Harmful effects of the microplastic pollution on animal health: a literature review

Invertebrates

The effects of MP exposure have been characterized in detail for a number of aquatic species. Haegerbaeumer et al. (2019) meticulously reviewed the studies on the impact of MP on the benthic marine and freshwater invertebrates including annelids, arthropods, ascidians, sea urchins, bivalve mollusks, and rotifers. Of the 28 studies assessing the effect of MP on mortality, only three revealed significant changes: the mortality increased in polychaetas Perinereis aibuhitensis (PS ø 8–12 μm, 100–1,000 particles/mL), shrimps Palaemonetes pugio (PE, PS, PP ø 30–165 μm, 50,000 particles/L), and copepods Tigriotopus japonicus (PS ø 0.05 μm, 1.25 μg/L). MP exposure promoted a decline in food activity and fertility; it also inhibited the larval growth and development and increased oxygen consumption and reactive form production. However, given significant variation of types, sizes, and concentrations of MP particles among the studies, as well as the use of different animal phyla, a dedicated comparative analysis of MP effects based on these studies will hardly make sense.

Fish

MP were found in 49% of the examined fish from the North East Atlantic Ocean (Dicentrachus labrax, Trachurus trachurus, and Scomber colias). Approximately 35% of the examined specimens had MP in the gastrointestinal tract (mean ± SD = 1.2 ± 2.0 items/individual), 36% had MP in the gills (0.7 ± 1.2 items/individual), and 32% had MP in the dorsal muscles (0.054 ± 0.099 items/g of tissue). The MP particles were mainly represented by 151–1,500 µm fibers and 100–1,500 µm shreds of PE and polyester. In fish, MP exposure leads to increased lipid peroxidation levels in the brain, gills and spinal muscles; it also stimulates the activity of brain acetylcholinesterase (Barboza et al., 2019).

Yong, Valiyaveetill & Tang (2020) conducted a meta-analysis of the experimental studies revealing significant toxic or pathological MP effects in fish. Toxic reactions generally arise from smaller particles (≤5 µm in diameter), whereas bigger PS particles (100 µm or more) have no significant effect. During the tests, MP particles are usually added to a water tank containing fish and roe at concentrations varying from 1 to 1,000 mg/L, most conventionally at 20 mg/L. The time of exposure varies from several hours to several months; the most conventionally used exposure time is 1 week. MP uptake has been shown to disturb the feeding behavior and motor activity in adult fish and fry, and also to negatively affect fertility. Maternal transmission of MP to offspring has been documented (Pitt et al., 2018a), along with an evidence that prenatal exposure of MP affects the early fry development (Wang et al., 2019). Increasing doses of MP exacerbate the degree of their accumulation in tissues and severity of the associated histological and biochemical changes (Lu et al., 2016; Wang et al., 2019; Pannetier et al., 2020).

The majority of experimental studies on MP in fish have been performed on zebrafish using PS or, less commonly, PE particles. The 25–70 nm PS particles penetrate the roe membrane, accumulate in the yolk sac, and move to the gastrointestinal tract. In fish embryos and fry, the nanoparticles are found in the intestines, gallbladder, liver, pancreas, heart, and brain. The transfer of PS nanoparticles from mother to the offspring has been confirmed experimentally. The nanoparticles have been shown to interfere with fry motility, causing either hyperactivity or weakness, boost cortisol levels, suppress glucose levels, and increase the rates of heartbeat, inflammatory reactions, and fatty degeneration of the liver. The nanoparticles also affect the antioxidant system by inhibiting glutathione reductase in the brain, muscles, and testes of the adult fish and their offspring; they interfere with synaptic transmission by suppressing acetylcholinesterase activity and inhibit the expression of cytoskeleton genes (Lu et al., 2016; Chen et al., 2017; Pitt et al., 2018a; Pitt et al., 2018b; Brun et al., 2019).

Larger PS particles, 5 μm in diameter, found in the gills, intestines, and livers of fry and adult fish, promote fatty degeneration of hepatocytes and inflammatory changes in the liver and intestines, alter the qualitative and quantitative composition of the intestinal microbiome, interfere with carbohydrate and lipid metabolism, and induce the oxidative stress-related changes in the antioxidant protection gene expression (Lu et al., 2016; Qiao et al., 2019; Wan et al., 2019).

