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Friday, November 07, 2003

Equine Infectious Anemia (EIA)

Description | Transmission | Clinical Signs | Detection | EIA Control Program | Recommendations | EIA Control Program Policy | Statistical Report

Description

Equine Infectious Anemia (EIA) or “swamp fever” is an infectious, worldwide disease of horses, donkeys and mules caused by a virus.  Once infected, animals carry the virus for the remainder of their lives.  There is currently no effective treatment or approved vaccine available in North America.  The EIA virus does not infect people or other animals outside the family Equidae.  In Canada, the disease was first recognized in Manitoba in 1881.

Transmission of disease

The natural spread of EIA is predominantly by biting insects, especially by horseflies and deerflies when their feeding is interrupted (by tail switching, etc), and they move from an infected horse to an uninfected horse.  The virus is thought to last approximately 30 minutes to four hours within these insects.  Horses can also become infected by the transfer of virus in blood-contaminated needles, blood-contaminated instruments and blood transfusions.  The EIA virus can survive for several months at room temperature in dried blood and serum.  In an infected pregnant mare, the virus can sometimes cross over to infect the foal before it is born.  Foals can also occasionally get the disease by drinking milk containing EIA virus from an infected mare.  Although uncommon, EIA can be spread by breeding infected animals to uninfected animals.

Clinical signs

EIA virus, once it establishes infection inside the horse, is thought to live and grow primarily in macrophages, a type of white blood cell used to help defend the body against disease.  An infected horse, in its attempts to control the virus, usually shows up as having one of four forms of the disease as follows:

1.      Peracute – overwhelming infection that causes few if any signs before the horse suddenly dies.  This form of the infection is not common.

2.      Acute – the virus, and the horse’s immune system called in to fight it, causes destruction of red blood cells, destruction of platelets (used to help clot the blood) and inflammation in various tissues.  The result is usually high fever, anemia (low numbers of red blood cells), edema (fluid build up under the skin), hemorrhages, loss of condition, jaundice (yellowing of the tissues), sometimes incoordination and often death.  Levels of virus are high in the blood and they can transmit the virus readily to other horses.

3.      Subacute to Chronic – continuous but milder signs as described in the acute form, or horses may cycle in and out of the acute stage and look relatively normal between episodes.  The subacute to chronic stage is the form that is usually seen in horses showing clinical signs of the disease.  Weight loss is often more evident and the horse may die during a more severe episode.  These horses have generally moderate levels of virus in the blood and are a major source of transmission to other horses.

4.      Subclinical or Inapparent – the horse appears well and may have never been noticed to be sick.  This is the most common form of the disease detected by routine testing.  Some of these horses may remain subclinical or inapparent carriers for much of their life.  Others have recurring lapses into the subacute to chronic form.  These lapses may be caused by stress, another illness, certain drugs such as steroids or by the virus mutating to a more damaging strain within the horse’s body. 

Subclinical or inapparent carriers generally have low numbers of virus in their blood and do not pose as great a threat for transmission to other horses.  There is however documented evidence of EIA being spread from these inapparent carriers to other horses by insects, infected mare’s milk and breeding.  Also, these inapparent carriers may sometimes relapse to the chronic or subacute form where they are more likely to spread the disease.  Although there are methods to monitor and evaluate these carriers (permanent identification as carrier, daily temperature taking, counts of iron-laden macrophages in blood stream, etc.), with our current technology it is cumbersome to do so.

Detection

As the virus itself is not easy to detect in infected horses, a test to find antibodies to the virus was developed in 1970 by Dr. LeRoy Coggins.  This type of test is based on the horse’s antibodies to the EIA virus in its serum (the clear fluid part of the blood), precipitating in a specific way in an agar gel with a known purified form of the virus.  This Agar Gel Immunodiffusion test or AGID is commonly called the Coggins test after Dr. Coggins.  It is a very specific test for the antibodies to the virus and is only done by accredited labs that have been trained by the Canadian Food Inspection Agency (CFIA) to perform the test.  These labs must undergo proficiency panels every year to keep their accreditation. 

