EPIZOOTIC HEMORRHAGIC DISEASE
Epizootic hemorrhagic disease (EHD) is a non-contagious viral infection spread by Culicoides midges, which act as vectors and reservoirs. This disease affects both domesticated and wild ruminants. It is in the World Organization for Animal Health (WOAH) Terrestrial Animal Health Code and must be reported in the European Union.
It can also be found in some wild deer species, which act as reservoirs.
Traditionally, epizootic hemorrhagic disease (EHD) has been isolated in North America, South America, Africa, Asia, the Middle East and Oceania. However, due to the effects of global warming, there have been outbreaks in European countries on the Mediterranean, especially Spain, France and Italy.
As it is spread by Culicoides midges that act as vectors, the main risk factors are directly related to conditions that make it easier for these insects to survive and multiply.
- Climate change: Changes in climate and temperature increases disrupt natural ecosystems and favor ideal conditions for the spread of carrier mosquitoes.
- Standing water: Areas with standing water are a perfect place for midges to breed.
- Environmental transformation: Human activities such as deforestation, road and dam construction, as well as intensive agricultural and animal production systems can create new habitats for pathogens and vectors or favor contact between pathogens and vectors and hosts.
Asymptomatic infected animals can pass the virus to midges. Once infected, they are carriers for life and can infect other ruminants.
Like the bluetongue virus, the EHD virus (EDHV) is spread through the bites of female midges of the Culicoides genus, which act as biological vectors. These female midges, once infected, are carriers of the virus and can spread it to ruminants within 10 to 14 days of the bite.
Most EHD transmission happens silently between host animals that have become resistant to the disease. Cattle are the main reservoir in most zones, often carrying the virus without showing any clinical symptoms and occasionally having viral load in their blood for a very long time.
When a midge bites an infected animal, it ingests the virus, which then infects its intestinal cells. The virus replicates in those cells, escapes into the insect’s body cavity (hemocoel) and infects and replicates in its saliva glands. After this process is complete, the midge can pass the virus onto a new host.
The virus multiplies faster in the vector at high temperatures (+25 ºC). As the temperature decreases, multiplication slows down, and below 12 ºC it does not multiply, but remains alive. The incubation period in ruminants ranges from 2 to 10 days and the duration of viraemia averages 30 days, but can last several months.
There is no known risk of EHD infection in humans. And it isn’t transmitted through contact with infected animals or eating meat or milk from them.
The clinical signs of EHD in wild ruminants and bovine livestock are similar to those of bluetongue in sheep and cattle. Although sheep and goats can also be infected, they generally don’t show clinical symptoms.
In cattle, the symptoms and effects of the disease can vary widely, without an exact clinical pattern. The symptoms can include a variety of manifestations, including:
- Dehydration
- Red or discolored peeling muzzle
- Constant nasal or ocular exudates
- Motor issues and lameness due to inflamed coronet
- Dysphagia (difficulty swallowing)
- Swollen tongue, eyelids and conjunctivas
- Erythema on vulva and/or udders (redness)
- Fever
- Diarrhea
- Anorexia
- In gestating females, it can cause abortions, stillbirths, mummification, weak calf syndrome, arthrogryposis and rear limb deformities
- Blindness
- Pulmonary edema
- Death
For a long time, it was generally believed that strains of the EHD virus (except serotype 2, which causes Ibaraki disease) didn’t cause any significant clinical signs in livestock. This meant it was considered a disease of little importance.
However, this perception changed after outbreaks in Réunion, Northern Africa, Turkey and Israel in the early 2000s, where the infected livestock showed decreased yield and increased mortality. EHD is currently an important concern, especially in Europe and the United States.
During the 2006 outbreak in Israel, milk yield dropped an average of 125 kg per cow per milking, associated with the infection and seroprevalence of the herd. The total losses for the dairy industry due to reduced milk yield and increased mortality were estimated at €2,330,000 ($2,491,000). The average loss per cow was €25 ($26.5) or 0.55% of the total average production value of a dairy cow in Israel.
Although most of the productivity effects associated with EHD infection are from the first incursion, the economic impact is still significant, especially if the losses are extrapolated to Europe or the United States. The ongoing presence of the virus and recurring outbreaks can seriously affect the livestock industry, not only due to direct losses but also the additional cost of control and prevention measures, and for treating affected animals:
- Increased mortality: Fatality rate of the disease is between 1 and 5%.
- Treatment and labor costs: Treatment to alleviate symptoms incurs additional costs for the farmer.
