– Michelle Arnold, DVM (Ruminant Extension Veterinarian, UKVDL)
Information regarding insect control in this article was provided previously by Lee Townsend PhD (Retired Extension Entomologist, UK Department of Entomology)
EHD has been correlated with droughts because deer tend to concentrate around the wet areas available and these are where the gnats breed.
Epizootic hemorrhagic disease (EHD) virus is a pathogen of wild and domestic ruminants, especially white-tailed deer. EHD occasionally causes serious epidemics in wild deer populations that can spill over into domestic animals, including cattle. Bluetongue (BT) virus is very similar to EHD although sheep are the most susceptible animal to bluetongue and it can cause tremendous losses to this industry. Cattle are susceptible to both EHD and BT infection, although it is generally a mild infection with fever and weight loss as the most common symptoms. Both viruses that cause EHD and BT belong to the genus Orbivirus, family Reoviridae, and the viruses are primarily transmitted through the bite of blood feeding gnats, or midges, of the Culicoides genus. Bluetongue significantly affects the cattle industry due to restrictions on the sale and movement of cattle that test positive for the virus.
EHD and BT have a very predictable pattern with cases concentrated in the months of August, September and October. Deer are often severely affected by EHD and epidemics result in significant death loss. Sick deer may be found unable to stand or be seen drooling, foaming, or bleeding from the mouth and nose. Dead deer may be found in or near water with no apparent wounds. EHDV-1 and 2 are the most common strains in the US with EHDV-2 most frequently isolated. Traditionally there is an outbreak every 5-10 years in Kentucky, but a new EHDV-6 serotype in the US has been isolated from deer every year since 2006. There is also an increase in frequency of EHD outbreaks in northern states where it was rarely seen previously and increasing reports of EHD in cattle around the world.
Both beef and dairy cattle are affected by EHD and both cows and calves may show signs of disease. Typical clinical signs in cattle include fever, oral ulcers, drooling, sloughing of the nasal mucosa, lameness associated with inflammation of the coronary band, and weight loss. In dairy cattle, udder lesions and a severe drop in milk production may occur. These same clinical signs, especially the oral ulcers and lameness, may be due to other notifiable diseases such as Foot and Mouth disease so it is extremely important to contact a regulatory veterinarian in suspected cases of EHD. Most cattle recover within 2-3 weeks but may lose substantial body condition during that period of time. Sheep rarely develop clinical signs when infected with EHD and goats are not considered susceptible.
Bluetongue virus causes severe disease in sheep, white-tailed deer, pronghorn antelope, bison, and llamas and alpacas. Cattle and goats usually have milder disease symptoms although there have been outbreaks with death loss in both species. Symptoms of bluetongue include fever, redness and swelling of the face, ears and tongue, ulcers in the mouth, salivation, ocular and nasal discharge, lameness due to inflammation of the coronary band, difficulty breathing, weight loss and death. The tongue may be swollen to the point of appearing blue due to lack of oxygen. In cattle, symptoms are less obvious but may include lethargy, fever, drooling, tearing, nasal discharge, swollen and scabby teats, and lameness. There may be abortions and/or congenital defects in the fetus if pregnant dams are infected with either virus between 70-120 days’ gestation. Calves may be born small, weak, deformed or blind. BT is also of regulatory significance to the cattle industry (reportable to State Veterinarian) because cattle are considered carriers of the virus which affects movement, especially internationally. Although biting midges are the primary mode of transmission, bluetongue virus can be transmitted from animal to animal by other routes including close contact, through the placenta to the fetus, venereal transmission, by blood-sucking insects, and iatrogenically through injections with dirty needles. Colorado Serum produces a modified-live Bluetongue vaccine only for use in sheep and goats.
EHD in deer has been correlated with droughts because the deer tend to concentrate around the few wet areas available and these are where the gnats breed. The gnats that transmit EHD and BT tend to feed on the underbelly and lower legs, areas in cattle that are hard to treat effectively with insecticides. Whole animal sprays of pyrethroids should provide the best protection – especially with coverage to under belly and legs where these gnats feed. Organophosphate insecticides are less desirable due to shorter residual life. The gnats are poor fliers. If possible, keep animals in open areas away (30’ or more) from edges of woods. Gnats tend to be in and around wooded areas and usually do not venture far from margins. Air movement will disrupt their flight. If practical, keeping cattle out of wooded areas, especially around wet spots can help decrease the risk of exposure. Building heavy use areas incorporating rock and geotextile fabric underlayment around watering points helps keep these areas dry and clean.
Diagnosis in cattle is accomplished through several types of blood tests. Virus isolation or real time PCR can detect the virus in whole blood (purple top tube). The UKVDL offers a BT/EHD PCR that detects virus antigen in 10 ml of whole blood (drawn in an EDTA or “purple top” tube), or from a swab or tissue samples (lung and spleen preferred) for $50 in-state + $10 Accession fee. There is a less expensive serologic test available for BT virus (BT virus ELISA $6) but a positive result only indicates exposure to the virus at some point in time and does not prove active infection.
There is no specific treatment for EHD or BT virus infections in cattle. Supportive treatment with anti-inflammatory drugs and supplemental feed may be helpful in affected animals showing severe lameness or those that go off feed. Animals recovering from the infection should be treated with an effective insecticide against biting midges because the virus remains in their blood for several weeks which aids in the continued transmission of the virus. However, the biting midges that spread the virus will cease activity after the first killing frost and transmission will stop.