Sarcoptic mange affects a large range of mammal species globally, with clinical manifestations of disease generally depending on the type of hypersensitiy response experienced by the host (Type I and IV). Thus, combining studies across host species with a meta-analysis provides a fuller understanding of the clinical pathology of sarcoptic mange. We brought together both narrative reviews on the topic, which help develop proposed immunological processes/hypotheses, and empirical studies, formally examined with the use of a meta-analysis. We showed which immunological parameters changed significantly during Type I and Type IV hypersensitivity responses, as well as oxidant/antioxidant balance, APPs, erythrocytic, hepatological and nephrological changes compared to controls. Overall, our results reveal the complex nature of immune and clinical pathological changes associated with scabies/mange. As anticipated, our empirical findings broadly align with narrative reviews (see Fig. 1), although they also simplify proposed processes. Our results provide evidence of interspecific consensus in 27 immunological and clinical pathology variables across Type I (6/26 parameters) and Type IV (3/20 parameters) hypersensitivity, and in oxidant/antioxidant balance (6/10 parameters), APPs (3/6 parameters), erythrocytic (4/7 parameters) and hepatological/nephrological (5/10 parameters) changes. The only parameter showing interspecific consensus in both Type I and Type IV hypersensitivity responses was neutrophils, which showed a similar increase in effect sizes across severity and in affected compared to controls.
Type I hypersensitivity-associated changes
The immediate immune response associated with Type I hypersensitivity reactions is considered to be primarily driven by a combination of IgE, mast cells and eosinophils . Activation of these immunological variables are confirmed in our meta-analysis. Eosinophils, which primarily help promote inflammation in response to a parasitic infection, increased in scabies/mange affected hosts relative to controls and across disease severity. IgE stimulates mast cells and basophils to secrete proinflammatory reagents in response to infection [54,55,56]. Our results indicated increased levels of IgE and mast cells with mange severity, which corresponds to what is described in the literature [57, 58]. There was evidence of interspecific consensus in the response of eosinophils and IgE, including humans, domestic dogs and pigs, and humans, bulls, rabbits and domestic dogs, respectively.
Looking at the effect sizes of changes in lymphocyte numbers, an increase was observed in moderate cases compared to controls, but not in other severity categories. Importantly, results in this moderate severity category only stem from a single study involving domestic dogs. Increased lymphocytes numbers could be due to the increased pruritus and the subsequent abrasion of the skin, which enhances microbial invasion leading to the increase of white blood cells . However, both intra- and inter-specific variation was evident in mild and severe cases of mange with both non-significant decreases and increases in effect sizes being documented. This suggests variation when it comes to lymphocyte numbers in response to S. scabiei infection. A potential explaination for this variation could be linked to the shift or overlap between Th1 and Th2 responses , discussed further below.
According to narrative reviews (illustrated in Fig. 1A) of Type I hypersensitivity response to S. scabiei infection affected hosts most often exhibit a predominant Th1 response. With regards to specific lymphocytes lineages, our meta-analysis provides some evidence of significant changes in response to disease. For example, a single study showed significant decreases in CD4+ T cells in diseased domestic dogs relative to controls indicating cell differentiation from naïve CD4+ T cells to specific linages such as Th1 and/or Th2 T cells . Contrastingly, we observed significant increases in cytokines that are linked to an upregulation of the Th1 response, such as IL-2 in dogs, as well as the Th2 response, such as IL-4 in dogs and humans [59, 61, 62]. We also observed increases in effect sizes for IL-5 in dogs and humans, and IL-10 and IL-13 in dogs, which have all been linked to both Th1 and Th2 cells in humans . While there remains uncertainty about the precise role of these cytokines in non-human mammals, the interspecific consensus between humans and dogs with regards to IL-4 and IL-5 could indicate a more intricate immune response with an overlap between Th1/Th2 response in sarcoptic mange affected hosts, or a shift from one response to another depending disease severity. Another surprising result was the fact that we did not observe a significant change or increase in the effect size for IFN-y in a study involving humans which is linked to a Th1 response in humans , perhaps providing further evidence of more mixed response. In order to fully understand the complex processes occurring, further research into the associated lymphocytes lineages and relevant cytokines within and across host species is warranted.
