Detection of Aleutian mink disease virus DNA and antiviral antibodies in American mink ( Neovison vison ) 10 days postinoculation

Abstract

Early detection of infection by the Aleutian mink disease virus (AMDV; Carnivore amdoparvovirus 1) by polymerase chain reaction (PCR) or counterimmunoelectrophoresis (CIEP) has important ramifications in virus eradication programs. A spleen homogenate containing a local isolate of AMDV was injected intraperitoneally into black (n = 44) and sapphire (n = 12) American mink (Neovison vison). Animals were euthanized 10 days postinoculation and anti-AMDV antibodies and AMDV DNA were tested in plasma and 7 organs by CIEP and PCR, respectively. Viral DNA was detected in the plasma, spleen, lymph nodes, bone marrow, and lung samples of all inoculated mink, but was not detected in some small intestine, kidney, and liver samples. In contrast, antibodies were detected in the plasma of 3 sapphire (25.0%) and 19 black (43.2%) mink but not in any of the organs. The sensitivity of the CIEP test on plasma samples was 39.3%, implying that low levels of antibodies during the early stages of virus exposure resulted in failure to detect infection by the CIEP test. We concluded that CIEP is not a reliable test for early detection of AMDV infection in mink and that there were considerable differences among mink of each color type for production of detectable levels of antibodies. PCR tests on samples of saliva, rectal swabs, and feces did not produce consistent and reliable results.

Introduction

Aleutian mink disease virus (AMDV; Carnivore amdoparvovirus 1, family Parvoviridae, subfamily Parvovirinae, genus Amdoparvovirus) ¹⁵ causes an immune-complex disease, characterized by high antiviral antibody titers, hypergammaglobulinemia, plasmacytosis, and progressive renal disease in some adult mink. ⁸ Aleutian disease (AD) reduces reproductive performance and increases adult and kit mortality.^7,11,22,30 There is no cure for, or vaccine against, AD, and the accepted strategy for controlling the disease is virus eradication by testing mink for anti-AMDV antibodies by counterimmunoelectrophoresis ¹³ (CIEP) and eliminating seropositive animals. The genotype of the mink, the virus strain, and the virus dose play important roles in establishment of infection, kinetics of viral replication, and antibody response,^8,29 which result in considerable variation among mink for the earliest time after exposure that antibodies reach a detectible level by CIEP and viremia can be detected by polymerase chain reaction (PCR). While anti-AMDV antibodies have been detected by CIEP as early as 5–7 days postinoculation (dpi) in some mink,^8,29 this period has been as long as 3–4 weeks in other cases.^21,25 Similarly, AMDV DNA was detected between 5 and 20 dpi by PCR,^24,25,29 indicating large differences among and within experiments for the earliest time that viremia can be detected.

More than 50% of mink pelts in Canada are produced in the province of Nova Scotia where AMDV is endemic, and more than 30 years of a test-and-removal strategy has failed in permanent eradication of the virus from many ranches. ²⁰ A large proportion of mink in Nova Scotia carries the dominant Jet-Black allele, which was discovered in the1960s in this province and has been widely used to improve fur quality (Bowness ER. History of the early mink people in Canada-Nova Scotia. Available from: http://jkcc.com/minkpeople/index.html). Information on the response of Jet-Black mink populations to infection with local strains of AMDV is needed to understand the reasons for the lack of success with the test-and-removal strategy in this province. The objective of this study was to examine variations among black American mink (Neovison vison) for the presence of anti-AMDV antibodies and viral DNA in blood and organs of mink at 10 dpi. This sampling point was chosen because AMDV replication reaches its peak around this time ⁸ and the virus is present throughout the body in most mink,^2,4,9 thus it is an optimal time for harvesting organs of infected mink as the source of AMDV. Furthermore, when animals are infected by means of intraperitoneal injection, AMDV DNA residues may be detected in tissues of some mink by PCR at least by 5 dpi. ²³ The sensitivity of detecting infection in live animals using noninvasive sampling methods, namely rectal swabs, saliva swabs, and fecal samples, were also evaluated.

