African horse sickness is a highly infectious, noncontagious, insect transmitted disease affecting all species of Equidae

1. AFRICAN HORSE
SICKNESS
NEERAJA E.S

2. AHS
 African horse sickness is a highly infectious, non-
contagious, insect-transmitted disease affecting all
species of Equidae
 Characterized by respiratory and circulatory damage
 Notifiable to the World Organisation for Animal Health
(OIE)

3. HISTORY
 First recorded reference of AHS occured in Yemen in 1327
 Disease first recognised 60 years after initial introduction of
horses to Africa in 1657
 First major outbreak in 1719- killed 1700 animals
 1854-1855- in South Africa- death of 70,000 animals
 In Asia 1959-1961(AHSV-9)- India, Pakistan, Afganisthan, Iraq,
Saudi Arabia- Death of 300,000 equids
 1965 Spain, Morocco, Algeria, Tunisia,
 1987- AHSV-4 IN Central Spain 1988-1990 new outbreaks
occured
 1989- Portugal

4. A map of AHS outbreaks that have occurred world wide during
the last century

5. TAXONOMY
 Domain: Virus
 Group: "Positive sense ssRNA viruses"
 Group: "RNA viruses"
 Family: Reoviridae
 Genus: Orbivirus
 Species: African horse sickness virus

6. ETIOLOGY
 Genome – 10 Ds RNA segments
encodes
 7- structural protein (VP1-7)
 4- non structural protein
(NS1-NS4)
 9 antigenic strains/ serotypes
recognised

7.  Structural proteins
 VP2 and VP5 form the outer capsid of the virion
 VP3 and VP7 are the major inner capsid proteins
 VP1, VP4, and VP6 constitute minor inner capsid proteins
 Non structural proteins
 NS3 proteins are the second most variable AHSV proteins
and are associated with viral release from cells and total viral
yield

8.  The serotypic differences are attributable to variations
in the capsid proteins- VP2 and to a lesser extent VP5
 VP2 contains the predominant neutralizing epitopes
 Antibodies to VP5 are one of the earliest serologic
markers of infection and have neutralizing activity
 Lineages are also evident within serotypes
 There are also variants of each serotype with attenuated
virulence

9. SUSCEPTIBILITY
 Inactivated by heating at
 50° C (122° F) for 3 hours
 60° C (140° F) for 15 minutes
 Inactivated by acid
 Inactivated in 48 hours by 0.1% formalin or phenol, sodium
hypochlorite, and iodophors
 Stable at 4° C (39° F)
 Survives for 37 days at 37° C
 Viable at pH of 6 to 12
 Putrefaction does not destroy the virus: putrid blood may
remain infective for >2 years

10. HOST ANIMALS
Animal name Context
Bactrian camel Domesticated host
Dromedary camel Domesticated host
Dogs Domesticated host, wild host
Equus quagga (Zebra)
Donkeys (Equus asinus) Domesticated host, wild host
Equus caballus(horses) Domesticated host, wild host
Mules Domesticated host
Elephantidae Wild host

11. IMPLICATED VECTORS OF AHSV
 Culicoides biting midges (Confirmed)
 Mosquitoes(Suspected, but low importance)
 Ticks (unknown, but low importance)
 Mechanical transmission (Suspected, but low
importance)

12. EPIDEMIOLOGY
 Enzootic in sub-Saharan Africa
 Regularly spreads to Southern Africa and occasionally to
Northern Africa
 Disease occurs from Senegal through sub-Saharan Africa to
Somalia and Ethiopia
 The virus occurs in the Middle East, including Saudi Arabia
and Yemen
 Outbreaks outside Africa occured
 Near and Middle East (1959-63)
 Spain (1966 (serotype 9), 1987-90 (serotype 4))
 Portugal (1989, serotype 4)
 Morocco (1989-91, serotype 4)
OCCURENCE

13. DISEASE OUTBREAK MAP-WAHID 2020
WAHID OIE 2020 from Jan 2020
Thailand

14. Recent outbreak in Thailand - serotype 1 had caused the outbreak; the first time
that this serotype has been seen outside of Africa. First that south-east Asia has
ever experienced.

