INTRODUCTION
Softwood shavings (pine and cedar) have been used as bedding for animals
because they are inexpensive, have natural insecticidal and barteriocidal
properties, and often effectively control odor. The
fact that these beddings contain chemicals capable of repelling or killing
pests (such as moths), however, is an indication that they might not be
safe as bedding for animals.
Chemicals responsible for the "good" properties of wood beddings also
cause:
- Respiratory tract damage and disease
- Liver damage and disease
Pine and cedar, and
products derived from them, are used by humans for a number of applications
as well. Therefore, hazards associated with contact of softwoods
are also a human health concern.
The toxic chemicals found in softwoods are aromatic (volatile) hydrocarbons.
The primary irritant in cedar is plicatic
acid, and the primary irritant in pine is abietic
acid. Plicatic acid is most concentrated in western red cedar
(Thuja plicata), but is also found in significant quantities in
eastern white cedar (Thuja occidentalis) and Japanese cedar (Cryptomeria
japonica). Abietic acid is found in varying quantities
in all pine trees (family Pinaceae) and is the main component of
colophony (rosin) which is a derivative of pine trees with a number of
industrial applications (Johnston 1995).
EFFECTS
OF SOFTWOODS ON
RESPIRATORY PHYSIOLOGY
AND FUNCTION
Evidence that Plicatic Acid is the Primary
Irritant in Cedar
Plicatic acid (PA) has been identified as the primary irritant in cedar
in a number of studies. Much of the research performed on PA has been with
sawmill workers because of the relatively high rate of western red cedar
asthma (WRCA) in the sawmill industry. In 1989, Chan-Yeung et al.
measured histamine and leukotriene E4 (LTE4) levels
in bronchial fluid of subjects exposed to PA. Workers with WRCA, atopic
asthma, and no asthma were tested. PA only induced a significant
increase in histamine and LTE4 levels in the WRCA patients meaning
that the WRCA patients were the only subjects sensitized to PA and PA is
the specific cause of their allergic reaction (Chan-Yeung et al. 1989) (Fig.
1). Similarly, Frew et al. (1993) found that PA caused release
of histamine from bronchial mast cells in WRCA patients, but not in patients
with atopic asthma. It has also been shown that sawmill workers with
WRCA do not have an allergic response to a WRC extract if the PA has been
chemically extracted (Chan-Yeung 1994).
Effects of Plicatic Acid
- Human
Studies
Western red cedar asthma affects between 4 and 13.5% of exposed populations,
such as sawmill workers
(Chan-Yeung 1994). Eastern white cedar (EWC) has identical effects,
but is only half as potent as western red cedar (WRC) because it contains
only half as much PA. WRCA and EWCA patients show cross-reactivity
with both cedar species which affirms that PA is the causal agent in both
cases (Cartier et al.1986).
The effects of PA are similar to other inflammatory an allergic reactions.
PA induces the release of
immunoglobulins , histamine, and leukotrienes, and causes an increase of
eosinophil and T-cell levels in
the blood. PA can cause both Type-I hypersensitivity, an immediate
increase in immunoglobulin E (IgE), and Type-IV hypersensitivity, a delayed
effect, of the respiratory system. WRCA patients may exhibit either
or both types (Johnston 1985).
In studies where WRCA patients have been removed from exposure to WRC,
and therefore PA, less that 50% of cases resolve (Choubrac 1991 and Rosenberg
1989). Similarly, in a study of 136 sawmill workers with WRCA, 60%
continued to have frequent asthma attacks after a year away from work (Chan-Yeung
1987). Although eliminating exposure to PA does not guarantee a restoration
of health, after a year of medical treatment, no WRCA patients recovered
when continuing to work in a sawmill (Cote 1990). In the same study, 10%
showed any improvement, 62% remained stable, and 38% showed a decline in
condition (Cote 1990). The finding that the disease does not resolve
after exposure to
PA is eliminated means that definite physiological changes have occurred,
such as in the function of the immune and respiratory systems.
