Bird-hunting dogs are at risk for shotgun injury as they work to retrieve, flush, or point to birds. Specific data regarding frequency of shotgun injury to dogs do not exist, but shotgun pellets are routinely detected in sporting dogs as an incidental finding during radiographic examinations. Shotgun pellets for hunting purposes were traditionally made of lead. Lead provokes minimal local reaction when embedded in tissues because of a rapid fibrous response, which effectively isolates lead projectiles.1 Surgical retrieval of embedded projectiles is considered traumatic and unnecessary because they are encapsulated and not considered harmful.2 Conversely, lead pellets implanted into a synovial joint induce substantial arthritis and, potentially, systemic lead toxicosis and are surgically extracted.2–4
Shotgun wounds in sporting dogs often occur when the dog is at the effective bird-hunting range of 20 to 40 yards.5 Shotgun injuries incurred at a distance > 7 yards are classified as type I or long-range shotgun injuries and result in many small, low-velocity puncture wounds that deposit pellets into underlying subcutaneous tissue and muscle.2 Such wounds are typically treated locally and with systemic administration of antimicrobials with no attempt to remove the pellets. Exploratory laparotomy may be required if abdominal cavity perforation occurs.2
Because of the detrimental environmental impact of lead shot, the US federal government enacted legislation that completely prohibited lead shot for hunting waterfowl species in 1991.6,7 Subsequently, many states have expanded this ban and prohibit use of lead shot for hunting upland birds, as well.8 The ban on lead shot for hunting has stimulated development of shot material considered nontoxic to the environment. These materials are intended to mimic the favorable ballistic properties of lead yet cause minimal toxicosis, as measured by use of ingestion studies.9,10 In an effort to adhere to federal and state regulations, development and approval of many shot materials such as steel, tungsten, bismuth, tin, nickel, zinc, alloys of these metals, and polymers containing these metals have occurred.
Depending on composition, a metallic foreign body embedded in tissue of humans or other animals may be susceptible to corrosion. Corrosion (breakdown of implanted metal by chemical means) induces an inflammatory reaction that may be detrimental to the animal.11
In a single report12 of the tissue effects of steel shot in dogs, it is hypothesized that long-term implantation may lead to deleterious effects including chronic wound problems and draining tracts. In that report, corrosion of implanted steel shot and severe tissue inflammation adjacent to the corroding metal are described. A short-term study13 in mallard ducks revealed minimal inflammatory response to tungsten-bismuth-tin and moderate to severe inflammatory response to steel shot implanted in breast muscles for up to 8 weeks. A similar study14 in mallard ducks investigating lead, iron, and bismuthtin shot revealed minimal tissue response to lead and bismuth-tin and a moderate amount of inflammatory response to embedded steel shot.
To the authors' knowledge, no studies have been conducted to evaluate the corrosion potential and tissue inflammatory effects of tungsten-, bismuth-, nickel-, tin-, or zinc-containing shot material in a mammalian species. Little is known about the short- and long-term effects of these shot types when embedded in mammalian tissue and what, if any, changes may be recommended for management of wounds with such shot.
The purpose of the study reported here was to investigate the effects of implanted metal shot, plated metal shot, and metal alloy shot on mammalian muscle by use of rats and to investigate the in vitro and in vivo corrosion potential of these materials and their effect on systemic heavy metal concentrations. We hypothesized that severity of inflammation and corrosion would differ substantially on the basis of material type and implantation duration and that systemic heavy metal concentrations would not be affected by IM implantation of shotgun pellets of various metals.
Materials and Methods
Animals—Twenty purebred 3-month-old female Sprague Dawley rats and two 4-month-old male Long Evans rats were used in this study. Sprague Dawley rats were obtained from a commercial source and acclimated to their environment for 1 week prior to beginning the study. All rats were housed individually in the same facility in solid-bottom cages on paper litter.