15. Horses kept in net after being vaccinated in Thailand(vaccination start from April
20th onwards in horses 50km with in affected farms )

16. TRANSMISSION
o Midges (Culicoides spp.)
o Ticks (Hyalomma dromadarii and Rhipicephalus
sanguineus)
o Mosquitoes (various species in laboratory studies)
Hematophagous insects

17. TRANSMISSION OF ASHV
Inefficient 1: 100-1000
Efficient 1:1
Intrinsic incubation period
2-4 days horses
Temperature independant
Extrinsic incubation period
3-28 days
Temperature dependant

18. TRANSMISSION....
 Midges are the most important vector in the spread of
the spontaneous disease
 Source of virus for midges is blood of infected horses,
donkeys, mules, and zebra
 Horses and mules have clinical signs of disease while
viraemic
 Donkeys & zebra are always, apparently uninfected
 Zebras may remain viraemic for 6 weeks, donkeys for 12
days, and horses for 18 to 21 days

19. TRANSMISSION.....
 Dogs are usually infected by eating infected animals,
although transmission to and from dogs by ticks can
occur
 Transmission of the virus to areas where it does not
usually exist occurs by
 Movement of infected animals, such as zebras and horses
 By transportation of midges by wind or in aircraft
 Mechanical transmission of the virus on contaminated
surgical instruments and needles should be considered a
possibility

20. TRANSMISSION.....
 Virus persists by cycling between zebra & vectors year round
 Can seroconvert during any month of the year
 Persistence of the virus is attributable to the
 Long period of viraemia in zebra
 The presence of a herd of sufficient size to support cycling
of infection among animals
 The minimum size of a zebra population to maintain an
enzootic infection is unknown
Zebra in enzootic area

21. TRANSMISSION.....
 Virus does not persist over the cooler winter months,
when viremic animals recover and the vectors die
 Concern exists that reintroduction of zebra to areas of
the country currently free of enzootic AHS might permit
reestablishment of the virus and disease in horses
Non enzootic area

22. TRANSMISSION..... MIDGES
 Virus infect & replicate in midges
 Rate of extrinsic incubation period is directly related to
temperature
 No transovarian transmission
 Infected for life
 C.imicola - primary vector in enzootic area
 C. bolitinos- vector in South Africa
 Other spp. Unlikely to be vectors- unable to maintain
infection with virus 10 days after ingesting blood

23. TRANSMISSION..... MIDGES
 Unclear which species, apart from C. imicola and C.
bolitinos, can be vectors for AHSV
 Capacity of these potential vectors to spread disease
 Midges must feed on horses with sufficient frequency to
spread the infection
 Introduction of midges to new areas occurs by
 Wind (up to 700 Km)or air craft
 Suitable niche for vector
 Result of climate change
 Human manipulation of local ecosystem- irrigation, alteration in
herbivore population

24. DISPERSAL OF CULICOIDES
 Long distance flight over sea (100’s km)
 Local movement over land
 Primarily short distance (< 100km)
 Occasional long distance (> 1 km)
 Movement in horse transport
 International movement

25. TRANSMISSION.... MIDGES
 Abundance of midges can be predicted from
 Measures of soil moisture content
 Land surface temperature
 Breed in damp soils that are rich in organic material-
irrigated pastures provide soil moisture adequate for
completion of the life cycle (at least 7–10 days)
 Higher temperatures
 Increase the rates of infection of midges
 Virogenesis within midges
 Transmission rate
 Decrease midge longevity

26. TRANSMISSION.... MIDGES
 Replication of AHSV in midges does not occur at
temperatures less than 15° C (59° F)
 Midges continue to be active at 12° C (54° F)
 The absence of AHSV in the midges during winter in
parts of South Africa can be ascribed to their
 Relatively low numbers
 Low infection prevalence
 Low virus replication rates
 Low virus titers in the potentially infected midges

