Author: ksubci

An old pest recently made news in November when the U.S. suspended Mexican cattle imports because a case of New World screwworm was identified in the southern part of Mexico. This pest used to be common in the southern U.S. but starting in the 1950s, the USDA, with the support of various cattlemen’s associations, worked to eradicate the fly from the U.S.; and then with partnerships with other countries, to push south through Mexico, Central America, and finally to Panama. The screwworm fly is still widespread in tropical areas of the Caribbean and South America.

A fly goes through four life stages: an adult fly lays hundreds of eggs, the eggs hatch into larvae (commonly called maggots), then they enter the pupa stage (cocoon-like form) where they grow into adult flies. For most flies, the adults lay eggs in fresh manure, decaying manure, decaying plant material, or occasionally decaying animal flesh (such as a wound with dead tissue). The screwworm is different in that the fly will lay eggs in any opening in otherwise healthy skin (such as the navel of newborn calves or a scratch or fresh wound), and the larvae (or maggots) will feed on the healthy tissue – causing severe damage or even death.

Before the fly was eradicated from the U.S., it caused millions of dollars of losses – and the damage extended beyond cattle and other livestock to any warm-blooded animal including wildlife, pets, and even people. The eradication of this parasite is an impressive success story that required years of effort and investment by the USDA and other federal agencies as well as cooperation with Mexico and other Central American and Caribbean countries. But, in order for the work and investment involved in the eradication effort to pay-off, a unique weakness in the screwworm fly lifecycle had to be exploited. Unlike other flies, a screwworm female only mates once. If she mates with a sterile male, she will never produce live offspring. Scientists at USDA used this information to begin looking for a method to sterilize male screwworms and found that a specific dose of radiation at a specific time in the pupa stage would sterilize the males while still allowing them to be able to mate.

By raising and sterilizing many millions of screwworm flies and then releasing them across the southern U.S. starting in the late 1950s, the screwworm was eradicated by 1966. However, keeping the screwworm fly out of the U.S. is a constant battle and several isolated outbreaks have occurred – most recently in 2016 in the Florida Keys. In order to reduce the likelihood of re-exposure, the U.S. has worked with Mexico and into Central America to use sterile flies to eradicate Screwworm flies all the way to a biologic barrier established in Panama.

Although the strategy to keep screwworms south of Panama has been mostly successful, the effort requires: continual release of sterile flies, restriction of animal movement, and rigorous monitoring for signs of screwworm infestation. The only current screwworm sterilization facility is in Panama and it produces about 20 million pupae per week. Because of the recent discovery of screwworms as far north as southern Mexico, the USDA and other agencies are working with Mexico and other countries in the area to re-establish an effective barrier in Panama.

Blackleg and other clostridial diseases are caused by a family of bacteria that can release large amounts of toxins that vary between the specific clostridial organisms. Different species of clostridial bacteria tend to cause disease in different ages of cattle, and the most common clostridial diseases of young calves are blackleg, clostridial enterotoxemia, and occasionally, tetanus.

Blackleg is caused by Clostridia chauvoei that enters cattle through the digestive tract and can be found in many tissues of healthy animals. The disease is created when the bacteria multiply rapidly in muscle and releases large quantities of a fatal toxin. Muscle trauma or bruising from handling, trucking, or normal animal interactions may be required for the bacteria to have a suitable environment to multiply rapidly, but the exact requirements for disease onset is not known.

Blackleg is most common in young calves up to 12 months of age, with animals more than 1 to 2 years of age rarely being affected. The disease attacks very rapidly and in many cases a dead calf is the first indication of a problem. Blackleg (like other clostridial diseases) is not considered a contagious disease, in that the disease does not pass directly from one calf to another – rather it is from soil to an animal. An outbreak may appear contagious in that a number of animals can be affected in a short period of time, usually following a soil disturbance. Any event that disturbs the soil, such as flooding, pond repair, bulldozer work, laying water pipe, etc, can initiate an outbreak of blackleg disease in pastures with heavy loads of blackleg organisms. Although vaccination is not 100% successful at protecting calves from blackleg disease (especially in young calves), vaccination will decrease the number of calves that are susceptible to the organism.

Clostridium perfringens or enterotoxemia is typically a disease of young calves. Most commonly, death following the onset of disease is so rapid that the first sign of a problem is to find a dead calf. Affected calves are typically suckling cows with high milk production and are often the healthiest, fastest-growing calves. Because the germ normally lives in the gut of healthy calves, two things must happen for the organism to multiply rapidly and produce large amounts of the toxin. First, the bacteria need an abundance of carbohydrates (which is present in milk), and secondly, the intestinal tract motility must be at least partially slowed (which occurs following a large meal). Following a large milk meal, high levels of toxin can be produced and death can occur rapidly.

Tetanus is caused by Clostridium tetani and cattle are not as susceptible as many other species – but deaths due to tetanus are occasionally seen. The organism can enter a wound and produce toxins that cause death. Many commercially available clostridial “Blackleg” vaccines do not include a component for protection against tetanus. Vaccines for cattle that provide protection against tetanus are available, and are commonly used when cattle are castrated with elastic bands as this method of castration is associated with some tetanus deaths.

Because the vaccine is fairly effective, a program where calves are initially vaccinated at 2 to 3 months of age with subsequent revaccination is recommended.