Long Evans rats were obtained from a breeding colony maintained at the Kansas State University Animal Resource Facility. These rats were born in the facility; housed individually after 4 weeks of age; and exposed only to bedding, food, and water. Two Long Evans rats that were being culled from this breeding colony and were not part of any other experimental protocol were used as control animals. Control rats were not exposed to anything other than food, water, and bedding within the facility.
All rats were fed identical commercial chowa and allowed water ad libitum throughout the study. The study protocol was approved by the Kansas State University Institutional Animal Care and Use Committee.
Study design—Sprague Dawley rats were randomly assigned to 4 time groups (2, 8, 16, and 26 weeks) composed of 5 rats each. Each rat could accommodate IM implantation of 6 pellets as follows: bilaterally in the trapezius muscle, bilaterally in the lumbar epaxial musculature, and bilaterally in the gluteal muscle mass. Pellet assignment for the 30 implantation sites/time group was determined by use of a block design. Determination of pellet type for each implantation site was assisted by use of a random number generator.b Briefly, 4 sets of unique numbers (1 through 8) were created for each time group, providing a list of 32 random numbers for each group. The last 2 numbers from each list were arbitrarily discarded to guarantee a minimum of 3 and a maximum of 4 numbers /time group. Each pellet type was given a randomly drawn unique random number (1 through 8). Rats in each time group were randomly assigned a position (1 through 5). Implantation sites (left trapezius, right trapezius, left epaxial, right epaxial, left gluteal, and right gluteal) were numbered 1 through 6, respectively. Therefore, rat No. 1 possessed implantation sites 1 through 6, rat No. 2 possessed sites 7 through 12, and so on. Pellets of the type assigned from the random number list for each time group were then implanted in sites 1 through 30.
Test materials—Eight test shot materials (bismuthtin alloy,c nickel-plated steel,d tungsten polymer,e tungsten-iron alloy,f tungsten-nickel-iron alloy,g zinc-plated steel,h lead,i and steelj) were selected for evaluation. Pellets were obtained by removal from store-purchased shotgun shells. Pellets of each material were weighed, stored, and catalogued with an identification number. Each pellet was individually wrapped in double-sealed pouches and sterilized in ethylene oxide gas. More than 48 hours elapsed after sterilization prior to implantation of any pellet.
Each pellet was weighed to the nearest microgram on a scientific scalek prior to sterilization. Pellets were reweighed after retrieval from implantation or immersion.
Four pellets of each material type were submerged in 500 mL of sterile saline (0.9% NaCl) solution. At each time interval (2, 8, 16, and 26 weeks), pellets were removed from the saline solution, dried, and weighed to the nearest microgram.
Anesthesia—Each Sprague Dawley rat was administered general anesthesia via chamber induction with 3.5% isoflurane in oxygen until laterally recumbent. Rats were then placed on a circulating warm water blanket, and anesthesia was maintained via inhalation of isoflurane in oxygen with a nonrebreathing face mask. Anesthetic depth was monitored by respiratory rate and toe pinch response.
Surgery—The hair was clipped from the dorsum of each rat with a No. 40 blade, and the skin was aseptically prepared with chlorhexidene solution and 70% isopropyl alcohol. By use of aseptic technique, a 1-cm longitudinal paramedian skin incision was performed to allow access to the underlying musculature at each site. A single stab incision into the muscle fascia allowed insertion of the appropriate pellet into the muscle. Pellets were handled carefully to avoid causing surface damage. Following insertion of the pellet into the muscle tissue, muscle fascia was apposed over the implanted pellet with a single interrupted suture of 4-0 polypropylene.l The skin at each site was apposed with a single interrupted suture of 4-0 nylon.m Buprenorphine hydrochloriden (0.05 mg/kg) was administered IM once for analgesia following the procedure. Rats recovered from general anesthesia breathing room air and were placed in individual cages when fully ambulatory.
