• Diagnosis
• Infection route
• Approach
• Treatment
• Prevention

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Susceptible chickens (and also turkeys) that come into contact with a pathogenic reovirus at a young age can develop tenosynovitis. Despite such early infection, the tenosynovitis often only becomes visible at 4 weeks of age or later. The chicks become lame, lose their appetite, lag behind in growth, and losses may be increased due to death or selection. Furthermore, tenosynovitis will result in increased condemnation of (parts of) the chicken upon slaughter.

On flock level the severity of the disease varies from a few affected animals to a strongly reduced uniformity, 100 to 200 grams of retarded growth at slaughter age, and strongly increased condemnations. The damage depends on:

• the pathogenic capacity of the reovirus strain in question;
• the animals’ age upon infection;
• the degree of protection due to the presence of maternally derived antibodies.

Tenosynovitis caused by reovirus infections was commonly seen in the 1970s. However, the number of cases was strongly reduced following development of a commercial vaccine for parent stock. In spite of this it has remained an important disease in broilers and the incidence of reoviral disease varies from year to year. The current situation in the Netherlands can always be found in the most recent monitoring report.

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Diagnosis

A presumed diagnosis of tenosynovitis caused by reovirus is often based on clinical signs. Tenosynovitis is generally only visible from the age of 4 weeks, but can be found as early as the second week of life. The chicks are lame and the gastrocnemius (hock) tendon is swollen; in severe cases it is thicker than the underlying bone. This will eventually affect the uniformity of the flock.

Photo: Leg of a broiler with tenosynovitis due to reovirus. The cross-section shows oedema of the tendon sheath and the surrounding tissue.

Reoviruses are extremely common in broilers, whereby up to 100 percent of the flocks can test positive over the course of the production round. However, the large majority of these reoviruses are not capable of causing tenosynovitis. Simply identifying the presence of reovirus is not sufficient to diagnose tenosynovitis as the result of reovirus, as some of the non-pathogenic reoviruses may also be found in the tendon sheath. The presence of the virus is therefore no guarantee that the problems are caused by reovirus. The same applies to the presence of antibodies to the reovirus.

The diagnosis of a pathogenic reovirus as the cause of tenosynovitis is made on the basis of the combination of two tests:

1. identification of reovirus in the tendon sheath, for example using a PCR test;
2. identification of viral inflammation symptoms in the tendon or tendon sheath by means of histological testing.

Tenosynovitis caused by reovirus must be distinguished from tenosynovitis caused by other viruses such as ANV3, or by bacteria such as Mycoplasma synoviae, E. coli or staphylococci. The gastrocnemius tendon may also become swollen due to trauma.

Genotyping

Once a reovirus has been identified in the tendon sheath, genotyping of the virus can take place. GD keeps track of the prevalence of the different genotypes for the Dutch animal disease monitoring. In the event of unusual developments, such as an increase in the number of reovirus cases, we report the results of the genotyping in the monitoring report, including our conclusions.

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Infection route

Transmission from parent stock to offspring (vertical infection)

Reovirus can be transferred from parent stock to offspring. This infection route is currently of limited importance in the Netherlands. During the production period, reovirus infections (both pathogenic and non-pathogenic) only occur very occasionally in parent flocks. Under experimental conditions, the virus was only transmitted to offspring within a short window of time (a few weeks) although this may be slightly longer under field conditions. It is often difficult to prove whether a pathogenic reovirus is actually transmitted from the parents. By the time tenosynovitis becomes visible in the offspring, the virus can often no longer be identified in the parent flock.

Transmission through introduction (horinzontal infection)

Transmission from flock to flock, between or within farms, is probably more common than vertical transmision. Reovirus can survive at least one day on cotton (galvanised or enamelled) metal, plastic, glass and rubber (boots), and can survive a number of weeks in manure or water. There have been no studies of how long a barn environment can remain infected. However, when considering the high prevalence of (often apathogenic) reovirus among broilers, it seems likely that reovirus remains present round after round.

Reovirus is a virus without an envelope, i.e. without an outermost mainly lipid layer. This is also referred to as a naked virus. Naked viruses cannot be killed by detergents and their susceptibility to disinfectants is limited, making it easier for them to survive cleaning and disinfecting, as well as persisting in an empty barn (see approach). Since the viruses survive more easily in the environment, they can be transferred more easily via all kinds of objects (clothing, equipment, etc.). Good farm hygiene is important to limit the introduction of reovirus. Effective cleaning and disinfecting once the barn is empty, in combination with good hygiene, can strongly reduce the risk of infection of a new flock, while also increasing the average age at which the animals come into contact with the virus. This is important to reduce clinical problems.

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Approach

Treatment options for reovirus infections are limited, with the emphasis lying on preventing vertical and horizontal infection in the next flock. Effective cleaning and disinfecting once the barn is empty, in combination with strict hygiene during the production round, plays an important role in tackling reovirus. Moreover, vaccination of the parent stock will provide the chicks with good inherited ‘maternal immunity’. These are antibodies which they receive from their mother via the egg. Such maternally derived antibodies can protect the chicks during the first period of life.

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Treatment 

There is no medication available for commercial poultry to cure chicks with tenosynovitis caused by reovirus. In severe cases, painkillers can be administered to reduce the clinical signs. Natural remedies, such as willow bark, are sometimes used as an alternative, but there is often no scientific evidence of their effectiveness.

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Prevention

Following an outbreak of reovirus tenosynovitis, it is important to take measures to protect the next flock. While it is difficult to eradicate reovirus from the barn entirely, effective cleaning and disinfecting can prevent early infections.

With regard to disinfecting following a reovirus infection, attention should be paid to the choice of disinfectant and the concentration in which it is applied as well as the cleaning protocol. All organic material must first be thoroughly removed, so that disinfectants can effectively tackle the virus.

The susceptibility of reovirus to disinfectants has only been studied under laboratory conditions. Reoviruses are susceptible to:

• glutaraldehyde, whether or not combined with quaternary ammonium compounds or chlorides;
• hydrogen peroxide, whether or not combined with acetic acid;
• formaldehyde (liquid and gas), whether or not combined with glutaraldehyde and chlorides;
• alcohol and iodine are also effective.

In practice, it should be taken into account that disinfectants may be less active at lower concentrations, shorter contact times, lower temperatures and in the presence of organic material such as manure. It is therefore prudent to check the effectiveness of cleaning and disinfecting. This can be done using the VIR check, developed specifically to assess the effects of cleaning and disinfecting on naked viruses.

Besides preventing early infections, it is important that newly introduced chicks receive optimal amounts of maternally derived antibodies. The vaccination of parent stock can result in high titres. Serological testing at GD can determine the values of the generated titres. Research into antibodies in parent stock has shown that the level of antibodies in the blood, and therefore in the eggs, varies greatly. Alternatively, vaccination of day-old chicks against reovirus is also possible. However, the vaccine must be administered by injection. Using such vaccines in broilers is costly and not common in the Netherlands.

 

Figure. Serological reactions four weeks after vaccination of SPF layers with a living (priming) and an inactivated (booster) reovirus vaccine at 9 and 13 weeks of age, respectively.