27. ENVIRONMENTAL RISK FACTORS
 Incidence is often seasonal because of
 Seasonal variations in the number of culicoides spp.
Present
 Other weather-related factors such as host (zebra)
behaviour
 Vector activity is favoured by temperatures between
12.5° and 29° C (54.5° and 84° F)

28. ENVIRONMENTAL RISK FACTORS......
 Several cool or cold episodes, rather than one “killing
frost,” are necessary to kill all or most vectors
 Disease has a geographic distribution
 Local factors- including topography, influence the
distribution of midges within their overall range
 Areas most severely affected are low lying and swampy

29. EPIZOOTIC OCCURENCE
 Occur in southern africa in association with variations in the
el niño/southern oscillation
 Epizootics of the disease occur in years in which the
oscillation produces drought followed by heavy rains
 Reason unknown but could be related to congregation of
zebra around water holes during the drought
 Congregation of large numbers of zebra might increase the
infection rate among midges
 Which then disseminate the infection when rains produce
widespread conditions favourable to their reproduction

30. ANIMAL FACTORS
 Natural infection occurs in Equidae
 Most severe disease occurring in horses
 Degree of susceptibility Mules > Donkeys > Zebras
 Risk of death is greatest in weanlings but not related to
sex of the animal
 The CFR varies depending on the severity of disease
 Can be as high as 90% in susceptible horses
 But it is lower in mules and donkeys

31. SEROPREVALENCE
 Elephants seroconvert when exposed to infection
 Not an important reservoir
 White rhinoceros sampled in Kruger National Park in
1989 had a 60% seroprevalence to AHSV
 Whereas in 2007 the seroprevalence was zero
 The reasons for this difference are unclear

32.  Vaccination is effective in reducing risk of the disease
(odds ratio for risk of death ~0.1 [0.04 to 0.4])
 After natural infection or vaccination, immunity to that
strain, but not to heterologous strains, is solid
 The development of immunity is slow and may require 3
weeks to be appreciable
 titers may continue to rise for 6 months after infection

33.  Foals from immune dams derive passive immunity
 The titer of which varies depending on the mare’s titer, the
serotype, and the time after ingestion of colostrum
 Mare titers before foaling and foal serum titers after suckling
are highly correlated regardless of serotype
 Mare serum titers for some serotypes (1, 4, 6, and 9) are
higher than for other serotypes, and this is mirrored in the
titers in foal serum
 Estimated mean half-life for neutralizing antibodies in foals to
all 9 serotypes was 20.5 days

34.  With a range from 15.4 days for serotype 8 to 22.6 days
for serotype 3
 The estimate for the mean time until the serum
neutralization test became negative at a 1 : 10 dilution,
considered absence of protection from infection, was 96
days for all nine serotypes, with a range from 62 days for
serotype 5 to 128 days for serotypes 3 and 4

35. ECONOMIC IMPORTANCE
 Costs associated with preventive measures in enzootic
areas
 Monitoring for introduction of disease in unaffected
areas
 Restrictions on importation of horses from countries in
which the disease is enzootic
 The high casefatality rate and morbidity of the disease in
outbreaks is another source of loss

36. ZOONOTIC DISEASE
 Caused encephalitis and chorioretinitis in eight workers
in an AHS vaccine factory
 Infection was likely be through inhalation of freeze-dried
virus

37. PATHOGENESIS
 Affects vascular endothelium & monocytes/macrophages
 The tissue tropism of the infecting serotype determines
which organs are most severely affected,
 All serotypes infect the heart and lungs and, to a lesser
extent, the spleen
 After infection, the virus multiplies in local lymph nodes,
and a primary viraemia occurs

38. PATHOGENESIS.....
 Dissemination of infection to endothelial cells and
intravascular macrophages of lung, spleen, and
lymphoid tissues
 Viral multiplication then results in a secondary cell-
associated (red cell and white cell) viremia in horses of
up to 9 days
 Fever and viremia occur at the same time
 Resolution of the viremia is associated with
defervescence

39. PATHOGENESIS....
 Localization of antigen depends on the form of the
disease horses with
 Horse sickness- most of the antigen in the spleen
 Cardiopulmonary form- abundant antigen in
cardiovascular and lymphatic systems

