Bacteria Lactococcus, D illustration

Reanalysis shows true identity of bacteria involved in lactococcosis cases

The bacterium Lactococcus petauri has played a more significant role in historical cases of the disease lactococcosis than was previously thought, new research suggests.

Lactococcosis affects numerous fish species but causes particular problems among rainbow trout, and high mortalities have been recorded at some aquaculture facilities.

Until recently, the disease had been predominantly associated with Lactococcus garvieae. However, since the isolation and description of L. petauri in 2017, scientists have acknowledged its key role in the development of clinical signs and mortality in many cases.

Now, a team of scientists led by Ana Isabel Vela at Universidad Complutense in Madrid have reanalyzed isolates from 48 cases thought to be L. garvieae, sampled between 1991 and 2019, largely from Mediterranean countries where lactococcosis is considered an endemic disease but also the US, Israel and others.1

Majority of historical identifications incorrect

Using the most up-to-date molecular methods, they found that  60.4% of the isolates were in fact L. petauri. Furthermore, they observed that L. petauri appears to be restricted to the Mediterranean and the US, whereas elsewhere in the world L. garvieae seems to dominate.

“These results confirm the implication of both species in the etiology of lactococcosis and suggest that L. petauri plays a significant role in the epidemiology of this disease,” the researchers wrote in the journal Aquaculture.

“Some of the isolates identified as L. petauri in the present study were isolated three decades ago, indicating that its association with lactococcosis is older than might be expected from the recent descriptions.”

Findings pose prevention questions

The team also observed changes over time. In Spain, L. petauri appears to have “replaced” L. garvieae in cases of lactococcosis around the turn of the century, they said, while the same may have happened in Turkey some years later.

With vaccination used widely in the aquaculture industry to help prevent serious economic and welfare impacts from diseases, the scientists underlined a need for further work to find out whether available commercial vaccines against lactococcosis can provide cross-protection against both bacterial species. There are also implications for the development of new products to prevent and treat the disease.

“In this sense, the correct discrimination between L. garvieae and L. petauri is a crucial issue,” they added.

You can read the full research report in the journal Aquaculture.




1 Vela AI, del Mar Blanco M, Colussi S, Kotzamanidis C, Prearo M, Altinok I, Acutis PL, Volpatti D, Alba P, Feltrin F. The association of Lactococcus petauri with lactococcosis is older than expected. Aquaculture. 2023;578(16):740057.