Vaccinations or other illnesses do not cause the specific lines of precipitation as found with EIA antibodies.  When an animal is found to have antibodies to EIA by this test, it is repeated by the lab to ensure the accuracy of the result.  Due to the serious consequences of a positive result, this positive sample is then sent to CFIA who repeat the test once again as another check.  Rarely does a horse give an atypical reaction on the test that is not clearly negative or positive.  These samples are also repeated by the accredited and CFIA lab; if they remain atypical the horse is retested in 30 days by which time they usually become clearly negative or positive.

Once a clear positive sample has been through all these checks, an owner can be very confident that his or her horse truly has antibodies to the EIA virus.  It then must be determined if there are any underlying circumstances which may cause the horse to have only antibodies to the virus but not the virus itself.  This occurs in two circumstances as follows:

a)      the horse is under approximately six months of age and may have only antibodies from the colostrum (first milk) of its dam and hopefully not the virus itself or,
b)     the horse has had a recent blood transfusion that had antibodies but little, if any, virus in its blood at the time the blood was drawn.

In a) or b) the horse is retested after a certain length of time in which the antibodies should have been eliminated from its system.  The initial presence of antibodies in these animals indicates still that there is an infected animal(s) out there that should also be tested (dam, donor, herd mates, etc.).

Once these factors (a and b) have been eliminated as possible causes of antibodies being present in the serum, a positive Coggins test is a very reliable marker that the horse truly has the virus somewhere in its body.

Unfortunately, the reverse is not always true; one negative Coggins test cannot always guarantee the horse does not have infection.  It takes approximately 14 to 42 days after infection by the virus for a horse to produce enough antibodies to detect by the Coggins test. During this time a horse may show up negative on the test.  If the test, however, is repeated after 60 days and kept in the company of other EIA negative horses or if the test is done 60 days after the end of fly season, you can be reasonably sure that the horse is free of EIA.

There are also a few reported cases of EIA infected horses that persistently have come up negative on the Coggins test.  Although these horses appear to be uncommon, they are a cause of concern.  More sensitive tests have been developed or are in the process of being developed to increase our ability to detect these horses.

EIA Control Program

EIA is a reportable disease of horses regulated by the federal Health of Animals Act since 1971.  Numbers of positive horses decreased under the policy from 2.9 per cent of 49,114 horses tested in 1972 to 0.09 per cent of 70,984 horses tested in 1992.  EIA statistics after 1992, broken down into provinces, are in Table Two. According to the CFIA, EIA has almost been eradicated from that segment of the horse industry that voluntarily requested testing.

The total number of horses tested in Western Canada is low compared to horses tested in Eastern Canada (Table Two).  Although testing for EIA is low in Western Canada, the percentage of positive horses is the highest.  An increase in surveillance of Western Canadian horses has the potential to significantly reduce the risk and spread of EIA in the region.

The increase in positive EIA horses has started a trend in 1999 to require EIA testing at exhibition grounds and fairs.  Some horse associations, fairs and exhibitions have funded clinics this year to help the horse industry increase the amount of EIA testing.

CFIA has implemented a revised EIA Control Program as of April 1, 1998.  Table One provides the policy in full.

Information on EIA and the Domestic Disease Control Policy is available from CFIA district offices located throughout the province.  A presentation on the disease is also available from CFIA district offices.