Yet bigger particles of PS and PE, 10–90 µm in diameter, accumulate in the intestines of fry and adult fish and in the gills of the adults. These particles cause changes in fish behavior and intestinal microbiome composition; they enhance the rates of neutrophil infiltration in the intestinal mucosa and the gills, interfere with lipid, carbohydrate, amino acid, and nucleic acid metabolism, affect the antioxidant defense mechanism, reduce the post-hatching survival, and inhibit regulatory genes involved in development and functioning of the nervous system (Lu et al., 2016; LeMoine et al., 2018; Limonta et al., 2019; Mak, Yeung & Chan, 2019; Malafaia et al., 2020; Wan et al., 2019).

The second most popular experimental models among fishes are rice fish, or medaka (Oryzias latipes and Oryzias melastigma). After exposure to PS particles (0.05 or 10 μm) or “natural” MP samples collected on the beach, the specimens presented with MP accumulations in the gills, intestines, and liver. The exposure led to increased mortality, a decrease in the average length and weight of fry and adult fish, oxidative stress and structural tissue damage at the sites of MP accumulation, increased content of reactive oxygen species and altered antioxidant enzyme activities, reproductive dysfunctions, and a decrease in roe production. Prenatal exposure to MP affected the early offspring development (Cong et al., 2019; Wang et al., 2019; Zhu et al., 2019; Pannetier et al., 2020).

Similar experiments involving other fish species (crucian carp, common carp, tilapia, catfish, perch, dorada, minnow) revealed the accumulation of MP (PS 0.025–15 μm; PE 40–100 μm, or PVC 40–200 μm) in the intestines, gills, liver, and brain. The exposure caused changes in feeding and shallow water behaviors, a decrease in swimming speed and range of movement, and fry growth retardation. At the tissue level, the exposure led to pronounced metabolic imbalance, with increased oxygen consumption and ammonia excretion, changes in the activity of liver enzymes, oxidative stress, structural damage to the liver, gills, intestines, skin and muscles, suppression of acetylcholinesterase activity in the brain, and altered expression of genes of the reproductive axis (Yong, Valiyaveetill & Tang, 2020).

It should be stressed that, in fish, MP accumulate mainly in the intestine, and in certain cases also in the gills and the liver, causing primary pathological changes in these organs. The compromised integrity of the epithelial barrier in the gut affects gene expression and protein production profiles, boosts the inflammation and oxidative stress levels, and alters gut microbiota. The signs of imbalanced lipid and carbohydrate metabolism and oxidative stress have been also observed in the affected fish livers (Fig. 1).

In addition, MP may enhance the toxic effects of certain pollutants/toxic substances in fish, including phenanthrene (Karami et al., 2016), mercury (Barboza et al., 2018), cadmium (Lu et al., 2018a; Banaee et al., 2019; Miranda, Vieira & Guilhermino, 2019), polychlorinated biphenyls (Rainieri et al., 2018), gold ions (Lee et al., 2019), and antibiotics (Zhang et al., 2019; Yong, Valiyaveetill & Tang, 2020).

Mammals

Studies focused on MP effects in terrestrial mammals were represented by 30 articles on the effect of MP particles in laboratory mice and rats listed in PubMed and Scopus databases. The earliest was published in 2017 and about half of the rest were published in 2021, which indicates the growing interest of the scientific community towards this problem.

It should be noted that species and strains of the animals, as well as doses, types, exposure lengths, and administration routes of the particles, varied considerably among the studies (Table 1). The majority of studies retrieved by the search (24 articles) involved mice, mostly ICR (CD-1) or C57Bl/6 and less often Balb/c or Swiss strains, whereas six of the studies were performed on Wistar and Sprague-Dawley rats. The administration routes were mostly oral, with the water from a drinking bowl (13 studies) or through a gastric tube (13 studies). In the rest few studies, MP were administered with food, by inhalation, intratracheally, or intraperitoneally. The particles were predominantly PS with diameters ranging from 0.02 to 500 μm (4.5 μm on average) (25 studies) and less often PE measured 0.4–150 μm (35 μm on average) (six studies).

The doses for oral exposure varied from 0.01 to 100 mg MP per kg of body weight a day. The daily doses of MP most typically used in mice were 0.024 or 0.24 mg/kg when added to a drinking bowl and 4 or 20–60 mg/kg when administered through a gastric tube; in rats, the typical doses were about 0.05, 0.5, and 5 mg/kg. When measured in MP particle number per kg of animal body weight a day, the doses ranged from 3.5 × 10³ to 1.5 × 10¹⁴, with the median of 2.5 × 10⁸ particles/kg/day. According to Cox et al. (2019), an average human consumes 39,000 to 52,000 particles/year with water and food, i.e., about 1.5–2 particles/kg/day. In the described experiments on mice and rats, the concentration of MP in drinking water varied from 7.2 × 10³ to 1.2 × 10¹⁸ particles/m³, with the median of 1.2 × 10¹¹ particles/m³. The “natural” MP concentrations in marine and fresh waters range from 0.1 to 2 × 10⁴ particles/m³ (Erni-Cassola et al., 2019; Sarijan et al., 2021). Thus, MP concentrations used in the published experiments involving mice and rats represented, on average, a 10⁶-fold excess compared with the MP content in natural waters and industrial foods.