- Animal Studies
A comprehensive study using guinea pigs (Cavia porcellus) by Salari
et al. (1994) provides additional evidence regarding the immune-mediated
allergic response to PA. Guinea pigs were sensitized to PA over a
period of six months via bi-weekly injections of PA. After three
months, PA-specific immunoglobulin G1 (IgG1) antibodies
were detected. After the six month injection period, assays were
performed to access the effects of PA with the following results:
(1) Lung mast cells and blood basophils released histamine, LTD4,
and prostaglandin D2 (PGD2) in response to PA in
vitro
(2) Harvested tracheal tissue contracted upon exposure to PA
(3) Untreated guinea pig tracheal tissue could be passively sensitized
with serum from PA-sensitized subjects and would contract in response to
PA
(4) Untreated guinea pig tracheal tissue passively sensitized with immunoglobulin-free
serum from PA-sensitized subjects did not contract in response to PA
These findings show
that: (1) the response to PA is immune-mediated and similar in nature
to other allergic/inflammatory reactions, (2) PA is the sensitizing irritant
causing asthmatic constriction in tracheal tissue, (3) the mediator responsible
for eliciting a reaction to PA is contained in blood serum, and (4) the
mediator responsible is an immunoglobulin. These findings agree with
a human study by Vedal et al. (1986) where PA-specific IgE was associated
with nonspecific bronchial hyper- responsiveness
(analogous to contraction in Salari et al. 1994).
Ayers et al. (1989) presents another comprehensive study, but where the
effects of PA and abietic acid (AA) were both examined. Monolayers
of rat type II and human A549 alveolar epithelial cells, intact rat lungs,
and rat tracheal explants were exposed to several concentrations of PA
and AA. Both PA and AA caused time and dose dependent lysis of rat
and human alveolar epithelial cells (Fig.
2). When PA and AA were injected into intact rat lungs, sloughing
of bronchial epithelial cells was observed upon examination of harvested
organs. There was also time and dose dependent sloughing of epithelial
cells in tracheal explants (Fig.
3). These findings show that PA and AA are cytotoxic and
damage alveolar, tracheal, and bronchial epithelial tissue of the respiratory
tract and therefore support that exposure to PA and
AA via cedar and pine is a major occupational hazard and neither is acceptable
for use as an animal bedding.
Evidence that Abietic Acid is the Primarily
Irritant in Pine
As discussed, Ayars et al. (1989) presents data identifying AA as the primary
irritant in pine. Like PA, AA caused time/dose dependent lysis of
rat and human alveolar cells (Fig.
2), sloughing of bronchial epithelial tissue in intact rat lungs,
and time/dose dependent sloughing of rat explant epithelial tracheal tissue (Fig.
3). In addition, AA also caused sloughing of the alveolar
epithelium in intact rat lungs. In this study, the effects of AA
were more pronounced; AA seemed to have a higher potency than PA which
is inconsistent with other studies where PA is identified as a more toxic
agent. The greater effect of AA in this study in the result of AA
and PA being tested at identical concentrations. In most other studies,
PA and AA extracts have been prepared from wood samples and because AA
is naturally present at much lower concentrations in pine than PA is in
cedar, the pine extracts are less concentrated as well.
Effects of Abietic Acid
- Human Studies
Abietic acid is found in colophony, a derivative of pine trees also called
rosin. Colophony has a number of industrial applications including
solder fluxes, paper sizing, adhesives, paints, varnishes, printing inks,
and plasticizers (Sadhra et al. 1994). The composition of colophony
is 90% resin acids and 10% component esters, aldehydes, and alcohols.
The resin acids are all isomers of AA (heat and oxygen can cause isomerization).
As it contains abietic acid, colophony is a major occupational health hazard
and is the third highest cause of occupational asthma. In the Guinea
Pig Maximization Skin Sensitivity Test, colophony is rated grade IV (strong),
but AA is rated III (moderate). This result led to the finding that
it is the oxidized form of abietic acid (15-hydroperoxyabietic acid)
that is the most active form. This finding is supported by the fact
that hydrogenation (reduction) of colophony gives it the same activity
as pure AA (Sadhra et al. 1994).
- Animal Studies
Far less research has been conducted regarding the respiratory effects
of AA than of PA; however, all the studies that have been performed show
that it is also a major health hazard (Ayars et al. 1989; Sadhra et al.
1994). Odynets et al. (1991) performed a detailed study comparing
pine, spruce, birch, aspen, and cellulose (control). In a toxicity
screen using Tetrahymena pyriformis, all woods inhibited growth
in comparison
to cellulose, but pine inhibited growth to the greatest degree (Fig.
4). Odynets et al. (1991) also performed a number of experiments
with mice:
- Immune Response
After being housed on a specific bedding for one month, Icr:Icl mice were
immunized with sheep erythrocytes and spleens were then examined for plaque
forming cells (PFC) after five days by addition of T-dependent antigen.
