African Horse sickness
Encyclopedia
African horse sickness is a highly infectious, and deadly disease
. It commonly affects horses, mule
s, and donkeys. It is caused by a virus
of the genus
Orbivirus
belonging to the family
Reoviridae
. This disease can be caused by any of the nine serotypes of this virus. AHS is not directly contagious
, but is known to be spread by insect
vectors.
AHS is known to be endemic
in Sub-Saharan Africa
, and has spread to Morocco
, the Middle East
, India
and Pakistan
. More recently, outbreaks have been reported in the Iberian Peninsula
. AHS has never been reported in the Americas, eastern Asia or Australasia
. Epidemiology is dependent on host-vector interaction, where cyclic disease outbreaks coincide with high numbers of competent vectors. The most important vector for AHS in endemic areas is the biting midge Culicoides imicola
, which prefers warm, humid conditions. Larvae do not carry the virus and long, cold winters are sufficient to break epidemics in non-endemic areas.
(midges) species
. However, this disease can also be transmitted by species of mosquitoes including Culex
, Anopheles
and Aedes
, and species of ticks such as Hyalomma
and Rhipicephalus
.
mortality of those affected, followed by mules (50%) and donkeys (10%). African donkeys and zebras very rarely display clinical symptoms, despite high virus titres in blood, and are thought to be the natural reservoir
of the virus. AHS manifests itself in four different forms: the pulmonary form, the cardiac form, a mild (horse sickness fever) form, and a mixed form.
Pulmonary form
The peracute form of the disease is characterized by high fever, depression and respiratory symptoms. The clinically affected animal has trouble breathing, starts coughing frothy fluid from nostril and mouth, and shows signs of pulmonary edema
within four days. Serious lung congestion causes respiratory failure
and results in death in under 24 hours. This form of the disease has the highest mortality rate.
Cardiac form
This subacute form of the disease has an incubation period
longer than that of the pulmonary form. Signs of disease start at day 7-12 after infection. High fever is a common symptom. The disease also manifests as conjunctivitis
, with abdominal pain and progressive dyspnea
. Additionally, edema is presented under the skin of the head and neck, most notably in swelling of the supra-orbital fossa
e, palpebral conjunctiva and intermandibular space. Mortality rate is between 50-70% and survivors recover in 7 days.
Mild or horse sickness fever form
Mild to subclinical disease
is seen in zebras and African donkeys. Infected animals may have low grade fever and congested mucous membrane
. The survival rate is 100%.
Mixed form
Diagnosis is made at necropsy
. Affected horses show signs of both the pulmonary and cardiac forms of AHS.
for detecting viral RNA, antigen capture ELISA
and immunofluorescence
of infected tissues. Serological tests are only useful for detecting recovered animals, as sick animals die before they are able to mount effective immune responses.
for AHS.
Control of an outbreak in an endemic region involves quarantine, vector control and vaccination
. To prevent this disease, the affected horses are usually slaughtered, and the uninfected horses are vaccinated against the virus. Three vaccine
s currently exist, which include a polyvalent vaccine, a monovalent vaccine and a monovalent inactivated vaccine. This disease can also be prevented by destroying the insect vector habitats using insecticide
s.
and is spread by the same midge (Culicoides species).
Disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...
. It commonly affects horses, mule
Mule
A mule is the offspring of a male donkey and a female horse. Horses and donkeys are different species, with different numbers of chromosomes. Of the two F1 hybrids between these two species, a mule is easier to obtain than a hinny...
s, and donkeys. It is caused by a virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
of the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Orbivirus
Orbivirus
The genus Orbivirus is a member of the Reoviridae family. This genus currently contains 22 species and at least 130 different serotypes. Orbiviruses can infect and replicate within a wide range of arthropod and vertebrate hosts...
belonging to the family
Family
In human context, a family is a group of people affiliated by consanguinity, affinity, or co-residence. In most societies it is the principal institution for the socialization of children...
Reoviridae
Reoviridae
Reoviridae is a family of viruses that can affect the gastrointestinal system and respiratory tract. Viruses in the family Reoviridae have genomes consisting of segmented, double-stranded RNA...
. This disease can be caused by any of the nine serotypes of this virus. AHS is not directly contagious
Contagious disease
A contagious disease is a subset category of infectious diseases , which are easily transmitted by physical contact with the person suffering the disease, or by their secretions or objects touched by them....
, but is known to be spread by insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
vectors.
Epidemiology
AHSV was first recorded south of the Sahara Desert in the mid 1600s with the introduction of horses to southern Africa. The virus is considered endemic to the equatorial, eastern, and southern regions of Africa. Several outbreaks have occurred in Equidaes throughout Africa and elsewhere.AHS is known to be endemic
Endemic (epidemiology)
In epidemiology, an infection is said to be endemic in a population when that infection is maintained in the population without the need for external inputs. For example, chickenpox is endemic in the UK, but malaria is not...
in Sub-Saharan Africa
Sub-Saharan Africa
Sub-Saharan Africa as a geographical term refers to the area of the African continent which lies south of the Sahara. A political definition of Sub-Saharan Africa, instead, covers all African countries which are fully or partially located south of the Sahara...
