	JAN	FEB	MAR	APR
BTV-2	7	32	44	24	29
BTV-10	–	100	–	–	–
BTV-11	19	100	100	98	95
BTV-17	17	45	–	–	–
EHDV-1	2	10	9	8	10
EHDV-2	19	100	100	100	88
– in progress

The number of cattle that tested positive for BTV and EHDV increased dramatically between early December and early January, reflecting active transmission of both viruses within the herd during the winter months (see Table 1). The phenomenon of winter seroconversion has been observed in previous years in another cattle research herd. The high degree of seroconversion seen during the winter months, reflecting active virus transmission, coincided with early gestation in pregnant cows at the Station.

To monitor the presence of potential insect vectors of BTV and EHDV at the site, two New Jersey light traps were operated nightly at the Piedmont Station from early October 1998 through the end of April 1999. During this period, 10 Culicoides species were collected (Table 2). Included was C. variipennis sonorensis, the principal known vector of bluetongue virus in North America. The presence of three other species that are suspected of playing a secondary role in the transmission of BTV and EHDV based on previous AAES studies in Alabama (C. debilipalpis, C. obsoletus-sanguisuga, and C. stellifer) also was confirmed.

This investigation of calving losses at the Piedmont Station indicates that all of the ingredients necessary for an outbreak of bluetongue disease in cattle are present—bluetongue virus, insect vectors, and susceptible animals. What is particularly significant is the high incidence of simultaneous exposure of the cattle to EHDV at the same time a major outbreak of epizootic hemorrhagic disease was occurring in white-tailed deer in the same area. High fetal mortality and deaths of newborn calves could be the result of simultaneous infections with BTV and EHDV during the early stages of gestation. The combined interaction of these two closely related insect-borne pathogens may represent a significant contributing factor to previously unexplained calving losses in Alabama cattle.