African Horse Sickness

African Horse Sickness



Basic Information image



Definition




• African horse sickness (AHS) is caused by a double-stranded, nonenveloped RNA virus that affects members of the family Equidae and is transmitted by midges (Culicoides spp.). Horses and mules are the most severely affected.


• There are nine distinct serotypes, with types 5 and 9 most commonly occurring outside of the endemic sub-Saharan Africa. Four major forms of the disease exist: peracute or pulmonary (95% mortality rate), subacute or cardiac (50% mortality rate), mixed (70% mortality rate), and horse sickness fever (recovery). The onset of disease in the severe forms is characterized by a high fever followed by respiratory failure with a frothy to serosanguineous nasal discharge or nondependent edema of the head and neck with petechiations of the conjunctiva. There is no effective treatment.


• A polyvalent vaccine exists that provides protection for all serotypes. Quarantine, constant monitoring, and vector control are necessary. This disease has not been documented in North America and is a World Organisation for Animal Health (OIE)–reportable disease.



Epidemiology



Species, Age, Sex




• Horses are most susceptible to infection, with a mortality rate of 70% to 95% depending on the form of the disease.


• Mules are slightly less susceptible, with a mortality rate of 50% to 70%.


• Donkeys and zebras develop subclinical disease.


• There does not appear to be an age predilection. Foals born to immune mares may receive passive immunity that disappears at age 4 to 6 months.


• There is no sex predilection.



Genetics and Breed Predisposition


Horses indigenous to certain areas of North and West Africa appear to have acquired resistance to infection with AHS. There is no breed disposition.



Contagion and Zoonosis


The only known incident of humans contracting AHS occurred in a laboratory setting. AHS is not considered a zoonotic disease in the natural setting; however, veterinarians dealing with suspected cases of AHS should still follow universal precautions.



Geography and Seasonality




• AHS is considered to be enzoonotic in eastern and central Africa, with yearly incursions of the disease into southern Africa. Documented and confirmed epidemics have occurred in North Africa, the Middle East, Turkey, Afghanistan, Pakistan, India, Spain, and Portugal. To date, the virus has never been documented in North America.


• Transmission


• AHS is biologically transmitted by midges of the Culicoides genera. Culicoides imicola and Culicoides bolitinos appear to be the most important vectors in Africa. Other vectors, including ticks and mosquitoes, have been shown to transmit AHS under experimental conditions; however, they have never been shown to transmit the virus under natural conditions. Mechanical transmission of the virus may occur through biting flies and iatrogenic incidents. In the United States, Culicoides sonorensis and Culicoides variipennis (which are responsible for blue-tongue virus transmission) have been shown to transmit AHS under experimental conditions.

• Transmission of AHS corresponds with peak vector activity. In Africa, this depends on climatic conditions, in which heavy rains followed by warm, dry periods favor the survival of Culicoides individuals. Cold weather and frosts kill Culicoides spp. and thus decrease or stop AHS transmission. This effect of climate on AHS transmission is especially evident in AHS epidemics, when the virus makes incursions into South Africa. The wet, warm months of February, March, and April have the highest case rates of AHS, with cases of AHS disappearing after the first frosts in April or May. Epidemics also occur when individuals having a high viral load are transported to countries with Culicoides vectors capable of transmitting the virus. Epidemics have also been reported with the windborne spread of infected vectors.

• Maintenance of AHS in the environment.


• The exact mechanism of the maintenance of AHS in the environment is uncertain. The short incubation period accompanied by the high mortality rate in horses and mules make it unlikely that these animals can serve as effective reservoirs for maintaining the virus.

• Continuous transmission between zebras and Culicoides midges was demonstrated in South Africa in Kruger National Park. Thus large populations of zebras and donkeys are likely to be one of the major reservoirs of AHS.

• Dogs have been shown to be susceptible to AHS when fed raw horse meat infected with AHS. Other populations of animals have yet to be investigated for their possible role in the maintenance of AHS.


Clinical Presentation



Disease Forms/Subtypes




• Pulmonary (“dunkop”): Peracute form


• Cardiac (“dikkop”): Subacute form


• Mixed: Most common form


• Horse sickness fever: Mildest form



History, Chief Complaint


Horses may present with a variety of clinical signs depending on the form of disease. These include a fever of 102° to 106° F, depression, supraorbital edema, and respiratory distress.

< div class='tao-gold-member'>

Related posts:

  1. Ultrasound
  2. Early Goal-Directed Therapy
  3. Nutrition
  4. Splenic Rupture

Stay updated, free articles. Join our Telegram channel
Tags:
Jul 24, 2016 | Posted by in SMALL ANIMAL | Comments Off on African Horse Sickness

Full access? Get Clinical Tree
Get Clinical Tree app for offline access