Postoperative monitoring—Rats were housed individually and monitored daily for 14 days for any tissue redness, swelling, discharge, or signs of self-mutilation associated with the implantation sites. Local infection marked by discharge from the wound, excessive redness, or separation of the wound edges would prompt removal of the affected animal from the study. Skin sutures were scheduled for removal at 14 days after surgery.
Specimen collection—Rats were euthanized at the appropriate time interval via carbon dioxide asphyxiation. Fluoroscopic examination of the 26-week time group was performed following euthanasia to aid in locating the pellets. Necropsy evaluation and gross examination of wound areas and internal organs were performed. Sites of implantation containing the implanted pellet were resected en bloc. The pellet from each site was carefully removed from any attached tissue. The pellet was weighed on the same scientific scale on which it was weighed prior to implantation. Skeletal muscle specimens were placed in neutral-buffered 10% formalin for fixation prior to histologic evaluation. Kidneys from all rats were harvested and stored at −70°C for heavy metal analysis.
Histologic evaluation—Skeletal muscle specimens were processed in a routine manner following fixation and embedded in parrafin blocks, sectioned at 4 μm, and stained with H&E. Each specimen was evaluated by a board-certified veterinary pathologist (KMP) who was unaware of group assignments. Inflammation at each site of implantation was graded and assigned a score (0 = no inflammation, 1 = mild inflammation, 2 = moderate inflammation, and 3 = severe inflammation.)
Special stains for calcium (von Kossa stain) and iron (Perl Prussian blue stain) were perfomed on 1 section from each skeletal muscle specimen. Degree of staining was graded and assigned a score (0 = no staining, 1 = mild staining, 2 = moderate staining, and 3 = heavy staining.)
Corrosion rate—Corrosion rate (in vivo and in vitro) of each metal type was calculated as described.15 Weight loss was calculated for each pellet. The weight loss for each pellet was used to calculate the corrosion rate with the following formula:
where K = 8.76 X 107 for corrosion rate (micrometers per year), W = mass loss (g), A = surface area (No. 4 shot = 0.3372 cm2; No. 5 shot = 0.2922 cm2), T = time of exposure (hours), and D = density (steel, nickel-plated steel, zinc-plated steel = 7.86 g/cm3; lead = 11 g/cm3; tungsten-nickel-iron = 12 g/cm3; tungsten matrix = 10.8 g/cm3; tungsten-iron = 10.3 g/cm3; bismuth-tin = 9.7 g/cm3).
Toxicologic analysis—Kidney specimens were evaluated by a commercial laboratory for concentrations of heavy metals (arsenic, barium, beryllium, cadmium, cobalt, chromium, copper, iron, mercury, manganese, molybdenum, nickel, lead, zinc, titanium, and vanadium) by use of inductively coupled plasmaatomic absorption.o
Long Evans rats were included in the study as an environmental control population after obtaining kidney heavy metal concentration results for the test animals. Control rats were born and raised in the Kansas State University Animal Resource Facility and were never exposed to any type of shot material. These rats were used because they were readily available and were exposed to the exact same enviroment as the test rats in the study. This testing was not part of the original study protocol but was included to evaluate the effect of the environment on kidney heavy metal concentrations.
Statistical analysis—Inflammation scores among metals within each time group were compared by use of a nonparametric C2 test. In vivo corrosion rates among metals within each time group were compared by use of ANOVA. Stain uptake scores for iron and calcium were compared among metals within each time group by use of a nonparametric C2 test. Heavy metal concentrations were compared among time groups by use of ANOVA. Control values for heavy metal concentrations were not included in statistical analyses. For all comparisons, P < 0.05 was considered significant. All statistical comparisons were performed with commercially available statistical software.p
Results
Animals—All anesthetic and surgical procedures were performed without complication. Rats tolerated pellet implantation well, and no wound complications warranting removal from the study occurred. All rats were euthanized at the appropriate time interval and were included in the final analysis.
