In our Animal Breeding series, we introduce new information about mouse breeding and health every week to provide education for aspiring mouse care experts. As most researchers know, the health of mice may affect the accuracy and reproducibility of experimental data when conducting experiments. However, there are many infectious organisms that may greatly impact the health of mice, which must be routinely tested for.  Today, we will introduce you to what should be done if the test result is positive for an infectious agent or pathogen.

 

Confirm and Analyze Positive Results

Why should we verify positive results?

Even with the best detection methods, false positive results can occur, either due to the method itself or because of user error. For example, ELISA method is more prone to false positives;

Verifying the positive result is especially critical in cases where the animals are held in distinct areas from the testing site, as the test results of different testing institutions may be different.

How do I effectively verify positive results?

Communicate with your testing institution. When the testing agency tells you a positive result, confirm with them what method was used to verify it.

Verify using multiple methods. For example, use both Enzyme-linked Immunosorbent Assay (ELISA) and Indirect Fluorescent Antibody (IFA) assay to verify the same sample. When the test results by the two detection methods are consistent, the results are verified as more credible. If the inspection agency uses the same method twice to verify detection, such a detection process is not credible.

It is recommended to send to different testing institutions for verification. Consistent test results from different testing agencies is another way to verify.

 

Confirm the Scope of Infection

After confirming that it is true that the mice are infected with the pathogen, we must rationally accept the facts and analyze the situation. Ideally, all information on animal import and screening processes, including pathogen screening, are regularly collected and maintained in an organized manner.  In any case, we have included tips for organizing pathogen data in the steps (below) to analyzing pathogen transmission:

Analysis of pathogen transmission routes

  • Collect and integrate relevant pathogen data, and find the infection route(s) according to the type of pathogen present.
  • Review all information, beginning with more recent data based on your pathogen testing schedule. List all the possibilities for the pathogen’s introduction to the facility and go through them one by one. It is recommended to refer to authoritative, primary sources of information to confirm the potential introduction event.
  • Pathogens may be transmitted in a number of ways: the fecal-oral route (ingestion), due to the lax operation of personnel (direct contact),  via aerosols (airborne), environmental contamination (fomite transmission), and some pathogens have a combination of transmission methods.

Based on the transmission route(s), analyze and infer the spread of the pathogen within the facility.

Speculate & Verify

  • The risk of infection throughout the facility needs to be considered with highly infectious pathogens, such as mouse norovirus, mouse hepatitis virus, ectoparasites, etc. 
  • Infection with low-infectivity pathogens, such as Helicobacter, may be limited to a few cages in a single rack in strictly managed facilities.
  • After estimating the spread of pathogens, different sampling ratios should be designed for specific areas to be sampled for inspection. After all, these transmission estimations need to be verified.

Results reassessed

Through the test results, the infection scope of the pathogen in the facility is reassessed. We won't be able to confirm the scope of infection until we get the test results.The facility is then divided into three areas:

  • Low-risk areas - Currently no positives detected, but require ongoing attention.
  • Medium-risk area - No positive test has been detected, but there is a risk of infection, so it is necessary to strengthen testing and continue to pay attention.
  • High-risk areas - Those that have tested positive and require further evaluation for treatment options.

 

Find the Source of Infection

Sort out all processes

Think and analyze all processes that may be relevant to the potential method(s) of transmission. Mind maps can be used to assist in sorting out thoughts in this case. 

Analyze positive samples

We need to analyze all positive samples and analyze whether there is any pattern between these positive samples. Sometimes, there is a connection between the breeding room, operator, and cage of these positive samples; or they may have no discernable relationships. This relationship between positive samples is also critical for analyzing the source of infection of the samples. 

Investigate the cause from the inside out

All areas must be checked: the source of the animal, the material contacted, the entry method of the items used, the operation of the personnel, the disinfection effect, etc., all need to be checked. After all these key factors have been checked, if the cause has not been found, you can expand the scope of checking, you can check warehouses, suppliers, testing agencies, etc. Look for every factor you can think of until you find the source of the infection.

