20.03.2017 change 20.03.2017

Fish can have a fever too. Researchers show why

Fever, which helps in the fight against disease, occurs not only in warm-blooded animals (such as mammals), but also in fish. And body temperature in fish depends on the ambient temperature. Researchers from the Jagiellonian University looked at the so-called behavioural fever in fish.

The Jagiellonian University reported on the research project on its website.

Fever, or body temperature increase, is a symptom of inflammation, which is a complex defence reaction of the organism in response to viral or bacterial infection. After pathogens enter our body, immune cells (leukocytes) secrete a number of agents, whose job is to stimulate the body to develop a proper immune response and eradicate the infection. Some of these agents such as cytokines - proteins that regulate the activity of cells struggling with inflammation - are also responsible for the fever. Cytokines (including TNF-α) are found not only mammals but also in other vertebrates, including fish.

Fish are ectothermic ("cold-blooded") organisms and, in most cases, their body temperature is similar to the temperature of the water in which they live. Interestingly, fish, like other ectothermic vertebrates (amphibians and reptiles), often migrate to places with higher temperature as a result of infection. This phenomenon is referred to as behavioural fever, because it leads to raised body temperature of the infected animal as a result of changes in its behaviour.

Behavioural fever in fish has been studied by Dr. Krzysztof Rakus from the Institute of Zoology, Jagiellonian University. His research - conducted in collaboration with Prof. Alain Vanderplasschen from the Belgian University of Liège - has been published in the journal "Cell Host & Microbe".

Many immune reactions are much more effective at higher body temperatures. At the same time, the multiplication of pathogens can be inhibited. Although fever is a very important process in the fight against pathogens, and has been perpetuated in the evolution of vertebrates, factors causing and inhibiting behavioural fever in fish were unknown until now.

When studying the mechanisms of behavioural fever in fish, Dr. Krzysztof Rakus and Prof. Alain Vanderplasschen used the model of infection of carp with the CyHV-3 virus (also called KHV). This pathogen causes mass mortality in carp. One important properties of this virus is that it can replicate and cause disease in carp at a temperature of 18 to 28 degrees C, but at temperatures above 30 degrees C fish do not show signs of infection.

"Current research has shown that allowing the free movement of fish between aquariums with different water temperatures (24, 28 and 32 degrees C) affects the inhibition of disease. It was found that healthy carp remained mostly in water at 24 deg. C, while the carp infected with CyHV-3 swam into the compartment with 32 deg. C temperature" - said Dr. Krzysztof Rakus.

As a result, all carp survived the infection with CyHV-3 virus, which may result in one hundred percent mortality in conditions where the fish are kept in tanks with constant water temperature at 24 deg. C.

During research on behavioural fever researchers also noted that the infected carp started to swim to the warmest aquarium quite late - when most of the other symptoms of the disease were already clearly visible. The carp, in which behavioural fever was observed, were already very sick.

Researchers began to wonder whether this virus could affect the behaviour of fish and delay the development of therapeutic behavioural fever. They looked at the virus genome. It turns out that it contains a gene, by means of which specific fish cytokines (TNF-α) are trapped. Researchers have produced a modified virus - without this gene. It turned out that carp infected with that virus migrated to the warmest aquarium earlier than carp infected with a virus containing the studied gene. It has been shown that the virus in fact affects the behaviour of the host and delays the development of behavioural fever.

The study published in "Cell Host & Microbe" not only indicates firmly established evolutionary mechanisms of the development of fever in fish and mammals, but also for the first time describes how a virus, through its single protein, modifies the behaviour of its host, delaying the development of behavioural fever. It is an extremely important ability for the virus, because the infected fish stay longer in a temperature that enables its replication, which allows the virus to more effectively spread in the carp population, moving from one fish to another.

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