Postoperative observations—Swelling of the implantation site 3 to 10 days after implantation was grossly observed at 21 of 120 (17.5%) implantation sites. Local swelling occurred with zinc-plated steel (3/16 sites), tungsten-nickel-iron (1/15 sites), nickel-plated steel (3/15 sites), steel (4/16 sites), bismuth-tin (4/13 sites), tungsten-polymer (3/16 sites), and tungsten-iron (3/15 sites). No gross local swelling was identified with any of the implanted lead pellets. All local reactions resolved within 10 days of detection without specific treatment in rats that survived > 2 weeks. Fluoroscopic examination of the rats in the 26-week group prior to necropsy identified 28 of 30 pellets.
Necropsy—Implantation sites were easily identified by identification of the nonabsorbable suture material placed in the muscle fascia at each site. In the 2-week time group, substantial subfascial swelling associated with the nickel-plated steel shot occurred with all 3 implants. Dissection of these areas revealed a fi-brous capsule encasing a white exudate material located around the implanted pellet. No other gross abnormalities were detected in any other implant sites in any time group. One hundred fourteen of 120 (95%) pellets were retrieved at necropsy. Two pellets each from the nickel-plated steel group from the 8-, 16-, and 26-week time groups were unaccounted for. All pellets of materials other than nickel-plated steel were recovered. Gross examination of the heart, lungs, liver, gall bladder, kidneys, digestive tract, and urinary bladder was unremarkable in all rats.
Histopathologic findings—Inflammation severity was greatest at 2 weeks for all pellet types. Median inflammation score decreased over the study period for all pellet types (Table 1). A significant (P = 0.014) difference between mean inflammation score for nickel-plated steel and bismuth was observed at 2 weeks after implantation (Figures 1 and 2). No other significant differences in inflammation score were detected in any other time group. Following 26 weeks of implantation, no significant differences for inflammation were present among pellet types. No evidence for neoplasia was identified in any of the implantation sites examined.
Variables associated with IM implantation of lead shot alternatives in rats.
| Pellet type | No. | Median inflammation score* | Median iron score* | Median calcium score* | Mean ± SD in vivo corrosion rate (μm/y) | In vitro corrosion rate (μm/y) |
|---|---|---|---|---|---|---|
| ZPS | ||||||
| Week 2 | 4 | 2 | 1 | 0 | 32.5 ± 12a,b,c | 240 |
| 8 | 4 | 0.5 | 0.5 | 0 | 7.5 ± 9 | 60 |
| 16 | 4 | 0 | 0.5 | 0 | 2.5 ± 5 | 1.4 |
| 26 | 4 | 0 | 2 | 0.5 | 2.5 ± 5b | 1.1 |
| TNI | ||||||
| Week 2 | 4 | 2 | 2 | 0 | 77.5 ± 45a,c | 50 |
| 8 | 3 | 1 | 2 | 0 | 16.7 ± 15 | 30 |
| 16 | 4 | 1 | 2 | 0 | 12.5 ± 5 | 10 |
| 26 | 4 | 1 | 3 | 0 | 5 ± 5b | 10 |
| NPS | ||||||
| Week 2 | 3 | 3a | 1 | 0 | 90 ± 85a | 850 |
| 8 | 4 | 2 | 0.5 | 1.5 | 2,390 ± 2,715 | 60 |
| 16 | 4 | 1 | 2 | 0 | 1,245.3 ± 1,409 | 10 |
| 26 | 3 | 0 | 3 | 0 | 1,003.3 ± 852a | |
| Steel | ||||||
| Week 2 | 4 | 2 | 3 | 0 | 15 ± 19b | 140 |
| 8 | 4 | 1 | 3 | 0 | 0 | 30 |
| 16 | 4 | 1 | 2.5 | 0.5 | 10 ± 20 | 0 |
| 26 | 4 | 1 | 3 | 0 | 0 | 0 |
