Colloidal gold immunochromatographic assay (GICA) is a new solid-phase immunolabeling technique developed on the basis of colloidal gold, immunochromatography, monoclonal antibodies, new materials, latex agglutination test, enzyme-linked immunosorbent assay and other methods . The technology was born in the early 1990s , and after more than 30 years of continuous development, the technology has been widely used in the fields of biomedicine, phytosanitary, food safety testing, agriculture and animal husbandry, heavy metal residue detection, and animal disease diagnosis . Compared with other technical fields, the application of GICA in the diagnosis of animal diseases started late but developed rapidly, and it has become one of the simplest, fastest and most sensitive detection techniques in the current diagnosis of animal diseases. Because this technology has the advantages of rapid and simple, high specificity, low cost, and intuitive result judgment, it is suitable for the majority of grassroots technicians to detect and census in large quantities, etc., and has great potential and broad prospects . In this paper, the principle, characteristics, classification and application of GICA in animal disease diagnosis are reviewed.

1 Colloidal Gold Immunochromatography

1.1 Principle of colloidal gold immunochromatographic technique

Colloidal gold immunochromatography is a new type of solid-phase immunolabeling technology that uses colloidal gold as a tracer marker to detect specific antigen or antibody. It is based on the principle of using nitrocellulose membrane (NC) as a solid-phase carrier, and the antigen or antibody labeled with colloidal gold is immobilized on the nitrocellulose membrane (NC). For detection, the sample to be tested is added to the sample pad and the sample moves forward in the direction of chromatography by capillary action. When the test substance in the sample to be tested swims to the colloidal gold binding pad, it specifically binds to the receptor (antigen or antibody) on the colloidal gold binding pad and forms an immune complex, and the immune complex is enriched or trapped in the detection line during the chromatographic process, and within a short period of time (5-10 min), the test result can be obtained visually by the color reaction of the colloidal gold .

1.2 Characteristics of colloidal gold immunochromatographic techniques

The main reason why colloidal gold immunochromatography can develop rapidly in diagnostic technology is that it has the following characteristics.

1.2.1 Low production cost

The structure of the test paper is simple, and the cost of equipment and consumables used in the production is low, which is relatively easy to manufacture and mass production.

1.2.2 Simple and fast operation

Colloidal gold immunochromatography is simple to operate, without professional instrumentation, professional personnel, and samples do not need to be processed. Under normal circumstances, the results can be obtained directly by visual inspection within 5-10 minutes.

1.2.3 Wide range of applications

Colloidal gold immunochromatography can detect a variety of samples, such as body fluids, saliva, urine, whole blood, plasma, serum, etc.; the technology can detect both antibodies and antigens, and is now widely used in the fields of biomedicine, phytosanitary, food safety testing, agriculture and animal husbandry, heavy metal residue detection, animal disease diagnosis, veterinary drug residue detection, etc.

1.2.4 High sensitivity and specificity

The technology uses high-quality monoclonal antibodies labeled colloidal gold, the lower limit of diagnosis up to 10 pg, and the non-specific adsorption effect on tissue cells is small, so its sensitivity and specificity is high.

1.2.5 Relatively stable, safe and environmentally friendly

The shelf life of colloidal gold immunochromatographic test strips is generally 12 to 24 months, usually without refrigeration. The reagents are relatively stable, and the test results can be stored at room temperature for a long time; there is no involvement of harmful substances such as o-phenylenediamine, which will not affect the health of the operator or pollute the environment.

1.2.6 Economical and practical, easy to transport

The test strips require less reagents and samples, including samples as low as 1-2 mL, and are small in size, making them easy to transport.

1.3 Classification of colloidal gold immunochromatographic techniques

Colloidal gold immunochromatography can be classified into indirect method, sandwich method and competitive method according to its principle. Among them, the sandwich method is divided into double antigen sandwich method and double antibody sandwich method. Indirect method is mostly used for the detection of antibodies, commonly used in the detection of corresponding serum antibodies to viruses or bacteria; Competition method is mostly used for the detection of small molecule antigens containing only one immune-binding site, such as morphine; Double-antibody sandwich method is mostly used for the detection of multi-epitope antigens, which is often applied to the detection of viruses; Double-antigen sandwich method is mostly used for the detection of corresponding antibodies to pathogens.

2 Application in the diagnosis of animal diseases

2.1 Application in the diagnosis of animal viral diseases

At present, according to relevant surveys and analyses, viral diseases are found to be the main cause of epidemics in animals in the animal husbandry industry . In the prevention and control of epidemics, accurate, rapid, simple and safe colloidal gold immunochromatography technology is particularly important, the technology has also been widely used in the diagnosis of viral diseases in animals.

