To date, despite advances in the identification of compounds with antiviral activity, the available repertoire of antiviral drugs is insufficient. Given the number of endemic viruses that impact the health of wild and domestic animals as well as human populations the need for effective and safe antivirals is urgent. The COVID19 pandemic has evidenced this need; since its start in January 2020, a steep rise in cases and associated mortality, mounting pressure on health services worldwide, and a marked damage to the economy of most countries [1, 2] warrants rigorous, unbiased investigation of compounds with potential antiviral action.
The surge in COVID19 cases and the lack of effective treatments have led to a growing number of informal reports and testimonials across social networks regarding the oral and intravenous use of chlorine dioxide (ClO2) solution as an efficacious treatment of COVID19. In an unprecedented move, the Senate of Bolivia authorized the extraordinary use of ClO2 to "manufacture, sell, supply and use ClO2 to prevent and treat COVID19" (law PL N° 219/2019-2020 CS approved 15 July 2020). This situation has sparked concern from health agencies of various countries regarding the potential toxicity of using this disinfectant as a treatment, given that ClO2 has strong oxidative properties. Chlorine dioxide has been known since 1946 for its broad antimicrobial properties [3] , and it is commonly used to purify water for human consumption, decontaminate produce and other food items [4] , and disinfect surgical instruments [5] . According to the Food and Drug Administration (FDA) of the US, consuming ClO2 can lead to adverse effects such as 'vomiting, diarrhoea, dehydration, abdominal pain, metahemaglobinemia and systemic failures that could potentially lead to death'. However, an impartial review of the scientific peer-reviewed literature reveals few case reports of human patients presenting adverse effects after consumption of ClO2, none of which were fatal, and reversed completely after treatment [6] [7] [8] [9] . Published studies on ClO2 toxicity have reported null to mild effects in adult humans and other animals [10] [11] [12] and have stressed that toxicity is strongly dose-dependent [12] [13] [14] .
There is evidence of virucidal activity of ClO2 against echovirus [15] , enterovirus [16] , poliovirus [17] , rotavirus [18] , norovirus [19, 20] , calicivirus [19] , and coronavirus [21] . However, there are few publications that explore antiviral effects in vitro and in vivo. One of the few published studies found that mice exposed to influenza A virus in an environment that contained aerosolized ClO2 had significantly lower mortality than mice that were solely exposed to Influenza A virus [22] . The authors reported that the antiviral effect was due to denaturation of hemagglutinin and neuraminidase glycoproteins, a finding that concurs with previous studies that explain the virucidal mechanism of action of ClO2 due to oxidation of amino acid residues that are key for cell entry [23] [24] [25] . More recently, the antiviral mechanism of action of ClO2 was investigated in vitro using pig alveolar macrophages and MARC-145 cells exposed to the porcine reproductive and respiratory syndrome virus (PRRSV1), finding that the chemical also inhibits the synthesis of proinflammatory molecules that contribute to the pathogenesis of this disease [26] . The authors concluded that viral synthesis of RNA and proteins was impeded by ClO2, leading to a reduction in viral replication. Despite these studies, to date there has been no published report of an in vivo assessment of the antiviral effect of ClO2. Here, we have investigated the effect of a ClO2 solution in 10-day old chick embryos inoculated with attenuated avian infectious bronchitis coronavirus (IBV) Massachusetts and Connecticut vaccine strains, which are pathogenic for birds during their embryonic stages of development [27] .
Avian coronavirus RNA was detected in all of the embryos that were inoculated with the vaccine (n=15, in all cases Cq <40). The viral load decreased significantly with increasing ClO2 (p=0.009 for linear trend; Fig. 1 ), doses being 2.4 times higher in the untreated embryos. The average viral load of ClO2-treated chicks was 10 4.3 /mL, range: 10 3.66 -10 5.03 and of untreated chicks was 10 4.83 /mL, range: 10 4.52 -10 5.01 , respectively (Tukey HSD, Group E vs. F, p = 0.03). There were no differences in the viral load between both ClO2-treated groups (p>0.05).
Mortality differed significantly between virus-inoculated (46.7%) and virus-free embryos (6.7%) (Fisher's exact p = 0.018), (Fig. 2 ). In the viral-inoculated groups, 3/10 (30 %) embryos treated with Table 2 ).
Microscopic lesions compatible with avian IBV infection were observed in various organs in all virus-inoculated groups (Fig. 4) . The severity of the lesions was either similar or slightly lower in the embryos that had been treated with ClO2 than in the embryos that did not receive any treatment (Table 3) . Two exceptions were the kidneys and the duodenum. In the kidneys, swelling and degeneration of renal tubular epithelium was more common and more severe in the infected chicks that were administered ClO2 than in the infected embryos that did not receive ClO2, (Table 3 ).