| BA | ||||||
| Week 2 | 4 | 1b | 1 | 0.5 | 0b | 0 |
| 8 | 3 | 1 | 0 | 0 | 0 | 0 |
| 16 | 3 | 1 | 0 | 0.5 | 0 | 10 |
| 26 | 3 | 1 | 0 | 0 | 0b | 0 |
| Lead | ||||||
| Week 2 | 4 | 2 | 1 | 0 | 0b | 0 |
| 8 | 4 | 1 | 1 | 0 | 0 | 0 |
| 16 | 3 | 1 | 1 | 0 | 0 | 0 |
| 26 | 4 | 0.5 | 0 | 0 | 0b | 0 |
| TM | ||||||
| Week 2 | 4 | 2 | 1 | 0 | 25 ± 10a,b,c | 60 |
| 8 | 4 | 1 | 0.5 | 0.5 | 17.5 ± 12 | 20 |
| 16 | 4 | 1 | 0 | 0 | 27.5 ± 5 | 10 |
| 26 | 4 | 0.5 | 0.5 | 0 | 37.5 ± 17b | 0 |
| TI | ||||||
| Week 2 | 3 | 2 | 3 | 0 | 0b,c | 230 |
| 8 | 4 | 1 | 3 | 0 | 0 | 50 |
| 16 | 4 | 1 | 2 | 0 | 0 | 10 |
| 26 | 4 | 1 | 3 | 0 | 0b | 0 |
Grading scale: 0 = absent, 1 = mild, 2 = moderate, 3 = severe. ZPS = Zinc-plated steel. TNI = Tungsten-nickel-iron alloy. NPS = Nickel-plated steel. BA = Bismuth alloy. TM = Tungsten matrix polymer. TI = Tungsten-iron alloy.
Within a column, values in the same time group with different superscript letters are significantly (P ≤ 0.05) different.
Photomicrograph of a portion of skeletal muscle in a rat 2 weeks after IM implantation of a nickel-plated steel shotgun pellet. IS = Implantation site. Arrow indicates area of severe inflammation adjacent to the implantation site. H&E stain; bar = 100 μm.
Citation: American Journal of Veterinary Research 68, 4; 10.2460/ajvr.68.4.446
Photomicrograph of a portion of skeletal muscle in a rat 2 weeks after IM implantation of a nickel-plated steel shotgun pellet. IS = Implantation site. Arrow indicates area of severe inflammation adjacent to the implantation site. H&E stain; bar = 100 μm.
Citation: American Journal of Veterinary Research 68, 4; 10.2460/ajvr.68.4.446
Photomicrograph of a portion of skeletal muscle in a rat 2 weeks after IM implantation of a nickel-plated steel shotgun pellet. IS = Implantation site. Arrow indicates area of severe inflammation adjacent to the implantation site. H&E stain; bar = 100 μm.
Citation: American Journal of Veterinary Research 68, 4; 10.2460/ajvr.68.4.446
Photomicrograph of a portion of skeletal muscle in a rat 2 weeks after IM implantation of a bismuth alloy shotgun pellet. Arrow indicates area of mild inflammation adjacent to the implantation site. H&E stain; bar = 100 μm. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 68, 4; 10.2460/ajvr.68.4.446
Photomicrograph of a portion of skeletal muscle in a rat 2 weeks after IM implantation of a bismuth alloy shotgun pellet. Arrow indicates area of mild inflammation adjacent to the implantation site. H&E stain; bar = 100 μm. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 68, 4; 10.2460/ajvr.68.4.446
Photomicrograph of a portion of skeletal muscle in a rat 2 weeks after IM implantation of a bismuth alloy shotgun pellet. Arrow indicates area of mild inflammation adjacent to the implantation site. H&E stain; bar = 100 μm. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 68, 4; 10.2460/ajvr.68.4.446
Special stains—Subjectively, there appeared to be increased iron staining at implantation sites of iron-containing pellets, compared with other types of pellets (Figure 3). The iron and calcium staining scores were not significantly different among metals within time groups or among time groups within metals (Table 1).
