By: Dr. Milad Ibrahim Areebi - Iraq
Many veterinarians are asking about Adenovirus, and in response, I will answer several questions about this topic, even if in a simple way to help everyone understand what Adenovirus is from my perspective.
How does Adenovirus spread?
Adenovirus can be transmitted to the embryo via the egg, and antibodies (Ab) can also be transferred. The virus remains dormant in chicks (broilers) until the maternal antibodies are cleared from their bodies, at which point the conditions that lead to its appearance, such as poisoning or immune-suppressing diseases, may cause it to manifest.
It is important to note that the presence of antibodies against Adenovirus cannot be detected by the ELISA test, because it is impossible to determine whether these antibodies are a result of infection or vaccination. Therefore, to diagnose Adenovirus in chicks, we must use both the ELISA and PCR tests. The ELISA test helps us determine the level of antibodies and the duration of their presence in the bird, enabling us to design a vaccination program for Adenovirus, regardless of whether the antibodies resulted from infection or maternal vaccination. The PCR test, on the other hand, helps diagnose whether the virus is present in the chicks, and if it is, it helps determine the viral load (CT value). We must also remember that the CT value is a good indicator of the virus's strength in the chicks.
Summary
When diagnosing Adenovirus in chicks, regardless of whether we know the vaccination status of the breeders and their vaccination program, we should:
Examine the chicks using the ELISA test to determine the level of maternal antibodies (Ab), and design a vaccination program for the chicks.
Test the chicks using PCR to determine the virus's CT value (virus load), which will help us accurately confirm whether the virus is present in the chicks.
Adenovirus Can Be Transmitted from Ducks to Chickens
There have been recorded cases of Adenovirus transmission from pigeons and sparrows to chickens. Therefore, I do not rule out the possibility of Adenovirus transmission from pigeons to chickens in facilities with poor biosecurity. As a result, we may wrongly blame egg and hatchery owners for spreading this disease, while in reality, they are not responsible for this occurrence.
Furthermore, based on my observations, wild birds may act as carriers of the virus without being infected, due to their consumption of poultry waste used by many farmers as fertilizer for agricultural land after being removed from poultry houses.
The maternal antibodies transferred to the embryo will not protect it from other types of Adenovirus. For example, if the mother was vaccinated against serotype FAdv8, the maternal antibodies will not protect the chick from an infection with serotype FAdv4, and so on for other types. This means the infection will give us antibodies against the specific serotype that infected the mother during the breeding period.
In my field observations, I have noticed some Adenovirus infections in chicks older than 24 days. The infection is typical in birds under stress, but the incidence is very low and not impactful. For example, in a poultry house with 6,000 birds, the daily incidence may be around 6 birds per day, with very low mortality. Upon tracing back to the breeder source, it was found that the breeder flock was infected with Adenovirus, and the antibody levels transferred to the chicks were over 8,000 titers as measured by ELISA. Therefore, the chicks had high levels of maternal antibodies, and the Adenovirus remained in a latent phase, unable to overcome the antibodies.
However, if the flock is exposed to diseases that cause immunosuppression, mycotoxins, or stress, the antibody titer will decrease, allowing the virus to activate and cause a severe infection.
Adenovirus can also be transmitted through the eggshell if infected birds lay in the same nest. It can also spread through feces or feathers contaminated with feces. After hatching, the virus can be transmitted through direct contact, ingestion, or inhalation, which is considered horizontal transmission. Therefore, eggs must be thoroughly disinfected before entering the incubators.
Adenovirus can be isolated on the first day of infection, but viral shedding peaks in the third week. When maternal antibodies (MDA) decline, shedding peaks between the fourth and sixth weeks, and can continue up to 14 weeks after infection.
I have read many reliable studies on Adenovirus infection and found that most cases are subclinical due to two important reasons: the presence of maternal antibodies and low virulence of some serotypes. These two factors are evidence that the virus exists but does not cause visible disease. Therefore, most experts agree that the majority of chicks tested carry Adenovirus.
In normal conditions, there are no issues with FAdv infections when maternal antibodies (MDA) are high. However, as the titer decreases, the virus may transition from its latent state to an active one, initiating primary replication, which could lead to cell-free viremia. This is a characteristic of Adenovirus, allowing the virus to spread throughout the body's organs during the replication stages, except for two organs:
The central nervous system (CNS) and the testes.
However, scientists have concluded that Adenovirus can be present in semen.
The primary sites of viral replication are the intestines and liver. Recent studies, based on samples sent for PCR testing, showed that the highest viral load (CT value) was found in the liver, cecal tonsils, and bursa. The CT value in these organs reached below 12.
The severity and strength of Adenovirus increase during peak production, particularly when the breeder birds are between 31 and 39 weeks old, due to increased steroid hormones and stress, which will trigger the virus to shift from latency to activation. This results in the virus being transmitted from the breeders to the embryos during the hormone increase and stress period, leading to immunosuppression and viral activation.
Lastly, detecting and diagnosing Adenovirus using PCR does not necessarily indicate clinical disease in the facility, because this virus can be isolated and diagnosed in healthy, non-infected chickens. It is naturally present in the environment where the chickens live, which is an important point for laboratory owners to consider.