, and has spread to Morocco
Morocco
Morocco , officially the Kingdom of Morocco , is a country located in North Africa. It has a population of more than 32 million and an area of 710,850 km², and also primarily administers the disputed region of the Western Sahara...
, the Middle East
Middle East
The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East...
, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
and Pakistan
Pakistan
Pakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan...
. More recently, outbreaks have been reported in the Iberian Peninsula
Iberian Peninsula
The Iberian Peninsula , sometimes called Iberia, is located in the extreme southwest of Europe and includes the modern-day sovereign states of Spain, Portugal and Andorra, as well as the British Overseas Territory of Gibraltar...
. AHS has never been reported in the Americas, eastern Asia or Australasia
Australasia
Australasia is a region of Oceania comprising Australia, New Zealand, the island of New Guinea, and neighbouring islands in the Pacific Ocean. The term was coined by Charles de Brosses in Histoire des navigations aux terres australes...
. Epidemiology is dependent on host-vector interaction, where cyclic disease outbreaks coincide with high numbers of competent vectors. The most important vector for AHS in endemic areas is the biting midge Culicoides imicola
Culicoides imicola
Culicoides imicola is a midge which transmits the bluetongue virus. Other suspected BTV vectors areCulicoides pulicaris and species in the Culicoides obsoletus complex....
, which prefers warm, humid conditions. Larvae do not carry the virus and long, cold winters are sufficient to break epidemics in non-endemic areas.
Host
The common hosts of this disease are horses, mules, donkeys and zebra. However, elephants, camels and dogs (after eating infected horsemeat) can be infected as well, but often show no signs of the disease.Transmission
This disease is spread by insect vectors. The biological vector of the virus is the CulicoidesCulicoides
Culicoides is a genus of biting midges in the subfamily Ceratopogonidae. Around 500 species of Ceratopogonidae are at present placed in the genus and this is split into many subgenera. Several species are known to be vectors of various diseases and parasites which can affect animals.-Notable...
(midges) species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
. However, this disease can also be transmitted by species of mosquitoes including Culex
Culex
Culex is a genus of mosquito, and is important in that several species serve as vectors of important diseases, such as West Nile virus, filariasis, Japanese encephalitis, St. Louis encephalitis and avian malaria....
, Anopheles
Anopheles
Anopheles is a genus of mosquito. There are approximately 460 recognized species: while over 100 can transmit human malaria, only 30–40 commonly transmit parasites of the genus Plasmodium, which cause malaria in humans in endemic areas...
and Aedes
Aedes
Aedes is a genus of mosquito originally found in tropical and subtropical zones, but now found on all continents excluding Antarctica. Some species have been spread by human activity. Aedes albopictus, a most invasive species was recently spread to the New World, including the U.S., by the used...
, and species of ticks such as Hyalomma
Hyalomma
Hyalomma is a genus of hard-bodied ticks, common in Asia, Europe, and Hyalomma is a genus of hard-bodied [[tick]]s, common in [[Asia]], [[Europe]], and...
and Rhipicephalus
Rhipicephalus
Rhipicephalus is a genus of tick.-Species:* Rhipicephalus annulatus Say, 1821* Rhipicephalus appendiculatus Neumann 1901* Rhipicephalus aquatilis Walker, Keirans & Pegram, 1993* Rhipicephalus armatus Pocock, 1900...
.
Clinical signs
Horses are the most susceptible host with close to 90%mortality of those affected, followed by mules (50%) and donkeys (10%). African donkeys and zebras very rarely display clinical symptoms, despite high virus titres in blood, and are thought to be the natural reservoir
Natural reservoir
Natural reservoir or nidus, refers to the long-term host of the pathogen of an infectious disease. It is often the case that hosts do not get the disease carried by the pathogen or it is carried as a subclinical infection and so asymptomatic and non-lethal...
of the virus. AHS manifests itself in four different forms: the pulmonary form, the cardiac form, a mild (horse sickness fever) form, and a mixed form.
Pulmonary form
The peracute form of the disease is characterized by high fever, depression and respiratory symptoms. The clinically affected animal has trouble breathing, starts coughing frothy fluid from nostril and mouth, and shows signs of pulmonary edema
Pulmonary edema
Pulmonary edema , or oedema , is fluid accumulation in the air spaces and parenchyma of the lungs. It leads to impaired gas exchange and may cause respiratory failure...
within four days. Serious lung congestion causes respiratory failure
Respiratory failure
The term respiratory failure, in medicine, is used to describe inadequate gas exchange by the respiratory system, with the result that arterial oxygen and/or carbon dioxide levels cannot be maintained within their normal ranges. A drop in blood oxygenation is known as hypoxemia; a rise in arterial...
and results in death in under 24 hours. This form of the disease has the highest mortality rate.