Only mice housed on pine bedding showed a PFC count significantly
higher (p = 0.05)
than the control group kept on cellulose bedding (Fig.
5).
- Relative Liver Mass
C57BL/6 and BALB/c mice were housed on a specific bedding from birth.
At 8 months, livers were collected and massed. Pine was the only
bedding that caused a significant increase (p = 0.05) in liver mass
(compared to cellulose group) in male and female groups of both breeds (Fig.
6).
- Evaluation of Reproductive Activity
C57BL/6 and BALB/c mice were housed on a specific bedding from birth.
K1 was devised as an index of reproductive rate, and K2
as an index of reproductive
productivity.
K1 = # offspring
weaned from female's first litter
period from female's birth to weaning of first litter
K2 = # offspring weaned from five litters
of one female
period from female's birth to weaning of five litters
Females of both breeds housed on pine showed a significantly (p
= 0.05) lower a K1 and K2 than females house on aspen
(Fig.
7).
- Evaluation of Bedding Choice
C57BL/6, BALB/c, and wild mice were placed in modified cages with a choice
of bedding (pine,
spruce, birch, aspen). All mice significantly preferred
aspen (p = 0.05),
and no mice chose pine (Fig.
8).
These results show
that pine does have a number of adverse physiological effects. In
addition, mice actually preferred the bedding that was least toxic, elicited
the lowest immune response, had the least effect on the liver, and gave
the highest reproductive capabilities.
EFFECTS
OF SOFTWOODS ON
LIVER PHYSIOLOGY
AND FUNCTION
- Human
Studies
The
effects of pine and cedar on human liver function has not been studied,
but the fact that small mammals, such as mice, rats, guinea pigs, and rabbits,
all show the same adverse liver reactions makes pine and cedar of direct
concern to humans. Animal testing is always meant to gain knowledge
regarding the function
of the animal being studied, but is also usually meant to serve as a model
for a human reaction. Extensive testing has shown that liver function
of small mammals is similar to that of humans,
and therefore it can be inferred that the same liver effects seen in small
mammals are likely to occur in humans (TeSelle, 1993). Because
humans are not forced to live directly in softwood shavings
the risk is reduced, however,
the subject deserves study.
- Animal
Studies
In one of the first studies regarding the effects of softwood beddings
on liver function, Vessel (1967) found that use of softwood beddings did
significantly increase the production of three liver microsomal enzymes.
The study was spurred by the discovery of an immediate decrease in Evipal
(hexobarbital) sleeping time after the introduction of softwood bedding
in a previous study. The factor responsible was not initially known
because bedding, cage type, and food had all been changed simultaneously;
however, each factor was examined independently and the softwood bedding
was identified as the cause.
Vessel (1967) carried out experiments with a number of strains of mice
and rats. In every case, sleeping time was defined as the time period
from intraperitoneal (IP) injection to the point of righting-response restoration.
At the restoration of righting-response, both livers and brains were harvested
for further study. Brain preparations were used to verify that the
same concentration of hexobarbital was present in animals with a decreased
sleeping time and normal sleeping time. Liver preparations were assayed
for enzyme activity. Enzyme activities (hexobarbital oxidase, aniline
hydroxylase, and ethyl morphine N-demethylase) were all significantly
higher for animals housed on softwood beddings (Fig.
9). When animals housed previously on softwoods were moved
to a hardwood bedding,
sleeping time and liver enzyme activity returned to normal levels (Fig.
10).
The results of Vessel's experiments identified a time dependent effect
on both sleeping time and activity of liver enzymes as well as a correlation
between the two:
- 24-hours post placement on WRC bedding mouse sleeping time decreased
by 33% and liver enzyme activity increased by 33%
- 48-hours post placement on WRC bedding mouse sleeping time decreased
by 66% and liver enzyme activity increased by 66%
Experiments were replicated
with white pine and ponderosa pine beddings and similar results were observed.
Interestingly, the toxic effects of softwood beddings can be reduced with
heat treatment and chemical
extraction of aromatic compounds:
- Mice housed on hexane-extracted and autoclaved WRC bedding for eight
days showed a 25% decrease in sleeping time as opposed to the 80% decrease
in mice housed on untreated bedding for the same time period (Vessel 1967).