Materials and methods

Animal handling

All protocols were performed according to the standards of the Canadian Council on Animal Care after approval by the institutional Animal Care and Use Committee. Animals were maintained according to the standard industry practices in 35.5 cm × 30.5 cm × 20.3 cm cages with a wooden nest box. Mink were individually caged, and the cages were separated by solid barriers to prevent direct physical contact. Prior to inoculation or blood sampling, animals were anesthetized by intramuscular injection of ketamine hydrochloride ^a and xylazine hydrochloride ^b at the rates of 10 mg and 2 mg per kg live weight, respectively. Euthanasia was performed by intracardiac injection of sodium pentobarbital ^a at the rate of 100 mg per kg body weight.

Source of the virus

A 5-month-old black mink from an infected commercial ranch in Cape Breton, Nova Scotia, which tested positive for AMDV infection by CIEP, was euthanized and its spleen harvested aseptically on the ranch. A 10% (w/v) spleen suspension (original stock) was prepared with sterile RPMI (Roswell Park Memorial Institute)-1640 ^c medium supplemented with 2% fetal bovine serum (FBS), ^d penicillin (100 U/mL), ^d streptomycin (100 μg/mL), ^d and amphotericin B (0.25 μg/mL). ^d Two black and 1 sapphire female mink from the Dalhousie University Fur Unit (Nova Scotia, Canada), which has been AMDV-free for at least 7 years, were transferred to a vacant shed on the same ranch where the virus originated, then sedated and given an intraperitoneal injection of 1.5 mL of the original stock prepared on the same day (Trial 1). One sapphire mink was kept as a control. The experimental animals were housed ~25 m from the nearest mink shed on this ranch. The control animal was kept in a cage adjacent to the inoculated mink. To reduce the chance of cross-contamination from the resident mink, experimental animals were fed first, beginning with the control animal. The animals were euthanized at 10 dpi, and the spleens of inoculated mink were harvested aseptically and transferred to a laboratory. A 10% (w/v) suspension was prepared from each animal with sterile RPMI-1640 ^c using a homogenizer ^e at 16,000 rpm for 3 pulses of 30 sec each, centrifuged at 1,125 × g ^f for 20 min, and antibiotics and FBS were added as explained above. After confirming the presence of AMDV by PCR ^g as previously described, ¹⁹ the suspensions were pooled (passage 1), aliquoted, and stored at −80°C.

Animal inoculation

Aleutian mink disease virus–free black and sapphire mink (n = 61) from the Dalhousie University Fur Unit were transferred to an enclosed building between March 2010 and July 2011 on 4 occasions (trials 2–5; Table 1). The building was designed to minimize the chance of viral introduction or escape. Mink on these trials were kept at least 30 cages apart from resident mink that had been inoculated with the same virus. One animal was kept as a control in each trial, and 56 mink were sedated and given an intraperitoneal injection of 1.0 mL of the passage 1 spleen suspension. Animals were inoculated after 1 week of adaptation during which their diet was gradually changed to a commercial dry feed. ^h Control animals were kept 2 cages apart from the inoculated mink, were fed first daily, and were sampled first to minimize the chance of cross-contamination.

Table 1.

Distribution of experimental animals by color and sex in each trial.*

		Inoculant		Black mink		Sapphire mink
Trial	Date	Source	Volume (mL)	M	F	M	F	Total
1	September 2009	Original	1.5		2		2†	4
2	March 2010	Passage 1	1.0	5		4†		9
3	June 2010	Passage 1	0.5	9†		8		17
4	June 2011	Passage 1	0.5		7†			7
5	July 2011	Passage 1	0.5		24†			24
Total				14	33	12	2	61

*

M = male; F = female.

†

One mink in this group was used as the control (total of 5).