40. CLINICAL FINDINGS
 Incubation period in natural infections is about 5 to 7 days
 3 or 4 clinical forms of the disease occur
 Acute or pulmonary form/ DUNKOP
 Cardiac or sub-acute form/ DIKKOP
 Mixed form
 Mild form/ Horse sickness fever
 An intermittent fever of 40° to 41° C (105–106° F) is
characteristic of all forms

41. 1. ACUTE/PULMONARY HORSE SICKNESS
(DUNKOP)/ CENTRAL FORM
 Most common form in epizootics
 A case-fatality rate of 95%
 Fever is followed by labored breathing,
 Severe paroxysms of coughing
 Profuse nasal discharge of yellowish serous fluid and
froth
 Profuse sweating, profound weakness, and a staggering
gait progress to recumbency

42. DUNKOP...
 Death usually occurs after a total course of 4 to 5 days
 It can be so acute as to be without observed
premonitory signs in some horses
 Severe respiratory distress persists for many weeks in
surviving animals
 Form of the disease that occurs naturally in dogs

43. Excessive nasal discharge, followed
by death from anoxia
Foam from the nares- pulmonary oedema

44. 2. SUBACUTE (CARDIAC) HORSE SICKNESS
(DIKKOP)/ PERIPHERAL FORM
 Most common in horses in enzootic areas
 A case-fatality rate of 50%
 Incubation period may be up to 3 weeks
 Disease has a more protracted course than does the
acute, pulmonary form
 Edema in the head, particularly in the temporal fossa,
the eyelids, and the lips, and the chest, which may not
develop until the horse has been febrile for a week

45. DIKKOP...
 Cyanotic oral mucosa
 Petechiae may develop under the tongue
 Examination of the heart and lungs reveals evidence of
hydropericardium, endocarditis, and pulmonary edema
 Restlessness and mild abdominal pain
 Paralysis of the esophagus, with inability to swallow
 Regurgitation of food and water through the nose, is not
uncommon
 Recovery is prolonged
 A fatal course may last as long as 2 weeks

46. Oedema of supraorbital
fossa
Supraorbital oedema

47. 3. MIXED FORM
 With both pulmonary and cardiac signs
 Evident as an initial subacute cardiac form that suddenly
develops acute pulmonary signs
 Primary pulmonary syndrome may subside, but cardiac
involvement causes death
 Mixed form is not common in field outbreaks

48. 4. HORSE SICKNESS FEVER
 Mild form of horse sickness fever, which may be easily
overlooked
 Common in enzootic areas
 The disease occurs in horses with
 Some immunity
 Infection by serotypes of low virulence
 This is the only form of the disease that occurs in zebras
 The temperature rises to 40.5° C (105° F) over a period of 1 to
3 days but returns to normal about 3 days later
 The appetite is poor, and there is slight conjunctivitis and
moderate respiratory distress

49. Congestion & oedema of the
conjunctiva

50. CLINICAL PATHOLOGY
 Characteristic of the acute forms of AHS
 Leukopenia with lymphopenia
 Neutropenia and a left shift
 Mild thrombocytopenia
 Hemoconcentration
 There is evidence of activation of coagulation cascade
and fibrinolysis, although disseminated intravascular
coagulation is unusual

51. CLINICAL PATHOLOGY......
 Serum biochemical abnormalities include- Increasesed
 Creatine kinase
 Lactate dehydrogenase
 Alkaline phosphatase activities
 Creatinine and bilirubin concentrations

52. NECROPSY FINDINGS
Gross findings in acute cases
 Severe hydrothorax
 Pulmonary oedema
 Moderate ascites
 The liver is acutely congested
 Oedema of the bowel wall

53. NECROPSY FINDINGS...
 The pharynx, trachea, and bronchi are filled with yellow
serous fluid and froth
 In cases of cardiac horse sickness
 There is marked hydropericardium, endocardial
hemorrhage, and myocardial degeneration
 Edema of the head and neck is common, especially of
the supraorbital fossa and nuchal ligament
 Microscopic lesions are minimal in the acute form