The meta-analysis also provided evidence of other changes among white blood cell types and immunoglobulins. Overall leukocyte numbers increased significantly in severe cases of mange compared to controls, however this was derived from a single dog study. Looking at the diseased relative to control category, the overall effect size was not significant but not surprisingly all eight studies (human n = 5, dog n = 3) showed a general increase indicating interspecific consensus of leukocyte numbers. In the remaining individual white blood cell types we observed a significant increase in effect sizes of neutrophils. Effect sizes from two studies (human and domestic dog) measuring disease severity showed an increase of neutrophils, but the relationship was less clear for the infected vs. control (n = 8). Looking at the individual studies in the diseased relative to controls, neutrophils in 6/8 studies significantly increased (n = 3 dog studies, n = 3 human studies), and two studies did not (one pig study, one dog study). This could indicate a more varied neutrophil response depending on host, however it could also depend on disease severity of the infected hosts in the diseased relative to controls category.
Only one study (dog) of hosts affected with sarcoptic mange measured the change in macrophages. In the study we observed a significant increase in the effect size in mild, moderate and severe cases, as well as with increased severity, consistent in other infections causing chronic inflammation [63, 64]. Macrophage levels or activation is linked with the host’s innate immune response in regards to the Th1/Th2 pathway activation . The cytokine TGF-ß increased in diseased relative to controls (from a single study in dogs). TGF-ß plays an important role in many regulatory functions, such as wound healing and immunoregulation. The increase observed in dogs could be linked to its chemoattractant properties for macrophages, as well as other key cells at the site of inflammation . In some instances IgG antibody production might increase and take over from IgE antibodies in high infestations, as seen in R. microplus studies in cattle . This could be the reason why we observed a significant increase in IgG effect size in diseased relative to control comparisons (n = 2, both humans). Since we do not have any information regarding changes with disease severity we can only speculate. IgM, associated with the immediate immune response to microbial infection  and the activation of the complement system , increased significantly in diseased relative to controls (n = 2, both humans). Again we do not have any information with disease severity.
Type IV hypersensitivity-associated changes
Type IV hypersensitivity is characterised by the activation of T cells by antigen-presenting cells, such as macrophages and dentritic cells. This promotes cytokine secretion  by T cells as well as T cell differentiation into helper T cell CD4+ and cytotoxic T cell CD8+, which further promotes cytokine secretion and recruits other white blood cell types such as neutrophils . This is somewhat evident in our meta-analysis results, however there were generally fewer parameters available for analysis in host exhibiting Type IV hypersensitivity than Type I hypersensitivity response, which limits interpretation.
Total T cells significantly increased in a single bare-nosed wombat study. T cells can futher be divded into two types; the helper T cells and cytotoxic T cells, which we have data on in one human study (significantly increased effect sizes). The activation of macrophages is a bit harder to quantify, as we do not have a direct measure of macrophage levels in hosts exhibiting Type IV hypersensitivity available. However, MCP-1, a key chemokine for regulation of migration and infiltration of monocytes, memory T lymphocytes and natural killer cells , from a human study suggested a potential increase in monocytes/macrophages at the inflammation site. This is further suggested by an increase in monocytes in diseased relative to control comparisons (n = 7, 5 animal species), as well as in mild and severe cases (n = 2, Iberian ibex, raccoon dog).
No change in lymphocytes was observed (diseased relative to controls, n = 8; severity range, n = 4). Looking at the individual studies the explaination for the non-significant effect sizes is probably due to interspecific variation. In disease relative to control comparison studies two studies (both BNW) decreased significantly, three studies (goat, san joaquin kit fox, southern hairy-nosed wombat) increased, and the last three studies (Iberian ibex, camel, BNW) non-significantly decreased. The same pattern is evident in studies across severity with species including raccoon dog, Iberian ibex and chamois. Another suprising result is that we only observe a significant increased effect size in neutrophils in mild cases of infection (n = 2, raccoon dogs and Iberian ibex). In both moderate and severe cases, as well as in diseased relative to control comparisons we observe a mixture of increased and decreased effect sizes indicating interspecies variation.
According to narrative review papers (Additional file 1: S1), it is most common to observe a non-protective Th2 dominant response in crusted scabies/mange [13, 72]. However, it is important to note that reviews describing this occurrence have been mostly human focused with a few exceptions (red fox, chamois, ibex and Scandinavian canids, Additional file 1: S1). A review focused on scabies in humans, argued that IL-1 and IL-6 promoted the secretion of IL-17 by T cells, and the increased expression of these cytokines could be contributing to the inflammation observed in crusted scabies patients . IL-8 has been found to be secreted during infestation of S. scabiei to promote the recruitment of neutrophils to the site of infection. Only IL-17 of these exhibited significant changes in our meta-analysis, and this was in a single study on humans. IL-1 and IL-6 did not significantly differ and neither did IL-8. This could be due to the nature of the studies involved in these results. To better understand if IL-1, IL-6 and IL-8 change in hosts affected by sarcoptic mange in situ (as oppose to in vitro) experimental studies would be valuable.