Sampling

The animals were euthanized at 10 dpi. Blood was collected by heart puncture and transferred to duplicate heparinized capillary tubes ⁱ for the CIEP test and in ethylenediamine tetra-acetic acid–coated capillary tubes ⁱ for viral detection by PCR. In trials 2, 3, and 5, saliva samples were collected by rubbing a sterile calcium alginate swab ^c inside the oral cavity. The same type of swab was used to collect samples from the rectum. The tip of the swab was cut off and dropped into a sterile 1.5-mL tube containing 0.6 mL of nuclease- and protease-free water. ^d The swab was vigorously agitated in a vortex, ^j and all liquid was squeezed from the swab using sterile forceps. In trials 3 and 5, a sheet of paper towel was put under each cage the day before sampling, and ~0.5 g of fresh feces on the paper towel was collected and transferred into a 1.5-mL tube. In trials 1 and 2, attempts were made to collect urine samples directly from the bladder by a needle and syringe after animals were sedated, but bladders were empty in all but 2 animals. Body fluids that were collected for DNA extraction were centrifuged at 16,000 × g ^k for 10 min, and the supernatant was frozen at −80°C. Weight of each fecal sample was determined, then 1 volume of nuclease- and protease-free water ^d was added, thoroughly mixed, centrifuged at 16,000 × g, ^k and the supernatant was saved for viral detection.

Samples of the spleen, lungs, liver, kidney, heart, mesenteric lymph nodes, bone marrow, and small intestine (duodenum) were collected for viral detection by PCR. Lymph nodes were stripped of fat before transferring to cryovials. Using a 3-mL syringe and 20-gauge needle, bone marrow was flushed out of the tibia with 0.5 mL of phosphate buffered saline (PBS). ^d Tissue samples, except blood, were kept frozen at −80°C. Samples of the lungs, liver, kidney, and heart were collected in 10% neutral buffered formalin, ^l and hematoxylin ^l and eosin ^l –stained sections of the tissues were examined under a light microscope ^m for AD lesions. Histologic lesions were subjectively scored by an experienced pathologist on a scale of 0 (no lesion) to 4 (very severe lesions of advanced AD). Scoring was based on the accumulation of plasma cells in the tissues with some associated lesions^26,28 that were not expected in healthy mink.

Laboratory procedures

The CIEP test ¹³ was performed on duplicate samples of plasma by the Animal Health Laboratory of the Nova Scotia Department of Agriculture (Truro, Nova Scotia, Canada), which is a laboratory accredited for this test by the Standards Council of Canada. The test was performed in agarose gels ⁿ using a cell-cultured antigen supplied by the Research Foundation of the Danish Fur Breeders Association. ^o Gels were soaked overnight in 2% sodium chloride to remove any nonspecific proteins and improve sensitivity. ¹⁶ The results were scored under a stereoscope, ^p and in cases where the bands were faint, the tests were recorded as inconclusive.

Cell-free suspensions were prepared from 0.25 g of each of the 7 organs in 750 µL of sterile PBS ^d and centrifuged at 16,000 × g ^k for 10 min. The supernatants were collected, and 50 µL was tested for anti-AMDV antibodies by CIEP in duplicate. DNA was extracted from 200 µL of cell-free tissue homogenates, saliva, rectal, and feces suspensions, and from 50 to 120 µL of plasma using a commercial kit according to the manufacturer’s protocol, ^q and eluted in 100 µL of elution buffer. The volume of proteinase K ^r (50 µL of 14–22 mg/mL) was not changed when less than 200 µL of cell-free medium was available.

Aleutian mink disease virus DNA was amplified by PCR using primers 60F and 60R as previously described. ¹⁸ Three PCR tests were performed on each sample using 1.5, 2.5, and 3.5 μL of DNA in 15-μL total volumes. Three DNA volumes were used because viral DNA concentration obtained from the cell-free medium was too low to be measured accurately by a spectrophotometer. This battery of tests was repeated when all samples were negative or produced faint amplifications. The tests were repeated for the third time when only 1 of the 6 reactions contained a faint band (≤9 amplifications/sample). A sample was considered positive when at least 1 reaction showed strong amplification and was considered inconclusive when at least 2 of the reactions resulted in faint amplifications. When only 1 of the 9 reactions produced a faint band, the sample was considered negative. All tests included a reaction containing DNA from a known AMDV-infected animal (positive control), a reaction containing DNA from an AMDV-free mink, and a blank reaction (negative controls). Sample preparation, DNA extraction, PCR amplification, and PCR product testing were performed in 4 different laboratories with unidirectional sample movement to avoid cross-contamination. Sterile, filtered pipette tips were used throughout the experiment.