The duration of exposure varied from a single injection of MP to a continuous exposure over three months; most typically, the exposure lasted four weeks. The high variability of experimental parameters questions the possibility of reliable comparison between the studies.

When administered orally, MP primarily act on the gastrointestinal tract. A number of studies demonstrate accumulation of 0.5–50 μm PS granules in the colon of mice, causing damage to the colon epithelial barrier, reducing the mucus production, inhibiting the expression of tight-junction proteins, and altering the intestinal microflora composition (Lu et al., 2018b; Jin et al., 2019; Li et al., 2020a; Qiao et al., 2021). MP were also detected in the rodents’ liver and kidneys, causing inflammatory changes and oxidative stress, a decrease in the relative mass of the liver, and energy and lipid metabolism imbalances (Deng et al., 2017; Deng et al., 2018; Yang et al., 2019; Deng et al., 2021; Wang et al., 2021). MP have been implicated in the development of cardiac fibrosis and impaired cardiac function by promoting cell death of cardiomyocytes through the induction of oxidative stress and mitochondrial damage (Li et al., 2020b; Wei et al., 2021). The mice administered with MP revealed abnormal patterns of skeletal muscle regeneration, with suppressed myogenic differentiation of myosatellite cells and their redirection towards adipogenic differentiation (Shengchen et al., 2021). MP also affect hematopoiesis by suppressing the leukocyte differentiation and altering expression of dozens of genes in the bone marrow cells (Sun et al., 2021). MP exposure has been shown to promote cognitive impairments and affect animal behavior (da Costa Araújo & Malafaia, 2021; Estrela et al., 2021), and also to cause reproductive consequences and developmental disorders in the offspring. In male mice, MP penetrated into the testes, caused damage to the epithelium of seminiferous tubules and apoptosis of spermatogenic cells, while reducing testosterone levels along with the number and motility of spermatozoa and increasing the number of dysfunctional deformed spermatozoa (Xie et al., 2020; Hou et al., 2021a; Jin et al., 2021; Li et al., 2021). In female mice, MP penetrated into the ovaries, causing granulosa cell death and fibrosis (An et al., 2021; Hou et al., 2021b). The offspring of mice that consumed plastics had reduced body weight, higher neonatal mortality rates, metabolic imbalances, and altered intestinal microflora composition (Luo et al., 2019a; Luo et al., 2019b; Park et al., 2020). MP also enhanced the toxic and pathological effects of organophosphates, antiperenes, phthalates, and sodium dextran sulfate (Deng et al., 2018; Deng et al., 2020; Deng et al., 2021; Zheng et al., 2021).

Two studies evaluated the effects of MP that entered the body through the airways by inhalation or intratracheal administration. The airborne MP significantly increased the relative mass of the heart while reducing the leukocyte and lymphocyte counts and shortening the inspiration time. The treatment also led to increased production of TGF-β and TNF-α cytokines in the lung. Moreover, the particles were effectively transmitted to the fetus: MP were detected in the placenta, as well as embryonic livers, lungs, hearts, kidneys, and brains (Fournier et al., 2020; Lim et al., 2021).

At the same time, several studies demonstrated the lack of significant effects of the MP exposure. According to Stock et al. (2019), mice treated with MP had unaltered body and organ weights and no signs of inflammation or oxidative stress in the intestine. Rafiee et al. (2018) observed the same results of neurobehavioral tests in the rats that consumed MP and control animals.

Overall, the experiments on laboratory rodents indicate effective spreading of MP, which enter the body with water, food, and/or air, to various organs and tissues. The MP particles were detected in the intestines, livers, kidneys, lungs, spleens, hearts, ovaries, and testes of the animals, causing biochemical changes, structural damage, and dysfunction. MP can effectively cross the placental barrier and interfere with offspring development. In addition, MP may absorb and carry various pollutants and enhance their negative effects (Fig. 2). However, due to the small number of available studies and significant variation in particle doses, sizes, and the exposure parameters, the data are still fragmentary and controversial. It should be noted that the doses of MP conventionally used in animal studies significantly exceed the content of MP in natural samples and consumer products.