Maggots in the brains of circling sheep not a myth


Sheep bound for export in Lamu County. Animals that show nervous signs should be slaughtered and inspected for maggots in the brain.

Photo credit: File | Nation Media Group

 Kimani from Kajiado County was excited when he stocked his farm for the first time with a flock of 25 Dorper sheep two months ago.

He had always wanted to keep some livestock but lacked land.

Kimani watched his animals tour his close to 20 acres he had bought over time in separate adjacent pieces.

The farm was fenced well with barbed wire and the runners were close enough to prevent even the smallest of the lambs from sneaking out.

The sheep appeared to be enjoying a self-tour of the farm.

When livestock are introduced to a new environment, they first walk or run around nipping at vegetation and scanning the area.

It is an in-built security behaviour for the animals to familiarise themselves with their new environment.

Territorial animals like male dogs, on the other hand, inspect the area and squirt jets of urine on physical objects.

I recall a farmer once asking me why dogs never empty their bladder fully.

The dog behaviour is a way of marking the area as its own territory, just the way we put beacons on our land.

Kimani’s Dorper sheep did well in their new environment until the end of the first week after arrival.

The worried farmer called and said one of his sheep was behaving in a way to suggest it was mad.

The animal would stop eating suddenly, vocaliSe and then walk with its head held up.

The situation deteriorated to the extent of the sheep sometimes walking in circles or circling at one spot.

Kimani wanted to know if the animal had rabies.

I told Kimani I did not think the sheep had rabies but definitely, something was amiss with the animal’s brain.

I explained to him the many reasons a sheep could behave like that.

There was need for a veterinary paraprofessional or paravet to visit his farm, examine the sheep and share the findings with me for further action.

He called Jane, a paravet from the nearby Isinya town.

Jane confirmed to me that all the vital parameters of the sheep were normal.

These are the body temperature, breathing and heart rates, movement of the stomach and blood flow.  Clinically, we determine there is normal blood flow when mucous membranes of the gums, eyes and vulva are a healthy pink colour.

We agreed with Jane that the most probable cause of the animal’s behaviour was fly maggots trapped in its brain.

I advised Kimani to sell the sheep for slaughter on condition that the head would be opened and inspected for maggots.

If maggots were confirmed at meat inspection, then the meat would be fit for human consumption and Kimani would salvage the cost of his investment in the sheep.

Sure enough, maggots were found nested in the brain of the sheep after the animal was slaughtered. Kimani was surprised and kept asking how the creatures find their way to the brain.

We had animals on our farm while growing up and always used to be told that every sheep has a big maggot in the brain.

It was said that if the animal did not sneeze out the worm, it would become mad and die.

We were advised to cover our noses when the sheep sneezed, otherwise the worms could get into our brains through the noses. That was scary.

Many years later when I studied parasitology in my Veterinary Medicine course, I learnt that there was some inkling of truth in the myth of the sheep-brain worm. But the science of the worm is a well-understood veterinary medical issue.

The worm is caused by the immature stages or larvae of the sheep nasal bot fly, scientifically called Oestrus ovis.

The adult fly is widely distributed throughout the planet but is more common in tropical countries.

Adult flies lay eggs around the nostrils of the sheep.

Mucous secretions

They hatch and the larvae crawl into the nostrils where they feed on the mucous secretions.

As the larvae grow older, they go deeper into the nasal cavity and securely lodge in the spaces of the nasal bones for weeks or months before migrating back into the nasal cavity.

In normal cycles, the larvae irritate the nostrils as they move from the nasal bones and cause the sheep to sneeze them out.

Outside the body, they form the pupa which matures into the adult fly, to begin a new cycle.

In rare cases, the immature worms miss direction and proceed through the nasal canal all the way to the brain.

Such larvae fail to mature. They cause damage to the brain, resulting in the neurological signs seen in Kimani’s sheep.

The most common signs are depression, staggering, circling, aimless wandering and bloody mucous discharge.

Some sheep lift their heads high, become blind or press their heads against objects.

The disease is prevented through regular washing of sheep with insecticides that remain for long periods on the skin of the animal, such as synthetic pyrethrins.

Sheep in affected flocks or those obtained from flocks with no known records of treatment should be treated with ivermectin injections.

Animals that show nervous signs should be slaughtered and inspected for the maggots in the brain.  It should be noted that the  damage done to the brain cannot be reversed. Jane treated the remaining sheep in Kimani’s herd with ivermectin injections and advised weekly washing with synthetic pyrethrins.

No other sheep on the farm had shown signs of the maggots in the brain by the time of publishing this article.