Statistical analysis

Data was analyzed using commercial software. ^s The likelihood ratio chi-square or Fisher exact test, when applicable, were used to analyze the difference among trials for the PCR test results of various organs, and CIEP results of plasma among trials and between color types. The agreement between the replicates of CIEP results was tested by kappa coefficient (PROC FREQ), which is expected to be a more robust measure of association than the simple correlation coefficient because it takes into account the agreement occurring by chance. Sensitivity and specificity of the CIEP relative to PCR on blood plasma, and sensitivity of the rectal swabs, saliva swabs, and feces relative to PCR results on plasma were calculated by the Clinical Research Calculator of VassarStats. ^t

Results

Aleutian mink disease virus DNA in plasma and organs

Plasma samples of 96.4% of 56 inoculated mink were PCR positive and the remaining 2 mink (1 sapphire in trial 1, and 1 black in trial 2) had inconclusive test results. AMDV DNA was detected by PCR in at least 4 organs of all inoculated animals. The virus was present in the spleen, lymph node, bone marrow, and lung of all inoculated mink, but it was not detected in 35.7% of the liver, 10.7% of kidney, and 1.9% of intestine samples (Table 2). None of the spleen samples showed inconclusive PCR test results, whereas this was observed in 14.3% of kidney and lung samples, and in less than 4% of the other organs. Differences among trials for the PCR test results were significant only for liver (χ² = 25.7, P < 0.01) and kidney (χ² = 34.7, P < 0.01) samples. AMDV DNA was not detected in plasma or any of the organs of the 5 negative control mink.

Table 2.

Percentage of polymerase chain reaction–positive, inconclusive, and negative tests of plasma and organs of Aleutian mink disease virus–inoculated mink.

Organ	No. of mink	Positive	Inconclusive	Negative
Plasma	56	96.4	3.6	0.0
Spleen	56	100.0	0.0	0.0
Lymph nodes	56	98.2	1.8	0.0
Bone marrow	29	96.6	3.4	0.0
Liver	56	62.5	1.8	35.7
Kidney	56	75.0	14.3	10.7
Lung	56	85.7	14.3	0.0
Intestine	53	94.3	3.8	1.9

Anti-AMDV antibodies

None of the 44 inoculated black mink was CIEP positive, but 19 (43.2%) showed weak precipitation (inconclusive; Table 3). Two of the 12 sapphire mink were CIEP positive and 1 showed inconclusive results (25.0%; trials 1 and 2). All 16 mink in trial 3 and the 5 control mink remained CIEP negative. Considering all control mink as non-infected and all inoculated mink as infected, the relative specificity of the CIEP tests was 100% in all trials, and the relative sensitivity of the CIEP test over all trials was 43.1% (95% confidence interval [CI] = 29.6–57.7%). The estimates of relative sensitivity were 50.0%, 100.0%, 0.0%, 20.0%, and 59.1% in trials 1–5, respectively.

Table 3.

Anti–Aleutian mink disease virus antibody detection by counterimmunoelectrophoresis in plasma of black and sapphire mink 10 days postinoculation.*

	Black			Sapphire
Trial	+	I	–	+	I	–	Total
1	0	0	2	1	0	0	3
2	0	5	0	1	1	1	8
3	0	0	8	0	0	8	16
4	0	1	5	0	0	0	6
5	0	13	10	0	0	0	23
Subtotal	0	19 (43.2)	25 (56.8)	2 (16.7)	1 (8.3)	9 (75.0)	56
Total		44		12			56

*

+ = positive; – = negative; I = inconclusive. Numbers in parentheses are percentages.