54. NECROPSY FINDINGS...
 Pulmonary edema may be present but no obvious
vascular injury
Histology of cardiac(subacute cases)
 Myocardial damage, including foci of necrosis,
hemorrhage, and mild leukocytic infiltrates
 An immunoperoxidase test is sensitive in detecting viral
antigen in formalin-fixed, paraffin-embedded tissues

55. SAMPLES FOR DIAGNOSIS
 Virology
 Chilled spleen, lung, lymph node (PCR, VIRUS
ISOLATION)
 Histology
 Fixed lung, heart (light microscopy,
immunohistochemistry)

56. DIAGNOSIS
Virus isolation
 Cell cultures- baby hamster kidney-21 (BHK-21), monkey
stable (MS) or African green monkey kidney (Vero) or
insect cells (KC)
 Intravenously in embryonated eggs
 Intracerebrally in newborn mice

57. DIAGNOSIS
Virus identification
 ELISA – rapid detection of AHSV antigen in blood, spleen and
supernatant from cell culture
 Virus neutralization (VN) – until recently the ‘gold standard’
for typing as well as identifying virus isolates, but takes 5 days
 RT-PCR is a highly sensitive technique that allows the
detection of a very low number of copies of RNA molecules
 Real-time PCR – detects all 9 serotypes

58. DIAGNOSIS
AHSV typing
 VN test- method of choice for typing as well as the ‘gold’
standard test for identifying AHSV’s isolated from the
field using type specific antisera
 Development of a type-specific gel-based RT-PCR and
real-time RT-PCR using hybridisation probes for
identification and differentiation AHSV genotypes
provides a rapid typing method for AHSV in tissue
samples and blood.

59. DIAGNOSIS
Serological diagnosis
 Horses that survive natural infection develop antibodies
against the infecting serotype within 8–12 days post-
infection
 Blocking ELISA (prescribed test in the OIE Terrestrial Manual)
 Indirect ELISA (prescribed test in the OIE Terrestrial Manual)
 Complement fixation (prescribed test in the OIE Terrestrial
Manual)

60. TREATMENT
 There is no specific treatment for AHS
 Supportive care and treatment of complication of the
disease should be provided.

61. CONTROL
 The principles of control in enzootic areas are
 Vaccination
 Reduction of exposure of horses to biting insects
 Non enzootic areas the aim is to
 Prevent introduction of the disease
 Eradication if introduced

62. CONTROL....
The objectives of a control program
 Prevention of introduction of infection by clinically ill or
apparently uninfected animals
 Slaughter of viraemic animals where animal welfare and
economic considerations permit this course of action
 Management changes to reduce exposure to midges
 Vector control
 Induction of active immunity in animals at risk of disease

63. 1. PREVENTION OF INTRODUCTION
 Infection can be introduced into an area free of AHSV by
infected animals or midges
 Infected animals can be
 Horses incubating the disease
 Clinically ill animals
 Donkeys and zebras, that have no clinical signs of
illness but are infected and viraemic

64. PREVENTION OF INTRODUCTION....
 Completion of a vaccination protocol effective against all
important serotypes at least 42 to 60 days before
introduction of the horse
 Positive identification of all horses by microchipping
 Passport documenting vaccination status
Control measures to prevent movement of animals at
risk of being infected

65. PREVENTION OF INTRODUCTION....
 Veterinary certificate confirming health and issued no
more than 48 hours before introduction
 Quarantine period of 60 days in insect proof enclosures
for equids from enzootic areas or from neighbouring
regions
 Vaccination of all equids within 10 miles (16 km) of
imported horses

66. 2. SLAUGHTER OF SICK / VIRAEMIC ANIMALS
 Slaughter- appropriate in controlling infection recently
introduced into areas previously free of the disease
 It is an effective adjunct in control of spread of
infection(as demonstrated in portugal)
 There are obvious economic, animal welfare, and public
relations aspects to this practice
(Especially in areas where horses have high intrinsic
worth or are companion animals)