Case study

A few years ago, Cyagen's animal samples were sent to testing institutions for Certificate of Analysis (COA) testing, where the PCR method detected beta-hemolytic streptococcus. Later, we traced that this bacteria was introduced from the fish meal in the feed. Since the feed is irradiated and the aseptic test of the feed is negative, there are only dead bacteria residues in the feed that will be excreted with the feces and have no physiological effect on animals. This reminds us that when a positive result is detected, it is not necessarily the animal itself that is infected.

 

Different Strategies for Positive Pathogens

There is no uniform standard for formulating treatment plans based on the impact of pathogens on facilities, animals, and experiments. Some general situations are provided below as guidance.

No need to clear

You know the existence of this pathogen and have contained the situation. After evaluation, the impact on animals is limited, and it is unnecessary to remove it or the cost of removal is too high, so the removal can be abandoned. The previously mentioned Proteus mirabilis is one example of this.

Test and cull

After extensive screening, all positive animals detected were eliminated. This requires the ability to self-evaluate the presence of this pathogen. Otherwise, it is very expensive to send for external testing. It also requires the ability to control pathogens within a certain degree. Otherwise, such as if the personnel operations are not standardized, the pathogens will be transmitted by personnel as a medium, which will cause serious losses.

Phase out and regroup

After evaluation, the pathogen has limited impact on the population and animal experiments. Considering the comprehensive reasons, the population may not be eliminated immediately. Make a phase-out plan for the infected population.

Emergency elimination, infection control, regrouping

After assessment, if the pathogen has a strong impact on the population (e.g., is highly infectious, or affects other animal populations in the facility) then we need to consider immediate elimination and re-establish the population. For example, the mouse hepatitis virus must be eliminated as soon as possible for production facilities and affected mouse lines must be re-established.

 

Ways to Clear Pathogens

Purification via In Vitro Fertilization (IVF)-Embryo Transfer (ET)

In Vitro Fertilization (IVF) is the most reliable purification method at present, but there are individual pathogens that cannot be removed by IVF purification. Examples include lymphocytic choriomeningitis virus (LCMV), polyoma virus (POLY) and mouse kidney parvovirus (MKPV). However, the relative prevalence of the first two viruses is extremely low. MKPV is a newly discovered virus, and some institutions have yet to pay attention. Monitoring using the sentinel animal method, which is exposed to dirty litter from a sample set representative of the entire facility,  may be considered.

Cesarean section purification

A cesarean section is a classic form of purification, but its reliability is slightly less than IVF. Some data show that it can effectively remove mouse parvovirus, Helicobacter pylori, and murine norovirus (MNV). In order to ensure the purification effect, two rounds of cesarean section purification may need to be considered.

Cross breastfeeding

Studies have shown that milk replacer can effectively eliminate a variety of common mouse pathogens, including MPV, MHV, MNV and Helicobacter pylori. Compared to caesarean section, it has the advantage of being less expensive and not requiring the euthanasia of valuable dams.

Test and cull

Studies have shown that this method can effectively remove Helicobacter pylori, MKPV, MHV, MNV, and mouse rotavirus. However, this method requires relatively high detection capabilities, facility management capabilities, and personnel operations.

Drug treatment

Drug treatment is unreliable and only applicable to individual pathogens. It can be considered for rare strains that are particularly valuable, but is not suitable for the treatment of large populations of animals.

Burn-out

“Burn out" in the population is a process that takes about 15 weeks. It has historically been successful, but is currently not recommended.Upon cessation of breeding and newly introducing animals, the virus is allowed to fully circulate in the existing immunocompetent animals, infect all animals, and clear the virus by the animal's immune system, so that the virus is in the existing animals. Because of this model, it is very difficult to replicate in our current facility and in the animal population we are raising.

 

 

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