Differences among the trials for the CIEP results (positive, inconclusive, and negative) were significant (χ² = 30.8, P < 0.01). There were significant differences between trials 2 and 3, in which only males were used, for the number of CIEP-positive animals (χ² = 19.7, P < 0.01) but not between the outcomes of trials 4 and 5 in which only females were used (χ² = 3.03, P = 0.08). Although the 2 CIEP-positive mink were both sapphires, color effects on test results in trials 1–3, in which both color types were used, were not significant (χ² = 4.4, P = 0.102). The relative sensitivity of the CIEP tests in trials 1–3 was 33.3% for black mink and 25.0% for sapphire mink.

Duplicate CIEP test results agreed in 94.1% of the cases. Duplicate tests were either both negative, positive, or inconclusive in 60.7% (n = 34), 1.8% (n = 1), and 32.1% (n = 18) of the 56 inoculated mink, respectively. The disagreements between the duplicate samples of individual mink included 2 mink that were negative in 1 replicate and inconclusive in the other replicate, and 1 mink that was positive in 1 replicate and inconclusive in the other replicate. No positive and negative results were observed in duplicate samples of the same animal. The simple kappa coefficient between the results of duplicate samples was 0.89 (95% CI = 0.78–1.0). Considering inconclusive tests as positives, simple kappa coefficient increased to 0.92 (95% CI = 0.82–1.0). Antibody was not detected in cell-free suspensions of any of the organs.

Histology

None of the mink showed any obvious signs of AD at autopsy. Histologic examination revealed mild lesions of AD in the liver of 1 and in the kidneys of 2 other black mink in trial 2, and in the liver and kidney of 1 sapphire mink in trial 3. No lesions of AD were observed in any of the heart samples. Lungs of most of the animals, including controls, had microscopic lesions that are not normally caused by AMDV infection, namely mild to moderate lymphoplasmacytic infiltrates around airways and blood vessels, and occasionally a few regions with macrophages in alveoli.

Aleutian mink disease virus DNA in saliva, rectal, and fecal samples

Saliva swabs, fecal samples, and rectal swabs were PCR positive or inconclusive in 49%, 41.0%, and 85.1% of the inoculated mink, respectively (Table 4). Although control animals remained virus negative at 10 dpi, faint PCR amplifications were observed in the rectal swab of 1 mink and in the feces sample of another, which could be the result of sample contamination with the virus present in the environment. Relative sensitivity of PCR tests for saliva and fecal samples were low (22.0–52.2%), but that of rectal swabs was 88.9%, when inconclusive test results were considered as positive. Sensitivity was reduced to 56.8% when inconclusive tests were considered as negative. Bladders of most mink that were tested in trials 1 and 2 were empty after sedation, and urine samples could be collected only from 2 mink, which were both negative for the virus. Urine collection did not continue after trial 2.

Table 4.

Percentage of Aleutian mink disease virus polymerase chain reaction (PCR) positive (+), inconclusive (I), and negative (–) tests of saliva, rectal, and fecal samples and relative sensitivity of the tests.

		Percentage			Inconclusive PCR results considered
Source	No. of samples	+	I	–	+	–
Saliva	49	24.5	24.5	51.0	52.2	26.1
Rectal	47	53.2	31.9	14.9	88.9	56.8
Feces	39	20.5	20.5	58.9	42.1	22.0