Photomicrograph of a portion of skeletal muscle in a rat 8 weeks after IM implantation of a nickel-plated steel shotgun pellet.A—Arrows indicate area of severe inflammation. H&E stain; bar = 100 μm. B—Arrows indicate area of intense iron staining adjacent to implantation site. Prussian blue stain; bar=100 μm. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 68, 4; 10.2460/ajvr.68.4.446
Photomicrograph of a portion of skeletal muscle in a rat 8 weeks after IM implantation of a nickel-plated steel shotgun pellet.A—Arrows indicate area of severe inflammation. H&E stain; bar = 100 μm. B—Arrows indicate area of intense iron staining adjacent to implantation site. Prussian blue stain; bar=100 μm. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 68, 4; 10.2460/ajvr.68.4.446
Photomicrograph of a portion of skeletal muscle in a rat 8 weeks after IM implantation of a nickel-plated steel shotgun pellet.A—Arrows indicate area of severe inflammation. H&E stain; bar = 100 μm. B—Arrows indicate area of intense iron staining adjacent to implantation site. Prussian blue stain; bar=100 μm. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 68, 4; 10.2460/ajvr.68.4.446
Corrosion rate—In vivo corrosion rate was greatest at 2 weeks for metals subject to corrosion and declined at subsequent time periods for all succeptible metals. Nickel-plated steel had a significantly greater corrosion rate than bismuth alloy, lead, steel, and tungsten-iron at 2 weeks (P = 0.042) and all other metals at 26 weeks (P < 0.001). Tungsten-nickel-iron had a significantly (P = 0.042) greater corrosion rate than bismuth alloy, lead, and steel at 2 weeks (Table 1).
Toxicologic findings—Heavy metal analysis was performed on the left kidney of all rats. Concentrations of barium, beryllium, cadmium, cobalt, chromium, mercury, molybdenum, nickel, lead, titanium, and vanadium were less than the minimum detection limit for all rats. Arsenic, copper, iron, manganese, and zinc were detected in all test rats. Significantly (P < 0.001) greater mean arsenic concentration was detected in the 8-, 16-, and 26-week groups, compared with the 2-week group. Significantly (P < 0.001) greater mean iron and copper concentrations were detected in the 26-week group, compared with the other time groups (Table 2). Arsenic concentrations were less than the minimum detectable limit in the control rats.
Mean ± SD heavy metal concentrations ($mUg/g [ppm]) in 2 control rats and 5 rats with IM implantation of various lead shot alternatives for 2 to 26 weeks.
| Week | Arsenic | Copper | Iron | Manganese | Zinc |
|---|---|---|---|---|---|
| 2 | 0.95 ± 0.21a | 5.82 ± 0.83a | 122.2 ± 13.39a | 0.75 ± 0.09 | 17.08 ± 1.25 |
| 8 | 1.83 ± 0.34b | 6.06 ± 0.68a | 141 ± 16.54a | 0.80 ± 0.12 | 17.44 ± 0.89 |
| 16 | 2.40 ± 0.37c | 6.16 ± 0.93a | 140.8 ± 12.87a | 0.75 ± 0.04 | 17.72 ± 1.30 |
| 26 | 3.41 ± 0.20d | 8.06 ± 1.1b | 171.4 ± 12.84b | 0.77 ± 0.07 | 17.86 ± 0.75 |
| Control | ND | 7.75 ± 3.18 | 111.2 ± 47.87 | 0.79 ± 0.04 | 18.75 ± 0.21 |
ND = None detected
Within a column, values with difference superscript letters are significantly (P 0.05) different.
Discussion
In vivo studies1,12–17 have evaluated local tissue response to pure metals such as lead, copper, iron, nickel, zinc, and titanium. The local reaction to such metals is variable in severity and ranges from a limited response (fibrous capsule formation around a lead implant) to a severe necrotic reaction observed following implantation of copper in rats.1 Pure nickel induces a severe inflammatory response when implanted into mammalian tissue.1,15,16 Tumor formation has been detected with IM implantation of nickel compunds and weapons-grade tungsten alloy in rats.18–20 The local reaction induced by lead shot alternatives evaluated in the present study was similar for up to 26 weeks after implantation, with no evidence of local tumor formation.