- Production reduction: Especially in the dairy herd, which can see a decrease in production of 125kg of milk per lactation.
- Lower fertility: Increased incidence of abortions, stillbirths and weak calves at birth. It can also produce infertility in bulls.
- Immunosuppression: May cause an increase in associated diseases due to a weakened immune system.
- Culling: Increases the replacement rate of animals.
- Restriction to animal movement.
How to control and prevent EHD?
There is no specific treatment for animals infected with the epizootic hemorrhagic disease, so prevention is key to fighting the spread of the disease.
In endemic zones, vigilance programs should be implemented to actively sample animals from sentinel herds to detect the presence of the virus. These programs, in combination with active vigilance of vector insects, make it possible to apply appropriate control measures like:
- Identification, vigilance and tracking of susceptible and potentially infected animals.
- Quarantine and/or restricted movement during insect activity periods.
- Identification of specific areas with high incidence rates.
- Insect control
Definitive diagnosis of EHD infection requires specific lab tests, which can be used for both domesticated and wild ruminants.
In vitro and in vivo cultures:
The virus is isolated from blood or tissue samples from viremic animals, from the spleen, lungs or lymph nodes, for example.
RT-PCR:
Identifica rápidamente el virus en la sangre y otros tejidos de los animales infectados. Es importante destacar que esta prueba detecta el ácido nucleico una vez el virus ya no es viable y, por lo tanto, un resultado positivo no necesariamente indica la presencia del virus infeccioso.
Tests like ELISA detect virus antibodies in serum or plasma samples. They are useful to control the infection status of a population during active vigilance programs.
A vaccine approved under emergency authorization against serotype 8 of epizootic hemorrhagic disease virus is now available. Vaccination against EHD is key, as the measures employed in the control of the disease have proven to be insufficient. Vaccination is used as the most effective and practical measure to control the virus, to minimize economic losses, to interrupt the cycle of infection and to allow safe movement of animals.
Bibliography
- Abdy, M.J., Howerth, E.E. & Stallknecht, D.E. (1999) Experimental infection of calves with epizootic hemorrhagic disease virus. American Journal of Veterinary Research, 60, 621–626.
- Batten, C.A., Edwards, L., Bin-Tarif, A., Henstock, M.R. & Oura, C.A. (2011) Infection kinetics of epizootic haemorrhagic disease virus serotype 6 in Holstein-Friesian cattle. Veterinary Microbiology, 154, 23–28. doi: 10.1016/j.vetmic.2011.06.018
- Jiménez-Cabello, L., Utrilla-Trigo, S., Lorenzo, G., Ortego, J. & Calvo-Pinilla, E. (2023) Epizootic Hemorrhagic Disease Virus: Current Knowledge and Emerging Perspectives. Microorganisms, 19;11(5):1339. doi: 10.3390/microorganisms11051339.
- Kedmi, M., Van Straten, M., Ezra, E., Galon, N. & Klement, E. (2010). Assessment of the productivity effects associated with epizootic hemorrhagic disease in dairy herds. Journal of dairy science, 93(6), 2486–2495. doi: 10.3168/jds.2009-2850
- Manual Terrestre de la OIE (2021) Enfermedad Hemorrágica Epizoótica (infección por el virus de la Enfermedad Hemorrágica Epizoótica. Capítulo 3.1.7. Recuperado de: www.woah.org/fileadmin/Home/esp/Health_standards/tahm/3.01.07_EHD.pdf
- Maurer, L.M., Paslaru, A., Torgerson, P.R., Veronesi, E. & Mathis, A. (2021) Vector Competence of Culicoides Biting Midges from Switzerland for African Horse Sickness Virus and Epizootic Haemorrhagic Disease Virus. Schweiz. Arch. Tierheilkd, 164, 66–70. doi: 10.17236/sat00337
- Maclachlan, N.J., Zientara, S., Savini, G., & Daniels, P.W. (2015) Epizootic haemorrhagic disease. Revue Scientifique et Technique, 34 (2), 341-351. doi: 10.20506/rst.34.2.2361
- Ministerio de Agricultura, Pesca y Alimentación. Enfermedad Hemorrágica Epizoótica. Recuperado de: https://www.mapa.gob.es/es/ganaderia/temas/laboratorios-sanidad-genetica/areas-actividad/diagnostico/enfermedad-hemorragica-epizootica.aspx
HEPIZOVAC*
Inactivated vaccine for active immunization of cattle to prevent viremia and to reduce clinical signs caused by the serotype 8 of epizootic hemorrhagic disease virus, in injectable suspension.