Can you say Piscirickettsiosis?

BLOOMINGTON, MN - Piscirickettsiosis is the kind of unpronounceable scientific name that even I don’t enjoy saying. But, at the latest American Fisheries Society-Fish Health Section (AFS-FHS) meeting, Dr. Michael Mauel repeated this fish disease name enough times and with so many great photographs that I felt you should hear it too.

Piscirickettsia-like bacteria are being recognized with increasing frequency in fish from aquaculture facilities around the world. Mortalities have occurred in white seabass, black seabass, tilapia, grouper, hybrid striped bass, and blue-eyed plecostomus since Piscirickettsia salmonis was described in salmonids in 1989 by Dr. John Fryer.

Tilapia morts in a Hawaiian fish pond, the result of a PLO outbreak in the mid-1990s. Losses exceeded 60 percent on some Oahu farms.
When the disease was first identified in Chile, Coho salmon, many near market-size, were the species suffering severe mortalities. The following year, Atlantic salmon losses from piscirickettsiosis were seen on some farms. Other salmonid species have become infected since then and the disease has been detected in salmon farming countries like Canada and Norway, though the outbeaks have never been as severe.

When fish health researchers around the globe started to look for similar Piscirickettsia-like organisms (PLO), it did not take long for them to find some. In 1992, PLOs were identified as the causative agent in a mass mortality among pond-reared tilapia in Taiwan. Also, PLO-related mortalities in juvenile European sea bass grown in sea cages have been reported along the French Mediterranean coast.

PLO problems

Another example of PLO killing tilapia occurred in 1994, when wild and farmed tilapia on Oahu, HI began to die of an unknown disease that was similar, but not identical, to piscirickettsiosis in salmonids – the disease Dr. Mauel had studied while at Oregon State University.

Only tilapia were affected and they were often pale and emaciated. The diseased fish swam erratically and had trouble staying at depth. Scattered skin hemorrhages and bulging eyes were also noted. Losses exceeded 60 percent on some Oahu farms.

Starting with the initial outbreaks in Chilean salmon and the subsequent cases in Hawaii, Dr. Mauel has continued to follow these fastidious intracellular organisms as they have emerged as a problem in many different species of fish from a variety of different aquatic environment

Two views showing normal vs. PLO-infected spleens in tilapia. Top, a healthy, normal spleen (see arrow) contrasted with the enlarged spleen dotted with white nodules (arrow) at below. The nodules are the fish’s response to the bacterial infection.This spread of piscirickettsiosis has been the focus of Dr. Mauel’s research at the University of Georgia and, most recently, as an assistant professor at the Mississippi State University (MSU) College of Veterinary Medicine and the Delta Research and Extension Center in Mississippi. In the past few years, he has examined tissues of tilapia from facilities in Jamaica, Indonesia, Florida, and southern California that were experiencing Piscirickettsia-like epizootics.

But at the AFS-FHS meeting in late July, Dr. Mauel focused on the new outbreaks in tilapia. He traveled from MSU to Minnesota to describe to his fellow fish disease workers his latest efforts in characterizing, say it with me now, “piscirickettsiosis.”

Tilapia mortalities

Tilapia is now the second most popular non-carp farmed fish family in the world after salmonids, with global production over 1.5 million metric tons in 2003. Today, the vast majority of world supply is still consumed in the producing countries of Asia and Latin America.

At the meeting, Dr. Mauel described mortalities at some Central American tilapia farms that approached 75 percent with average losses closer to 30 percent. The disease was associated with stress due to rapid temperature changes and poor water conditions.

In addition to the signs and lesions listed above, Dr. Mauel also observed infected tilapia acting lethargic and he usually found necrotic gills on these specimens with a patchy white and red appearance. But in some cases, the tilapia were found dead from piscirickettsiosis without any external signs.

When he cut these sick fish open, the swollen spleens appeared rough and granular instead of smooth, and the kidneys had irregular white nodules throughout.

These nodules are the fish’s response to the bacterial infection. They are similar to the granulomas you see in striped bass with mycobacterial infections. Inside the nodules are the bacteria surrounded by the inflammatory cells that the fish use to try to fight the disease.

These granulomas can be seen in the gills, muscle, and gonads, as well as in the spleen and kidney, but rarely in the liver of infected tilapia. The presence of these granulomas in the fillets has led to increased processing costs.

Above, gross and microscopic lesions in tilapia muscle. PLO-infected fish may exhibit lethargic behavior and, on closer inspection, reveal skin lesions and necrotic gills. But in some cases, tilapia have been found dead from piscirickettsiosis without any external signs.When blood smears are examined under the microscope, you sometimes see moderate to large numbers of Gram-negative, pleomorphic (capable of assuming different shapes), intracellular bacteria in the circulating white blood cells.

Piscirickettsia-like organisms are members of a group of microorganisms that, like viruses, require other living cells for growth but, like bacteria, use oxygen, have metabolic enzymes and cell walls, and are susceptible to antibiotics.

Disease control

Dr. Mauel and others believe piscirickettsiosis and syndromes caused by similar bacteria are an emerging disease complex that will increasingly inhibit fish production. These pesky agents can be transmitted horizontally by cohabitation (i.e. by living in the same tank together). Cold stress induces the disease in juvenile tilapia at farms where the PLO are endemic.

The oxytetracycline (OTC) trial conducted in Hawaii and published in the Diseases of Aquatic Organisms in 2003 indicated that feeding tilapia OTC-medicated feed can dramatically improve survival of the fish when the tilapia are exposed to PLO. This is especially true if the treatment is started prior to the onset of mortality.

It should be pointed out that no drugs have been approved for the treatment of piscirickettsial infections of fish in the US.

Dr. Mauel and his co-authors also showed in this 2003 manuscript that these PLOs are clearly not the same species as the salmonid isolate, Piscirickettsia salmonis.

Intervet’s Aquatic Animal Health Newsletter from November 2004 (www.intervet.com/AAH/bin/AAHnews9.pdf) has some good information on how the fish farming industry is fighting piscirickettsiosis.

In the management and prevention section, the newsletter describes in vitro experiments that have shown P. salmonis is sensitive to a variety of antibiotics.

An electron microscope image of a Piscirickettsia-like organism (PLO) in tilapia.
The treatments of choice for rickettsial infections in Chile are oxolinic acid and flumequine. However, the use of medicated feed to control intracellular pathogens, including P. salmonis, has been largely unsuccessful, possibly because antibiotic levels may not reach sufficient concentrations within the host cells in vivo.

Injection of broodstock with antibiotics before leaving seawater in order to control the typical “summer outbreak” is common.

Although commercial vaccines against P. salmonis have very recently become available, there is little published information or field experience on their efficacy or economic value. However, several institutes and pharmaceutical companies have active research programs directed towards developing efficacious vaccines.

Dr. Rod Getchell works in the Aquatic Animal Health Program at the Cornell University College of Veterinary Medicine.