Cardiac form
This subacute form of the disease has an incubation period
Incubation period
Incubation period is the time elapsed between exposure to a pathogenic organism, a chemical or radiation, and when symptoms and signs are first apparent...
longer than that of the pulmonary form. Signs of disease start at day 7-12 after infection. High fever is a common symptom. The disease also manifests as conjunctivitis
Conjunctivitis
Conjunctivitis refers to inflammation of the conjunctiva...
, with abdominal pain and progressive dyspnea
Dyspnea
Dyspnea , shortness of breath , or air hunger, is the subjective symptom of breathlessness.It is a normal symptom of heavy exertion but becomes pathological if it occurs in unexpected situations...
. Additionally, edema is presented under the skin of the head and neck, most notably in swelling of the supra-orbital fossa
Fossa (anatomy)
In anatomical terminology, fossa has come to mean a depression or hollow, in general, in a bone. Other parts of the body may be involved, for example, the antecubital fossa...
e, palpebral conjunctiva and intermandibular space. Mortality rate is between 50-70% and survivors recover in 7 days.
Mild or horse sickness fever form
Mild to subclinical disease
Subclinical infection
A subclinical infection is the asymptomatic carrying of an by an individual of an agent that usually is a pathogen causing illness, at least in some individuals. Many pathogens spread by being silently carried in this way by some of their host population...
is seen in zebras and African donkeys. Infected animals may have low grade fever and congested mucous membrane
Mucous membrane
The mucous membranes are linings of mostly endodermal origin, covered in epithelium, which are involved in absorption and secretion. They line cavities that are exposed to the external environment and internal organs...
. The survival rate is 100%.
Mixed form
Diagnosis is made at necropsy
Autopsy
An autopsy—also known as a post-mortem examination, necropsy , autopsia cadaverum, or obduction—is a highly specialized surgical procedure that consists of a thorough examination of a corpse to determine the cause and manner of death and to evaluate any disease or injury that may be present...
. Affected horses show signs of both the pulmonary and cardiac forms of AHS.
Diagnosis
Presumptive diagnosis is made by characteristic clinical signs, post mortem lesions and presence of competent vectors. Laboratory confirmation is by viral isolation, with such techniques as Real Time PCRReal-time polymerase chain reaction
In molecular biology, real-time polymerase chain reaction, also called quantitative real time polymerase chain reaction or kinetic polymerase chain reaction , is a laboratory technique based on the PCR, which is used to amplify and simultaneously quantify a targeted DNA molecule...
for detecting viral RNA, antigen capture ELISA
ELISA
Enzyme-linked immunosorbent assay , is a popular format of a "wet-lab" type analytic biochemistry assay that uses one sub-type of heterogeneous, solid-phase enzyme immunoassay to detect the presence of a substance in a liquid sample."Wet lab" analytic biochemistry assays involves detection of an...
and immunofluorescence
Immunofluorescence
Immunofluorescence is a technique used for light microscopy with a fluorescence microscope and is used primarily on biological samples. This technique uses the specificity of antibodies to their antigen to target fluorescent dyes to specific biomolecule targets within a cell, and therefore allows...
of infected tissues. Serological tests are only useful for detecting recovered animals, as sick animals die before they are able to mount effective immune responses.
Treatment and prevention
There is currently no treatmentPharmacotherapy
Pharmacotherapy is the treatment of disease through the administration of drugs. As such, it is considered part of the larger category of therapy....
for AHS.
Control of an outbreak in an endemic region involves quarantine, vector control and vaccination
Vaccination
Vaccination is the administration of antigenic material to stimulate the immune system of an individual to develop adaptive immunity to a disease. Vaccines can prevent or ameliorate the effects of infection by many pathogens...
. To prevent this disease, the affected horses are usually slaughtered, and the uninfected horses are vaccinated against the virus. Three vaccine
Vaccine
A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weakened or killed forms of the microbe or its toxins...
s currently exist, which include a polyvalent vaccine, a monovalent vaccine and a monovalent inactivated vaccine. This disease can also be prevented by destroying the insect vector habitats using insecticide
Insecticide
An insecticide is a pesticide used against insects. They include ovicides and larvicides used against the eggs and larvae of insects respectively. Insecticides are used in agriculture, medicine, industry and the household. The use of insecticides is believed to be one of the major factors behind...
s.
History
African horse sickness was diagnosed in Spain in 1987-90 and in Portugal in 1989, but was eradicated using slaughter policies, movement restrictions, vector eradication and vaccination.Related Diseases
AHS is related to bluetongue diseaseBluetongue disease
Bluetongue disease or catarrhal fever is a non-contagious, non-zoonotic, insect-borne, viral disease of ruminants, mainly sheep and less frequently cattle, goats, buffalo, deer, dromedaries and antelope...
and is spread by the same midge (Culicoides species).
External links
- CIDRAP Overview on African Horse Sickness
- http://www.oie.int/eng/maladies/fiches/A_A110.HTM
- http://www.promedmail.org/pls/otn/f?p=2400:1001:2914773925093697