For antibody studies, we detected increased effect size in IgG in both diseased relative to control comparisons (n = 4) and in severe cases (n = 3). Species covered in these results are chamois, Iberian ibex and human indicating interspecific consensus. In single species studies we also observed a decrease in IgA (Table 2) linked to the Th2 response in type IV hypersensitivity, and an increase in IgM.
In general, for Type IV hypersensitivity, we observed an overrepresentation of human only studies (e.g., all cytokines, interleukines and antibodies, except for IgG). We also observed many effect sizes could only be calculated for single studies (e.g., CD4+, CD8+, IgA, IgM, IL-17 and MCP-1). Expansion of immune investigations association with Type IV hypersensitivity to other host species would be beneficial for a general understanding of Type IV immunological response to S. scabiei. The variable results in Table 2 might also be due to the range of severity that can occur during type IV hypersensitivity responses.
The oxidative/antioxidant balance impacts the progression of inflammatory diseases including sarcoptic mange. If antioxidant defenses during inflammation are insufficient or inadequate it can lead to cell death and damage due to excessive amount of free radicals/reactive species , shifting the balance towards potential oxidative stress and if left unchecked exacterbation of disease development . Oxidant/antioxidant balance during sarcoptic mange has been investigated in several species, such as dogs, pigs, water buffalo, camels, and humans. We found evidence of an altered antioxidant defence in hosts affected by sarcoptic mange, evidenced by increased levels of lipid peroxidation (LPO), total oxidative stress (TOS) and malondialdehyde (MDA), as well as decreased levels of catalase (CAT). Vitamin C, an antioxidant , also decreased, in diseased relative to control comparisons, suggesting increased free radicals leading to further oxidative stress. This is supported by the decrease of trace minerals such as zinc in moderate and severe cases of sarcoptic mange, as well as copper which is linked to both the acute phase protein ceruloplasmin and the body’s antioxidant defence in combination with zinc and superoxide dismutase (Cu–Zn-SOD) . The decreased effect size of vitamin C in combination with decreased levels of free glutathione (GSH) could also indicate potential sepsis in mange affected hosts, as observed in critically ill patients [74, 77]. All of the significant parameters covered several species each, suggesting interspecific consensus among these.
Surprisingly, superoxide dismutase (SOD) did not provide any significant results, both in diseased relative to control and across severity, despite there being a general consensus of decreased levels of SOD in mange affected hosts (8/10 studies). The general decrease (albeit not significant) does follow the expected trend if the oxidant/antioxidant balance were shifting towards oxidative stress.
Acute phase proteins
Acute phase proteins (APPs) can have many different functions under infection, inflammation, or stress, which can be species-specific . In response to infection or inflammation the liver produces several APPs while consecutively reducing others, subsequently classifying them into two categories; negative APPs and positive APPs. Negative APPs include albumin and transferrin, whereas positive APPs include serum amyloid A (SAA), ceruloplasmin, haptoglobin and AGP . The increase observed in our results, in all but albumin, come mostly from research on Alpine ibex (Capra ibex)  and Iberian ibex (Capra pyrenaica)  with a small contribution of a pig study  in transferrin and ceruloplasmin. The increases in effect sizes of ceruloplasmin, AGP and SAA corresponds to what is expected in the host response to infection as illustrated in dogs, pigs and cattle . SAA is secreted during the acute phase of inflammation with a key role of recruiting white blood cell types to the site of inflammation . Surprisingly, we only observed a significant effect size in severe cases of mange in Iberian ibex relative to controls. This is particularly surprising as we would expect APPs to be highest in early stages of infection (mild and moderate cases) and slowly wane with chronic disease (e.g. severe cases) , however chronic expression of SAA has been observed in rats causing systemic amyloidosis  indicating that it is likely species and/or condition specific APP responses. Another unexpected result is the lack of significance in haptoglobin. Haptoglobin is known to increase several fold during an inflammatory event and the levels (e.g. high or low of haptoglobin subtypes) of haptoglobin has an important role in the disease development for other parasitic diseases in regards to exacterbated oxidative stress . Looking at the individual studies’ effect sizes the two studies with Type I hypersensitivity response (pig and alpine ibex) have significantly increased effect sizes, whereas the Iberian ibex study (involving both severity range and diseased relative to control) exhibiting Type IV hypersensitivity had non-significantly increased effect sizes. To understand if this difference is due to Type I or Type IV hypersensitivity responses or due to species variation or lastly a function of the assays used in the analysis, more research is needed.