Discussion

Presence of AMDV DNA in the plasma of 96.4% of inoculated mink 10 dpi is consistent with the peak of virus replication around this time. ⁸ This number is within the range of values for the proportion of viremic mink tested by PCR by 10 dpi in a few published reports. In a prior experiment using 3 mink per treatment, AMDV DNA was detected by PCR in blood of sapphire and pastel mink inoculated with the pathogenic Utah-1 strain at 5 dpi, but only 1 of the 3 sapphire and pastel mink inoculated with the low-pathogenic Pullman strain was PCR positive by 5 and 10 dpi. ²⁹ In a previous experiment, leukocytes from 5 dark mink were tested for AMDV DNA by PCR at 3, 7, and 10 dpi, ²⁴ and 4 of the mink (80%) were PCR positive at 10 dpi. One mink remained PCR negative on all sampling occasions but became PCR positive at 20 dpi, which was attributed to the slow virus replication in this individual. ²⁴ Two of 5 mink were PCR positive at 3 dpi, but not at 7 dpi, which was attributed to the difference in kinetics of virus replication between animals. ²⁴ AMDV DNA residues from the high amount of inoculum (2 mL), which was injected intraperitoneally into mink in that experiment, ²⁴ would be a more logical explanation, because it has been shown that virus residues can be detected by PCR 5 days after inoculation. ²³ One of the 5 mink in the above experiment ²⁴ tested positive by PCR at 7 dpi and remained positive at 10 dpi, suggesting that day 7 was the earliest time for detecting the virus in the blood of 20% of animals that were inoculated with a high amount of the virus. On the other hand, AMDV was previously detected by PCR in the blood of 24.1%, 44.8%, and 31.0% of 29 wild-type mink inoculated with a Danish AMDV strain at 1, 2, and 3 weeks after inoculation, respectively. ²⁵

In addition to the genotype of the mink, differences among the above experiments in the proportion of animals that were PCR positive by ~10 dpi were likely caused by differences in virus pathogenicity ²⁹ as well as DNA extraction and PCR methods. The high proportion of mink that were PCR positive in the current experiment, which was conducted on a relatively large number of animals, strongly suggests that AMDV DNA can be detected by PCR in black and sapphire mink inoculated with this local strain of the virus by 10 dpi using the DNA extraction kit and the PCR method that were employed. The inconclusive PCR test results in the plasma of 2 mink were likely due to the slow rate of virus replication in these individuals. Nine PCR tests were performed on each of these 2 samples, eliminating the effect of any PCR amplification error. Variations in the sequence of the primer target sites were not the reason because virus DNA was amplified in at least 3 organs of the same individuals. Although errors inherent in sampling and sample processing cannot be ruled out, the most plausible explanation for the apparently slow rate of virus replication and inconclusive PCR tests in the current experiment, and PCR negative results observed in other experiments,^24,25,29 seems to be the differences in mink genotypes because other factors were identical in each experiment.

The presence of AMDV DNA in the organs of all mink confirmed that all inoculated mink were infected, with the virus being particularly present in the 3 lymphoid organs (lymph nodes, spleen, and bone marrow), which are the primary sites of virus replication and sequestration. ⁸ Detection of AMDV DNA in so many organs was expected because AMDV antigens and AMDV DNA reach their peaks around 10 dpi in various organs of infected mink,^2,4,9 and the virus is present throughout the body, primarily complexed with anti-AMDV antibodies. ⁸ Blood is also a source of the virus in samples from various organs, ¹⁷ particularly during the early period after infection when animals are viremic.

In cases where AMDV was present in lymphoid tissues early after inoculation, the virus was also detected in samples from the liver,^9,21,31 kidneys,^9,31 and intestine ⁹ as well, although it was not detected in the kidney and intestine in a previous experiment. ²¹ It is reported that the amount of AMDV DNA in the liver of infected mink at 10 dpi, measured by fluorescent antibody testing, was the same as that in the spleen, but was lower in kidney. ⁹ This finding could imply that failing to detect viral DNA in the liver of 35.7% and in kidneys of 10.7% of the mink (Table 2), and significant differences among trials only for these 2 organs, was not because of the absence of the virus; rather, it was most likely because of the high amounts of PCR inhibitors ³⁵ in these organs, which were not removed by the DNA extraction kit.