Ding Haojie et al  developed colloidal gold immunochromatographic test strips for the detection of PCV2-dCap antibody using recombinant capsid protein (dCap) of porcine circovirus type 2 (PCV2). The results showed that the method has good specificity, sensitivity, stability and reproducibility. The results showed a positive compliance rate of 90.5%, a negative compliance rate of 100%, and an overall compliance rate of 91% when compared with ELISA. Yu  et al. established a colloidal gold immunochromatographic test strip for the rapid detection of duck tambucus virus (DTMUV) by using an antibody specific for the E protein of DTMUV, and tested 50 clinical samples, which was in good agreement with RT-PCR. The compliance rate with RT-PCR was 93.9%. Cheng Tingting  et al. established a colloidal gold immunochromatographic test strip for the rapid detection of antibodies against Brucella abortus by using recombinant proteins of OMP31 and BP26 as detection antigens. The test strip had no cross-reactivity with other unrelated diseases and high specificity. The test strip was tested to have a 92% compliance rate with the amber plate test method, indicating that it has good value for use and can be popularized for use at the grassroots level. Li Xin et al used double antibody sandwich method to establish the detection of foot and mouth disease O, A, Asia1 type virus colloidal gold immunochromatographic quantitative test card, which is not only in the O, A, Asia1 serotypes of foot and mouth disease virus without cross-reactivity, and other non-foot and mouth disease virus also has no cross-reactivity, and can be applied to the rapid identification of foot and mouth disease virus antigens, quantitative detection of 146S and serotyping.

2.2 Application in the diagnosis of animal bacterial diseases

In recent years, animal bacterial diseases have become more frequent in some areas of China, which to a certain extent has seriously affected the development of the breeding industry. Colloidal gold immunochromatography has played a great role in the diagnosis, monitoring and detection of bacterial diseases such as brucellosis.

Based on BP26 protein, Fifi Xu et al  developed a gold standard test strip for detection of Brucella abortus antibody in sheep by encapsulating BP26 protein in the detection line (T) and rabbit BRU-BP26 antibody in the quality control line (C). The specificity and sensitivity of the method were 94.44% and 90.62%, respectively, and the overall compliance rate with the tube agglutination test was 92.69%. In addition, there was no cross-reactivity with six bacteria, including Escherichia coli O:157, indicating that the test strip is an ideal method for the diagnosis of brucellosis in sheep. Zhao Yue et al  established a colloidal method for the detection of antibodies against Mycobacterium bovis paratuberculosis using the recombinant epitope protein map0862-2154c as antigen. The results of serum detection in clinical samples using this method showed that its sensitivity was 91.86%, specificity was 94.23%, and the compliance rate was 93.38%, which provided a simple, highly sensitive, and highly specific means of detecting serum antibodies to bovine paratuberculosis. Zhao Xiao et al  developed a colloidal gold immunochromatographic test strip for Mycoplasma pneumoniae antibody in pigs that can simultaneously detect Mhp serum antibody and mucosal antibody. The test results were obtained in 10 min, and the compliance rate with the commercial Mhp ELISA kit was 96.1%, which provided a rapid detection technique for the diagnosis of Mycoplasma pneumoniae in pigs.

2.3 Application in the diagnosis of parasitic diseases

Compared with other diseases, the application of colloidal gold immunochromatography in the diagnosis of parasitic diseases is relatively few, but there are more research results in this area.

Ding Haojie et al established a colloidal gold test strip detection method based on anti-Schistosoma japonicum egg antigen nano-antibody by labeling VHH-48 and VHH-52 antibody proteins with colloidal gold and preparing a gold-labeled pad, and then using soluble egg antigen (SEA) of Schistosoma japonicum as a detection line encapsulant and sheep anti-mouse IgG as a quality control line encapsulant. The results showed that the specificity and sensitivity of the method were good. Kou Jinhua et al  prepared colloidal gold test strips for immunization of porcine Toxoplasma gondii serum circulating antigen using colloidal gold-labeled anti-Toxoplasma gondii surface antigen SAG3 monoclonal antibody. The test strips had a sensitivity of up to 1:160, and in terms of specificity, there was no cross-reactivity with porcine Toxoplasma gondii positive sera. The test strips can be stored for more than 1 month at room temperature and have good stability. Yang Sheng et al  established a method for the detection of antibodies to Neospora caninum-infected bovine sera. The test strip was prepared with rabbit anti-SPA encapsulation as the quality control line and Neospora caninum P40 protein encapsulation as the detection line. The test strip was tested to be specific, sensitive and suitable for monitoring Neospora caninum infection in cattle.

3 Summary

To date, colloidal gold immunochromatography has been one of the most successful commercial POCT products due to its many advantages, and has been a hot topic of research and application in many fields. As a result, it is constantly being innovated and optimized. Currently, colloidal gold immunochromatography is relatively mature and has a broad development prospect. However, it still has some shortcomings, such as the quality control system is not strict enough; compared with other techniques, the sensitivity is low, which is easy to lead to false positives; it is difficult to realize quantitative detection; and it can not realize the simultaneous detection of antigen and antibody, and so on. Therefore, colloidal gold immunochromatography should fully utilize its own advantages, and develop towards the realization of high-throughput detection, improvement of sensitivity, quantitative detection, and simultaneous detection of antigens and antibodies, so as to give full play to the maximum value of this technology, and continue to escort the development of China's aquaculture industry.