67. 3. REDUCE EXPOSURE TO BITING MIDGES
 Horses should be housed in insect-proof buildings
 Limit exposure of horses to midges by closure of doors
and covering of windows with gauze
 Impregnation of gauze with an insecticide
 Stables should be situated in areas, such as on hilltops
or well-drained sites, that have minimal midge
populations

69. 3. REDUCE EXPOSURE TO BITING MIDGES....
 Habitat alteration- to reduce midge numbers in individual
farms (areas of damp, organically enriched soils are
eliminated)
 Widespread use of insecticides is unlikely to be
environmentally acceptable
 Housing of horses during the crepuscular periods and at night
will significantly reduce biting rates
 Horses kept at pasture should have insect repellents applied
regularly and especially to provide protection during periods
of highinsect- biting activity

70. 3. REDUCE EXPOSURE TO BITING MIDGES....
 DEET (N,N-diethyl-mtoluamide) is the only commercially
available repellent with documented activity against
Culicoides spp.
 Installation of alphacypermethrin impregnated mesh to
jet stalls reduced the attach rate of culicoides species by
6- to 14-fold

71. 4. VACCINATION
 Effective in reducing both morbidity and mortality in
enzootic areas and to control epizootics of the disease
 Used in two circumstances
 Areas in which the disease is endemic
 In regions with an epizootic of the disease

72. 4. VACCINATION...
 Can be used in enzootic or neighbouring regions to provide
active immunity of all resident equids because of the
continual risk of the disease in these areas
 Initiated as soon as foals no longer have passive immunity to
the virus, and it continues annually throughout the horse’s life
 Can be used in the face of an epizootic to induce active
immunity in horses in contact or in regions surrounding the
outbreak
 Vaccination is stopped when the infection is eradicated from the
area

73. VACCINES
Attenuated virus vaccine
 Early attenuated virus vaccines, although effective in
preventing AHS, were associated with encephalitis
 Virus attenuated by passage through tissue culture are
effective in preventing disease but do not prevent
viraemia
 Currently available vaccines are polyvalent or
monovalent preparations containing attenuated strains
of the virus

74.  Protection against heterologous serotypes is usually
weak, and most vaccines are polyvalent
Polyvalent vaccine
 Contain serotypes 1, 3, and 4 or serotypes 2, 6, 7, and 8
 AHSV-9 is not included because serotype 6 is cross-
protective
 Vaccination of foals with either monovalent or
polyvalent vaccine did not affect the serologic response
to each serotype

75.  Foals have markedly varying serologic responses to
differing serotypes, similar to the situation in adult to
horses and they fail to develop protective immunity to
some serotypes
Inactivated vaccine
 Effective in preventing viremia in most animals and
disease without adverse effects
 No longer available

76.  A number of recombinant canary-pox or vaccinia
subunit vaccines have been trialed experimentally
 Provide protective immunity against challenge exposure
of horses
 Appear to be effective using guinea pig models of the
disease
 The remaining challenge is to ensure that vaccines
provide protection against all 9 serotypes

77.  All race horses shall be vaccinated using a registered,
nonexpired, polyvalent horse sickness vaccine
 Two times as foals between the ages of 6 and 18 months, not
less than 90 days apart and between june 1 and october 31
 Thereafter every year between june 1 and october 31
 Foals are not vaccinated until they are at least 6 months of
age to prevent any effect of colostral passive immunity on
efficacy of vaccination.
 Horses resident in the AHS controlled area may not be
vaccinated without written permission from authorities.
Vaccination program for horses in South Africa

78.  Immunity after vaccination is protective for at least 1
year
 Annual revaccination of all horses, mules, and donkeys is
recommended

79.  There is concern over the use of attenuated virus vaccines in
epizootic situations
 The lack of vaccines approved for use in the European Community
 Availability of only two types of polyvalent vaccines and one type of
monovalent vaccine
 Delays in availability of vaccine for emergency vaccination
 Introduction of the virus, even attenuated virus, into regions in
which it is not present
 Attenuated-virus viremia in some vaccinated horses
 Reversion of vaccine strains to virulence

80. A summary of the various vaccine strategies against African horse sickness