Metal alloys available for use by hunters have been evaluated on a limited basis. Nickel-plated steel was not included as a test metal in previous studies. In our study, nickel-plated steel incited the most severe gross and microscopic reaction, with the greatest effect at 2 weeks after implantation. Corrosion rate was significantly greater for nickel-plated steel, compared with the other metals in this study. It is possible that nickel in close contact with iron in a conductive environment caused galvanic corrosion. Galvanic corrosion occurs when 2 conductive metals are in contact in a conductive environment.11 A galvanic process may have accelerated the corrosion of nickel-plated steel.21 Nickelplated steel pellets were the only material that was not recovered as intact pellets in the present study and were unaccounted for during fluoroscopic examination in the 26-week group. These findings suggest corrosion of some of the nickel-plated steel pellets continued until the pellet was completely corroded and absorbed. This observation was supported by the local uptake of iron at the implantation sites of nickel-plated steel pellets. Radiographic examinations of the rats to verify location of the implanted pellets were not performed. Routine radiographic examinations could have been used to determine the location and possibly the incremental dissolution or corrosion of implanted pellets.
Tungsten alloys have been tested for use in munitions by the US military and by manufacturers of so-called green bullets.22 Tungsten is a hard, dense metal considered desirable for use as a nontoxic projectile because of its limited toxic effects when ingested or inhaled. Interestingly, results of previous in vivo and in vitro studies indicate the possible carcinogenic potential of tungsten alloys in rats18 and human osteoblast cells,23 respectively. In the present study, no evidence of neoplasia was identified following implantation of tungsten-nickel-iron, tungsten-iron, or tungsten-polymer for up to 26 weeks.
Corrosion of steel, tungsten, and nickel has been detected following implantation into animals including dogs,12 rats,1 and rabbits.24 Calculation of corrosion rate on the basis of weight loss is considered an acceptable method to determine metallic corrosion. In biological systems, the amount of weight loss may be too small to permit accurate calculation of corrosion rate. Weight loss was not recorded for all the recovered metals even though corrosion was highly likely, as indicated by results of special stains for iron. The likely reason for this finding was the inability to safely remove all tissue adhered to the pellet prior to obtaining the postimplantation weight. Aggressive cleaning of the retrieved pellets was avoided because it may artificially lower the weight by removing metal. Possibly, use of gentle enzymatic or ultrasonic cleansing of retrieved pellets to remove all tissue and limit metal removal would have eliminated this source of error. Scanning electron microscopy25 or x-ray scanning analytic microscopy26 to evaluate metallurgic corrosion may also be a more sensitive method to detect minimal corrosion in biological systems.
Iron is a heavy metal that is necessary for hemoglobin function in mammals. Acute iron toxicosis has been reported following ingestion of large amounts of iron-containing dietary supplements or fertilizers by dogs.27 Ingestion of large quantities of these materials may cause acute vomiting and diarrhea with potential progression to circulatory collapse and death.27 In the present study, local iron concentration adjacent to embedded iron-containing pellets was subjectively increased, compared with local iron concentration adjacent to pellets without iron. Iron from such sites may be directly absorbed into the systemic circulation or possibly distributed by inflammatory cells, contributing to increased systemic iron concentrations over time.12 Although kidney iron concentration was significantly increased in the 26-week group, compared with the other groups and control rats, adverse affects were unlikely.