None of the black mink was CIEP positive at 10 dpi, although 43.2% had inconclusive test results (Table 3), apparently because of low antibody titers. This figure is in the lower limit of the percentage of CIEP-positive cases reported for inoculated mink around 10 dpi,^13,14,16,24 except in an experiment where antibodies were detected by CIEP in pastel mink inoculated with the mildly pathogenic Pullman strain at 28 dpi, but not on days 8, 10, 12, or 14. ²¹ Several factors could be responsible for such a low incidence of CIEP-positive mink in the current experiment, which deserve attention because CIEP is the official test for the AMDV eradication program in Nova Scotia. First, the black mink used in this study were genetically different from animals that were used in most other experiments, which were often sapphire and pastel color types.^1,13,22,30 The effect of mink genetics on antibody production is apparent in the current experiment where a larger proportion of sapphire mink were clearly CIEP positive (16.7%) compared with the black mink (0%). It is well known that Aleutian-type mink produce higher levels of antibodies that can be detected by CIEP earlier after inoculation than in non-Aleutian mink.^10,29 The proportion of CIEP-positive sapphire mink by 10 dpi in the current study was lower than that in another experiment, ²⁹ suggesting that the genetics of the mink was not the only reason for the low incidence of CIEP-positive cases and that other factors may have been involved.

Second, pathogenicity of the virus strain profoundly modulates the time of establishment of infection, the kinetics of antibody production, and the time when antibodies reach a detectible level by CIEP. ²⁹ Antibody titers were shown to be several fold higher at 10 dpi in sapphire mink inoculated with the pathogenic Utah-1 strain than in those inoculated with the less pathogenic DK strain. ² Pathogenicity of the virus used in our study has not been established, but it seems to be mildly pathogenic (unpublished data).

Third, virus dose modulates animals’ response to infection and determines antibody profile. It was shown that inoculation of neonatal mice with low doses of the Murine leukemia virus induced a protective cytotic T lymphocyte (CTL) response whereas high doses of the virus resulted in development of non-protective Th2 cytokines, failure to develop effective CTL, and subsequent clinical disease. ³³ When sapphire and pastel mink were inoculated intraperitoneally with 10-fold serial dilutions of the Pullman strain of AMDV, both color types became CIEP positive by 2 weeks postinoculation when inoculated with 30,000 LD₅₀ of the virus, only sapphires became CIEP positive when inoculated with 3,000 LD₅₀, and neither color type was CIEP positive by 2 weeks postinoculation when inoculated with 30 LD₅₀. ¹⁰

Fourth, the increase in antibody titer after infection is gradual, ⁸ the pattern of increase varies among individual mink, and the earliest time after infection that antibody can be detected depends largely on the kinetics of antibody production as well as the sensitivity of the laboratory method used. The CIEP ¹³ test is widely used for testing mink for infection with AMDV because of its low cost, high specificity, and acceptable sensitivity.^1,3,5,17 Methods of detecting anti-AMDV antibodies with higher sensitivity than CIEP have been developed,^1–3,5,6,34 which could increase the chance of detecting infected animals at early periods after infection. These tests are more complex and more costly than CIEP and are not used for routine herd screening. CIEP was used in our study because it is the only antibody test used, at date of publication, by mink ranchers in Nova Scotia.

Until 1983, the CIEP test was based on organ-produced virus antigen, ¹³ usually the Utah-1 strain, which was changed to in vitro–grown, laboratory-adapted AMDV-G antigen (B. Aasted, personal communication, 2014). A comparison between the 2 sources of antigen showed that antibody titer measured by the organ-produced antigen was higher than that by the in vitro–grown antigen. ³ This change in the source of antigen could be the reason for the decrease in sensitivity of CIEP after 1983. For instance, antibodies were detected by organ-produced CIEP in 1 of the 8 inoculated mink at 7 dpi, all 4 mink at 9 dpi, and 5 of the 6 mink at 11 dpi, ¹⁶ as well as in all 19 mink tested at 7 dpi and in another 97 mink tested at 10 dpi. ¹⁴ In contrast, the CIEP test based on cell-cultured Danish antigen resulted in detecting antibodies in 0%, 48.3%, and 48.3% of 29 wild-type mink inoculated with a Danish AMDV strain at weeks 1, 2, and 3 postinoculation, respectively, and 1 mink remained CIEP negative by the third week postinoculation. ²⁵ Five of the 10 inoculated mink were CIEP positive at 10 dpi using organ-produced antigen, ¹⁶ whereas all 5 dark mink were CIEP positive at 10 dpi using the in vitro–grown antigen, ²⁴ indicating the variability among different experiments because of factors other than the source of antigen. There seems to be a need for antigen-producing companies and CIEP testing laboratories to use a common standard, without which such variations make it difficult to compare the results of different experiments. This situation reinforces the need for performing the current experiment. Significant differences among the trials for the proportion of CIEP-positive animals, and the observation that none of the 8 black and 8 sapphire mink were CIEP positive in trial 3 (Table 3), may be due to environmental factors or to variations in test conditions even within a single laboratory.

