EHP is caused by a spore forming microsporidian, Enterocytozoon hepatopenaei. The spores are how it infects. When the spore is ingested, directly by consuming infected tissues or feces or indirectly through the water, it germinates by infecting specific types of cells in the hepatopancreas and intestinal tract. It uses the metabolic machinery of the cell to make more spores until the infected cell ruptures, killing it and releasing many more spores to spread throughout the animal. Many microsporidians have intermediate hosts for some of the life stages and it has been theorized that this could help to explain why live feeds can be carrying the spores. No one has of yet identified an intermediate host.
EHP affected shrimp (A) versus non affected shrimp (B)
Despite the availability of common-sense measures to mitigate the spread of this obligate pathogen of P. vannamei (as well as others) it continues to spread. Its impact is significant and growing. We are seeing increasingly frequent reports of serious impacts. It takes an accumulation of spores in the animal for the symptoms to reach the point where the syndrome is noticeable, and the impact becomes obvious. The animals grow slowly if at all and continue to consume feed. They do not grow uniformly, and they are increasingly susceptible to opportunistic pathogens.
There appears to be a relationship between rearing densities and the severity of the disease with low density production paradigms in general experiencing less of a problem. Higher density systems have worse problems. It stands to reason that in environments where the shrimp are closely packed there is a greater potential for the spores to pass between animals.
Animals can be carrying a fairly high level of spores before they are affected. Sluggish growth and excessive feed consumption follow. While for some types of microsporidians certain drugs work to suppress them, EHP is refractory to them (as most microsporidians are). The only way to control this pathogen is to exclude it and control the levels through common sense biosecurity measures. Some of these are:
Use our proprietary blend of bacteria, tableted for ease of use, PRO 4000X, to reduce accumulated organic matter during the cycle.
Anything and everything you can do to lower the spore loads is the only path towards minimizing the impact for now. Genetic selection could give us shrimp that are refractory to spore infection or even resistant. This could turn out to be easy or impossible. Only time will tell. There are no drugs that could clear the animal of the very heavy spore loads that can accumulate. There are no quick fixes that would meet regulatory approval in the buyers’ countries. The only way to deal with this for now is to push it back as far as you can, i.e., control the levels of spores at all phases of the process and take the steps needed to ensure this.
Amplification of pathogens via broodstock through PLs onto farms has cost the shrimp farming industry tens of billions of dollars over the last three decades. For shrimp farming to have a chance at sustainability this cycle needs to be broken. Progress is being made but there is more work to do. Greater oversight of broodstock production in many countries would be a good start. Screening for pathogens needs to be comprehensive, not focused on what regulators have determined to be solely of concern. Many pathogens are missed because of this. The presence or absence of a pathogen in captive broodstock held in a biosecure environment should not be based on population sampling. Every individual brood animal needs to be tested. The technology exists today to do this economically, although it will double the price of most commercial broodstock. Given the losses and the role of pond reared broodstock in ensuring the continued increase in the incidence and severity of this disease this is easily economically viable. This is the only path that will lead to the cessation of this endless cycle of profit limiting diseases that have impacted shrimp farming since its inception.
The fact that a pathogen like EHP is thriving despite the known role of broodstock, inadequate pond preparation, infected larvae and PLs in the process is not a positive statement about shrimp farming. The industry is poorly regulated and more than likely there will continue to be dogmatic approaches with attention to myth rather than the science of what is taking place even with added oversight. EHP can be dealt with. Until there are truly resistant animals this disease is here to stay. Even in countries where there seems to be little impact at this time, ignoring the commonsense measures could eventually result in the disease becoming problematic.
There are two ways to make sure that broodstock are free of EHP spores to start with. These are screening and following the performance of the animals in the field, history. Screening is essential. RT PCR is a powerful tool, but it has serious limitations much as all population-based testing of PCR does. Primers must be specific or there will be false positives. Tissues being sampled must contain the organism of interest. Too small of a sample or targeting tissue that is not infected early in the disease process can lead to false negatives. Even if these were not issues, PCR use in population testing is a statistical exercise. You take a sample of animals and test them. Most of the animals in the population are not tested. One can never be 100% sure using this approach that EHP is not present at some level. For PLs this is of course understandable. Less so for broodstock. Following the larva and PLs from each spawn is essential. If they are PCR positive as determined by routine testing, they should be destroyed and the presence of the pathogen in the broodstock should be considered. If they are “clean” then performance on the farm will provide additional clues. If it is a serious problem, how early it occurs will give some indication of the spores loads. Soon after stocking suggests that the spore load is high to start with.
Proper animal husbandry is essential for sustainability. If the above practices were routine, the impact of WSSV and many other diseases would be reduced considerably. Perhaps what is the most important message to take home here is that by ignoring these practices, taking short cuts, hoping it will go away or desperately seeking magic bullets, the industry is guaranteeing that disease problems will be a constant in shrimp farming. Shrimp farming will not be sustainable if this is the case.
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