There were no observable alterations to the architecture and integrity of the tissues of noninfected embryos that were administered ClO2 (Experimental groups B and C), nor was any difference in the area (µm 2 ) of the thyroid (p>0.1). ClO2-treated groups showed a slightly higher myeloid to erythroid ratio in the bone marrow than the experimental control group and the viral control group, although the difference was not statistically significant (ANOVA; F5,15 = 2.33, p = 0.094).
We have investigated the antiviral effect of chlorine dioxide (ClO2) using avian infectious bronchitis coronavirus (IBV)-infected chick embryos as models. At the concentrations used (30 ppm and 300 ppm), both below the reported NOAEL [13] , we observed a reduction in viral titre in infected embryos that were treated with ClO2. Furthermore, mortality decreased substantially in the ClO2treated embryos, although alterations to chick development were prevalent regardless of treatment.
Virulent avian IBV strains typically have a burst size of 10 to 100 infective units per cell [28] , which can reach peak virus titres of 10 6.5 -10 8.5 TCID50 after 36 hours [29] . The vaccine strains used here have lower replication efficiencies as they are attenuated [30] , but they are capable of replicating and causing damage in chick embryos [27] . With 2,000 infective units (200 µl of 10 4 infective units/mL) inoculated into each embryo, the ClO2 treatments decreased viral load 2.4-fold compared to the infected non-treated embryos, representing an average difference of 42,711 infective units. This result could be explained by two non-mutually exclusive mechanisms. Firstly, direct destruction or neutralization of the virions exposed to ClO2 could have occurred by denaturing of envelope glycoproteins following oxidation of amino acid residues [22, 23, 25] .
Secondly, viral replication efficiency could have decreased due to ClO2-induced biochemical changes in the extra-or intracellular milieu, impeding the synthesis of viral RNA and proteins [26, 31] .
The effects of reduced viral titres in the infected embryos were evident, with a 50 to 75% reduction in mortality in the treated embryos treated with 300 ppm and 30 ppm of ClO2, respectively. However, developmental abnormalities were observed in the majority of the infected embryos, including those that received ClO2 treatment. Namely, dwarfing, assessed by body mass, axis length, and femur length were significantly lower in all infected embryos, as expected to occur in IBV infections [32] . In contrast, curling [33, 34] was virtually absent in the embryos that received ClO2 after infection. Lesions associated con IBV infection [35, 36] were observed in most of the infected embryos, including those that were administered ClO2. However, for most of these lesions, severity was lower or similar in the ClO2-treated groups. One exception was in the kidneys, where nephrosis was more severe and frequent in the IBV-infected embryos treated with ClO2.
Nephrosis is unlikely to have occurred due to ClO2 treatment per se, given that none of the uninfected embryos that were administered ClO2 showed kidney abnormalities. Similarly, atrophy of the duodenal villi was highest in the inoculated group that was administered a high dose of ClO2, but absent in the non-infected groups that only received ClO2. If duodenal atrophy and nephropathogenicity in IBV-infected embryos are mitigated by inflammatory responses [37] , it is possible that the observed damage is due to ClO2-driven downregulation of acute inflammation that allowed virion replication in the tubular epithelium and in the duodenal villi. This scenario could be plausible if we consider that ClO2-treated pig alveolar macrophages and African green monkey kidney cells infected with the porcine reproductive and respiratory syndrome virus (PRRSV1) downregulate pro-inflammatory cytokines IL-1, IL-6 and TNF-α [26] . There are two drugs, based on chlorite, whose describe mechanism of action might be similar to that elicited by ClO2.
One is the drug NP001 (a pH-adjusted stabilized form of NaClO2) exerts strong anti-inflammatory responses by inhibiting macrophage activation in humans, even after a single dose [38] . In turn, WF10 (tetrachlorodecaoxyde, Cl4H2O11), an oxidizing agent that contains 63.0 mmol/L chlorite and 56.4 mmol/L chloride [39] induces apoptosis of inflammatory cells and downregulates proinflammatory genes [40, 41] . More studies will be needed to establish whether the effect of ClO2 on IBV-infected chicks is due to similar mechanisms. Interestingly, the affected renal tubules of the chick embryos had mitotic cells, suggestive of regeneration as a reparative response to damage of the renal tubular epithelium [42] [43] [44] . This possibility will need to be explored further in the animal model used in our study. Unfortunately, knowledge of IBV pathogenesis, immune responses and tissue reparation in the embryonated egg is limited. In hatched birds, IBV can impact lymphocyte populations by inducing apoptosis, thus impeding virus clearance [45] . We found some evidence of this effect, as the virus-inoculated embryos had a reduced percent of bursal lymphoid tissue.