Since the time of publication of Vessel's study, the effects of aromatic
hydrocarbons in softwoods on
liver function has been studied in detail. It has been established,
as in respiratory irritation, that the active irritants in cedar and pine
are PA and AA respectively (Törrönen et al. 1989). In a
comprehensive experiment by Törrönen et al. (1989), in vivo
and in vitro studies were performed with rats and mice to test differences
between softwood, hardwood, and inert cellulose bedding materials.
- In vivo experiment
Rats were housed without bedding for two weeks, and then moved to:
- softwood bedding: pine (Pinus silvestrus)
- hardwood bedding: alder (Alnus incana) or aspen (Populus tremula)
- no bedding: control
for an additional
two weeks prior to harvesting of livers. Although eight liver enzyme
levels were assayed, a significant increase in activity
(indicating a change in liver function) was only seen for aldehyde dehydrogenase
in the pine and alder groups.
- In vitro experiment
The mouse hepatoma cell-line, Hepa-1, was used in order to design a more
controlled study of the beddings. Cell cultures were exposed to acetone
extracts of:
- softwood bedding: pine (Pinus silvestrus) or pine/spruce (Picea
abies)
- hardwood bedding: alder (Alnus incana) or aspen (Populus tremula)
- inert bedding: cellulose material (control)
In assays for cytotoxicity
and induction of liver enzyme production (aryl hydrocarbon hydroxylase
and aldehyde dehydrogenase), softwoods produced a much stronger effect.
The fact that hardwood (aspen and alder) extracts also showed a degree
of cytotoxicity and liver enzyme induction means that they too should be
avoided as bedding materials.
Although cedar and pine have been eliminated from laboratories and ongoing
research continues to understand their harmful effects, both materials
continue to be used as bedding for companion animals. The Guide
for the Care and Use of Laboratory Animals (1996), an official guide
released by the National Research Council, explicitly states that softwood
beddings have been shown to induce liver microsomal enzyme production,
increase incidence of cancer, and are cytotoxic. Pine and cedar beddings
continue to be available because consumers are misinformed, a result of
ignorance and lack of concern in the pet industry. Many organizations
have become aware of the dangers of softwood bedding and now work to publicize
the associated dangers. For example, the House Rabbit Society, a
nation-wide organization that harbors abandoned rabbits and promotes their
proper care, was one of the first companion animal groups to recognize
the harmful effects of softwood beddings. In an informal study in
1989, the HRS determined that otherwise healthy rabbits, if housed on pine
or cedar bedding, showed elevated liver enzymes in blood tests, and showed
a higher rate of liver disease and cancer (Harriman 1989). They also
found that the rabbits, if moved to inert bedding materials, showed normal
liver enzyme levels after one month (Harriman 1989).
Softwood beddings are generally less expensive than safer alternatives,
and, ironically, the aromatic hydrocarbons making them dangerous also make
them attractive in terms of deodorization. The new
generation of veterinarians specializing in exotic animals (which includes
the small mammals), animal advocacy groups, and better distribution of
information to pet owners through pet stores are all helping to stop the
use of pine and cedar as companion animal beddings.
ADDITIONAL
HARMFUL
CHEMICALS IN SOFTWOODS
In a 1974 publication, Schoental (1974) studied two aromatic hydrocarbons, coniferaldehyde
and sinapaldehyde,
found in pine and cedar respectively. Both compounds are normal constituents
of wood lignin and can be isolated by thin layer liquid chromatography.
Schoental found that both compounds, and particularly sinapaldehyde, readily
isomerize and undergo redox reactions to produce carcinogenic
compounds. For example,
3,4,5-trimethoxycinnamaldehyde
(TMCA), the p-O-methyl derivative of sinapaldehyde,
was shown to cause many types of tumors in rats, including squamous-cell
carcinomas. An oxidation product of sinapaldehyde present in wood,
2,
6-dimethoxy-1,4-benzoquinone, was shown to induce sarcomas in rats
following subcutaneous injection. The presence of additional hazardous
chemicals in softwoods gives even greater reason to avoid them as bedding
materials for companion animals.
ALTERNATIVE
BEDDINGS
FOR COMPANION
ANIMALS
As is evidenced in the studies described, no wood bedding is without risk,
although hardwoods, such as aspen, are highly preferable to softwoods.
A number of safer alternative beddings are available for companion animals.
In choosing an appropriate bedding for companion animals, consider the
particularly needs of your animal.