Albumin is the most commonly studied APP in our analysis incompassing 12 different species in total. Despite its antioxidant properties that could influence oxidative status, as a negative acute phase protein its decreased levels are most-likely explained by reduced hepatic synthesis  and catabolic processes during sarcoptic mange [35, 83]. Unexpectedly, the effect size of transferrin, in diseased relative to control comparisons, showed an increase. This contradicts other studies into immune-modulated inflammation, as these found that transferrin, which is a negative APP associated with the innate immune response , typically decreases during inflammation [84, 85]. Increased levels of transferrin could be due to overload of free iron in the blood which could be indicative of anaemia . In general, more studies and species are needed in regard to acute phase protein responses to obtain a comprehensive understanding the APPs role in sarcoptic mange affected hosts.
Erythrocytic, hepatological and nephrological changes
The occurance of anaemia can have many underlying causes with for example low levels of blood cells or iron being linked to side effects of prolonged/chronic disease (e.g. anaemia of chronic disease), including increased oxidative stress due to cell death . Anaemia is usually confirmed by decreased levels of total red blood cells (RBC), haemoglobin and haematocrit [87, 88]. Our meta-analysis provided evidence of anaemia in diseased relative to control comparisons in mange affected hosts with decreasing effect sizes in all three parameters covering 10 to 12 animal species respectively, with additional evidence of anaemia in severe cases of mange with significant decreased effect sizes in haemoglobin and RBC levels.
Another factor complementing RBC results were the decreased effect sizes of iron in both diseased relative to controls and in severe cases of disease, as well as increased levels of transferrin as discussed above. Surprisingly haematocrit were only significantly decreased in mild and moderate cases of mange, not in severe. However, breaking down the individual results we do observe a general decrease of haematocrit values in severe cases of mange across all studies and species (n = 5) suggesting aneamia of chronic disease. MCV and MCHC, sometimes used to classify anaemia, exhibited variable relationships. MCHC decreased significantly in severe cases of mange which might be connected with iron-deficiency anaemia , but showed no significant results in any other categories. MCV decreased significantly in mild cases, but increased in severe cases of mange with no significant results in diseased relative to controls despite covering more studies (n = 5). These patterns could be due to variability between hosts, as anaemia was a consequence of sarcoptic mange in some hosts [19, 33, 39], but not others [32, 90], independent from the hypersensitivity reaction that the host exhibited. Although with parameters such as haemoglobin, RBC and haematocrit incompassing more studies, development of anaemia in sarcoptic mange affected hosts is a relatively certain conclusion to draw from these results.
Hepatic and renal function cannot be directly or solely quantified using the parameters reported in the studies analysed by this meta-analysis and must be assessed on an individual basis. Instead, with regards to the liver, we are able to provide potential evidence of general hepatocellular damage, evidenced by increased ALT in diseased relative to controls and increased levels of GGT in severe cases, and use proxy measures such as BUN and APPs to infer overall hepatic function. Regarding the latter, the absence of significant decreases in BUN and significant increases in positive APPs may provide a gross, albeit highly insensitive, indication of appropriate liver function. In light of these findings, it is deemed unlikely that the observed decreases in albumin are a result of hepatopathy , although the complex physiological relationship of this parameter with multiple body systems hinders accurate interpretation.
While the observed increases in BUN are not suggestive of hepatic dysfunction, in combination with increased BUN:creatinine ratio these changes may indicate a degree of renal compromise among study subjects . However, in the absence of urine analysis and with only 1–2 studies per parameter our ability to interpret these findings is also severely limited. Decreased or altered kidney function in mange is usually described in combination with post-streptoccocal glomerulonephritis (see Fig. 1A) caused by group A streptococci infections associated with secondary infections [92, 93]. Despite the presence of neutrophilia, more specific research into measures of renal function alongside microbiological culture of urine would be required to rigorously investigate the impacts of mange on the kidneys. Additionally, renal parameters (e.g., urinary urea nitrogen and creatinine ratio (UN:C)) together with organ-fat scores could provide indication of starvation or loss of appetite associated with sarcoptic mange [35, 37].