It is plausible that even though ClO2 limited viral replication, given that the embryos were only administered a single dose of ClO2 solution, not all virions were eliminated. The viruses that remained viable after administration of the ClO2 were able to replicate, yielding lower viral titres, and the damage that they caused to the embryos was less severe and ultimately led to less mortality than in the untreated infected embryos. Repeated administrations of ClO2 solution might have reduced the viral load further, plausibly leading to even less virus-induced damage, a possibility that we could not explore in this model. However, an important result of our study was the lack of evidence of tissue damage caused by ClO2 itself. Only one (20%) of the uninfected embryos treated with the high dose of ClO2 died, and this difference was not statistically significant to the experimental control. It is possible that the death was due to other causes, as a 20% embryo mortality rate is considered normal in aviculture [46, 47] .
Chlorine dioxide had an unequivocal antiviral effect in the model system used in this study, with a reduction in viral RNA, virus-induced lesions and mortality and no evidence of toxicity at the doses used herein. Further studies should aim to test the antiviral effect of ClO2 in hatched chicks, which have a more mature immune system, and where repeated administrations are easier to procure. However, it is promising that we found an evident effect against IBV without any significant adverse effect or evidence of toxicity to the chick embryos. Much research needs to be done before it is possible to generalize and extrapolate our findings to other viruses and animal hosts.
We aimed to examine whether chlorine dioxide elicits a measurable antiviral effect in vivo, and investigate toxicity associated with the administration of this chemical compound. For this, we The solution was kept at 4 °C and was used within 5 days of production. Two working solutions were prepared with sterile 0.9% saline solution: a high dose (HD, 300 ppm) and a low dose (LD, 30 ppm). Final concentration was verified using test strips (LaMotte Insta-Test 3002) which allowed qualitative detection between 0 and 500 ppm.
We used 30 10-day old SPF RossxRoss chick embryos (Pilgrims, Mexico) incubated at 38°C and 65-70% humidity [48] . Five chick embryos were assigned to each of the six experimental groups at random and treated according to their group (Table 1) . Total sample size in the experimental group was determined in order to achieve power 80% (type II error: 0.20) with allocation ratio 2/1, effect size of 0.7 and a significance level < 0.05. Embryos were placed at random in the incubator to avoid potential variations in temperature and humidity between the edges and the centre of the incubator. All embryos were inoculated via the chorioallanotic sac, as recommended for avian coronavirus replication [49, 50] . Details on inoculation can be seen as Supplementary Materials.
Seven days after inoculation, embryos were sacrificed according to guidelines on humane treatment and euthanasia of chick embryos over 13 days of age (AVMA 2020). The embryos were placed at 4 °C for 4 h to ensure blood coagulation and avoid contamination of the allantoic fluid [48] . The eggshell over the air chamber was disinfected with 70% ethanol and 3.5% iodine [48] and removed together with the chorioallantoic membrane. Allanotic fluid (750 µl) was collected and stored at −70 °C.
Each embryo was cleaned with distilled water to remove any remaining albumin, vitellus and amniotic fluid. Body mass was determined with a precision scale (±0.1 mg). Embryonic axis length
was measured with digital callipers (± 0.1 cm). The body was examined for macroscopic lesions typically caused by avian coronavirus, such as cutaneous haemorrhages, stunting, curving, urate deposits in the kidney and feather alterations [49, 51] . Chick embryos were considered to have died during the experiment when there was evidence of disconnected or detached blood vessels of the chorioallantoic membrane or their organs exhibited imbibition of haemoglobin. Further evidence of mortality was determined by examining the presence of autolysis at microscopic examination.
Both femurs were dissected and their length was determined and averaged with a digital calliper as a surrogate measure of chick development. Samples of all organs and tissues were collected, including those known to be affected by the Massachusetts and Connecticut strains of avian coronavirus, namely trachea, lung, proventricle, duodenum, liver, kidneys and the Bursa of Fabricius [27, 49, 52, 53] , and those reportedly affected by exposure to ClO2 [13, 54, 55] .
Samples were cut longitudinally, immersed in 10% buffered formalin (pH 7.4), paraffin-embedded and sectioned at 3 µm, stained with haematoxylin-eosin and observed under the microscope at 40X and 100X. To assess haematopoietic status, bone marrow preparations were used for a 200cell differential count to classify the marrow precursors and to determine the myeloid:erythroid (M/E) ratio for each embryo. This ratio was obtained by dividing the sum of all the granulocytic cells by the sum of all the erythrocytic cells [56] . 
The authors declare they do not have any conflict of interest regarding this submission. Table 1 . Table 1 . Table 1 . 