Recommendations to owners

For an individual owner, some of the control measures to prevent EIA from coming into their herd are as follows:

  1. Any horse, donkey or mule that is brought onto the farm should have a negative EIA test prior to coming onto the premises.  Depending of the circumstances and source of the animals, you may wish to keep this animal isolated and retest it after 60 days in case it is incubating the disease.  As the biting insects involved in spread of the disease usually will not travel more than 200 yards if their feeding is interrupted (otherwise they return to the first animal), 200 yards is likely adequate separation from other horses.
  2. Retest all horses, donkeys and mules every year; depending on the circumstances, a minimum of 60 days after the end of fly season might be most appropriate.
  3. Wherever possible, try to put fly control measures into place.
  4. All surgical instruments, tattoo equipment, teeth floats, etc., should be thoroughly disinfected after each use.  For surgical instruments, sterilization by boiling for 15 minutes or autoclaving at 6.6 kg pressure is recommended.  Chemical disinfection of instruments (tattoo equipment, etc.) can be done by first removing any dirt or manure (organic matter) and then immersing for 10 minutes in a phenolic disinfectant.  For personal disinfection, sodium hypochlorite, ethanol or iodine compounds are said to be safe.  Where organic matter is not removable, agents such as chlorhexidine or phenolic compounds combined with a detergent are said to be satisfactory.  Only disposable needles should be used and should be discarded after use on a particular animal.
  5. If you have a horse, donkey or mule going to an organized event during fly season, ensure the organizers are requiring a negative EIA test for all animals at the event.  Testing should have been done at least 60 days after the previous fly season.

Table One

EIA Control Program Policy as of 1998

EIA is reportable under the Health of Animals Regulations

An EIA accredited veterinarian is authorized to perform duties related to routine domestic testing and for export to the USA and Mexico.  The tests are conducted by laboratories accredited by CFIA.

The Retrovirology Centre of Expertise confirms all positive and atypical blood test results from EIA accredited laboratories.

Following notification of a positive diagnosis:

1)     A CFIA veterinarian visits the premises where the positive (reactor) animals were sampled and declares the premises an infected place.

2)     Owners are advised in person by CFIA of their legal responsibilities under the Health of Animals Act.  A CFIA veterinarian visits the premises and conducts a clinical examination of all reactor animals and animals exposed to the reactor animals.

3)     A CFIA veterinarian issues a destruction order for all positive (reactor) animals that are exhibiting clinical signs and for any symptomatic positive (reactor) animals that are not put into isolation under official CFIA surveillance.

4)     A CFIA veterinarian awards compensation for animals that are ordered destroyed.

5)     A CFIA veterinarian or an “EIA accredited veterinarian under contract” collects blood samples for all retests of positive (reactor) and atypical animals.

6)     A CFIA veterinarian issues licenses for the removal of any susceptible animals from or into an infected place including reactors.

Slaughter and Disposal:

7)     Confirmation of the slaughter of a reactor sent to a federal abattoir is by a CFIA veterinarian.

8)     A CFIA veterinarian or inspector or an ‘accredited veterinarian under contract’ supervises the destruction of a reactor and the disposition of that animal’s carcass.

Follow-up Testing:

9)     A CFIA veterinarian or an ‘accredited veterinarian under contract’ collects blood samples from all test eligible animals on an infected place at no charge to the owners.

10)   All blood samples are collected by CFIA inspectors or ‘accredited veterinarians under contract’ at no charge to the owners. 

In the case of a clinical reactor, all test eligible animals on the premises must undergo two blood tests 40 days apart with negative results before CFIA issues a declaration that the premises is not an infected place.

In the case of an asymptomatic reactor, all test eligible animals on the premises must undergo one blood test, after the removal of the reactor, with negative results before CFIA issues a declaration that the premises is not an infected place.

11)  A CFIA veterinarian or an ‘accredited veterinarian’ under contract collects blood samples from all animals that were exposed to or have been in direct contact with a reactor animal within 30 days previous to the reactors sampling date.  The owners do not pay for the testing.

12)  A CFIA veterinarian only permits asymptomatic EIA reactors to remain in isolation and confinement on an infected place.  The reactor animals may not be used for breeding purposes.

Enforcement and Compliance:

13)  Each reported suspected violation of the Health of Animals Act must be investigated by a CFIA inspector.

Communication:

14)  A structured communication plan exists whereby CFIA notifies horse industry officials and veterinarians of EIA outbreaks that occur in new area.