Results of our study clearly demonstrated differences among individual mink for antibody profiles. Consistent with the results of the current experiment, animals of the same color type that received the same amount of a specific strain of a virus and were managed under the same conditions showed different antibody titers ²⁷ and variable CIEP test results^25,29 in other experiments. These differences implicate host factors as the primary source of variation for the kinetics of antibody production. Virus characteristics were not likely the cause of these differences because it is unlikely that variants of inoculated virus with different replicative capabilities could have emerged during a 10-day period, and slight differences in the amount of each inoculum could not have caused such differences.

Significant differences among the trials for CIEP test results, which caused a wide range of CIEP sensitivity (0.0–87.5%), were unexpected. The design of the experiments did not permit detailed assessment of the factors that contributed to the differences among the trials, but it was not likely the result of differences in mink color types, as color types, when tested in trials 1–3 in which both color phases were used, was not significant. The high level of concordance between the duplicate CIEP test results and the absence of a negative and a positive result in any duplicate test is evidence for the high reproducibility of the CIEP test, as was previously reported. ¹⁸ The presence of only 2 mink with negative and inconclusive results in duplicate CIEP tests implies that little accuracy would be gained by testing each sample twice. The results also suggest that the inconclusive test result is likely an indication of infected animals with low antibody titer and must be considered as positive cases in eradication programs.

Despite the fact that almost all animals were viremic and the virus was widespread in organs, sensitivity of viral DNA detection by PCR using saliva swabs and fecal samples were low (<25%; Table 4). The results are in agreement with another report showing inconsistent results for oronasal and fecal samples from inoculated mink. ²⁵ The low amount of the virus in saliva may be due to the fact that the mouths of sedated animals became very dry. Collecting saliva samples from un-sedated mink was performed by offering a hard plastic rod to the mink to take a bite, inserting a sterile swab in the oral cavity, and rotating the swab a few times. This procedure, which could have produced more accurate results, was abandoned after a few attempts because it caused a great amount of discomfort for the animal and increased the chance of sample contamination. The presence of 20.5–31.9% inconclusive PCR tests on saliva, rectal, and fecal samples (Table 4) is indicative of a low number of viruses, or the presence of inhibitors such as bile salts and polysaccharides in feces ³² and polysaccharides in saliva, ¹² which could impact PCR amplification by impeding cell lysis, causing degradation of target DNA or primers, or inactivating the polymerase enzyme, ³⁵ which was not removed by the DNA extraction kit. Observing faint PCR amplifications in the rectal swab of 1 control mink and in the feces sample of another indicates the possibility of sample contamination in an infected environment. Collecting urine samples was found to be difficult and was prone to contamination from the surrounding environment. It was concluded that saliva, fecal, and urine samples are not suitable for detection of infection with AMDV in live animals. The use of rectal swabs, only second best to the blood, for detection of the virus warrants further investigation.

In conclusion, although the CIEP is a reliable test for detecting antibodies against AMDV, it fails to detect infection within 10 days after mink are exposed to the virus. PCR can accurately detect the virus in blood and lymphoid organs at this time. The results of the present study and published data suggest that the genotype of individual mink plays an important role in establishment of infection, early virus replication, viremia, virus distribution among tissues, and particularly antibody production. Understanding the relationship between immunologic responses of individual mink during the early days after infection with the development of progressive immune complex disease is paramount to designing programs for selecting tolerant mink.