Vertical Transmission of Pathogens

Pathogenic microbes – both viral and bacterial – can be spread in a number of ways. Horizontal transmission refers to the spread between community members, while vertical transmission involves the passing of a pathogen between mother and offspring. In people, that involves microbes that infect the fetus in utero or during childbirth. Chickens, on the other hand, can spread disease between flock mates or from the hen to her chicks via the egg, which is referred to as transovarian infection.

The goal of most chicken keepers is to keep our birds healthy and we’re often on the lookout for signs of illness, concerned about things that can spread through our flock. Some of us breed birds, or hatch out chicks with eggs from our own, or other, hens. Some pathogens only spread through birds post-hatch, but there are a few that can affect embryos.

I am curious as to how roosters might transmit pathogens to their offspring via infected semen. Unfortunately I found very little on what is an interested subject. Campylobacter, E.coli, Mycoplasmas, Salmonella, Staphylococcus and Streptococcus have all been found in chicken semen, but I found no information about vertical transmission involving roosters.

This post is not intended to be comprehensive, rather a heads up if you’ve had any of these pathogens in your flock to be cautious about breeding affected birds as their offspring may become inadvertently infected.

Here’s a synopsis of some pathogens to be aware of:

Avian Encephalomyelitis (AE) is a viral infection of the central nervous system affecting various species of poultry: chickens, turkeys, Japanese quails and pheasants.

The disease most often affects chicks up to the age of six weeks. Symptoms usually appear at 7-10 days, although they may be present at hatching or delayed for several weeks. After 6-weeks of age, chicks are usually protected due to their increased immune system response. In adults, symptoms are usually milder.

Avian Leukosis refers to several leukemia-like diseases caused by the Avian Leukosis Virus (ALV). Lymphoid Leukosis is the most common type of cancer and occurs in chickens four months of age or older. Tumors often develop in the liver, spleen, and bursa of Fabricius and less commonly, the kidney, lung, gonad, heart, mesentery, and bone marrow.

Campylobacter jejuni is a bacterium found in chickens that usually doesn’t cause illness in them. However, it’s a zoonotic disease that can spread to people via live birds or contaminated meat. You don’t hear much about it, but it’s one of the leading causes of food poisoning in people from raw or under cooked poultry. Another form of the bacteria, Campylobacter hepaticus, is associated with Spotty Liver Disease in chickens.

Infectious Anemia develops when a chicken is experiencing external or internal blood loss, or their blood lacks enough healthy red blood cells. Blood requires hemoglobin for transporting oxygen and carbon dioxide within the body’s tissues. Chickens that are anemic do not have the ability to replace the cells that die out as quickly as normal.

Infection of young birds results in anemia, decreased weight gain and increased mortality. Chicks immune systems can become compromised, making them vulnerable to secondary infections.

Mycoplasma are tiny bacteria that cause compromised immune systems and respiratory disease in their affected hosts. They’re unique in the world of bacteria because they lack a cell wall, which makes them resistant to many common antibiotics that work by targeting cell wall synthesis. The Mycoplasma family includes more than 100 types, each infecting a specific animal species: 17 species are found in poultry; two of which cause disease in chickens.

Mycoplasma gallisepticum (MG) is primarily a respiratory infection and is becoming more common with the increased popularity of backyard flocks. It often occurs in chickens that are co-infected with other pathogens (i.e. E. coli or Infectious Bronchitis); birds that are stressed (e.g. changes in the pecking order, integrating into a new flock); nutritionally deficient; or live in coops with high levels of ammonia or dust. Chickens can become infected several days after exposure, but remain asymptomatic for months.

Mycoplasma synoviae (MS) was historically seen as a disease which affected the synovial membrane of joints and tendons, but is increasingly affecting multi-age flocks, presenting as an upper respiratory tract illness. Birds can be infected and appear healthy until stressed, which then triggers systemic symptoms.

Newcastle Disease (ND) is a highly contagious viral disease that affects both domestic and wild birds worldwide. NDV strains were originally classified into three groups depending on their virulence.

Different strains affect the nervous, respiratory, or digestive systems. The most common signs are related to central nervous system damage: loss of coordination,  stumbling, abnormal gait, wryneck, and paralysis.

The severity of symptoms varies and is dependent on the strain of the virus, the age of the chicken (young chicks are more vulnerable), co-infection with other pathogens, stress and immune status.

Salmonella pullorum and Salmonella gallinarum are bacterial diseases that can be carried in various species of birds, but is most often seen in chickens. It can be passed from rodent droppings, flies, fecal contaminated feeders and waterers, inhaling dust and dander. Hens can pass it to their chicks as the egg is being formed and those chicks, when hatched, can spread it through their droppings to their hatch mates.

In chicks, the symptoms are lethargy, white diarrhea, pasty vents and high mortality. Adults can often carry a small bacterial load without symptoms. Some may have watery diarrhea, a drop in egg production, weight loss, increased thirst, decreased food intake and a bluish comb. On post-mortem damage to the liver, spleen, kidney, heart and ovaries are common.

Increasing the overall vigour of our flocks will have a direct impact on the outcomes of their offspring: healthy parents produce robust chicks.

Credits: Merck Veterinary Manual; NCBI; The Poultry Site; Research Gate.