Table Two

Equine Infectious Anemia Statistical Report
Calendar Years 1993 to 2000

Region

# of Horses Census Years1
Year
# of Horses Tested
% of Horses Tested
# of Horses Positive
% of Horses Positive

Atlantic

8,290
 
 
7,808
93
94
95
96
97
98
 99
00
01
2,286
2,267
2,579
2,572
2,477
2,390
2,453
 1,981
2,538
27.6
27.3
31.1
32.9
31.7
30.6
31.4
25.4
32.5
0
0
0
0
0
0
0
0
0
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000

Quebec

25,787
 
 
25,108
93
94
95
96
 97
98
99
 00
01
17,604
16,754
15,470
15,250
14,888
14,831
14,872
 14,562
12,853
68.3
65.0
60.0
60.7
59.3
59.1
59.2
58.0
51.2
12
8
17
6
4
4
1
2
0
0.068
0.048
0.110
0.039
0.027
0.027
0.007
0.014
0.000

Ontario

75,132
 
 
76,553
93
94
95
96
 97
98
99
 00
01
29,924
30,265
28,124
29,036
28,238
27,641
25,784
27,859
25,097
39.8
40.3
37.4
37.9
36.9
36.1
33.7
36.4
32.8
13
1
0
1
0
1
0
0
1
0.043
0.003
0.000
0.003
0.000
0.004
0.000
0.000
0.004

Manitoba

44,130
 
 
68,783
93
94
95
96
97
98
 99
 00
01
5,060
6,220
4,659
5,261
5,216
4,732
4,418
4,595
4,020
11.5
14.1
10.6
 7.6
 7.6
 6.9
 6.4
 6.7
5.8
225
7
25
2
4
1
1
4
0
4.447
0.113
0.537
0.038
0.077
0.021
0.023
 0.087
0.000

Saskatchewan

53,843
 
 
66,372
93
94
95
96
97
98
99
 00
01
2,036
3,562
4,119
8,201
5,302
3,460
8,933
7,662
5,524
 3.8
 6.6
 7.7
12.4
 8.0
 5.2
13.4
 11.5
8.3
0
1
43
6
18
17
48
8
15
0.000
0.028
1.044
0.073
0.339
0.491
0.537
 0.104
0.272

Alberta

109,237
 
 
149,960
93
94
95
96
 97
98
 99
00
01
6,926
4,565
4,002
6,766
6,976
7,898
14,559
19,159
17,905
6.3
4.2
3.7
4.5
4.7
5.3
9.7
12.8
11.9
8
9
31
139
60
49
329
283
86
0.116
0.197
0.775
2.054
0.860
0.620
2.260
1.477
0.480

British Columbia

39,785
 
 
49,305
93
94
95
96
97
98
99
00
01
3,715
3,723
4,019
4,724
5,250
5,225
6,203
8,399
7,365
 9.3
 9.4
10.1
 9.6
10.6
10.6
12.6
17.0
14.9
6
3
19
25
25
24
131
87
59
0.162
0.081
0.473
0.529
0.476
0.459
2.112
1.036
0.801

Canada

 356,204

 


 443,889 

93
94
95
96
97
98
 99
00
01
67,551
67,356
62,972
71,810
68,347
66,177
77,222
84,217
75,302
19.0
18.9
17.7
16.2
15.4
14.9
17.4
19.0
16.9
264
29
135
179
111
96
510
384
161
0.391
0.043
0.214
0.249
0.162
0.145
0.660
 0.456
0.214

[1] From Agriculture Canada’s 1991 and 1996 Agriculture Census

(The following are CFIA statistics on test results in Saskatchewan from Sept. 1998 to April 1999, as a result of testing traceout positive horses: 153 owners; 1652 horses tested; 40 horses tested positive; on 15 premises.)

Material presented in this publication was written by: Dr. Shelagh Copeland and Dr. LeeAnn Forsythe, Inspection and Regulatory Management – Saskatchewan Agriculture, Food and Rural Revitalization.

 

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