Arsenic is a heavy metal that is ubiquitous in the environment. It is associated with mining and smelting of many metals, including lead and tungsten.28 It is also used in the manufacturing of lead and bismuth shot to facilitate hardening and sphere formation of the metal alloy.29 Arsenic toxicosis results from inhibition of sulfhydryl enzyme systems essential to cellular metabolism.28 Chronic arsenic poisoning is associated with nonspecific signs such as lethargy and more specific dermatologic and neurologic syndromes.28 Most animal cases consistent with arsenic poisoning are associated with kidney arsenic concentrations of at least 10 ppm.28 In the present study, all treatment group rats had detectable concentrations of arsenic in their kidneys, which increased significantly over time.
Following the discovery of sequentially increased concentrations of arsenic in the kidneys of test rats, heavy metal concentrations of rats raised in the same facility, housed in identical conditions, and fed the same diet as the test rats were evaluated. This was carried out to evaluate the possible contribution of environmental factors to kidney arsenic concentrations. Arsenic was not detected in kidneys of the control rats. This result essentially ruled out the environment as a source of arsenic and confirmed the implanted pellets as the source of arsenic in the test rats. Specific information regarding the potential differences in metabolism of heavy metals by Sprague Dawley and Long Evans rats is not available. Because these rats are strains of the same species (Rattus norvegicus), the authors believed that any potential differences between the strains were not important for this purpose.30 The same strain of rat would have been ideal for use as an environmental control population for this study but was not available.
Arsenic has been identified in bismuth alloy and lead shot materials.29 Intramuscularly implanted bismuth alloy and lead shot materials induced minimal inflammation in the rats in this study. Therefore, it is hypothesized that arsenic in the implanted pellets leached into the tissues of the rats over time. Distribution of arsenic by inflammatory cells into the systemic circulation may also occur but may be less important because inflammation induced by implantation of these pellets was mild. Because of the randomized nature of the pellet implantation, it was impossible to determine which pellets may be more likely to result in increased kidney arsenic concentrations. The effects of heavy metals in experimental animals have been considered as a predictor of potentially toxic or carcinogenic effects in humans.31 The possibility of arsenic leaching into the systemic circulation from embedded shotgun pellets may warrant further investigation.
On the basis of results of the study reported here, a profound acute inflammatory response may be expected in a dog or person wounded by a large number of nickel-plated steel pellets. In the short term, veterinarians and physicians may need to monitor these wounds more closely than usual or consider more aggressive debridement and retrieval of pellets. The clinical consequences of high kidney arsenic concentrations in the rats reported here are not known, but routine monitoring of systemic arsenic concentrations may be indicated in the long-term management of patients wounded with shotgun pellets.
LabDiet 5001, Purina Nutrition International, St Louis, Mo.
Research randomizer, Geoffrey C. Urbaniak and Scott Plous, Middletown, Conn. Available at: www.randomizer.org. Accessed Oct 3, 2005.
Bismuth Shotshells, Bismuth Cartridge Co, Van Nuys, Calif.
Fiocchi Golden Pheasant-GPX shotshells, Fiocchi Ammunition Co, Ozark, Mo.
Impact tungsten matrix shotshells, Kent Cartridge Co, Kearneysville, WV.
Federal premium tungsten iron shotshells, Federal Cartridge Co, Anoka, Minn.
Hevi-shot, Environmetal Inc, Sweethome, Ore.
Federal premium Ultrashok zinc-plated steel shotshells, Federal Cartridge Co, Anoka, Minn.
Winchester super-X heavy field loads, Winchester Ammunition, East Alton, Ill.
Winchester Xpert steel shot waterfowl loads, Winchester Ammunition, East Alton, Ill.
Accu-224 electronic top loading balance, Fisher Scientific International Inc, Hampton, NH.
Prolene, Ethicon Inc, Sommerville, NJ.
Ethilon, Ethicon Inc, Sommerville, NJ.
Buprenex injectable, Reckitt Benckiser Pharmaceuticals Inc, Richmond, Va.
Extended heavy metal screen, California Animal Health and Food Safety (CAHFS) Laboratory System, Davis, Calif.
Winks statistical data analysis software, version 4.0, TexaSoft Statistics Software, Cedar Hill, Tex.
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